1- General Properties of Viruses

1- General Properties of Viruses

- cannot be seen with OLM - obligate intracellular parasites (replicate inside host cell) - contain only one kind of nucleic acid ( RNA or DNA) as their genome. - N/a is encased in a protein coat (capsid) - may be surrounded by a lipid containing membrane (envelope). - The capsid contains many identical repeating subunits known as capsomeres. Virion -The complete infectious virus particle including envelope.

Classification of viruses

1. NA composition. 2. Size and morphology - symmetry, no capsomeres, presence of membranes. 3. Mode of replication. 4. Susceptibility to Physical & Chemical agents. 5. Presence of specific enzymes: reverse transcriptase. 6. Mode of transmission 7. Host tissue and cell tropisms 8. Immunologic response 9. Pathogenesis: inclusion body formation 10. Symptomatology

VIRAL REPLICATION:

1.Attachment, Penetration & Uncoating 2. Gene expression & Genome Replication 3. Assembly & Release RNA viruses synthesize mRNA in 4 different ways: a. ss RNA of positive polarity use their genome directly as mRNA b. ss RNA of negative polarity has own RNA dependent RNA polymerase (host cell has no RNA polymerase) to synthesize mRNA c. reovirus carries its own polymerase to transcribe ds RNA into mRNA d. ssRNA of positive polarity that is transcribed into ds DNA by RNA dependent DNA polymerase ( reverse transcriptase) in retroviruses.

CULTIVATION OF VIRUSES : 3 METHODS

1.Live animals. 2. Embryonated eggs Yolk sac, Chorioallantoic cavity, Chorioallantoic Mem, Amniotic cavity. 3. Tissue & Cell culture - Primary cell cultures: primary human amnion,monkey kidney - Secondary cell cultures - Continuous cell lines Virus can replicate independently.

OUTCOMES

A. CHRONIC-CARRIER : significant amounts of viruses produced. B. LATENT INFECTIONS :The patient recover from initial infection and virus production stops. May recur by the production of virus C. SLOW VIRUS INFECTIONS: Prolong period between the initial infection and the onset of disease. LABORATORY DIAGNOSIS: 5 approaches 1. identification of the virus in cell culture. 2. microscopic identification directly in the specimen. 3. serologic procedures to detect rise in antibody titre or presence of IgM antibody. 4. detection of viral antigens in blood or body fluids. 5. detection of viral nucleic acid in blood or the patient's cell.

Virus capsid & symmetry

Capsid - subunits ,c/d capsomeres. 1. cubic/icosahedral symmetry. 2. helical /cylindrical capsid symmetry. 3. Complex structure e.g. poxviruses If virion possess outer membrane - called enveloped virus, if not -known as naked virus Virus capsid proteins: protect the genetic material and mediate attachment to specific receptor on the host cell surface. Contain important antigen. Envelope: The envelope is lipoprotein membrane composed of lipid derived from the host cell membrane and protein that is virus specific. Spike like glycoprotein may be present.

Atypical virus like agents

Defective Virus : deficient for replication. It needs helper virus for replication. e.g. parvoviridae, Hepatitis D virus. Pseudovirions: contain host cell DNA instead of viral DNA within the capsid. Viroids : disease of plants. It is a single molecule of circular RNA without a protein coat or envelope. Prions (PrP:1920s)- The small proteinatious infectious particle and is sensitive to protein denaturing substances. No detectable nucleic acid. Resistant to nucleases.

STRUCTURE OF VIRUSES

Size & shape: Smallest: poliovirus. Largest: small pox. Viral nucleic acids- genome either DNA or RNA encodes information for replication of the virus. NA: single or double stranded, circular or linear and segmented or non-segmented. Most of RNA single stranded except the reoviruses. Most of DNA viruses double stranded except parvoviruses.

VIRAL PATHOGENESIS: 1. Changes within infected cell 2. Changes in the infected patient

Transmission - Horizontal: person to person. - Vertical: mother to offspring. - Direct: animal to human (bite). - Indirect: through insect vector. Persistent viral infections: 1. integration of a DNA provirus into host cell DNA e.g. retroviruses 2. immune tolerance 3. formation of virus- antibody complexes 4. location within an immunologically sheltered area (hidden, inactivated) e.g. the brain 5. rapid antigenic variation 6. spread from cell to cell - no exposing to antibody 7. immunosuppression e.g. AIDS

Viruses causing perinatal infections

Transplacental transmission: Cytomeglovirus, Parvovirus, Rubella virus, HIV, HBV, Herpes At the time of birth: HIV, HBV, HCV, Herpes simplex virus -2, human papillomavirus During breast feeding: HIV, HBV, Cytomegalo virus, Human T-cell leukemia virus