Chapter 9 General Virology PDF

Kingdom of Saudi Arabia ‫اﻟﻣﻣﻠﻛﺔ اﻟﻌرﺑﯾﺔ اﻟﺳﻌودﯾﺔ‬ Ministry of Education ‫وزارة اﻟﺗﻌﻠﯾم‬ Qassim University ‫ﺟﺎﻣﻌﺔ اﻟﻘﺻﯾم‬ Coll...

Kingdom of Saudi Arabia ‫اﻟﻣﻣﻠﻛﺔ اﻟﻌرﺑﯾﺔ اﻟﺳﻌودﯾﺔ‬ Ministry of Education ‫وزارة اﻟﺗﻌﻠﯾم‬ Qassim University ‫ﺟﺎﻣﻌﺔ اﻟﻘﺻﯾم‬ College of Medical Rehabilitation ‫ﻛﻠﯾﺔ اﻟﺗﺎھﯾل اﻟطﺑﻰ‬ Microbiology [EDPT 211] Prof. Dr. Noha Mohammed Afifi Chapter 9 General Virology VIRUSES Can Infect All Organisms in Nature  1- Bacteriphages: are Bacterial Viruses.  2- Plant viruses.  3- Animal viruses: Infect vertebrates including man. Properties that differentiate Viruses from Bacteria: 1) Size: Viruses are very small in size (ranging from 20-300 nm), SO: - They can be seen only under the electron microscope (E.M.), EXCEPT Pox viruses. - They can pass through bacterial filters, need Ultrafiltration to be retained. 2) Genome: Viruses contain only one type of nucleic acid (either DNA or RNA), never both. 3) Metabolically: Viruses have NO metabolic activity outside the host cells. They do not have ribosomes or protein-synthesizing apparatus. Prof. Noha Afifi 89  (Viruses are Obligate Intracellular Parasites; can replicate only inside living cells and do not divide by binary fission. When viruses enter a host cell, they introduce their genetic material, DNA or RNA, inside the host cell. The virus directs the host cell metabolism to form new copies of viral genes and viral proteins). 4) Culture: Viruses can not be cultivated in the laboratory on artificial culture media; however, they can be grown on tissue culture. 5) Susceptibility to Antibiotics: Viruses are Not susceptible to antibacterial antibiotics. STRUCTURE of VIRUSES The typical complete virus particle, called Virion, consists of: 1. A core of nucleic acid (genome) 2. A protein coat (capsid) Both nucleic acid and the protein coat form the Nucleocapsid. 3. Some viruses, called Enveloped viruses, have an outer lipid-containg Envelop, while others are Non-enveloped viruses (Naked). Prof. Noha Afifi 90  Viral Nucleic acid (genome) It is either DNA or RNA. All DNA viruses are double-stranded (ds) except Parvoviruses, while All RNA viruses are single-stranded (ss) except Reoviruses. The viral genome is responsible for virulence, i.e. it is the infectious part of the virus. Therefore, Coreless Viruses are non- infectious. Capsid (Protein Coat): It is the protein coat that encloses the genome and it is made up of many small protein subunits called capsomeres. Capsid has the following functions: 1- It protects the genetic material from the effect of Nucleases. 2- It mediates the attachment of the virus to specific receptors on the host cell surface in non-enveloped viruses. 3- It is Antigenic----includes important antigens, which induce neutralizing antibodies and activate cytotoxic T lymphocytes (CTLs) to kill virus-infected cells. Prof. Noha Afifi 91 Viral Envelope: It is a lipoprotein membrane surrounding some viruses.  The lipid component is derived from the host cell membrane and the protein component is virus specific.  Frequently, the virus may have glycoprotein spikes on the surface of the envelope.  Importance of the glycoprotein spikes:  They mediate the attachment of the virus to specific receptors on the host cell surface in enveloped viruses. Therefore, dissolving the envelope inhibits attachment and the virus loses its infectivity.  They are Antigenic: include important antigens against which the host elicits its immune response to viruses.  Enveloped Viruses are less stable than naked Viruses; destroyed by heat, detergents and lipid solvents (as alcohol or ether) than non- enveloped viruses. Prof. Noha Afifi 92  Classification of VIRUSES A) Classification by Symptomatology (Tropism) It is the old classification based on diseases that viruses produce; i.e. Tropism e.g. neurotropic viruses (rabies), enteroviruses…. B) Hierarchial Virus Classification (Viral Taxonomy) The scheme classifying viruses into orders, families and subfamilies is based on: - Nature of the nucleic acid; RNA or DNA genome. - Virus replication strategy. - Presence or absence of an envelope. CORONAVIRUSES What are Coronaviruses? Coronaviruses (CoV) are a large family of viruses that cause illness ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV). Covid-19 is also known as SARS-CoV2. What is (COVID-19)? It is the new coronavirus and most cases appeared in the Chinese city, Wuhan at the end of December 2019 in the form of acute pneumonia, then became pandemic. Mode of transmission of COVID-19: The virus can spread from the infected person to another person through close contact by droplet infection. Coronavirus structure: COVID-19 is a spherical enveloped particle containing single-stranded (positive-sense) RNA surrounded by a capsid. The envelope bears glycoprotein spikes. Prof. Noha Afifi 93  VIRAL REPLICATION Viruses are metabolically inert, unable to replicate on their own, because they lack the genes and enzymes necessary for energy production. Therefore, replication depends on living host cells & is directed by the viral genome to produce the virus components. Virus Replication Cycle It can be divided into the following stages: 1. Attachment: This is a specific interaction where the virus attaches by viral surface proteins to specific receptors on the host cell plasma membrane (e.g. hepatotropic viruses and neurotropic viruses). 