Pathogenesis of viral infection
Viral Pathogenesis Viral pathogenesis is the process by which a viral infection leads to disease. Viral pathogenesis is an abnormal situation of no value to the virus. The majority of viral infections are subclinical. It is not in the interest of the virus to severely harm or kill the host. The consequences of viral infections depend on the interplay between a number of viral and host factors.
Factors in Viral Pathogenesis Effects of viral infection on cells (Cellular Pathogenesis) Entry into the Host Course of Infection (Primary Replication, Systemic Spread, Secondary Replication) Cell/Tissue Tropism Cell/Tissue Damage Host Immune Response Virus Clearance or Persistence
Routs of infection Inhalation ;e.g influenza viruses Ingestion : polio viruses Parentral : AIDs Transplacental, cytomegalovirus,rubella. Following infection the virus is transmitted by blood, cells, along nerves and become localized in certain tissue which it prefer (tropism) e.g. polio, rabies etc
Routes of entry: ingestion inoculation sexual Inhalation Blood organ t/plant Congenital / vertical
Mechanisms of viral injury 1- Inhibits host cell DNA, RNA or protein synthesis e.g. Poliovirus. 2- Direct cell killing by damaging host cell membrane e.g. Rhinoviruses. 3- Induce Immune reaction e.g type III &VI hypersensitivity reaction in respiratory cyncytial viruses, arbovirus encephalitis.
Mechanism of viral injury (cont) 4- Damage host defence mechanism e.g. respiratory epithelium predisposes to the pneumonia by Staph. Pneumoniae &Haemophilus influenza. Immune system in AID 5-Induce cell proliferation & transformation result in neoplasia e.g. HBV, EBV.
Microscopical picture of viral infection Mononuclear infiltration Tissue necrosis Giant cell formation Inclusion body formation.(intranuclear or intracytoplasmic)
Viral inclusion bodies viral particles aggregate within the cells they infect & form characteristic ( Inclusion bodies) nuclear inclusion surrounded by a clear halo as in Herpes virus cytoplasmic inclusions as in small pox and rabies virus Many viruses do not give rise to inclusions e.g. EBV.
Lymphocytic infiltrate within myocardial fibers in viral myocarditis.
Individual hepatocytes are affected by viral hepatitis. A large pink cell undergoing "ballooning degeneration" is seen below the right arrow. At a later stage, a dying hepatocyte is seen shrinking down to form an eosinophilic "councilman body" below the arrow on the left.
Viral pneumonia due to respiratory syncytial virus. Note the mononuclear interstitial infiltrate with scattered giant cells, The pink, rounded intracytoplasmic inclusion in the giant cell of the inset at the upper right represents viral particles.
This is cytomegalovirus (CMV) infection in the lung. Note the very large cells that have large violet intranuclear inclusions with a small clear halo. Basophilic stippling can be seen in the cytoplasm.
Some examples of generalised viral infections:chicken pox characterized by generalized pastular erruption.
herpes virus tends to stay dormant within ganglion of cranial nerve and reactivated ++++++++++ after a latency period +++ leading to appearance of vesicles in the dermatome supplied by that nerve
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