AB2 Virology Handbook 2024 PDF

2024 Animal Body 2 VIROLOGY PAUL DIGARD, ELEANOR GAUNT, RUTE PINTO, CHRISTINE TAIT- BURKARD, NEIL MABBOTT & FINN GREY Contents LECTURE 1: Virus classification............................................................................................................... 3 WHAT IS A VIRUS?............................................................................................................................... 3 SOME PERTINENT VETERINARY VIRUS EXAMPLES.............................................................................. 4 LECTURES 2 AND 3: Virus replication I & II........................................................................................... 10 CELL ENTRY........................................................................................................................................ 10 BALTIMORE CLASSIFICATION............................................................................................................ 11 TRANSCRIPTION AND REPLICATION.................................................................................................. 12 VIRUS ASSEMBLY AND RELEASE........................................................................................................ 12 LECTURE 4: Virus-host interactions I..................................................................................................... 16 CYTOPATHIC EFFECT......................................................................................................................... 16 MECHANISMS TO INDUCE CPE......................................................................................................... 16 CPE AS A MEANS TO TURN THE CELL INTO A VIRUS DELIVERY VEHICLE.......................................... 20 CPE AS A CELLULAR EFFORT TO COMMIT SUICIDE........................................................................... 23 LECTURE 5: Virus detection................................................................................................................... 25 VIRUS QUANTIFICATION................................................................................................................... 25 IMPORTANCE OF DIAGNOSTICS........................................................................................................ 29 LECTURE 6: Virus infection and pathogenesis I.................................................................................... 31 INFECTION ROUTES........................................................................................................................... 31 SYSTEMIC SPREAD............................................................................................................................. 35 LECTURE 7: Immune responses to virus infections............................................................................... 40 INNATE VERSUS ADAPTIVE RESPONSES............................................................................................ 40 VACCINE INDUCED IMMUNITY......................................................................................................... 46 BATS.................................................................................................................................................. 47 LECTURE 8: Viral pathogenesis II.......................................................................................................... 49 IMMUNE-MEDIATED VIRAL DISEASE................................................................................................ 49 IMMUNE TOLERANCE....................................................................................................................... 51 VIRULENCE........................................................................................................................................ 52 INFLUENZA A VIRUS HAEMAGGLUTININ AS A VIRULENCE PARADIGM............................................ 54 LECTURE 9: Mechanisms of virus persistence I..................................................................................... 57 VIRUS PERSISTENCE.......................................................................................................................... 57 THE BASIC REPRODUCTION RATIO.................................................................................................... 57 THE DIFFERENT TYPES OF PERSISTENCE........................................................................................... 58 LECTURE 10: Virus persistence II........................................................................................................... 63 CRITICAL COMMUNITY SIZE.............................................................................................................. 63 1 WITHIN HOST PERSISTENCE.............................................................................................................. 64 LECTURE 11: Viruses and cancer........................................................................................................... 72 VIRUSES AND CANCER...................................................................................................................... 72 RETROVIRUSES.................................................................................................................................. 73 RETROVIRUS ONCOGENESIS............................................................................................................. 75 ENDOGENOUS RETROVIRUSES......................................................................................................... 78 LECTURE 12: Prions and prion diseases................................................................................................ 79 PRION CHARACTERISTICS.................................................................................................................. 79 THE PRION HYPOTHESIS: TRANSMISSION......................................................................................... 80 PRION PATHOGENESIS...................................................................................................................... 80 PRION SPREAD.................................................................................................................................. 81 EXAMPLES OF PRION DISEASES........................................................................................................ 83 2 LECTURE 1: Virus classification LEARNING OBJECTIVES: 1. Understand what differentiates a virus from other organisms 2. Describe the basic components of a virus 3. Outline the classification of viruses based on structure WHAT IS A VIRUS? A virus is a microscopic infectious agent that can only replicate inside the cells of living organisms. It consists of genetic material, either DNA or RNA, enclosed in a protein coat called a capsid. Some viruses also have an outer envelope made of lipids. Viruses are considered obligate intracellular parasites because they cannot carry out metabolic processes or reproduce on their own. Instead, they infect a host cell and hijack its cellular machinery to replicate and produce new virus particles. This process often damages or destroys the host cell in the course of viral replication. Viruses are incredibly diverse and can infect all forms of life, including animals, plants, fungi, and bacteria. What makes viruses unique? Here are some key ways they differ from other organisms: 1. Non-cellular structure: Unlike bacteria, fungi, plants, and animals, viruses are not composed of cells. Instead, they consist of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. 2. Obligate intracellular parasites: Viruses cannot replicate on their own. They must invade a host cell and hijack its cellular machinery to replicate and produce more virus particles. 3. Genetic material: While most organisms have either DNA or RNA as their genetic material, viruses can have either. This genetic material can be single-stranded or double-stranded, linear or circular. 4. Size: Viruses are typically much smaller than bacteria, ranging from 20 to 300 nanometers in diameter. 5. Mode of reproduction: Virus replication involves injecting their genetic material into a host cell, hijacking the cell's machinery to replicate the viral components, and assembling new virus particles. How do viruses reproduce? The virus replication cycle involves several stages, each crucial for the virus to successfully infect a host cell, replicate its genetic material, and produce new virus particles. The eclipse phase is a significant stage in this cycle. All viruses follow this general pathway of replication: 1. Attachment: The virus attaches to specific receptor molecules on the surface of a host cell. This interaction is often highly specific and determines the host range of the virus. 2. Entry: The virus gains entry into the host cell, either through direct fusion with the cell membrane or by being engulfed into a vesicle through endocytosis. 3. Uncoating: Once inside the host cell, the virus must release its genetic material from its protective protein coat (capsid). 4. Eclipse phase: This phase marks the period during which the virus's genetic material is replicated and viral components are synthesized within the host cell. However, during this phase, new virus particles are not yet assembled or released from the host cell. 3 5. Replication and transcription: The virus utilizes the host cell's machinery to replicate its genetic material (DNA or RNA) and produce viral proteins. This often involves hijacking the cell's enzymes, ribosomes, and other cellular components. 6. Assembly: Newly synthesized viral components, including genetic material and proteins, are assembled into new virus particles (‘virions’). 7. Release: Newly formed virions are released from the host cell. This can occur through various mechanisms, including cell lysis (bursting of the cell) or budding (where the virus particles are released gradually without killing the cell). After release, the newly formed virus particles can then infect other cells and initiate new replication cycles. To envelope or not? A virus envelope is a membranous outer layer that surrounds the capsid of certain viruses. Not all viruses have an envelope. The envelope is typically derived from the host cell's membrane during the process of viral replication or budding. Key features of a virus envelope include: 1. Lipid bilayer: The envelope is composed of a lipid bilayer, similar to the lipid membranes found in cells. This lipid bilayer is often derived from the host cell's plasma membrane. 2. Embedded proteins: Proteins, including viral glycoproteins, are embedded within the lipid bilayer of the envelope. These proteins play various roles in viral attachment and entry. 