Re: how long does the polio virus usualy stay in its host???

Jeremy,

Thanks for the interesting and challenging question.

More than 20 years ago it would have been really easy to answer your question. Since Polio has an incubation period of up to 35 days. Incubation period refers to the time from first exposure to the virus to the first appearance of symptoms.

The incubation period may be quiet for the host, but it is a busy time for the virus as it crosses the intestinal lining, becomes activated and initiates metabolic events in host cells that enable it to replicate itself, increase in number and break out to begin to infect the rest of the hosts body, especially parts of the nervous system.

Once symptoms appear the virus is expected to hang around and or be shed in gastrointestinal secretions for up to 14 days, at least this is what school health officials decided was the period of time you were supposed to stay out of school if you had polio. Therefore, 14 + 35 = 49 days. Under normal situations the virus should be cleared from the body by cellular and antibody immunity that develops. Some virus might persist in neural ganglia or in the spleen but not in a form that would be infectious.

The average time the virus may be excreted in secretions and its persistence in the body has lengthened during the past 20 years. This increase is due to infections occurring in individuals with Acquired Immunodeficiency (AIDS for instance) and other causes of immunodeficiency.

If the immune system does not make an adequate cellular and antibody immune response to the vaccine or the disease causing polio virus, the virus will persist and will be continuously shed in an infectious form. Studies of Polio persistence by scientists interested in the global irradication of polio have been instructive.

Cases of poliovaccine virus persistence among patients with vaccine- associated polio reported in the United States during 1975 1997 were reviewed and followed by virological testing. Less than a quarter of these subjects excreted poliovaccine viruses for up to 6 months. Poliovirus persistence declined over time with the greatest reduction happening between 5 and 10 years after the last oral vaccine dose. Although patients with poliovirus persistence because of immunodeficiency have chronically excrete virus, this group appears to be an unlikely source of poliovirus reintroduction in developed countries because of the rarity of the condition and its high fatality rate.

So your answer isn't so straight-forward but hopefully the attempts to eliminate polio from the globe will be successful and the rightness or wrongness of this answer will not be an issue for public safety.

Here are some URLs with more detailed information about polio.

http://www.journals.uchicago.edu/JID/journal/issues/v188n12/30918/30918.html

http://aje.oxfordjournals.org/cgi/content/abstract/150/10/1001

http://www.health.vic.gov.au/ideas/bluebook/poliomyelitis.htm

I hope this helps.