Viral Infections

Mechanisms of viral injury: 

 Viruses damage host cells by entering the cell and replicating at the host’s expense.

Viral tropism -in part caused by the binding of specific viral surface proteins to particular host cell surface receptor proteins.  

The second major cause of viral tropism is the ability of the virus to replicate inside some cells but not in others. For example, JC papovavirus, which causes leukoencephalopathy is restricted to oligoderdroglia, in the CNS. 

-  Once attached the entire viron or a portion containing the genome and  the essential polymerase penetrate into the cell cytoplasm in one of the three ways 

1)  Translocation of the entire virus across the plasma membrane

2)  Fusion of viral envelop with the cell membrane  or

3)  Receptor -mediated endocytosis of the virus and fusion with endosomal

membranes 

Within the cell, the virus uncoats separating its genome from its structural component and

losing its infectivity.  Viruses also use host system for viral synthesis. 

-  Newly synthesized viral genome and capsid proteins are then assembled into progeny

virons in the nucleus or cytoplasm and are released directly (unencapsulated viruses)

or bud through the plasma membrane (encapsulated viruses) 

-  Viral infection can be abortive with incomplete replicative cycle 

-  Latent in which the virus (eg herpes zoster) persists in a cryptic state within the dorsal

root ganglia and then present with painful shingles 

-  Or persistent in which virons are synthesized continuously with or without altered cell

function (eg. Hepatitis B) 

-  Viruses kill host cell in a number of ways 

-  Viruses infect host cell DNA ,RNA or protein synthesis, ex. Poliovirus

-  Viral proteins insert into the host cells plasma membrane and directly damage its

integrity or promote cell fusion ex HIV ,measles herpes viruses (direct cytotopathic effect).

-   Viruses replicate effiently and lyse host cell ex yellow fever virus in liver and neurons by poliovirus. 

-  Viral proteins on the surface of the host cell are recognized by the immune system, and the host cytotoxic lymphocytes then attack the virus-infected cells ex hepatitis B virus infection, and respiratory synaytial virus.

-  Viruses damage cells involved in host antimicrobial defense leading to secondary infection for example viral damaged respiratory epithelium allows subsequent bacterial pneumonias and HIV depletes CD4+ T cell predisposing to opportunistic infection. 

-  Viral killing of one cell type causes the death of other cells that depend on them, Example poliovirus cause motor neuron injury and atrophy of distal skeletal muscle.

-  Slow virus infection cause in severe progressive disease after a long latency period for example sub-acute pan encephalitis caused by measles virus. 

-  Some viruses induce cellular proliferation and transformation example HBV, HPV, EBV which result in neoplasm.

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.