3. Vulnerability: Despite its protective role, the envelope also makes enveloped viruses more vulnerable to certain environmental conditions, such as exposure to heat or drying. This vulnerability can affect the stability and infectivity of the virus particles. Examples of enveloped viruses include the influenza virus and coronaviruses (including SARS-CoV-2, the virus responsible for COVID-19). SOME PERTINENT VETERINARY VIRUS EXAMPLES Ovine herpesvirus 2 Ovine herpesvirus 2 (OvHV-2) primarily affects sheep. It is a member of the Herpesviridae family. OvHV-2 is closely related to Alcelaphine herpesvirus 1 (AlHV-1), which primarily infects wildebeest. Ovine herpesvirus 2 is causes malignant catarrhal fever (MCF) in sheep. MCF is a severe and often fatal viral disease that primarily affects domestic and wild ruminants, including sheep, cattle, deer, and certain species of antelope. The disease is characterized by fever, respiratory signs, ocular and nasal discharge, oral lesions, and sometimes neurological symptoms. OvHV-2 is typically carried asymptomatically by healthy carrier sheep. Transmission to susceptible animals occurs through close contact with carrier sheep or exposure to their bodily fluids. While carrier sheep may not show signs of disease, when susceptible species are infected, MCF can be devastating, leading to significant economic losses in affected livestock populations. Orf virus Orf virus, also known as contagious ecthyma or contagious pustular dermatitis, is a parapoxvirus that primarily affects sheep and goats. It can also infect other ruminants and occasionally humans. Orf virus causes a contagious skin disease characterized by the formation of lesions, typically on the lips, muzzle, face, and occasionally on the udders and legs of affected animals. Here's what Orf virus does: 1. Lesion formation: Orf virus causes the formation of characteristic lesions on the skin of infected animals. These lesions eventually crust over and form scabs. The lesions can be painful and may cause discomfort to the animal, leading to decreased feed intake and weight loss. 4 2. Contagiousness: Orf virus is highly contagious among susceptible animals. It can spread through direct contact with infected animals or their contaminated environments. The virus can persist in the environment for a considerable period, facilitating transmission between animals sharing the same living spaces. 3. Zoonotic potential: While Orf virus primarily affects animals, it can also infect humans. In humans, the disease is usually self-limiting and presents as localized lesions on the skin, similar to those seen in animals. 4. Economic impact: Orf virus can have significant economic implications for sheep and goat farming operations due to decreased productivity, treatment costs, and potential trade restrictions imposed to control its spread. Management of Orf virus typically involves implementing strict biosecurity measures to prevent the introduction and spread of the virus within animal populations. Vaccination is also available in some regions including the UK. Lumpy skin disease virus (LSDV) LSDV is a member of the Capripoxvirus genus, which primarily affects cattle. The disease caused by LSDV is known as lumpy skin disease (LSD). Here's what LSDV does: 1. Skin lesions: The hallmark feature of lumpy skin disease is the development of firm, raised nodules or lumps on the skin of affected cattle. These lesions can vary in size and may appear anywhere on the body, including the head, neck, limbs, and trunk. Over time, the nodules may ulcerate, leading to the formation of scabs and crusts. 2. Fever: Infected cattle often develop a fever as a result of the viral infection. 3. Reduced milk production: Lumpy skin disease can lead to a temporary decrease in milk production in dairy cattle. 4. Secondary infections: The skin lesions caused by LSDV can predispose affected cattle to secondary bacterial infections. 5. Transmission: LSDV is primarily transmitted between cattle through direct contact with infected animals or contaminated environments. Insect vectors, such as certain species of biting flies and mosquitoes, can also play a role in the transmission of the virus. 6. Trade restrictions: Lumpy skin disease is considered a significant transboundary animal disease (TAD) and can have trade implications for countries affected by outbreaks. 7. Control measures: Control of lumpy skin disease typically involves implementing strict biosecurity measures to prevent the introduction and spread of the virus within cattle populations. Vaccination is also used as a preventive measure in regions where the disease is endemic or during outbreaks to limit its impact. Lumpy skin disease is not typically fatal in adult cattle, but it can cause significant economic losses due to reduced productivity, trade restrictions, and the costs associated with disease management and control. Avian influenza Avian influenza virus (AIV), commonly known as ‘bird flu’, can cause a range of symptoms and outcomes in chickens. Here's what avian influenza virus can do in chickens: 1. Respiratory symptoms: Avian influenza virus commonly affects the respiratory system of chickens, leading to symptoms such as coughing, sneezing, nasal discharge, and difficulty breathing. 2. Drop in egg production: Infected laying hens may experience a drop in egg production or produce eggs with abnormal shells. 3. Decreased feed intake and weight loss: Infected chickens may exhibit reduced appetite, leading to decreased feed intake and weight loss. 5 4. Morbidity and mortality: The severity of avian influenza virus infection can vary widely, ranging from mild or subclinical cases to severe outbreaks with high morbidity and mortality rates. Certain strains of AIV, such as highly pathogenic avian influenza (HPAI) viruses, can cause severe disease with high mortality rates in chickens. 5. Neurological signs: In some cases, avian influenza virus infection may lead to neurological signs in chickens, such as tremors, paralysis, or other abnormalities of movement. These signs are more commonly associated with highly pathogenic strains of AIV. 6. Transmission to humans: While avian influenza viruses primarily circulate among birds, some strains have the potential to infect humans. In humans, avian influenza infection can range from mild respiratory illness to severe pneumonia and, in some cases, death. Control and prevention of avian influenza virus in poultry populations typically involve implementing strict biosecurity measures to prevent the introduction and spread of the virus, as well as surveillance, early detection, and rapid response to outbreaks. Vaccination may also be used in some cases as a preventive measure in regions where the disease is endemic or during outbreaks to limit its spread. Rinderpest virus Rinderpest circulated in Africa and Asia in cattle and other cloven-hoofed animals, such as buffalo and yak, but was the first virus to be eradicated through vaccination efforts in a non-human animal. Here's what rinderpest virus does: 1. Clinical signs: Rinderpest virus infection typically results in a range of clinical signs, including high fever, nasal and ocular discharge, oral lesions, diarrhoea, and dehydration. 2. High mortality: Rinderpest is known for its high mortality rates, especially in susceptible populations of cattle and wildlife. In severe outbreaks, mortality rates can reach up to 90% in affected herds. 3. Transmission: Rinderpest virus is highly contagious and spreads rapidly between susceptible animals through direct contact with infected individuals or their bodily fluids, as well as through contaminated feed, water, and fomites (inanimate objects). 4. Impact on livelihoods: Rinderpest has historically had a profound impact on agriculture and food security, particularly in regions where cattle rearing is a primary livelihood. Outbreaks of rinderpest have led to widespread famine and social upheaval in affected areas. 5. Eradication efforts: Rinderpest was one of the most devastating animal diseases throughout history. However, concerted international efforts led to the successful eradication of rinderpest, with the last known case occurring in 2003. The eradication of rinderpest represents one of the most significant achievements in veterinary medicine and public health. 6. Prevention and control: Prevention and control of rinderpest primarily involves vaccination of susceptible animals, strict quarantine measures, surveillance, and rapid response to outbreaks. Following the successful eradication of rinderpest, continued vigilance and maintenance of surveillance systems are essential to prevent the reintroduction of the virus. Newcastle disease virus Newcastle disease virus (NDV) is a highly contagious viral pathogen that primarily affects poultry. Here's what Newcastle disease virus does: 1. Respiratory and nervous system symptoms: Newcastle disease virus can cause a range of symptoms in infected birds, including respiratory signs such as coughing, sneezing, nasal discharge, and difficulty breathing. In severe cases, the virus can also affect the nervous system, leading to neurological signs such as tremors, paralysis, twisting of the neck (torticollis), and circling behaviour. 2. Decreased egg production: In laying hens, Newcastle disease virus infection can result in a temporary decrease in egg production, as well as the production of eggs with abnormal shells. 6 3. Morbidity and mortality: The severity of Newcastle disease can vary depending on factors such as the strain of the virus, and the age and species of the affected birds. In severe outbreaks, Newcastle disease can cause high morbidity and mortality rates in affected poultry flocks. 4. Transmission: Newcastle disease virus is highly contagious and spreads rapidly between birds through direct contact with infected individuals, as well as through contaminated feed, water, equipment, and fomites. 5. Zoonotic potential: While Newcastle disease primarily affects birds, some strains of the virus have the potential to infect humans. In humans, Newcastle disease infection typically causes mild conjunctivitis or flu-like symptoms. 6. Prevention and control: Prevention and control of Newcastle disease primarily involve vaccination of susceptible birds, strict biosecurity measures to prevent the introduction and spread of the virus, surveillance, and rapid response to outbreaks. Canine distemper virus Canine distemper virus (CDV) is a highly contagious viral pathogen that affects a wide range of domestic and wild carnivores, particularly dogs. Here's what canine distemper virus does: 1. Respiratory signs: Canine distemper virus commonly causes respiratory signs in infected dogs, including coughing, sneezing, nasal discharge, and difficulty breathing. 2. Gastrointestinal signs: Canine distemper virus can also cause gastrointestinal signs, such as vomiting, diarrhoea, and decreased appetite. 3. Neurological signs: Canine distemper virus has a predilection for the central nervous system, leading to a range of neurological signs including tremors, seizures, muscle twitching, weakness, incoordination, and paralysis. In severe cases, dogs may develop progressive neurological deterioration and ultimately succumb to the disease. 4. Ocular signs: Canine distemper virus can also affect the eyes, leading to conjunctivitis, discharge from the eyes, redness, and inflammation of the eyelids. In some cases, dogs may develop corneal ulcers or opacity of the cornea. 5. Dermatological signs: Skin lesions, such as hyperkeratosis (thickening of the skin) on the footpads and nose, may develop in some dogs with canine distemper virus infection. These skin lesions are more commonly observed in the later stages of the disease and may contribute to the characteristic appearance of "hard pad disease". 6. High mortality: Canine distemper virus can be a severe and often fatal disease, particularly in young puppies and unvaccinated dogs. Vaccination is highly effective in preventing canine distemper virus infection and is considered a core component of routine canine vaccination protocols. Feline infectious peritonitis virus (FIPV) Feline Infectious Peritonitis (FIP) is a severe and often fatal disease in cats caused by certain strains of feline coronavirus (FCoV). Here's what FIPV does: 1. Systemic infection: FIPV infects macrophages, leading to a systemic infection that can affect multiple organs and systems in the body. 2. Two forms of disease: FIP can manifest in two main forms: the effusive or ‘wet’ form, and the non-effusive or ‘dry’ form. The effusive form is characterized by the accumulation of fluid in body cavities, such as the abdomen or chest, leading to symptoms such as abdominal distension or difficulty breathing. The non-effusive form primarily affects organs such as the kidneys, liver, or central nervous system, leading to symptoms such as weight loss, jaundice, neurological signs, or ocular abnormalities. 