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ScienceShot: Displaced Snakes Slither Back
Roy Wood/U.S. Geological Survey

Vipers Go Viral

Every year as the days grow warmer, the Eastern equine encephalitis virus (EEEV) reemerges along the eastern coast of the United States, where it causes devastating disease in horses and, more rarely, humans. Scientists have long wondered how the virus, which is transmitted by the bite of an infected mosquito, survives the cold, mosquito-killing North American winters. Now, a new study suggests that snakes harbor the virus through the winter, but experts disagree on whether the finding clinches the question for good.

"This is very, very impressive work," says Scott Weaver, a virologist at the University of Texas Medical Branch in Galveston, who has worked extensively on EEEV but was not involved in the study. "It makes a very strong case that snakes play an important role in maintaining the virus."

EEEV is deadly: Anywhere between 60 and 712 horses have been infected with the virus each year since 2003, according to U.S. Department of Agriculture statistics, and most infected horses die. Human cases are rare, about five to 10 cases a year, but 35% to 50% of those with the disease die, and more than a third of its survivors have lasting neurological damage. Most cases are concentrated along the Gulf Coast and Eastern Seaboard.

Because there is no vaccine for humans, efforts to curb the disease's spread focus on controlling mosquitoes and their infection rates. But that's where researchers have run into a mystery: How does the disease come back year after year, when wintertime temperatures in most of the country are cold enough to kill the mosquitoes that carry it?

The question caught Thomas Unnasch's attention more than a decade ago. Unnasch, an infectious disease researcher at the University of South Florida in Tampa, and colleagues thought the virus's cold-weather hideaway might be a specific host—a bird or reptile—that sticks around during the winter. Their previous studies steered them away from birds, which rid themselves of EEEV too quickly to harbor the virus for long, and from most types of reptiles—but not from snakes.

So the team collected snakes of several different species in Alabama's Tuskegee National Forest—near where EEEV is known to strike—and tested their blood for signs of the virus. It looked for antibodies (proteins the immune system produces to fight off an infection) against the virus or traces of the virus's RNA or genetic code, which can indicate an active infection.

The researchers found the virus in two species of snakes, the cottonmouth and copperhead—two venomous vipers that are common in the Tuskegee National Forest. More than 35% of the cottonmouths had antibodies against the virus, and 22.2% had bits of the virus's RNA in their blood.

Experiments with nonvenomous garter snakes—selected because "we were nervous about bringing an animal with a really bad attitude, a pair of poisonous hypodermic needles in its mouth, and a deadly viral infection into the lab," Unnasch says—showed that the virus persisted even during the snakes' hibernation and remained active even after 30 days of cold-induced sleep. Those results combined with the findings in the forest-dwelling snakes suggest the virus can persist through and after hibernation in the wild, the team reports online this week in The American Journal of Tropical Medicine and Hygiene.

The study's one weakness, Weaver says, is that Unnasch's team isolated only pieces of the virus's RNA and not the active virus itself from the wild snakes. He notes, however, that the freezing and processing required to study the blood samples often causes degradation that makes RNA easier to find than the virus.

But Laura Kramer, a virologist for the New York State Department of Health's Wadsworth Center in Albany, isn't so quick to dismiss that concern. "Having RNA is not the same as having actively infectious virus," she says. "I think this is an exciting study and I do think snakes could be part of the transmission cycle for this virus. But now they have to show in the field that snakes are infecting mosquitoes that are spreading disease—that's the last little piece of this puzzle."

*Correction 4:25 p.m., 5 October: This item corrects the number of horses infected with EEEV each year, and the percentage of cottonmouth snakes that were found to have the virus's RNA in their blood.