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Questions and Answers
What shape does an icosahedral capsid have?
Which characteristic is NOT associated with the biological role of the capsid?
How are viruses classified?
What is a key feature of enveloped viruses?
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What is a non-enveloped virus?
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What type of nucleic acid do viruses possess?
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Which of the following is a method of viral acquisition?
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What is the first step in the viral replication process?
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Which virus is associated with the disease Chickenpox?
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Why must viruses invade host cells?
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Which of the following statements about viral infections is true?
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What is a common characteristic of Hepatitis C as a viral infection?
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Which method is NOT used in the laboratory diagnosis of viral infections?
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Which statement accurately describes the role of molecular testing in diagnosing viral infections?
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What is one of the main challenges in clinically diagnosing viral infections?
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What initiates the viral attachment process to a host cell?
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Which method is used for internalizing the virus into the host cell?
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During which stage of the viral life cycle is the genome made available to the host cell?
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Where does protein synthesis occur during the viral life cycle?
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What happens to non-enveloped viruses after assembly?
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What is one possible effect of viral pathogenesis on host tissue?
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Which mode of viral spread involves transmission through peripheral nerves?
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What occurs in nuclear spread during viral infection?
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What is a major disadvantage of viral culture?
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Which microscopy technique is used for rapid detection of viral antigens?
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What is the primary purpose of performing serology testing?
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What does the presence of cytopathological effects indicate in a viral culture?
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What is a notable limitation of using viral culture for viral diagnosis?
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How are viruses classified?
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What is the significance of taking paired sera samples in serology?
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What is the primary characteristic of viruses?
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Which method is often used for rapid diagnosis and quantification of viral infections?
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What is a characteristic feature of chronic viral infections?
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Which statement best describes the role of serological tests in diagnosing viral infections?
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Which type of viral infection is most likely to be severe in vulnerable individuals, like transplant patients?
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What is the main challenge in the clinical diagnosis of viral infections?
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What is essential for a virus to replicate within a host?
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Which of the following viral acquisition methods is NOT correctly paired with its associated viruses?
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Which step comes after penetration/entry in the viral replication process?
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Which of the following accurately describes the nucleic acid classification of viruses?
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What is the main reason that viruses need to invade a host cell?
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What is a key feature of viruses that distinguishes them from living organisms?
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Which component is responsible for protecting the virus's genetic material?
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What type of nucleic acid can be found in viruses?
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What is the term for the total genetic material and protein coat of a virus?
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Why are viruses considered obligate intracellular parasites?
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What does the presence of an envelope in a virus typically indicate?
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What happens to viruses once they infect a host cell?
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What is the primary method through which viruses enter the host cell?
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What is the role of capsomers in a virus?
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What is the purpose of uncoating in the viral life cycle?
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During the synthesis stage, where is viral protein synthesis always located?
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Which process do enveloped viruses primarily use to exit infected host cells?
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What interaction can allow a virus to establish a systemic infection in the host?
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What effect does cytopathogenicity have on the host tissue?
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What is one way that viruses can escape the host's immune response?
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How can viruses spread from one infected cell to another intracellularly?
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What is a major disadvantage of using viral culture for diagnosing viral infections?
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What is the primary advantage of viral culture in laboratory diagnosis?
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Which microscopy technique utilizes commercially produced antibodies for detection?
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What does the rise in antibodies in paired sera samples indicate?
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Which of the following viruses cannot typically reproduce within standard cell cultures?
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What does the 'cytopathological effect' (CPE) indicate in viral culture?
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Which technique is NOT involved in confirming the identification of a virus in viral culture?
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What is the role of electron microscopy in viral diagnosis?
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What must enveloped viruses do to survive during transmission?
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Which of the following characteristics does NOT apply to the capsid of viruses?
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How are viruses primarily classified according to their properties?
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What is a key implication of the statement that viruses are technically not living entities?
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What characteristic of enveloped viruses contributes to their vulnerability to environmental conditions?
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Which diagnostic method is primarily used to detect and quantify viral nucleic acids?
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What is a common challenge faced in the clinical diagnosis of viral infections?
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Which virus is known for causing intermittent reoccurrences in patients?
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What type of viral infection is particularly severe in immune-compromised individuals, such as transplant patients?
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Which process occurs immediately after uncoating in the viral replication cycle?
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What is the purpose of qualitative testing in laboratory diagnosis of viral infections?
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What is the primary reason viruses cannot replicate independently?
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Which mode of viral acquisition is associated with the transmission of HIV?
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Which type of viral nucleic acid is associated with the Influenza virus?
