الرحيم الرحمن الله بسم

General Virology(contin.) PATHOGENESIS OF VIRAL DISEASES

PATHOGENESIS OF VIRAL DISEASES It is the interaction of viral and host factors that lead to disease production.

Steps of Viral Pathogenesis 1. Entry into the host 2. Local or primary replication 3. Viral spread to target organs and cell tropism 4. Effect of the virus on the host cell 5. Host immune response and recovery from infection 6. Fate of the virus after clinical recovery 7. Virus shedding into the environment

1. Entry into the host: Through 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 bite of an arthropod vector or an animal.

2. Local or primary replication Viruses usually replicate at the primary site of entry. Some viruses produce disease at the site of entry where they spread locally over the epithelial surfaces with no invasion of the underlying tissues or spread to distant sites, e.g. Influenza virus causes respiratory tract infection and Rotavirus causes gastrointestinal tract (GIT) infection.

3. Viral spread to target organs and cell tropism Many viruses produce disease at sites distant from site of entry, e.g. enteroviruses enter via the GIT and after primary replication, they spread within the host to produce central nervous system (CNS) disease. Routes of spread: a. Through blood (viremia) or lymphatics. b. In few cases neuronal spread occurs, as in rabies.

Mechanisms of cell tropism (viral affinity for specific body tissues) a. Presence of specific cell surface receptors for that virus. Poliovirus selectively infects a nerve cell with a high concentration of surface receptors for poliovirus. HIV infects cells with CD4 receptors.

b. Presence of certain proteolytic enzymes Certain paramyxoviruses are not infectious until their envelope glycoprotein undergoes proteolytic cleavage.

c. Local temperature, pH and oxygen tension e.g. rhinoviruses multiply exclusively in the upper respiratory tract, as they are adapted to multiply at low temperature and high oxygen tension.

4. Effect of the virus on the host cell a. Cell death The infection is lethal and kills the cell causing a cytopathic effect (CPE). The virally infected cells may also be killed by cytotoxic T lymphocytes (CTL) and NK cells. Cell injury produced by the virus causes lesions and dysfunction (symptoms and signs) in organs concerned. Some tissues rapidly regenerate e.g. intestinal epithelium, while others do not e.g. brain.

b. Cell transformation The cell is not killed but is changed from a normal cell to a cell with the properties of malignant or cancerous cell.

5. Host immune response and recovery from infection This involves humoral and cell mediated immune response, interferons and other host defense factors.

6. Fate of the virus after clinical recovery It is either one of two outcomes i-Complete resolution Most viruses do not remain in the host after clinical recovery being completely eliminated from the host body (viral clearance).

ii-Persistent viral infections Sometimes the virus persists in the host producing either chronic infection or latent infection. a. Chronic infection: Virus is continuously detected with mild or no clinical symptoms, e.g. HBV b. Latent infection: Virus persists in an occult form inside the infected cell. During latency, no viral markers could be detected. Intermittent flare up (reactivation) may occur where the virus can be recovered, e.g. Herpes viruses become latent in sensory ganglia.

7. Virus shedding into the environment It represents the time at which the infected person becomes infectious to contacts. Shedding usually occurs from body surfaces involved in viral entry. In some viral infections, the human represents dead-end host and shedding does not occur i.e. human is not infectious to others e.g. Rabies.

Types of Viral Diseases 1. Systemic Diseases : The virus spreads widely and invades many tissues and organs because of viremia. Usually associated with long incubation period. In most cases, it produces long lasting immunity. 2. Localized Diseases : The virus invades only tissues adjacent to the site of entry. Associated with short incubation period. It produces transient immunity.

LABORATORY DIAGNOSIS OF VIRAL INFECTION

LABORATORY DIAGNOSIS OF VIRAL INFECTION I-Direct Demonstration of the Virus or Its constituents in Clinical Specimens II-Virus Isolation III- Serological Detection of Specific Antibodies

I-Direct Demonstration of the Virus or Its constituents in Clinical Specimens A. Detection of viral particles by Electron Microscopy (E/M): It is required for detection of viruses of special morphology e.g. Herpes simplex viruses and Rota viruses.

