Viral Pathogenesis PDF

VIRAL PATHOGENESIS Prepared By Dr. Mervat Abdel-Aziz Medical Microbiology, Immunology & Infection Control  PATHOGENESIS OF VIRAL DISEASES 1 I. Entry of viruses Viruses enter the body either by inhalation, ingestion, contact (urogenital system) and through...

VIRAL PATHOGENESIS Prepared By Dr. Mervat Abdel-Aziz Medical Microbiology, Immunology & Infection Control  PATHOGENESIS OF VIRAL DISEASES 1 I. Entry of viruses Viruses enter the body either by inhalation, ingestion, contact (urogenital system) and through skin (injections, blood transfusion, insect and animal bites). Then replicate in the primary site of entry. Some viruses produce disease at the portal of entry (local infections), others have to spread to distant organs either via the blood (viraemia), or by other means, e.g. along nerves and produce systemic or deep viral infections.  Differences between local and systemic viral infections Local Systemic infections infections Specific disease Respiratory Measles example (common cold e.g. rhinovirus) Site of pathology Portal of entry At distant sites Incubation period Relatively Relatively long short Viraemia Absent Present Duration of immunity Usually short Usually life-long Role of secretory IgA important Usually not important  II. Immunity to Viruses Viruses are obligatory intracellular parasite , after replication inside host liberate to infect another cell A- Innate Immunity NK cells: Cytotoxic killing of virally infected cells by NK cells early in the course of infection. Type I interferon: Virally-infected cells produce type I interferon (IFN-α and IFN-β): ◦ They inhibit viral replication and induce an antiviral state. ◦ They activate NK cells. ◦ They increase the expression of MHC I molecules, allowing better presentation of viral peptides.  B- Specific Immunity Humoral immune response Specific antibodies are important in defense against viruses before they enter their target cells, and against viruses released from infected cells. Antibodies’ functions against viruses: 1. Neutralizing antibodies bind to viruses and prevent viral attachment and entrance into host cells. 2. Opsonizing antibodies enhance phagocytosis. 3. Activation of complement. Secretory IgA is important in neutralizing viruses that enter through the mucosa, while circulating antibodies (IgM & IgG) are effective against viruses which pass through the blood stream before reaching their target cells. B- Specific Immunity (Cont.) T cell response Cytotoxic T cells: The main mechanism of specific immunity against established viral infections is killing of infected cells by Tc cells. Helper T cells: These contribute by secretion of cytokines. The Th1 cytokines IL-2 and IFN-γ are most important in this respect: ◦ IL-2 promotes proliferation and activation of Tc cells and also activates NK cells. ◦ IFN-γ activates NK cells.  III. Fate of viral infections 1. Inapparent or subclinical viral infections: Viral infection without overt signs and symptoms. 2. Apparent infections (disease): This may be local or systemic with the appearance of clinical signs and symptoms. 3. Persistent viral infections (chronic): In this form, the virus is continuously detected with mild or no clinical symptoms, e.g. chronic hepatitis B.  II. Fate of viral infections 4. Latent viral infections: The virus persists in a dormant form and may flare up intermittently to produce disease, e.g. herpes viruses. 5. Slow virus infections: Virus infections with long incubation periods (months or years). They are caused by two types of infectious agents: - Conventional viruses, e.g. a variant of measles virus which causes subacute sclerosing panencephalitis (SSPE). - Unconventional agents (prions).  Viral zoonosis Rabies Influenza A Viruses: Viral haemorrhagic fever viruses as » Yellow fever virus » Dengue fever » Rift valley fever virus » Ebola virus and murburg virus  CLASSIFICATION OF VIRUSES 1 A- Classification by symptomatology: It is the old classification based on diseases they produce, i.e. tropism, e.g. neurotropic viruses, enteroviruses,... etc.  CLASSIFICATION OF VIRUSES 2 B- Classification of viruses into families, genera, and species, based on: 1. Type of nucleic acid (DNA or RNA). 2. Virus size. 3. Virus symmetry. 4. Presence or absence of envelope. 5- virus replication strategy RNA VIRUSES DNA VIRUSES  Diagnosis of viral infections A- Direct methods B- Indirect methods I. Direct II. Isolation of I. Serology II. Skin tests detection of viruses viruses and / or their components Dr /MERVAT ABDELBASEER TOHAMY