3. Clinical signs: Clinical signs of FIP can vary depending on the form of the disease and the organs affected. Common symptoms include fever, lethargy, anorexia, weight loss, jaundice, difficulty 7 breathing, abdominal distension, vomiting, diarrhoea, neurological signs (such as seizures or paralysis), and ocular abnormalities (such as uveitis or blindness). 4. High mortality: FIP is typically fatal, with a poor prognosis for affected cats. The disease can progress rapidly, leading to severe illness and death within weeks to months of onset. 5. Transmission: FIP is caused by certain mutations of FCoV, which are believed to arise within the host cat's body. The virus is primarily transmitted through the faecal-oral route. 6. Diagnosis: Diagnosis of FIP can be challenging due to the variable clinical presentation and the lack of a definitive diagnostic test. Veterinarians may use a combination of clinical signs, blood tests, imaging studies (such as radiography or ultrasound), and analysis of fluid samples to help support a diagnosis. 7. Treatment and management: Treatment is primarily supportive and aimed at managing clinical signs and improving quality of life. The efficacy of currently available vaccines against FIP is limited, and they are not routinely recommended for all cats. Porcine reproductive and respiratory syndrome virus (PRRSV) Porcine Reproductive and Respiratory Syndrome (PRRS) is a significant viral disease affecting pigs worldwide. PRRSV belongs to the Arteriviridae family. PRRSV manifests in two distinct forms: reproductive and respiratory. 1. Reproductive form: This form primarily affects pregnant sows, leading to reproductive failures such as late-term abortions, stillbirths, and weak piglets. 2. Respiratory form: This form mainly affects pigs of all ages and is characterized by respiratory symptoms such as coughing, sneezing, nasal discharge, and difficulty breathing. PRRSV can spread rapidly within pig populations through various means, including direct contact between pigs, contaminated equipment, aerosols, and via infected semen or foetal tissues. Management of PRRS involves implementing strict biosecurity measures, such as controlling pig movements, disinfecting facilities, and screening incoming animals. Vaccines are available, but their efficacy can vary. Jaagsietke virus The Jaagsiekte sheep retrovirus (JSRV) is a retrovirus that primarily affects sheep, causing a contagious and often fatal lung disease known as ovine pulmonary adenocarcinoma (OPA), also called Jaagsiekte. Here's what the Jaagsiekte virus does: 1. Respiratory symptoms: JSRV primarily targets the lungs of infected sheep, leading to respiratory symptoms that progressively worsen as the disease advances, until it is ultimately fatal. 2. Pulmonary adenocarcinoma: The hallmark of JSRV infection is the development of OPA, a type of lung cancer. The virus induces the transformation of lung epithelial cells, leading to the formation of tumours. These tumours can disrupt normal lung function, leading to respiratory distress and eventual death. 3. Contagious nature: JSRV is highly contagious among sheep and can spread rapidly within infected flocks through respiratory secretions, close contact between animals, and environmental contamination. The virus can persist in the environment for extended periods. 4. Diagnostic Challenges: Diagnosis of JSRV infection can be challenging due to the nonspecific nature of respiratory symptoms and the need for specialized diagnostic tests, such as PCR or immunohistochemistry. 5. Control Measures: Control of JSRV infection typically involves implementing strict biosecurity measures to prevent the introduction and spread of the virus within sheep populations. Additionally, culling of affected animals may be necessary to prevent further transmission and reduce the prevalence of the disease within infected flocks. 8 Foot and mouth disease virus (FMDV) Foot-and-mouth disease (FMD) is a highly contagious viral disease that affects cloven-hoofed animals, including cattle, pigs, sheep, and goats. Here's what FMDV does: 1. Clinical signs: FMDV infection causes a range of clinical signs in affected animals, including fever, lameness, reluctance to move, drooling, loss of appetite, and the formation of painful blisters or vesicles on the feet, mouth, tongue, and other areas of the body. These blisters can rupture, leading to the formation of erosions and ulcers. 2. Decreased productivity: FMD can have significant economic implications for the livestock industry due to decreased productivity, including reduced milk production, weight loss, decreased growth rates, and reduced reproductive performance in affected animals. 3. Highly contagious: FMDV is extremely contagious and spreads rapidly between susceptible animals through direct contact with infected individuals, as well as through indirect contact with contaminated objects, feed, water, and fomites (inanimate objects). The virus can also be transmitted by airborne droplets over short distances. 4. Global trade restrictions: FMD outbreaks can have significant trade implications, as affected countries may impose trade restrictions on livestock and animal products to prevent the spread of the virus to disease-free regions. 5. Prevention and Control: Prevention and control of FMD primarily may involve vaccination of susceptible animals, strict biosecurity measures to prevent the introduction and spread of the virus, surveillance, and rapid response to outbreaks. In 2001, the UK experienced a devastating outbreak of FMDV. The outbreak began in February 2001 when the disease was identified on a farm in Essex, England. The virus quickly spread to other parts of the country, facilitated by movements of infected animals, contaminated vehicles, and airborne transmission. The government implemented strict measures to control the outbreak, including the culling of millions of animals, movement restrictions, and disinfection procedures. The impact on the farming industry was severe. Farmers faced financial losses due to the loss of livestock, restrictions on animal movements, and the closure of export markets for British meat. The tourism industry also suffered as access to rural areas was restricted. The handling of the outbreak by the government was widely criticized. There were allegations of delays in implementing control measures, inadequate biosecurity measures, and controversies surrounding the mass culling of animals, including healthy animals on farms near infected premises. The crisis led to inquiries and reviews of the government's response and ultimately resulted in changes to disease control policies and procedures. The last outbreak of FMD in the UK occurred in 2007, with only a few cases reported in Surrey, England, resulting from a ‘lab leak’. The authorities quickly implemented control measures to contain the disease, including culling infected animals, movement restrictions, and disinfection protocols. The outbreak was effectively contained within a short period. 9 LECTURES 2 AND 3: Virus replication I & II LEARNING OBJECTIVES: Describe how viruses recognise and enter cells Outline the principle of the Baltimore Classification. Describe examples of how different classes of virus replicate their genome and produce mRNA Describe how viruses are assembled and exit the cell CELL ENTRY Overall steps of a virus lifecycle The life cycle of a virus can be broadly divided into several stages, including attachment and entry, genome replication and gene expression, assembly, and release. However, it is important to note that the specifics of each stage can vary depending on the type of virus and its mode of replication. Here's a general overview of the life cycle of a virus: Attachment and Entry: The life cycle begins when the virus encounters a susceptible host cell. Viruses typically bind to specific receptors on the surface of host cells through interactions between viral surface proteins and host membrane-containing cell receptors. After attachment, the virus may enter the host cell through various mechanisms, including receptor-mediated endocytosis or fusion with the host cell membrane. Genome Replication and Gene Expression: Once inside the host cell, the virus delivers its genetic material (genome) into the host cell cytoplasm and/or nucleus (depending on the virus). The viral genome serves as a template for transcription, leading to protein synthesis, as well as genome replication during which more genomic copies are synthesised. Assembly: After genome replication and gene expression, newly synthesized viral components, including the viral genome and structural proteins are assembled into complete virions (viral particles). Release: Once assembled, mature virions are released from the host cell to infect new target cells and reinitiate a new infectious cycle. Viral release can occur through various mechanisms, including cell lysis (lytic release), budding from the host cell membrane (enveloped viruses), or exocytosis. Virus receptors A virus receptor is a molecule on the surface of a host cell that specifically interacts with viral structural proteins in order to facilitate the attachment and entry of the virus into the cell. Examples of virus receptors include CD4 for HIV, ACE2 for SARS-CoV-2, sialic acid for influenza virus and nicotinic acetylcholine receptor for rabies. Cell receptors can determine the following aspects of a virus behaviour: Host range: If a given species does not express/expresses a distant orthologue of the virus receptor, that species is less likely to be a host for that particular virus. Cell tropism: The presence of specific virus receptors on certain cell types determines the tropism of the virus, or its ability to infect those cells. For example, viruses that infect 10 respiratory epithelial cells may interact with receptors specific to those cells, while viruses that infect immune cells may interact with receptors present on immune cells. Virus co-receptors Virus co-receptors are additional cell surface molecules that collaborate with virus receptors to facilitate viral entry and infection. While virus receptors mediate the initial attachment of the virus to the host cell, virus co-receptors often enhance viral binding, promote conformational changes in viral surface proteins, or facilitate viral fusion and entry into the host cell. Examples of virus co- receptors include CD163 for Porcine reproductive and respiratory syndrome virus (PRRSV), CXCR4 and CCR5 for HIV, and integrins for adenoviruses and certain picornaviruses. Virus entry Virus entry via endocytosis is a process by which viruses exploit the host cell's mechanisms for internalizing extracellular material. Endocytosis is a fundamental cellular process where cells engulf substances from their external environment by forming vesicles from the plasma membrane. Viruses can hijack this process to entry into host cells and initiate infection. a. Attachment: The first step involves the attachment of the virus to specific receptors on the surface of the host cell. This attachment is mediated by viral surface proteins interacting with cellular receptors. b. Internalization: The virus is then engulfed by the host cell by clathrin-mediated, caveolin- mediated endocytosis or micropinocytosis. The specific mechanism used is virus dependent. c. Formation of Endocytic Vesicle: As the plasma membrane invaginates around the virus particle, it forms a