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What is the last step in the viral replication process before new virions are released?
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During viral attachment, which component interacts with the host cell receptor?
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What is the primary purpose of uncoating in the viral life cycle?
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Which statement accurately describes protein synthesis during the viral life cycle?
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What distinguishes the process of budding in enveloped viruses from the lysis of non-enveloped viruses?
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Which of the following describes how viruses can escape immune responses?
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What is a characteristic feature of latent viral infections?
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What form of spread involves the virus using circulatory systems to reach distant sites?
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Which of the following mechanisms does NOT describe how viruses can spread from one host cell to another?
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What does the presence of haemoabsorption in viral culture typically indicate?
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Which limitation of viral culture is primarily due to the condition of the specimen?
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What is a major reason that certain viruses, like Hepatitis B, cannot be cultured in cell cultures?
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Which microscopy technique is best suited for examining virus-induced masses within host cells?
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Why is it necessary to take paired serum samples for serological testing?
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What can be inferred if no cytopathological effects are observed in a viral culture?
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Which of the following statements about immunofluorescence microscopy is true?
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What is a unique advantage of using electron microscopy over other microscopy techniques in viral diagnosis?
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Study Notes
Capsid Morphology
- Capsid protects viral genome
- Different shapes: Icosahedral, Helical, and Complex
- Icosahedral capsids have 20 faces, each an equilateral triangle
Biological Role of the Capsid
- Introduces viral genome into host cells by interacting with host cell receptors
- Contains antigens that stimulate the host immune system
- Resistant to drying, heat, detergents, acids, and proteases
Viral Classification
- Viruses are not living entities
- Classified based on:
- Phenotypic characteristics like morphology
- Genotypic characteristics such as nucleic acid type
- Mechanism of replication
- Hosts they infect
- Diseases they cause
The Envelope
- Lipid bilayer surrounding the capsid composed of phospholipids and glycoproteins
- Derived from host cell membranes by budding
- Contains viral proteins that attach the virus to receptors on susceptible host cells
- Viruses without an envelope are called ‘non-enveloped’ viruses
Properties of Enveloped Viruses
- Confers a distinctive serological characteristic for diagnosis
- Surface polypeptides bind specific receptor sites on the host cell
- Viral antigens stimulate host immune responses
- Sensitive to drying, heat, detergents, and acid
Genetics
- Viral genome is composed of circular or linear nucleic acid.
Genetic Classification
- DNA or RNA, never both
- Single-stranded or double-stranded
- Sense
Viral Acquisition
- Modes of acquisition include vectors, inhalation, fecal-oral, bloodborne, sexual, and congenital.
Examples of Common Viruses
-
DNA Viruses:
- Herpesvirus family: Herpes simplex (cold sores), Varicella zoster (chickenpox, shingles), Epstein-Barr (infectious mononucleosis)
- Hepadnavirus family: Hepatitis B
- Orthomyxovirus family: Influenza
-
RNA Viruses:
- Picornavirus family: Polio, Hepatitis A
- Retrovirus family: HIV
Viral Replication
- Viruses reproduce only within an appropriate cell
- Do not possess the necessary machinery to replicate independently
- Invade cells and utilize the cell's reproduction system to make copies
Viral Replication Steps
- Attachment/Fusion
- Penetration/Entry
- Uncoating
- Synthesis/Replication
- Assembly
- Release
Attachment and Penetration
- Viral attachment occurs through capsid protein/glycoprotein binding to host cell receptors
- Virus internalized into the cell through endocytosis or envelope fusion with the plasma membrane
Uncoating
- The viral nucleic acid is released into the cytoplasm or nucleus, making the genome available for replication.
Synthesis and Assembly
- Viral nucleic acid and proteins are synthesized using various strategies, including nucleic acid synthesis in the nucleus or cytoplasm and protein synthesis always in the cytoplasm.
Assembly and Release of New Virus from Cell
- Viral components assemble into complete viruses.
