Sean Graham / American Society of Tropical Medicine and Hygiene
A cottonmouth, also known as a water moccasin. A new study shows cottonmouths and other vipers can harbor easterm equine encephalitus virus
Snakes may provide a winter hiding place for a virus that’s causing an unusually severe outbreak in the U.S. northeast this year, and this could be good news for control efforts, researchers said on Monday.
They found eastern equine encephalitis virus (EEE for short) in cottonmouths and copperhead snakes and said it’s likely the reptiles incubate the virus while they hibernate over the winter. When they come out in spring, mosquitos feast on the snakes and then pass it to birds.
The study, published in the American Journal of Tropical Medicine and Hygiene, explains why EEE keeps coming back in northern states where mosquitos hunker down for the winter and where many birds take off for warmer climes. “There must be a way that the virus manages to overwinter. We think it’s the snakes, because they do overwinter in these sites,” said Dr. Thomas Unnasch of the University of South Florida, who led the study.
Eastern equine encephalitis is rare but highly deadly – it kills about a third of victims who develop the worst symptoms -- and there’s no cure for it. Survivors often have permanent brain damage. It infects people, horses and birds along the Atlantic and Gulf coasts, Michigan and Ohio. Usually there are only about six human cases a year according to the Centers for Disease Control and Prevention, but seven people in Massaschusetts have been affected this year alone. While there is a vaccine for horses, there is not one for people, and two have died this year from EEE.
It’s a year-round threat in Florida but further north it doesn’t start affecting horses or people until late summer. Unnasch and colleagues have been trying to figure out why for years.
“You know how they used to say follow the money? We followed the mosquito,” Unnasch told NBC News. Female mosquitoes spread the virus as they look for blood meals to nourish their eggs.
In the summer, it’s clear, the virus infects birds. They don’t get sick and their bodies get rid of the virus after a few days, but because there are so many birds the mosquitoes keep it circulating. “You have a pretty good, high concentration of mosquitoes then that have this infection,” Unnasch says.
“Now all the birds decamp for the winter. They migrate. You have these ladies running around, these mosquitoes, and they can’t find their favorite hosts, the birds and so they feed on whatever they can find.” That's when humans and horses are at risk.
Scientists couldn’t figure out what other animals were harboring the virus, so they tested the blood in infected mosquitoes. It turned out that mosquitoes infected with EEE had often just feasted on snake blood. How? “Right around their eyes,” Unnasch said. The membrane that covers a snake’s eyes is soft and mosquitoes can feed there.
Tests on cottonmouths in the Tuskeegee National Forest in Alabama confirmed a fairly high percentage were infected with EEE, as well as at least one copperhead. It doesn’t make the snakes sick and they seem to stay actively infected for months on end, Unnasch said.
So when it gets cold up north and the migrating birds leave, the snakes are still infected with EEE and they hibernate. In the spring, newly hatched mosquitoes feed on the snakes and get infected. “They act as the Typhoid Mary of the virus,” Unnasch said.
Copperheads are related to rattlesnakes and there are plenty of both in the north, Unnasch noted.
This is actually good news for control efforts, Unnasch says. “What usually happens is we pretty don’t worry about this too much until it jumps out and starts infecting humans and horses,” he said. That’s usually late in the summer, when the mosquitoes get desperate enough to bite people and horses.
Public health officials may be waiting until too late to try to control the outbreaks, Unnasch said. “They’ll go out spray like crazy and try to control the mosquitoes.” But it’s expensive then.
But if the virus is only beginning to escape in spring, it might be cheaper and easier to go to the swampy areas where the snakes are hibernating, and spray for the relatively few mosquitoes that are around then. “You might be able to interrupt the viral amplification cycle,” Unnasch said, “You might be able to do ounce of prevention, which is better than a pound of cure.”
Is there a lesson here for West nile virus, another mosquito-borne infection that's causing an unusual amount of trouble this year? CDC reports 3,545 cases of West Nile virus disease in people, including 147 deaths. Unfortunately not, says Unnasch -- West Nile is not related to EEE and while it passes from birds to mosquitoes to people, it has a different cycle of infection.