virus-containing vesicle called an endosome. d. Endosomal Trafficking: The endosome undergoes a series of maturation steps, during which it moves deeper into the cell and undergoes changes in its composition and pH. e. Release of Virus Contents/Penetration: Upon reaching certain acidity level, fusion of the endosomal and viral (when present) membranes is triggered, releasing the viral genome or capsid into the cytoplasm of the host cell. Alternatively, some viruses can disrupt the endosomal membrane to release their contents into the cytoplasm. A fundamental difference between enveloped and non-enveloped viruses: Enveloped enter by membrane fusion Non-enveloped: pore formation or membrane disruption A fundamental similarity between enveloped and non-enveloped viruses: Penetrating the cell membrane takes energy This energy is stored in the virus particle Virus entry is a “one shot” process BALTIMORE CLASSIFICATION The Baltimore classification system is a framework used to classify viruses based on their genome type and replication strategy. Developed by Nobel laureate David Baltimore in 1971, this 11 classification system organizes viruses into seven groups (I to VII) based on common features of their genetic material and replication processes. Here's an overview of each group: Group I: Double-stranded DNA viruses (dsDNA): Herpesviruses (e.g., ovine herpes virus 2), adenoviruses (e.g., Canine adenovirus 1), and poxviruses (e.g., Lumpy Skin Disease Virus). Group II: Single-stranded DNA viruses (ssDNA): These viruses have single-stranded DNA genomes that serve as templates for complementary strand synthesis. Examples include parvoviruses (cause various diseases in animals, including canine parvovirus) and circoviruses (cause diseases in birds and pigs). Group III: Double-stranded RNA viruses (dsRNA): Viruses in this group have double-stranded RNA genomes. Examples include reoviruses (e.g., Bluetongue virus) and rotaviruses (cause of severe diarrhoea). Group IV: Single-stranded positive-sense RNA viruses (ssRNA+): These viruses have single-stranded RNA genomes that can serve as mRNA upon entry into the host cell. Examples include picornaviruses (e.g., poliovirus and hepatitis A virus), flaviviruses (e.g., Zika virus and West Nile virus), and coronaviruses (e.g., Infectious bronchitis virus). Group V: Single-stranded negative-sense RNA viruses (ssRNA-): Viruses in this group have single- stranded RNA genomes complementary to mRNA. Examples include paramyxoviruses (e.g., Hendra and Nipah viruses), orthomyxoviruses (e.g., influenza virus), and rhabdoviruses (e.g., rabies virus). Group VI: Retroviruses: These viruses have single-stranded RNA genomes that are reverse transcribed into DNA upon entry into the host cell. The resulting DNA is then integrated into the host cell genome. Examples include feline immunodeficiency virus (HIV) and Feline leukemia virus (FeLV). Group VII: Reverse-transcribing DNA viruses: This group includes viruses with reverse-transcribing DNA genomes that replicate through an RNA intermediate. Examples include hepatitis B virus (HBV), which has a partially double-stranded DNA genome that is transcribed into RNA during replication. The Baltimore classification system (covered in Lecture I) provides a systematic way to understand the diversity of viruses based on their genetic material and replication strategies, for example: DNA viruses (classes 1 & 2) can use cellular DNA synthesis machinery. They therefore tend to evolve relatively slowly and can be large. RNA viruses (classes 3, 4, 5) have to encode their own RNA-dependent RNA polymerase. They tend to evolve quickly (lack of many proofreading mechanisms) and have small genomes. Viruses with a reverse transcriptase (classes 6, 7) evolve fast and integrate their genomes into the cell’s resulting in latent reservoirs. TRANSCRIPTION AND REPLICATION In addition to using the viral polymerase, viruses can hijack the host cellular machinery to replicate their genetic material. This often involves the production of viral enzymes, such as polymerases, helicases, and proteases, which are essential for replicating viral nucleic acids and processing viral proteins. Although viruses encode their own polymerase to transcribe and/or replicate their genomes (which polymerase type differs depending of the group of viruses), a commonality between all viruses is that they all use the cellular ribosomal machinery to translate their proteins. VIRUS ASSEMBLY AND RELEASE 12 Assembly and Maturation: Once the viral components - viral proteins, viral genome and a membrane (only for enveloped viruses)- are produced within the host cell, they must be assembled into new virus particles. This process often involves the coordinated assembly of viral nucleic acids and proteins to form complete virus particles. Exit Mechanisms: After assembly, viruses need a mechanism to exit the host cell to infect new cells and spread the infection. Some (usually non-enveloped) viruses achieve this through cell lysis, where the infected cell ruptures, releasing virus particles. Others use non-lytic mechanisms, such as budding through the host cell membrane (usually enveloped viruses), where virus particles are released without causing immediate cell death. Cell lysis Viroporins are a class of small, hydrophobic viral proteins that have the ability to form ion channels or pores in host cell membranes. These proteins play diverse roles in the viral life cycle, including facilitating virus entry, assembly, release, and modulation of host cell functions. Viroporins insert themselves into host cell membranes and form pores or channels and alter the ion balance across the cell membrane by allowing the unregulated flow of ions (such as calcium, potassium, and sodium) This disruption in ion homeostasis can trigger various cellular responses, including activation of cell death pathways and membrane damage, contributing to cell lysis. Viroporins can also aid in the release of newly assembled virus particles from infected cells. By inducing membrane permeabilization, viroporins promote the release of viral progeny into the extracellular environment, facilitating viral spread and transmission to other cells. Examples of viroporins include rotavirus NSP4 and Foot-and-mouth disease virus (FMDV) 2B protein. Virus budding Virus budding is a process by which enveloped viruses (e.g., influenza virus, FIV, and herpesviruses) acquire their lipid membrane envelope as they exit the host cell. Viral components such as the viral genome (DNA or RNA), structural proteins are assembled and organised beneath the host membrane. Viral glycoproteins are also incorporated into the membrane and interact with cellular factors and lipids, inducing membrane curvature. As the curvature increases, the viral envelope proteins interact with the budding site's membrane, causing the viral envelope to bud outward from the cell surface, resulting in the acquisition of the viral lipid membrane envelope. The newly formed virus particle, is then released from the cell surface. Membrane scission Another example on how viruses exploit cellular machineries is the use of membrane scission, the physical separation of membrane-bound structures, such as vesicles or viral particles, from the parent membrane. Viruses can hijack cellular machinery involved in membrane scission to facilitate their egress. For example, the endosomal sorting complexes required for transport (ESCRT) machinery, which is normally involved in cellular membrane remodelling and vesicle formation, can be recruited by certain viruses to catalyse membrane scission. The ESCRT machinery constricts and sever the neck of the budding vesicle, resulting in membrane scission and the release of the vesicle into the extracellular space. These virions can then go on to infect other cells and propagate the viral infection. 13 Once the virion is budding, there’s a chance that newly formed virus particles bind back to the cell that just produced them. In order to avoid that: Viruses with specific protein receptors will often downregulate them Viruses with ubiquitous ones will degrade them For example: influenza neuraminidase (NA) protein: Influenza NA is an enzyme found on the surface of influenza viruses that lays a crucial role in the viral release by cleaving sialic acid residues from glycoproteins and glycolipids on the surface of infected cells and newly formed virus particles. Without neuraminidase activity, virus particles harbouring HA would bind membrane sialic acid and therefore remain attached to the cell surface, forming aggregates that limit their ability to spread and infect neighbouring cells. Moreover, neuraminidase activity also allows the virus to overcome barriers to infection, such as mucus layers in the respiratory tract to enhance viral spread. Replication sites Sites of replication within an infected cell vary between different viruses. Often, replication occurs at a location that is distinct from the site of budding. For example: Influenza replicates in the nucleus, buds at the plasma membrane Newcastle disease virus replicates in the cytoplasm, buds at the plasma membrane SARS-CoV-2 replicates in the cytoplasm, buds through the ER/intermediate Golgi Complex (ERGIC) Herpes viruses replicate in the nucleus, bud first through the inner nuclear membrane then through the Golgi. Virus spread Viruses can spread from cell to cell using various mechanisms, depending on the virus type, host cell type and tissue environment. 1. Cell-Free Diffusion: In this mechanism, viruses are released from infected cells into the extracellular space, where they diffuse freely and infect neighbouring cells. This process occurs when viruses are released from the host cell through mechanisms such as cell lysis or exocytosis. Cell-free diffusion is common among non-enveloped viruses and some enveloped viruses that are released from the host cell without requiring direct cell-to-cell contact. Exit then entry as described above may be inefficient and expose the new virus particles to be intercepted by antibodies. 2. Direct Cell-to-Cell Spread: As an alternative, some viruses fuse the membranes of infected cells with neighbouring uninfected cells to allow direct cell-cell spread, providing the advantage that a complete capsid does not need to be made. Syncytia Formation: Fusion of infected cells with neighbouring uninfected cells, forming multinucleated syncytia allowing viral to spread without exposure to the extracellular environment (e.g., paramyxoviruses). 14 Tunnelling Nanotubes (TNTs): Thin, filamentous membranous structures that connect neighbouring cells, allowing the direct transfer of cellular components, including viruses, between cells (e.g., herpesviruses and influenza virus). Cellular Protrusions and Filopodia: Formation of cellular protrusions, such as filopodia or membrane extensions, which facilitate direct physical contact between infected and uninfected cells. These cellular extensions serve as conduits for virus transmission and allow viruses to move efficiently between cells (e.g., retroviruses). 3. ‘Surfing’. Some viruses remain bound to the surface of the cell they are exiting from, effectively ‘surfing’ to adjacent uninfected cells. This is efficient, but requires a mechanism to block re-entry into the already infected cell, and these can still be targeted by antibodies. 