B. Detection of viral inclusion bodies (IB) by light microscopy (L/M): These are eosinophilic or basophilic bodies stained with hematoxylin and eosin (H&E). They may be intracytoplasmic (e.g. rabies virus), intranuclear (e.g. herpes viruses), or both (e.g. measles virus).

Inclusion bodies represent: aggregation of mature virions or areas of altered staining at sites of viral synthesis or degenerative changes in the host cell.

C. Detection of viral antigens: Viral antigens are detected by enzyme linked immunosorbant assay (ELISA), immunofluorescence (IF) tests or radioimmunoassay (RIA) e.g. hepatitis B virus.

D. Detection of viral genome by molecular techniques 1. Polymerase chain reaction (PCR): It is a highly specific test. It detects minimal amounts of viral nucleic acid "DNA or RNA" by amplification of the specific nucleic acid sequences of the virus.

2. Hybridization reaction: Labeled nucleic acid probes are used to detect virus genome in tissues.

II-Virus Isolation A. Tissue culture Virus isolation needs living cells: On inoculation of the virus on a living susceptible cell culture, the virus infects the cell and replicates. Viral growth in infected cells is recognized by: Cytopathic effects induced by virus growth in tissue culture.

i. Cytopathic effects induced by virus growth in tissue culture a. The virus kills the cells: which detach from the wall of the tissue culture flasks. b. The virus causes fusion of tissue culture cells producing syncytia (multinucleated giant cell), e.g. respiratory syncytial virus. c. The virus causes rounding of cells, e.g. herpes viruses, or grape like cells e.g. adenoviruses.

Cytopathic effects induced by virus growth in tissue culture

2. Hemadsorption: It is the adsorption of erythrocytes to the infected cells due to the presence of viral- encoded hemagglutinins (influenza, parainfluenza) in cellular membrane.

4. Inclusion body formation: Detection of inclusion bodies is performed by L/M examination of virus-infected cells stained with H&E, e.g. Negri bodies in rabies virus infection.

3. Transformation: This is produced by oncogenic viruses that may lead to uncontrolled cell growth, loss of contact inhibition between infected cells and piling up of cells with formation of discrete foci.

5. Detection of viral antigens: Viral antigens can be detected on the surface of virus-infected cells by direct immunofluorescence.

6. Detection of viral nucleic acid: Molecular-based assays such as PCR or hybridization techniques provide rapid, sensitive and specific methods for detection.

7. Interference: Non-cytopathic viruses (e.g. rubella) interfere with replication and appearance of CPE produced by cytopathic-viruses, e.g. echovirus, which is added to the cell culture as an indicator.

Identification The isolated virus is identified by: 1. Neutralization of CPE with standard antiviral serum 2. Inhibition of hemagglutination or hemadsorption with standard antiviral serum. 3. Immunofluorescence with standard monoclonal antibody. 4. Electron microscopy.

B. Chick embryo Rarely used for diagnosis. It is used mainly for multiplication of viruses for production of vaccines. The virus is inoculated on: yolk sac or the chorioallantoic membrane or the amniotic sac. Identification: Viral growth causes death of the embryo, production of hemagglutinin or formation pocks.

C. Laboratory animals Some viruses can be only detected by inoculation in laboratory animals (e.g. mice, rabbits or monkeys). Virus growth results either in disease development or death of the laboratory animal. Identification: Neutralization of viral pathogenicity by standard antisera.

III- Serological Detection of Specific Antibodies Detection of antibody in patient's serum is the most widely used method of diagnosis. The following tests can be used:

1. ELISA. 2. Indirect immunofluorescence (IIF) test. 3. Radioimmunoassay (RIA). 4. Complement fixation test (CFT). 5. Hemagglutination inhibition (HI) test. 6. Neutralization test (Nt).

Types of specific antibodies detected by serology: 1. Detection of IgM: It appears early in infection and is therefore used for diagnosis of recent or acute infection. 2.Detection of IgG: A four-fold rise in IgG titer in paired serum samples with an interval of 10-14 days between the first and second sample, is diagnostic of acute infection.

If paired sera are not available, demonstration of IgM antibodies to the virus even in the first sample taken is diagnostic.

Thank You