Why did it have to be snakes?
Turns out that I do greatly empathize with Indiana Jones. Via the New York Times, and reporting research published in the American Journal of Tropical Medicine and Hygiene from the University of South Florida and Auburn University, a study that describes the detection of Eastern Equine Encephalitis Virus in hibernating snakes. EEEV is one of a family of closely related viruses (West Nile Virus is a member of this family) that causes fatal disease in horses and about a dozen humans annually. Like West Nile, the disease is spread via a mosquito vector, and so it shows a seasonal variation that reflects the life of the insect. The normal host of these viruses are in birds, where they tend to not cause very significant levels of disease. When they jump to mammals, the disease is much more severe.
This brings up a conundrum as to where the virus resides during that portion of the year when the vector is essentially absent. West Nile Virus and Equine Encephalitis Virus were thought to be maintained in avian reservoirs over the winter, and that a further explanation for why the incidence of disease is essentially zero during the winter is because not only is the vector (mosquito) not present during the winter months, the reservoir (birds) fly south for the winter.
It was quite surprising then when the researchers in this study collected hibernating snakes and mosquitoes from swamps in Alabama. They identified snake blood in the digestive tracts of the mosquitoes and found molecular signs of Equine Encephalitis Virus in the snakes. This data clearly indicated that the virus can be propagated in a reptile reservoir as well as in birds. Viral titres peaked in the snakes in the spring and fall–in the summer the snakes innate immune response works better with the higher temperature, and are able to clear the viral infection. The presence of the virus in reptile reservoirs, which do not migrate for the winter, helps to explain how mammalian infections begin to rise so quickly in the spring before all of the birds return to the area. This research can likely be extended to other Encephalitis Viruses, and other potential viral reservoirs.