4. Migration. Some viruses cause active migration of infected cells to uninfected areas (e.g., poxviruses). Clinical implications of understanding the life cycle of each virus Understanding the molecular details of virus tropism and pathology allows prediction of threats Understand host range, e.g. different influenza strains have different preferences for a-2,3 and a-2,6 sialic acid receptors Vaccine design based on viral anti-receptor Targeting neuraminidase with antiviral drugs, such as neuraminidase inhibitors like oseltamivir and zanamivir, can effectively inhibit viral replication and reduce the severity and duration of influenza infections. Genome edit the host to make it resistant – e.g. pigs lacking domain 5 of CD163 (PRRSV scavenger receptor) 15 LECTURE 4: Virus-host interactions I To be able to define the term “Cytopathic Effect (CPE)”. To be able to describe mechanisms by which viruses induce CPE. To be able to define the term “Syncytium”. To be able to describe the mechanism by which a syncytium is formed. To be able to describe what an “inclusion body” is. CYTOPATHIC EFFECT Cytopathic effect (CPE) refers to any abnormality induced by a viral infection. Often, this refers to the visible changes that occur in cells as a result of infection with a virus or exposure to other pathogens or toxic agents. These changes can be observed under a microscope and serve as an indicator of cellular damage or dysfunction. Here are some common types of cytopathic effects: Cell Morphological Changes: Infected cells may undergo morphological changes, such as rounding up, shrinkage, or elongation. These changes can alter the overall appearance and structure of the cell. Cell Lysis: Some viruses cause infected cells to undergo lysis or rupture, leading to the release of viral particles and cell debris into the surrounding environment. Syncytium Formation: Certain viruses, for example, some strains of paramyxoviruses and retroviruses, induce the fusion of infected cells, resulting in the formation of multinucleated giant cells called syncytia. Syncytium formation can lead to the spread of infection to neighbouring cells. Inclusion Bodies: Some viruses cause the formation of characteristic inclusion bodies within infected cells. These inclusion bodies are often composed of viral proteins or nucleic acids. Cell Death: Infection with certain viruses can induce programmed cell death, or apoptosis, in infected cells. Changes in Growth and Proliferation: Viral infection can alter the growth and proliferation of host cells, leading to changes in cell size, proliferation rate, and cell cycle progression. The induction of CPE can have multiple purposes: Turn the cell into a virus factory Turn the cell into a virus delivery vehicle Cell’s attempt at suicide to limit virus production We will look at examples of each of these. MECHANISMS TO INDUCE CPE Shutdown of host protein synthesis Many cytocidal viruses encode proteins that shutdown host protein synthesis by a variety of mechanisms, direct or indirect. Shutdown can be rapid (as with picornaviruses, herpesviruses and some poxviruses) or more gradual (e.g. adenoviruses). With non-cytocidal viruses (e.g. some arenaviruses) there is no generalised shutdown. Some viruses ( e.g. some flaviviruses) are cytocidal but do not shutdown host protein synthesis. 16 Indirect shutoff – influenza A virus Influenza cap snatching is a mechanism used by influenza viruses to acquire host mRNA caps for the initiation of viral mRNA synthesis. The process involves the cleavage and "snatching" of capped RNA fragments from host pre-mRNAs by the virus. By cap-snatching from cellular mRNAs, the cellular transcripts are no longer viable, meaning the cellular proteins are no longer produced. Here's how influenza cap snatching works: 1. Recognition of Host Pre-mRNAs: Influenza viruses replicate their genomes and transcribe viral mRNAs in the nucleus of infected host cells. The viral polymerase complex, which consists of three subunits (PB1, PB2, and PA), binds to the 5' end of host pre-mRNAs in the nucleus. 2. Cleavage of Host Pre-mRNAs: The PA subunit of the viral polymerase possesses endonuclease activity. It cleaves host pre-mRNAs near the 5' cap structure, typically 10-13 nucleotides downstream of the cap. 3. Snatching of Capped RNA Fragments: After cleavage, the viral polymerase complex releases the capped RNA fragment, which contains both the 5' cap and a short portion of the host pre-mRNA sequence. This capped RNA fragment is then used as a primer for viral mRNA synthesis. 4. Initiation of Viral mRNA Synthesis: The capped RNA fragment serves as a primer for viral mRNA synthesis by the viral RNA-dependent RNA polymerase (RdRp). The viral RdRp adds nucleotides to the 3' end of the capped RNA fragment, synthesizing viral mRNA transcripts. 5. Transcription of Viral mRNA: The newly synthesized viral mRNA transcripts are then exported from the nucleus to the cytoplasm, where they are translated by host ribosomes to produce viral proteins. These proteins are essential for viral replication and assembly. Direct shutoff – influenza A virus The influenza PA-X protein plays a crucial role in modulating host cell translation during influenza virus infection. PA-X has the ability to suppress host protein synthesis by inducing endonucleolytic cleavage of host mRNA transcripts. Here's how influenza PA-X shuts off host cell translation: Endonucleolytic Cleavage: PA-X translocates to the nucleus, where it exerts its effects on host cell translation. PA-X possesses endonuclease activity, allowing it to cleave host mRNA transcripts at specific sites. The cleavage sites targeted by PA-X are typically located near the 3' end of host mRNA transcripts. Downregulation of Host mRNA: Cleavage of host mRNA transcripts by PA-X results in the degradation of these transcripts, leading to a decrease in the abundance of host mRNAs available for translation. This downregulation of host mRNA levels ultimately leads to the inhibition of host protein synthesis. Selective Inhibition of Host Translation: PA-X appears to selectively target a subset of host mRNAs for cleavage, particularly those involved in antiviral responses and immune regulation. The influenza NS1 protein (non-structural protein 1) is a multifunctional protein that plays a crucial role in modulating host cell translation during influenza virus infection. NS1 exerts its effects through various mechanisms, including: Interference with Host mRNA Processing and Export: NS1 interferes with host mRNA processing and export from the nucleus to the cytoplasm. It binds to and inhibits the function of the cellular 17 mRNA processing and export machinery, including factors involved in mRNA splicing, polyadenylation, and nuclear export (particularly CPSF30). This results in the retention of host mRNAs in the nucleus and reduces the availability of mature mRNAs for translation in the cytoplasm. Inhibition of Host mRNA Translation: NS1 directly inhibits the translation of host mRNAs in the cytoplasm. It binds to components of the host translational machinery, including the translation initiation factor eIF4GI, and disrupts the formation of the translation initiation complex. Promotion of Host mRNA Degradation: NS1 promotes the degradation of host mRNA transcripts in the cytoplasm. It interacts with cellular factors involved in mRNA degradation pathways, such as the RNA exosome complex, and enhances the degradation of host mRNAs by recruiting them to cellular RNA degradation machinery. This leads to a decrease in the stability and abundance of host mRNAs available for translation. Direct shutoff – picornaviruses Picornaviruses use a non-conventional mechanism for translation of their proteins, negating the requirement for an mRNA cap. This means that they can use unique strategies to target capped transcripts for degradation. The picornavirus IRES, or internal ribosome entry site, is a structured RNA element located within the 5' untranslated region (UTR) of picornavirus genomes. IRES elements play a critical role in initiating translation of viral proteins in a cap-independent manner, allowing picornaviruses to efficiently translate their RNA genomes even in the presence of host cell translation shutoff mechanisms. Here's how picornavirus IRES works: Cap-Independent Translation Initiation: Unlike cellular mRNAs, which typically initiate translation at the 5' cap structure, picornavirus mRNA utilizes an alternative mechanism for translation initiation mediated by the IRES element. Structural Organization: Picornavirus IRES elements are highly structured RNA sequences that adopt complex secondary and tertiary structures. These structures consist of stem-loop motifs, pseudoknots, and other RNA elements that facilitate binding and recruitment of the host cell translational machinery, including ribosomes and initiation factors. Recruitment of Ribosomes: The picornavirus IRES interacts with ribosomes and initiation factors to form a ribonucleoprotein complex that positions the ribosome at the initiation codon of the viral mRNA. Once the ribosome is recruited to the initiation codon, translation initiation occurs, leading to the synthesis of viral proteins. Efficiency and Specificity: Picornavirus IRES elements are highly efficient in driving translation initiation, allowing for rapid and robust synthesis of viral proteins. Additionally, the IRES element enables specificity for viral mRNA, ensuring preferential translation of viral proteins over host cell proteins. Role in Viral Replication: The efficient translation initiation mediated by the IRES element is essential for picornavirus replication and propagation within infected cells. By allowing cap- independent translation of viral proteins, the IRES element ensures the synthesis of viral proteins even in the presence of host cell translation shutoff mechanisms, facilitating efficient viral replication and spread. 18 Picornaviruses employ various mechanisms to shut off host cell translation, which helps facilitate viral replication and propagation. Here's an overview of how picornaviruses achieve this: Cleavage of Translation Initiation Factors: Picornaviruses encode proteases that are capable of cleaving essential components of the host cell translational machinery. For example, the viral proteases 2A and L cleave eukaryotic initiation factors (eIFs) involved in cap-dependent translation initiation, thereby inhibiting the translation of host mRNAs while allowing viral mRNA translation to proceed. Sequestration of Host Cell Factors: Picornaviruses can also sequester host cell factors involved in translation initiation, further inhibiting host protein synthesis. For example, the viral protein 3CD binds to and sequesters the poly(rC)-binding protein (PCBP), which is essential for the translation of cellular mRNAs. Induction of Host Cell Shutoff: Picornaviruses induce a host cell shutoff response, leading to the degradation of host cell mRNAs and inhibition of host protein synthesis. This is mediated in part by the activity of viral proteases, which cleave cellular proteins involved in mRNA stability and translation. Manipulation of Cellular Signaling Pathways: Picornaviruses can manipulate cellular signaling pathways to inhibit host protein synthesis. For example, some picornaviruses activate the protein kinase PKR (double-stranded RNA-activated protein kinase), which phosphorylates and inactivates the translation initiation factor eIF2α. Phosphorylation of eIF2α inhibits translation initiation and reduces the overall rate of protein synthesis in the host cell. Why does a virus shut off host protein synthesis? Viruses shut off host protein synthesis for several reasons, all of which contribute to the successful completion of their replication cycle and the establishment of infection: 1. Resource Redistribution: By shutting off host protein synthesis, viruses redirect cellular resources, including ribosomes, tRNAs, and energy, towards the translation of viral proteins. 