22 September 2010
Incubation time
Once a human is infected by a pathogen, the dendritic cells present the antigen to the T cells and the whole cascade results in the adaptive immune response. Why is it that in some virus infections the "incubation time" is longer than others before the clinical manifestation is seen?
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I am guessing that it's the way the virus infects and grows that is the major determinant of incubation time. For example, rabies is so slow to initiate symptoms that you have time to begin and complete immunization against it after the bite or other exposure. But why? Does it grow only within neurons, and very slowly? Any virologists out there?
ReplyDeleteYou are probably correct.I know that in chickenpox,the best time to detect virus in blood in the day before the rash. I also wonder if the virus tries to suppress the immune response. I know that herpes viruses do down regulate MHC expression.
ReplyDeleteA lot of studies are being done now to investigate why particular viruses have such a long incubation period, rabies, for example, whereas others are evident within a day or two. There have even been a handful of cases where HIV virus was contracted but showed no clinical manifestations (in blood tests) for over 2 years. But in general, the latency of viruses is still not totally understood.
ReplyDeletei just got back from the Neurovirology meeting. One of the hypothesis for long incubation time is the time taken for specific pathogens to overcome the innate immunity. Atleast this has been proposed for chickenpox virus.
ReplyDeleteAs a guess, is it possible that incubation period is related to the evolution of the virus? Influenza is spread by being aerosolized and coming in to contact with surfaces with the virus. It would be beneficial to induce symptoms quickly to infect other hosts. As in HIV, the virus would want to hide symptoms because this virus requires contact to spread. If the host does not know that they have the virus, they are more likely to engage in risky behavior (intercourse) that would increase the chances of finding a new host.
ReplyDeleteThere is also an interesting section in the book cited below that mentions incubation time. You can find it on google books. It mentions that viruses that can replicate at the infection site have shorter incubation times compared to viruses that require specific cell types to replicate.
Evans, Alfred S., and Richard A. Kaslow, eds. Viral Infections of Humans: Epidemiology and Control. 4th ed. New York: Springer, 1997.