- Release from the host cell:
- Non-enveloped viruses: Lysis or exocytosis
- Enveloped viruses: Budding
Viral Pathogenesis
- Interaction with the target tissue: Local or systemic infection (viremia)
- Cytopathological activity: Effect on host tissue
- Immune response: Virus escape and type of immune response initiated
- Latent infection: Cell becomes malignant or pre-malignant
- Transformation: Subsequent reactivation
- Death: Cytopathogenic effect
Viral Spread
- Intracellular (cell to cell): Intercellular bridges, like Herpes Simplex Virus (HSV)
- Extracellular: Released from infected cells and spreads locally and distally (haematogenous/lymphatic cells), like Influenza
- Neural: Peripheral nerves to CNS, like Rabies and HSV
- Nuclear: Viral genome incorporated into host genome, passed onto succeeding cell generations, like HIV
Outcome of Viral Infections
- Many viral infections are mild and self-limiting: Coronaviruses causing the common cold
- May be severe in vulnerable patients: Cytomegalovirus in transplant patients
- May be silent: Hepatitis C
- May reoccur intermittently: Herpes simplex virus (cold sores)
- May be fatal: Rabies
- Infections can be acute or chronic
Clinical Diagnosis
- Difficult to distinguish from bacterial infection
- Sometimes obvious, based on symptoms
Laboratory Diagnosis of Viral Infection
-
Qualitative:
- Detect and identify the virus
- Determine if it is viable
-
Quantitative:
- Determine the quantity of the virus
Methods of Laboratory Diagnosis
-
Molecular Methods:
- Detect viral DNA or RNA
- Examples: PCR and RT-PCR
-
Serological Methods:
- Detect viral antigens or antibodies
- Examples: ELISA
-
Microscopy:
- Visualize the virus (electron microscopy - EM) or effects of the virus (light microscopy - LM)
Molecular Methods
-
PCR and RT-PCR:
- Fast
- Sensitive
- Allow for quantification
- Used for rapid diagnosis, treatment guidance (HIV viral load), and detection of drug resistance
Microscopy
-
Immunofluorescence (Microscope):
- Rapid detection of viral antigens in specimens using antibodies
-
Light Microscope - Inclusion Bodies:
- Virus-induced masses within the cytoplasm of cells
- Can be pathognomonic without need for culture (Negri bodies in hippocampal cells for rabies)
-
Electron Microscope (EM):
- Specialized
- Can give quick answers to clinical questions
Viral Culture
- Viruses replicate only within living cells
- Living human or animal cells are grown in artificial culture
- Examination of cells for the effect of viral growth (cytopathological effect)
- Confirmation of virus ID by immunofluorescence, neutralization, or haemadsorption inhibition
Disadvantages of Viral Culture
- Slow (up to 4 weeks)
- Poor sensitivity
- Susceptible to bacterial contamination
- Sensitive to toxic substances
- Many viruses will not grow in cell culture
Major Advantage of Cell Culture
- Determines viability – Cytopathological effect (CPE) indicates viable virus
Serology
- Testing for viral antigens (e.g. HBsAg - Hepatitis B surface antigen)
- Testing for the presence of specific antibodies produced against viral antigens
- Requires paired sera: one at the onset of illness and another 7-14 days later
- Rise in antibodies confirms infection
Summary
- Viruses are composed of nucleic acid, a protein coat (capsid), and occasionally an envelope
- Depend on host cells for replication
- Classified based on shape, nucleic acid, disease, host, and replication mechanism
- Cause a diverse range of infections
- Diagnosis can be made clinically
- Laboratory diagnosis can be qualitative or quantitative and involves molecular, serological, microscopy, and culture techniques.
Introduction to Viruses and Viral Infections
- Viruses are small infectious agents containing either DNA or RNA, never both.
- They are totally dependent on a host cell for replication, relying on the cellular processes of their hosts to reproduce.
- Viruses are strict intracellular parasites.
- They are different from bacteria in many ways, including their structure, replication, and size.
Viral Morphology
- Virions are the complete, infectious viral particles.
- Capsid: A protein coat made of capsomers that encloses the nucleic acid.
- Nucleic acid: DNA or RNA, never both.
- Nucleocapsid: The combination of the nucleic acid and capsid.
- Envelope: A membrane surrounding the nucleocapsid, not always present.
- Capsomers: Protein subunits that make up the capsid.
- Viral nucleic acid can be circular or linear.
- Genetic classification: Based on DNA or RNA, single-stranded or double-stranded, and sense.
Viral Replication
- Viral replication occurs within an appropriate host cell.
- Viruses lack the necessary machinery to replicate independently, relying on the host cell's machinery.
-
Viral replication steps:
- Attachment/Fusion: The virus binds to the host cell receptor.
- Penetration/Entry: The virus enters the host cell through endocytosis or envelope fusion.
- Uncoating: The viral genome is released into the cytoplasm or nucleus.
- Synthesis/Replication: Viral nucleic acid and proteins are produced using host cell machinery.
- Assembly: New viral components assemble into new virions.
- Release: New virions are released from the host cell through lysis, exocytosis, or budding.