2. Avoidance of Competition: Viruses compete with host cell machinery, including ribosomes and translation factors, for the resources necessary for protein synthesis. By shutting off host protein synthesis, viruses eliminate competition for these resources, ensuring that the translation of viral proteins proceeds unimpeded. 3. Evasion of Host Cell Apoptosis: Host protein synthesis shutoff can also prevent the expression of cellular proteins involved in apoptosis (programmed cell death). Viruses often inhibit apoptosis to prolong the survival of infected cells and create a more favourable environment for viral replication and spread. 4. Reduction of Antiviral Responses: Host protein synthesis shutoff may reduce the expression of cellular factors involved in innate and adaptive immune responses to viral infection. This includes cytokines, chemokines, and antiviral proteins that are normally produced in response to infection. By inhibiting the production of these factors, viruses can dampen host antiviral defences and promote their replication and spread. Inclusion bodies Inclusion bodies are specialized structures that can form within cells because of various cellular processes, including viral infection, protein aggregation, or accumulation of cellular debris. These 19 structures are often visible under a microscope and can vary in size, composition, and appearance depending on the underlying cause. Viral Inclusion Bodies: Simply speaking, inclusion bodies in viral infections are non-membrane- bound organelles comprised of virus components. Many viruses induce the formation of inclusion bodies within infected cells as part of their replication cycle. These inclusion bodies can contain viral proteins, nucleic acids, and other viral components and may serve as sites of viral replication and assembly. Examples include Negri bodies in rabies virus-infected neurons and viral factories in poxvirus-infected cells. Inclusion bodies are membrane-free compartments, often given integrity through liquid-liquid phase separation. Liquid-liquid phase separation (LLPS) is a phenomenon observed in biological systems where certain proteins and nucleic acids can undergo a transition from a soluble state to a liquid-like droplet or compartment within the cytoplasm or nucleus of cells. These liquid droplets, often referred to as biomolecular condensates or membrane-free organelles, have distinct properties that allow them to concentrate specific molecules and perform specialized cellular functions. CPE AS A MEANS TO TURN THE CELL INTO A VIRUS DELIVERY VEHICLE Syncytium formation Syncytium formation is a phenomenon where multiple cells fuse together to form multinucleated giant cells, known as syncytia. Syncytium originates from the Greek word for “together”. Viruses can induce syncytium formation through various mechanisms, primarily involving the viral envelope proteins and their interactions with host cell receptors and fusion machinery. Here's how viruses induce syncytium formation: 1. Viral Envelope Proteins: Many viruses, particularly enveloped viruses, express specialized viral envelope proteins that mediate membrane fusion between the viral envelope and host cell membrane. 2. Receptor Binding: The first step in syncytium formation involves the binding of viral envelope proteins to specific host cell receptors. These receptors can vary depending on the virus and may include proteins, carbohydrates, or lipid moieties present on the surface of host cells. 3. Membrane Fusion: Once viral envelope proteins bind to host cell receptors, they undergo conformational changes that facilitate membrane fusion between the viral envelope and host cell membrane. 4. Cell-Cell Fusion: In some cases, the fusion of viral and cellular membranes can lead to the fusion of multiple host cells, resulting in the formation of multinucleated syncytia. This occurs when infected cells expressing viral envelope proteins interact with neighbouring cells that also express the appropriate receptors. The fusion of infected cells with neighbouring cells leads to the mixing of cytoplasmic contents and the formation of syncytia. 5. Syncytium Expansion: Once formed, syncytia can continue to expand through the fusion of additional infected and uninfected cells, leading to the formation of large multinucleated structures. Syncytium expansion can facilitate viral spread within host tissues and enhance viral replication by allowing the virus to evade host immune responses and access additional cellular resources. Filopodia formation 20 Virus-induced filopodia (Latin: Thread protrusion) formation is a phenomenon observed during viral infection where infected cells extend long, slender protrusions called filopodia. These filopodia are rich in actin filaments and serve various functions during viral replication and spread. Here's how viruses induce filopodia formation: Viral Attachment and Entry: The process of virus-induced filopodia formation often begins with the attachment of viruses to specific receptors on the surface of host cells. This attachment can trigger signalling pathways that lead to cytoskeletal rearrangements, including the polymerization of actin filaments and the extension of filopodia from the cell surface. Viral Replication and Assembly: Some viruses exploit filopodia to concentrate viral components and facilitate their transport to specific cellular compartments involved in viral replication and assembly. Cell-Cell Transmission: Filopodia induced by viral infection can facilitate cell-cell transmission of viruses. Infected cells can extend filopodia to contact neighbouring cells and transfer viral particles directly to adjacent cells, bypassing the need for extracellular diffusion. This cell-cell transmission mechanism can enhance viral spread within tissues and evade host immune responses. Tunnelling nanotube formation Virus-induced tunnelling nanotube (TNT) formation is a phenomenon observed during viral infection where infected cells extend long, thin membrane protrusions called tunnelling nanotubes to establish direct intercellular connections with neighbouring cells. These tunnelling nanotubes facilitate the transfer of various cellular components, including proteins, lipids, and organelles, between infected and uninfected cells, thereby promoting viral spread and evasion of host immune responses. Here's how virus-induced tunnelling nanotube formation occurs: Viral Attachment and Entry: The process of virus-induced tunnelling nanotube formation often begins with the attachment of viruses to specific receptors on the surface of host cells. Viral attachment can trigger intracellular signalling pathways that lead to cytoskeletal rearrangements and the formation of tunnelling nanotubes. Cytoskeletal Remodelling: Upon viral attachment, infected cells undergo cytoskeletal remodelling, including the polymerization of actin filaments and microtubules. These cytoskeletal elements play a critical role in the formation and extension of tunnelling nanotubes from the infected cell surface. Extension of Tunnelling Nanotubes: Infected cells extend long, thin membrane protrusions called tunnelling nanotubes towards neighbouring cells. These nanotubes can vary in length and diameter and are enriched in actin filaments, microtubules, and cellular organelles. Transfer of Cellular Components: Once formed, tunnelling nanotubes allow for the transfer of various cellular components between infected and uninfected cells. This includes proteins, lipids, organelles, and even entire viral particles. Enhancement of Viral Pathogenesis: Virus-induced tunnelling nanotubes can contribute to viral pathogenesis by facilitating viral spread within host tissues and evasion of host immune responses. The direct intercellular communication mediated by tunnelling nanotubes allows for efficient transfer of viral components between infected and uninfected cells, leading to more severe disease outcomes. 21 Cell motility Viruses can alter cell motility through various mechanisms, including direct interactions with cellular components involved in cytoskeletal dynamics, signalling pathways, and cell adhesion molecules. Cell proliferation Viruses can alter cellular proliferation, or the process of cell division and growth, through various mechanisms, often involving manipulation of the host cell's molecular machinery. Here are several ways viruses can influence cellular proliferation: Activation of Cell Cycle Machinery: Many viruses encode proteins that interact with and manipulate key regulators of the cell cycle, such as cyclins, cyclin-dependent kinases (CDKs), and tumour suppressor proteins. By modulating the activity of these cell cycle regulators, viruses can promote cell cycle progression and stimulate cellular proliferation. Inhibition of Cell Cycle Checkpoints: Some viruses can disrupt cell cycle checkpoints, which are regulatory mechanisms that ensure the fidelity of cell division and prevent the proliferation of damaged or abnormal cells. Induction of Cellular Transformation: Certain viruses have the ability to transform infected cells into a proliferative state resembling that of cancer cells. This transformation is often mediated by viral oncoproteins, which can dysregulate cellular signalling pathways involved in cell growth, survival, and differentiation. Modulation of Apoptosis: Viruses can manipulate programmed cell death, or apoptosis, to promote cellular proliferation. Some viruses encode anti-apoptotic proteins that inhibit apoptotic signalling pathways and prevent infected cells from undergoing cell death. By blocking apoptosis, viruses can prolong the survival of infected cells and promote their continued proliferation. Cellular proliferation – papillomaviruses Papillomaviruses (PVs) are a family of DNA viruses known to induce cell proliferation primarily through the actions of two viral oncoproteins: E6 and E7. These proteins interact with various cellular factors leading to dysregulation of host cell proliferation control mechanisms. Here's how papillomaviruses induce cell proliferation: 1. E6 Protein: Inactivation of p53: The E6 protein of papillomaviruses interacts with the tumour suppressor protein p53 and promotes its degradation. Inactivation of p53 prevents infected cells from undergoing cell cycle arrest or apoptosis, allowing infected cells to continue proliferating. Dysregulation of Cellular Signalling: E6 can also interact with and modulate the activity of cellular signalling molecules involved in cell proliferation and survival. 2. E7 Protein: Inactivation of pRb: The E7 protein of papillomaviruses interacts with the retinoblastoma protein (pRb), a key regulators of the cell cycle. This interaction results in the stimulation of cell cycle entry and progression from the G1 to S phase. Induction of DNA Replication: E7 can also interact with cellular factors involved in DNA replication and repair, promoting the replication of viral DNA and cellular DNA. By stimulating DNA replication, E7 promotes cell cycle progression and cellular proliferation. 3. Cellular Signalling Pathways: 22 Activation of Growth Factor Signalling: Papillomaviruses can activate cellular signalling pathways involved in cell growth and proliferation, enhancing cell growth and survival, contributing to the proliferation of infected cells. Modulation of Immune Responses: Papillomaviruses may also modulate host immune responses to promote cellular proliferation. For example, viral proteins can inhibit the production of cytokines that suppress cell proliferation, allowing infected cells to proliferate unchecked. 