2. Penetration: occurs by two mechanisms:  In Non-Enveloped Viruses: Endocytosis: the virus invaginates the host cell.  Fusion may occur between the viral envelope and host cell membrane in case of enveloped viruses. 94 3. Uncoating: The protein coat (capsid) of the virus is removed. Nucleic acid is released & becomes available for replication. This is mainly carried out by host cell lysosomal enzymes. 4. Replication of viral genome & synthesis of viral proteins: The virus directs host cell to synthesize: - Viral genome is replicated (DNA or RNA). - Viral proteins (Capsid proteins) using host Ribosomes. 5. Assembly: Newly synthesized protein coats enclose the replicated nucleic acids to form new viruses (virion). Eclipse period This period from uncoating until assembly of mature viruses. During this phase, no infectious viruses can be detected in the host cell. Latent period Some viruses do not initiate synthesis and remain latent within the host cell for variable periods. 7. Release: The new viruses are released from the cell by one of two ways:  Rupture of the host cell (Cytolysis): usually occurs in non enveloped viruses.  Budding (Exocytosis): through the cell membrane, in case of enveloped viruses. Prof. Noha Afifi 95  Pathogenesis of Viral Diseases It is the interaction of viral and host factors that lead to disease production. Entry of Viruses into the host Viruses enter the body either by inhalation, ingestion, sexual contact or vertical transmission from a mother to her baby through the placenta, birth canal or breast feeding. Some viruses can enter the body through skin abrasions, injections or insect and animal bites. Viral infection may be Local or Systemic  Local Infection: Virus produce disease at the portal of entry.  Systemic or deep infection: the virus spreads to distant sites, either by the blood stream (Viremia) or by other means e.g. along nerves. Differences between Local and Systemic Viral Infections Local infections Systemic infections Example Common cold Measles (e.g. rhinovirus infection) Site of pathology Portal of entry At distant sites Incubation period Relatively short Relatively long Viraemia Absent Present Duration of Usually short Usually lifelong immunity Immunoglobulin Secretory IgA IgM and IgG Prof. Noha Afifi 96  Fate of Viral Infections 1. Inapparent / Subclinical / Silent / Abortive viral infections: - Viral infection without overt signs and symptoms. 2. Apparent / Overt / Clinical infection (Disease): - Local or systemic viral infections with the appearance of clinical signs and symptoms. 3. Persistent viral infection: Sometimes the virus persists in the host producing either chronic infection or latent infection: a) Chronic infection: Includes: - Chronic Active (has symptoms all the time) and - Chronic Carrier (No symptoms). In both types, the virus is continuously detected on investigations e.g. chronic hepatitis B. b) Latent infection: The virus persists in a dormant form without replication and may flare up intermittently to produce disease (as during stress), e.g. herpes simplex viruse remain dormant in trigeminal ganglia. The virus is NOT detected on investigations Prof. Noha Afifi 97  LABORATORY DIAGNOSIS of VIRAL INFECTION Laboratory diagnosis of viral infection involves 2 main diagnostic methods A) Direct methods: which depend on either the detection of viruses and/or their components in the patient's speimens OR on isolation of viruses. B) Indirect methods: which depend mainly on the detection of antibodies against the suspected virus in patient's serum, OR on Skin tests. The different techniques used in diagnosis of viral infections are discussed in the practical lab. TREATMENT OF VIRAL INFECTIONS  Viruses can not be treated with antibiotics because they lack the structural targets on which antibiotics can act. Viruses are obligate intracellular parasites, so anti-viral drugs must selectively inhibit viral replication, without causing damage to host cells. The number of anti-viral drugs is little, compared to anti-bacterial drugs. Prof. Noha Afifi 98