Viral Acquisition
- Viral infections are acquired through various routes, including:
- Vectors (animals or insects): Examples include rabies and dengue. - Inhalation: Examples include influenza, coronaviruses, and rhinovirus. - Faecal-oral: Examples include rotavirus, norovirus, hepatitis A, and E. - Bloodborne: Examples include hepatitis B and C, HIV. - Sexual: Examples include hepatitis B and C, HIV. - Congenital: Examples include rubella, CMV, hepatitis B and C, and HIV.
Viral Pathogenesis
- Viral pathogenesis describes the effects of viruses on the host.
- Cytopathological activity: The damage caused by the virus to host tissue.
- Immune response: The host's immune system response to the viral infection.
- Immunopathology: Immune-mediated damage to the host tissue as a result of the viral infection.
- Viral infections can be:
- Local: Infection confined to one area.
- Systemic: Infection that spreads throughout the body.
- Latent: Virus remains dormant in the host but can reactivate later.
- Transforming: The virus causes the infected cell to become malignant or pre-malignant.
- Persistent: The virus persists in the host for an extended period without causing symptoms.
Viral Spread
- Viruses spread through different mechanisms:
- Intracellular: From one cell directly to another.
- Extracellular: Released from infected cells and then spread to other cells locally and distally.
- Neural: Through peripheral nerves to the central nervous system.
- Nuclear: Viral genome incorporates into host genome, and is passed on to succeeding cell generations.
Outcomes of Viral Infections
- Viral infections can range in severity from mild and self-limiting to fatal.
- Many viral infections are mild and self-limiting, like coronaviruses causing the common cold.
- Some viral infections can be severe in vulnerable patients, like cytomegalovirus in transplant patients.
- Some viral infections are silent, like hepatitis C.
- Some viral infections can reoccur intermittently, like herpes simplex virus (cold sores).
- Some viral infections are fatal, like rabies.
- Viral infections can be acute or chronic.
Viral Diagnosis
- Viral infections can be difficult to distinguish from bacterial infections clinically.
-
Laboratory diagnosis:
- Qualitative: Determines the presence, identity, and viability of a virus.
- Quantitative: Measures the quantity of a virus.
-
Laboratory techniques:
- Molecular techniques (PCR and RT-PCR): Rapidly detect and quantify viral DNA or RNA, used for diagnosis, treatment guidance, and resistance detection.
-
Microscopy:
- Immunofluorescence microscopy: Rapid detection of viral antigens in a specimen by using commercially produced antibodies.
- Light microscopy: Examine inclusion bodies (viral-induced masses within cytoplasm) and may be pathognomonic.
- Electron microscopy: Provides a quick answer to clinical questions.
- Viral culture: Uses living cells to grow viruses to detect the 'cytopathological effect' or the ability of cells to stick to mammalian red blood cells (hemadsorption).
- Serology: Detects viral antigens or antibodies produced against them.
Advantages and Disadvantages of Diagnostic Techniques
- Molecular techniques: Fast, sensitive, and quantitative.
- Microscopy: Rapid for immunofluorescence, specialized for electron microscopy.
- Viral culture: Can determine viability.
- Serology: May require two samples, one at the onset and one later, to confirm infection.
Disadvantages of Viral Culture:
- Slow, may have poor sensitivity, susceptible to bacterial contamination, and many viruses will not grow in cell culture.
Capsid Morphology
- Capsids, the protective protein coats of viruses, come in various shapes and symmetries.
- Icosahedral capsids have 20 faces, each an equilateral triangle.
- Helical capsids have a cylindrical or rod-like shape.
- Complex capsids have irregular shapes and are not easily classified.
Biological Role of the Capsid
- Introduces the viral genome into host cells through interactions between capsid proteins and host cell receptors.
- Contains antigens that stimulate the host immune system.
- Resistant to drying, heat, detergents, acids, and proteases.
Viral Classification
- Viruses are not considered living organisms and can only infect host cells.
- Classified based on phenotypic characteristics (e.g., morphology, presence of an envelope), genotypic characteristics (e.g., nucleic acid type), replication mechanism, hosts they infect, and the diseases they cause.
Viral Envelope
- Composed of a lipid bilayer derived from host cell membranes during a process called budding.
- Contains host cell phospholipids and glycoproteins as well as viral proteins.
- Provides a distinctive serological characteristic for antigen identification.
- Surface polypeptides bind to specific receptor sites on host cells.
- Stimulates host immune responses.
- Envelopes are sensitive to drying, heat, detergents, and acids.
- Enveloped viruses must stay wet during transmission and cannot survive in the gastrointestinal tract.
- They do not always need to kill cells to spread.
Viral Genetics
- Composed of either circular or linear nucleic acid (DNA or RNA).
- Never both DNA and RNA.
Viral Acquisition Routes
- Vectors: Animal or insect transmission (e.g., rabies, dengue).
- Inhalation: Airborne viruses (e.g., influenza, coronaviruses, rhinovirus).
- Faecal-Oral: Spread through contaminated food or water (e.g., rotavirus, norovirus, hepatitis A&E).
- Blood-borne: Transmitted through blood contact (e.g., hepatitis B & C, HIV).
- Sexual: Transmission through sexual contact (e.g., hepatitis B & C, HIV).
- Congenital: Passed from mother to child during pregnancy or childbirth (e.g., rubella, cytomegalovirus, hepatitis B & C, HIV).
Viral Replication
- Viruses can only reproduce within a suitable host cell.
- They lack the necessary machinery to replicate independently.
- Requires invading a host cell and utilizing the cell's reproductive system for self-replication.
Viral Replication Stages
- Attachment/Fusion: The virus binds to a host cell receptor.
- Penetration/Entry: The virus enters the cell through endocytosis or envelope fusion with the plasma membrane.
- Uncoating: The viral genome is released from its capsid into the cytoplasm or nucleus.
- Synthesis/Replication: The virus synthesizes its nucleic acid and proteins using the host cell’s machinery.
- Assembly: Newly synthesized viral components assemble into new viral particles.
- Release: New viruses are released from the cell through lysis, exocytosis, or budding.
Viral Pathogenesis
- Interaction with target tissue can lead to local or systemic infections (e.g., viremia).
- Cytopathological activity: The effect the virus has on host tissue.
- Immune response: The host's response to the virus (can the virus escape the immune system?).
- Immunopathology: The damage caused by the immune response.
Effects of Viruses on Cells
- Latent infections: The virus remains dormant in the cell (e.g., herpesviruses).
- Transformation: The virus alters the cell, potentially leading to cancer (e.g., papillomavirus).
- Death: The virus kills the infected cell (e.g., many viruses).
- Infection: The cell becomes actively infected and produces new viruses.
Viral Spread
- Intracellular (cell to cell): Occurs through intercellular bridges (e.g., HSV).
- Extracellular: Released from infected cells and spreads to other cells locally and systemically through the bloodstream or lymph.
- Neural: Along peripheral nerves to the central nervous system (e.g., rabies, HSV).
- Nuclear: Viral genome incorporated into the host genome and passed onto succeeding generations of cells (e.g., HIV).
Outcomes of Viral Infections
- Many infections are mild and self-limiting (e.g., coronaviruses causing the common cold).
- Severe in vulnerable populations (e.g., cytomegalovirus in transplant patients).
- Silent (e.g., hepatitis C).
- Intermittent (e.g., herpes simplex virus causing cold sores).
- Fatal (e.g., rabies).
Viral Diagnosis
- Clinical diagnosis: Often difficult to distinguish from bacterial infections.
- Laboratory diagnosis: Uses various techniques to detect and identify viruses, such as molecular, serological, microscopy, and culture.
Laboratory Diagnosis Techniques
- Molecular (e.g., PCR, RT-PCR): Rapid, sensitive, and can quantify viral load (e.g., HIV viral load monitoring).
- Serological: Detects viral antigens or antibodies (e.g., HBsAg for hepatitis B).
- Microscopy: Uses methods like immunofluorescence, light microscopy (for inclusion bodies), and electron microscopy to visualize viruses or their effects.
- Viral culture: Used to determine viral viability. Viral components are grown in a cell culture, and the cytopathological effect is examined.
- Cell culture advantage: Can determine the viability of a virus. If cytopathological effect (CPE) is observed, the virus is viable.
Limitations of Viral Culture
- Slow: Result takes up to 4 weeks.
- Low sensitivity: Depends on the specimen condition.
- Contamination: Susceptible to bacterial or toxic substances.
- Not all viruses grow in cell culture.
Summary
- Viruses are composed of nucleic acid, a protein coat (capsid), and sometimes an envelope.
- They are totally dependent on host cells for replication.
- Classified by shape, nucleic acid, disease, host, and replication mechanism.
- Cause a diverse range of infections.
- Diagnosis can be made clinically or through laboratory techniques.
- Laboratory diagnosis can be qualitative or quantitative using methods like molecular, serological, microscopic, and culture techniques.
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Description
This quiz explores the intricate structures of viruses, focusing on capsid morphology and the role of the viral envelope. You'll learn how these components aid in viral classification and interaction with host cells. Test your knowledge about the properties and biological importance of viral structures.