4. Cellular Transformation: Induction of Cellular Transformation: Persistent infection with high-risk papillomaviruses can lead to the transformation of infected cells into a proliferative state resembling that of cancer cells. This transformation is characterized by dysregulated cell proliferation, loss of growth control, and acquisition of other cancer-related phenotypes. CPE AS A CELLULAR EFFORT TO COMMIT SUICIDE Programmed cell death Programmed cell death, or apoptosis, can occur through both caspase-dependent and caspase- independent mechanisms. These pathways involve distinct molecular mechanisms and regulatory factors. Here's an explanation of the differences between caspase-dependent and caspase- independent programmed cell death (PCD): PCD type I: Apoptosis Caspase-Dependent Programmed Cell Death: In caspase-dependent apoptosis, the activation of caspases is central to the execution of cell death. Caspases are typically activated through proteolytic cleavage in response to extrinsic or intrinsic apoptotic signals. Caspase activation is initiated by signals such as DNA damage or mitochondrial stress. Apoptosis is characterised by chromatin condensation, DNA fragmentation, cell shrinkage, and shedding of apoptotic bodies. PCD type II: Autophagy Caspase-Independent Programmed Cell Death: In caspase-independent apoptosis, cell death is initiated by factors and pathways that do not involve caspase activation, for example cathepsins. Cathepsins are a family of proteases that play essential roles in protein degradation, lysosome function, and other cellular processes. Cathepsins can also contribute to cell death, leading to a form of cell death known as cathepsin- dependent cell death. Cathepsin-mediated cell death is characterised by formation of autophagosomes. PCD type III: Necroptosis RIP3 and MLKL-mediated RIP3 (Receptor-interacting protein kinase 3) and MLKL (Mixed lineage kinase domain-like protein) are key components of a programmed cell death pathway known as necroptosis. Necroptosis is a form of regulated cell death that occurs independently of caspases and is characterized by 23 cellular swelling, plasma membrane rupture, and release of intracellular contents, leading to inflammation and tissue damage. RIP3 and MLKL mediated cell death is characterised by swelling of organelles and the cell, rupture of plasma membrane and cell lysis. 24 LECTURE 5: Virus detection LEARNING OBJECTIVES: To understand the basic techniques for virus quantification. To be able to calculate virus titre using data from a plaque assay. To be able to plot and interpret data from assays. To understand fundamentals of virus identification and diagnostics. Why do we need virus diagnostics? Virus diagnostics are essential tools for identifying and characterizing viral infections, enabling veterinary professionals to make informed decisions regarding care, treatment strategies, and infection control measures. Here are several reasons why virus diagnostics are critically important: 1. Early Detection and Diagnosis: Virus diagnostics allow for the early detection and diagnosis of viral infections, often before the onset of symptoms or when symptoms are mild. Early diagnosis enables prompt initiation of appropriate treatment and implementation of infection control measures to prevent the spread of the virus to others. 2. Management and Treatment: Different viruses may require different treatment approaches, such as antiviral medications, supportive care, or vaccination. Additionally, knowing the viral aetiology can help prevent unnecessary antibiotic use, which can contribute to antimicrobial resistance. 3. Infection Control and Prevention: Rapid identification of viral pathogens allows for timely implementation of infection control measures, such as isolation precautions, contact tracing, and vaccination campaigns, to contain outbreaks and prevent further transmission of the virus within communities. 4. Surveillance and Epidemiology: Virus diagnostics are essential for surveillance and epidemiological monitoring of viral diseases at local, national, and global levels. Surveillance systems rely on accurate and timely diagnostic data to track the prevalence, incidence, distribution, and trends of viral infections, identify emerging pathogens, and assess the effectiveness of control measures and public health interventions. 5. Research and Development: Virus diagnostics contribute to advancements in scientific research and the development of new diagnostic technologies, therapeutic interventions, and preventive measures. Diagnostic testing plays a crucial role in clinical trials, vaccine development, and epidemiological studies aimed at understanding the biology of viral pathogens, evaluating treatment efficacy, and identifying potential targets for intervention. VIRUS QUANTIFICATION Viruses can be quantified using various methods that measure viral particles, viral genome copies, or viral infectivity. Here are some common techniques used for virus quantification: Virus infectivity Practically, this is done by preparing serial (usually) 10-fold dilutions of your sample, infecting susceptible cells in dishes for several cycles of virus replication (typically 24-48 h, can be longer). Next, you formaldehyde fix and stain cells then count foci of CPE at a sensible dilution, and back calculate to obtain a virus titre. Measuring virus infectivity is the gold standard in virus diagnostics. 25 E.g. Plaque Assay: Plaque assays are commonly used to quantify viruses that form visible plaques, including some animal viruses. In a plaque assay, serial dilutions of the virus sample are prepared and inoculated onto a monolayer of susceptible host cells. After incubation, the cells are fixed and stained, and plaques (zones of cell death) formed by viral infection are counted to determine the viral titre. E.g. Tissue Culture Infectious Dose (TCID50) Assay: The TCID50 assay is a quantitative method for estimating the infectious titre of a virus. In this assay, serial dilutions of the virus sample are inoculated onto a monolayer of susceptible host cells in multiple replicate wells. After incubation, the presence or absence of cytopathic effects (CPE) is observed, and the TCID50 value is calculated based on the dilution at which 50% of the wells show CPE. Viral genomes PCR is used for DNA viruses, and RT-PCR for RNA viruses. ‘RT’ here stands for ‘reverse transcription’, i.e. copying the virus RNA genome into DNA. Detection by PCR methods can be quantitative (how much?) or qualitative (have I got any? / yes/no). In all cases, PCR requires that you know the virus sequence to be able to design primers. PCR tells you whether/ how much viral nucleic acid is there, not whether it is infectious. E.g. Polymerase Chain Reaction (PCR): PCR is a molecular technique used to determine whether there is any viral genome present in a sample. In PCR, specific are used to amplify and amplify viral nucleic acids. It allows researchers to make millions of copies of a particular DNA fragment, even if it is present in a very small amount. The PCR process begins with denaturation, where the double-stranded DNA template is heated to a high temperature (typically around 94-98°C). This causes the hydrogen bonds between the two complementary DNA strands to break, resulting in the separation of the DNA into two single strands. The temperature is lowered to allow the primers to anneal to the complementary sequences on the single-stranded DNA template. Primers are short, single-stranded DNA oligonucleotides that are designed to flank the target DNA sequence to be amplified. The temperature is then raised to the optimal temperature for the DNA polymerase enzyme (usually around 72°C). DNA polymerase is then able to synthesize new DNA strands by extending the primers along the template DNA. The denaturation, annealing, and extension steps are repeated for several cycles (typically 20-40 cycles) to amplify the target DNA sequence exponentially. Each cycle doubles the amount of DNA present, resulting in millions of copies of the target sequence after just a few cycles. The presence of DNA can be detected by agarose gel electrophoresis. E.g. Quantitative Polymerase Chain Reaction (qPCR): In qPCR, specific primers and fluorescent probes are used to amplify and detect viral nucleic acids as above, but with increasing amounts of DNA corresponding with increased fluorescent signal. By comparing the fluorescence signal to a standard curve generated from known concentrations of viral DNA or RNA, the viral genome copy number in the sample can be quantified. E.g. Reverse Transcription Quantitative PCR (RT-qPCR): RT-qPCR is a variation of qPCR used to quantify RNA viruses. In RT-qPCR, viral RNA is first reverse transcribed into complementary DNA (cDNA) using reverse transcriptase. The cDNA is then amplified and quantified using qPCR (or PCR) as described above. Virus antigen The aim is to detect virus (especially in tissue). Most commonly, detection of virus antigen (proteins) is achieved using specific antibodies. Alternatively, the receptor binding activity of the virus can be tested, e.g. agglutination assays (very commonly used for influenza). Less common is 26 detection of virus genome using nucleic acid baits. Virus antigen assays can be quantitative (how much) or qualitative (have I got any? Yes/no). Virus antigen assays require that you know the virus to be able to choose appropriate antisera or assay design. E.g. Hemagglutination Assay: Hemagglutination assays are commonly used to quantify certain types of viruses, such as influenza viruses. In this assay, viral particles are mixed with red blood cells (usually from chickens or turkeys), causing them to agglutinate due to binding of viral surface proteins to sialic acid receptors on the red blood cell surface. The titer of the virus is determined by the highest dilution that still causes hemagglutination. Detecting virus antigen through a hemagglutination assay Fresh RBCs are typically obtained from a suitable source, such as chickens, turkeys, or humans. The RBCs are washed to remove plasma proteins and other contaminants and suspended in a buffered saline solution to maintain their viability and stability. The viral sample, which may contain infectious virus particles or viral proteins, is two-fold serially diluted in a microtiter plate or test tubes. This allows for the testing of a range of virus concentrations to determine the titre or potency of the virus. Each dilution of the viral sample is mixed with a standardized volume of RBC suspension. The mixture is then incubated for a specified period (~30 minutes). After incubation, the wells or tubes are examined for the presence of agglutination, which appears as a lattice. Lattice formation indicates that the virus present in the sample has bound to the RBCs and caused them to be ‘held apart’ in lattice formation. If the RBCs sink to the bottom of the well to form a ‘button’, there is no virus present. The degree of agglutination is usually graded based on a scale from 0 to 4, with 0 indicating no agglutination and 4 indicating complete agglutination. The highest dilution of the viral sample that still causes agglutination is recorded as the hemagglutination titre. The hemagglutination titre is calculated based on the reciprocal of the highest dilution of the viral sample that produces agglutination. For example, if agglutination is observed up to a dilution of 1:128, the titre would be recorded as 128. To ensure the accuracy and reliability of the assay, appropriate controls are included, such as positive and negative controls. Positive controls consist of known virus samples that are expected to agglutinate RBCs, while negative controls are typically buffer-only or uninfected samples that should not cause agglutination. Was an animal previously infected? Assessed through a hemagglutination test Virus antigen can also be assayed retrospectively – i.e. was an animal previously infected? An example of this is the haemagglutination inhibition assay. This assay is particularly useful for measuring the presence of specific antibodies against viruses that agglutinate red blood cells (RBCs), such as influenza viruses. Here's how a hemagglutination inhibition assay works: Fresh RBCs are typically obtained from a suitable source, such as chickens, turkeys, or humans. The RBCs are washed to remove plasma proteins and other contaminants and suspended in a buffered saline solution to maintain their viability and stability. The viral antigen used in the assay is typically a standardized preparation of inactivated virus particles or viral proteins, such as hemagglutinin (HA) for influenza viruses. The viral antigen is diluted to a standardized concentration for use in the assay. 