A mosquito-borne virus that is rare but kills some horses and humans along the Eastern Seaboard every year almost certainly survives winters inside snakes, a new study has found.

The Eastern equine encephalomyelitis virus is found from Florida to Massachusetts and in parts of Latin America. The virus causing the disease normally circulates only in birds, but when it jumps to mammals, it kills 90 percent of infected horses - and about a third of the 10 or so Americans who get it each year.

But birds fly away in winter and, unlike West Nile virus, the "triple E virus" cannot survive in hibernating mosquitoes.

Curious scientists at the University of South Florida and Auburn examined mosquitoes' guts and found snake blood. The researchers infected garter snakes in the lab with the virus and then made them "hibernate" in refrigerators. The virus lived.

So the scientists went to the Tuskegee National Forest swamp in Alabama, vacuuming mosquitoes out of beaver dens and drawing blood from poisonous cottonmouths.

They found that virus levels in the snakes peaked in spring and fall, said Thomas R. Unnasch, lead author of the study.

"Snakes' immune systems work better when it's warm," he said, so they do not clear the virus in cool weather. In spring, when they venture out to warm up, mosquitoes pick up the virus again by biting snakes.