BIOLOGY 2D03 Lecture Notes - Poliomyelitis, Viremia, Herpesviridae

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28 Jan 2013
Virus-Host Interactions (Lecture 23)
Viruses propagate locally in the host at or near the site of entry into susceptible
cells. Spread to distant sites via blood stream or lymphatics.
Virulence – the capacity of a virus, compared to other closely related viruses, to
produce disease in a host
Neurovirulence – the ability of a virus to cause disease in the brain or nervous
Even the most virulent viruses can cause subclinical, inapparent infections that
outnumber the cases of symptomatic illnesses (handled by innate or specific
immune response)
oInnate immune responses – non-specific
oAdaptive immune responses – (specific) anti-viral antibodies and cytolytic
T-cell responses
Determinants of tissue and cell susceptibility
Each virus has a characteristic host range evident at 2 levels:
oThe degree of species specificity
oCell or tissue specificity the major determinant of the pattern of disease
observed in the individual organism. Can be determined at several stages
of the virus multiplication cycle.
Diverse life cycles characterized in cell culture comprise patterns of infection
Acute infections – virus produced 7-14 after infection, symptoms appear and virus
is cleared in a month.
oInfluenza virus, rhinovirus (common cold), and mumps virus
oClearance of infection is due to vigorous immune responses to the virus as
well as lack of persistent or latent infections caused by these viruses.
Persistent infections – virus infection is not cleared either due to poor immune
response, lack of access of the immune response to the infectious site (such as in
the brain), or rapid changes in the virus so that the virus evades the host immune
oVirus and viral proteins continually produced throughout life of host.
oSymptoms may or may not appear at any time
oHepatitis and HIV
Latent infections – result after an initial acute infection when the virus in not
cleared from infected cells.
oUsually characterized by transient activation of the virus at a local site.
oSymptoms may or may not accompany reactivation
oHerpes viruses (Herpes simplex, varicella zoster, Epstein-Barr,
cytomegalo virus), polyomaviruses (SV40 and JC), HIV, and Hepatitis
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