27 Serum samples containing antibodies against the virus of interest are serially diluted in a microtiter plate or test tubes. This allows for the testing of a range of antibody concentrations to determine the antibody titre. Each dilution of the serum sample is mixed with a standardized volume of the viral antigen solution. The mixture is incubated at room temperature or 37°C (body temperature) for a specified period to allow the antibodies in the serum to bind to the viral antigen. After the pre-incubation period, a standardized volume of RBC suspension is added to each well or tube containing the serum-viral antigen mixture. The RBCs are added in excess to ensure that any remaining free viral antigen will bind to the RBCs and cause agglutination. The wells or tubes are examined for the presence of hemagglutination. The absence of hemagglutination indicates that the antibodies in the serum were able to neutralize the viral antigen, preventing it from binding to the RBCs and resulting in RBCs settling to the bottom of the well to form a ‘button’. The highest dilution of serum that completely inhibits hemagglutination is recorded as the hemagglutination inhibition titre. This titre represents the concentration of specific antibodies in the serum that can neutralize the viral antigen and inhibit its hemagglutination activity. To ensure the accuracy and reliability of the assay, appropriate controls are included, such as positive and negative controls. Positive controls consist of known serum samples containing antibodies against the virus, while negative controls are typically serum samples from uninfected individuals or animals. Virus neutralisation test A virus neutralization test (VNT), also known as a virus neutralization assay or serum neutralization test, is a laboratory technique used to assess the ability of antibodies in serum or plasma samples to neutralize the infectivity of a virus. This assay is commonly used to measure the presence and potency of neutralizing antibodies against specific viruses, such as influenza viruses, coronaviruses, and flaviviruses. Here's how a virus neutralization test works: A standardized preparation of infectious virus is propagated in susceptible cells and titrated to determine the viral titre. The virus stock is typically diluted to a standardized concentration for use in the assay. Serum or plasma samples containing antibodies against the virus of interest are serially diluted. This allows for the testing of a range of antibody concentrations to determine the neutralizing antibody titre. Each dilution of the serum sample is mixed with a standardized volume of the viral stock solution containing a known concentration of infectious virus. The mixture is incubated to allow the antibodies in the serum to interact with the virus. After the serum-virus mixture has been incubated, a monolayer of susceptible host cells is inoculated with the mixture. After a suitable incubation period, the cells are examined for evidence of virus infection, such as cytopathic effects (CPE). These changes indicate that the virus has successfully infected and replicated within the cells. The neutralizing antibody titre is determined based on the highest dilution of serum that completely inhibits virus infection and prevents the development of cytopathic effects. This is 28 typically assessed by comparing the degree of cell damage or virus-induced changes in the presence and absence of serum. The neutralizing antibody titre is reported as the reciprocal of the highest dilution of serum that exhibits complete virus neutralization. For example, if complete neutralization is observed at a serum dilution of 1:80, the neutralizing antibody titre would be recorded as 80. To ensure the accuracy and reliability of the assay, appropriate controls are included, such as positive and negative controls. Positive controls consist of known serum samples containing neutralizing antibodies against the virus, while negative controls are typically serum samples from uninfected individuals or animals. IMPORTANCE OF DIAGNOSTICS On the example of a hypothetical Lumpy skin disease virus and the Schmallenberg virus outbreak in 2011 The choice of sample and test is typically guided by clinical presentation – i.e., “common things happen commonly”. However, what if something is not routine, e.g. exotic incursions of emerging viruses? Imagine the scenario: 1. Farmer calls vet out 2. Common culprits excluded or signs of exotic disease 3. Vet calls expert in government lab 4. Expert decides over sampling (type) 5. Sample arrives at lab 6. PCR test for the genus of a suspected pathogen e.g. capripox – LSDV, goat pox, sheep pox a. +ve – second PCR to identify virus species b. +ve – notification to Chief Veterinary Officer, declare outbreak Follow up – isolation of virus, reconfirmation of PCR tests, full-genome sequencing of the virus, serology on the herd. An example of when this happened was the outbreak of Schmallenberg virus in 2011. The Schmallenberg virus (SBV) outbreak in 2011 was a significant event in the European livestock industry, particularly affecting sheep and cattle. Here's an overview of what happened: The outbreak was first identified in November 2011 in the German town of Schmallenberg, which gave the virus its name. It was initially detected in dairy cattle exhibiting symptoms of fever, reduced milk production, and diarrhoea. The disease rapidly spread across Europe, affecting several countries including Germany, the Netherlands, Belgium, France, and the United Kingdom. It primarily affected sheep and cattle, although other ruminant species such as goats were also susceptible. The usual potential suspects were excluded, e.g. BVDV, BHV, FMDV. Blood samples from three affected animals and one healthy animal was taken, and deep sequencing was performed. In this way, the novel virus was identified and characterised very rapidly. Schmallenberg virus infection in livestock is characterized by a range of clinical signs, including fever, decreased milk production, diarrhea, and reproductive disorders such as abortion, stillbirths, and congenital malformations in newborn animals. 29 One of the distinguishing features of SBV infection is the occurrence of congenital malformations, such as arthrogryposis (joint deformities), hydranencephaly (fluid-filled brain), and brachygnathia (shortened jaw), in newborn animals born to infected mothers. These malformations were observed in lambs, calves, and goat kids born to infected dams during the outbreak. Schmallenberg virus is primarily transmitted to livestock through the bites of biting midges (Culicoides spp.), which serve as vectors for the virus. These insects become infected after feeding on viremic animals and can transmit the virus to susceptible hosts during subsequent blood meals. The virus can also be transmitted vertically from infected pregnant animals to their offspring, resulting in congenital malformations and reproductive losses. To confirm that Schmallenberg was the causative agent of disease, experimental infections of healthy animals with this new virus were performed, and clinical progression was confirmed. It was established that Schmallenberg did not pose a zoonotic threat, and wouild not be a long-term problem as illness was mild in adult animals and congenital infection was not a problem in immune mothers. A sheep vaccine is now available. 30 LECTURE 6: Virus infection and pathogenesis I LEARNING OBJECTIVES: Outline the routes by which a virus can enter a host. Describe the spread of virus through the host with particular reference to the cell types encountered. Outline the possible outcomes of these virus:cell interactions. Define the term “Viremia”. Describe the pathogenesis of virus infections of different systems. INFECTION ROUTES Viruses can infect animals through various routes. Here's a description of the primary routes by which they can infect: 1. Respiratory Route: Inhalation of Respiratory Droplets: Many viruses, including influenza virus and SARS-CoV-2 are primarily transmitted through respiratory droplets generated when an infected person coughs, sneezes, or talks. These droplets can be inhaled by nearby individuals, allowing the virus to enter the respiratory tract and establish infection. Aerosol Transmission: Some viruses can remain suspended in the air as aerosolized particles for an extended period, particularly in indoor environments with poor ventilation. Aerosol transmission can occur over longer distances. 2. Direct Contact: Skin-to-Skin Contact: Certain viruses, such as herpes simplex virus (HSV), varicella-zoster virus (VZV), and human papillomavirus (HPV), can be transmitted through direct skin-to-skin contact with an infected individual. Contact with infectious lesions, mucous membranes, or secretions containing the virus can facilitate transmission. Fomite Transmission: Viruses can also be transmitted indirectly through contact with contaminated surfaces or fomites, such as doorknobs, countertops, and shared objects. Touching a contaminated surface and then touching the eyes, nose, or mouth can introduce the virus into the body. 3. Faecal-Oral Route: Ingestion of Contaminated Food or Water: Some viruses, including norovirus, rotavirus, hepatitis A virus, and enteroviruses (e.g., poliovirus), can be transmitted through the faecal-oral route. Contaminated food, water, or surfaces can serve as vehicles for viral transmission, particularly in settings with poor sanitation and hygiene practices. 4. Blood-Borne Transmission: Parenteral Transmission: Blood-borne viruses, such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV), can be transmitted through exposure to infected blood or blood products. This can occur through needlestick injuries, transfusion of contaminated blood products, or sharing of needles or injection equipment among injection drug users. Reuse of needles by vets to vaccinate multiple animals is another example. 31 Vertical Transmission: Some viruses, including cytomegalovirus (CMV), HIV, hepatitis B virus (HBV), Schmallenberg and Zika virus, can be transmitted from mother to foetus during pregnancy, childbirth, or suckling. This route of transmission is known as vertical or perinatal transmission. 5. Vector-Borne Transmission: Vector-borne viruses are transmitted to humans through the bite of infected arthropods, such as mosquitoes, ticks, and sandflies. Examples of vector-borne viruses include dengue virus, Zika virus, chikungunya virus, West Nile virus, and Schmallenberg virus. The vector serves as a biological (when the virus replicates in the insect) or mechanical (when it is simply carried on the mouthparts of the insect) carrier of the virus, facilitating transmission from one host to another. 6. Sexual Transmission: Certai

AB2 Virology Handbook 2024 PDF

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue