Viral pathogenesis.pdf

Dr. Fatima Salah Mohammed  Pathogenesis is the process by which a virus infect and leads to disease  Viral tropism: is the viral affinity for specific body tissue and is determined by cell receptors ,cell transcription factors, local environment  Congenital infection: is infection of the fetus as...

Dr. Fatima Salah Mohammed  Pathogenesis is the process by which a virus infect and leads to disease  Viral tropism: is the viral affinity for specific body tissue and is determined by cell receptors ,cell transcription factors, local environment  Congenital infection: is infection of the fetus as a target organ  Incubation period: is the time between exposure to virus and onest of disease  Viruses effects on the host can be divided into :  Effects at the level of infected cell  Effects at the level of infected person When viruses infect any cells one or more of the following changes are observed: 1. Cell death : due to inhibition of synthesis of cellular proteins by viral proteins synthesis eg. Rabies virus 2. Fusion of cells to form multinucleated cells: due to cells membranes changes eg. Herpes viruses 3. Malignant transformation: the cell characterized by prolong survival and unstrained growth 4. No apparent morphological and functional changes : the cell survives and multiplies despite viral replication  1. 1. 2. 3. 4. Implantation of virus at the portal of entry Many different routes of transmission eg. Oral route, respiratory route, Parenteral route Viral local replication Spread to target cells and organs :infection can be localized or disseminated Cell damage and death and induce host immunity (Pathogenesis and immunopathogenesis ) is the balance between virus and host defense Spread to the sites of shedding of virus into the environment: The signs and symptoms of most viral diseases are the result of direct cell damage by one of the following mechanisms:  Diversion of cell energy  Shutoff cellular macromolecular synthesis  Competition of viral promoters and transcriptional factors the end result is cell killing by virus results in a loss of function and in the symptoms of disease.  For example, when poliovirus kills motor neurons, paralysis of the muscles innervated by those neurons results.  There are some diseases that are not caused by the virus damaging or killing the infected cell.  For example rotavirus-induced diarrhea is caused primarily by stimulation of the enteric nervous system., resulting in excess fluid and electrolyte secretion into the bowel lumen.  There are other diseases in which cell killing is mediated by immunologic attack Both cytotoxic T cells and antibodies play a role in immunopathogenesis  In certain instances the virus persist for prolong period of time  There are three types of persistent viral infection: 1. Carrier state: virus is produced continuously eg. Hepatitis c 2. Latent infection: the virus is not produced but can be reactivated at a subsequent time eg. herpes simplex virus ,Varicella zoster virus 3. Slow virus infection: there is prolong period between the initial infection(incubation period) and onset of disease eg  1. 2. 3. 4. Incubation period in which the patient is asymptomatic Prodromal period in which nonspecific symptoms occur like mild fever, myalgia ,flu like illness,…. Specific illness period Recovery period in which the illness regress and the patient regain good health  Host defenses against viruses fall into two major categories:  (1) nonspecific, of which the most important are physical barriers, interferons and natural killer cells  (2) specific, including both humoral and cell-mediated immunity.  Mutations in the viral genetic material  Viruses have several ways by which they evade our host defenses , These processes are often called immune evasion. Two very important processes are :  (1) synthesis of receptors for immune mediators  (2) reduction of expression of class I major histocompatibility complex (MHC) proteins  Several factors influence host defenses eg.:  Age :general, infections are more severe in neonates and in the elderly than in older children and young adults.  Increased corticosteroid levels predispose to more severe infections  Malnutrition leads to more severe viral infections