Snakes Linked to Spread of Equine Encephalitis Virus

A horse, mosquito, and snake walked into a bar. The bartender looks up and says, "Is this some kind of joke?" Turns out, the bartender knows those three animals shouldn't be fraternizing because he read a recent article by Thomas Unnasch, PhD, proving snakes can harbor Eastern equine encephalitis virus (EEEV) and could play an important role in transmitting this deadly virus.

Like the West Nile virus, mosquitoes become infected with EEEV after a blood meal from an infected bird. If that mosquito then feeds off a horse, the EEEV can be transmitted to the horse.

"Certain areas in the northeastern United States are 'hot spots' for EEEV," explained Unnasch, a researcher from the Global Health Infectious Disease Research Program, Department of Global Health at the University of South Florida, in Tampa. "Because there are no mosquitoes in those areas of the U.S. in the winter and few birds, it wasn't obvious how the virus over-wintered in those areas."

He added, "Previous research found that certain mosquito species feed off of reptiles as well as birds and horses, suggesting that hibernating snakes infected with the EEEV via mosquitoes could explain how the virus survives the winter."

To obtain further data on the potential for snakes to harbor EEEV, Unnasch and colleagues from the Department of Biological Sciences at Auburn University in Auburn, Ala., wrangled snakes at the Tuskegee National Forest (an EEEV endemic area in east-central Alabama). The researchers collected blood samples from the snakes and tested them for antibodies (infection-fighting proteins) against EEEV and genetic material specific to EEEV by a laboratory process called quantitative reverse transcription polymerase chain reaction (qRT-PCR).

Key findings of the study were:

  • Cottonmouths were the most frequently caught snake;
  • Sixty-six cottonmouths did not have antibodies to EEEV, indicating they were neither exposed nor infected with the virus;
  • Fifty-four cottonmouths did have antibodies to EEEV, and 22.22% of those snakes also tested positive for EEEV via qRT-PCR;
  • The highest levels of EEEV genetic material measured by qRT-PCR occured in the spring months; and
  • EEEV could not be cultured (grown in the laboratory) from any blood samples.

"This is the first study to clearly demonstrate that snakes are competent hosts for the EEEV as the snakes in this study had EEEV circulating in their bloodstreams," explained Unnasch. "Previous studies have only found that snakes have antibodies to EEEV."

He added, "The fact that wild-caught snakes have EEEV circulating in their bloodstreams and that the highest number of snakes with evidence of viremia was in the spring months supports the hypothesis that snakes play an important role in over-wintering and the early spread and amplification of EEEV."

Angela M. Pelzel-McCluskey, DVM, Western region epidemiologist with the USDA Animal and Plant Health Inspection Service, previously told The Horse magazine that snakes were not able to transmit diseases to horses. According to Pelzel-McCluskey, this is still true.

"The snakes themselves do not transmit infection directly to horses," she clarified. "The mosquitoes acquire the virus from feeding on the snakes, just as mosquitoes can acquire the West Nile virus from feeding on birds. After feeding, the mosquitoes can then transmit the virus to horses and/or people. EEEV is still a 'mosquito-to-horse transmission.'

"Prior to Unnasch's research, however, we didn't know that snakes could harbor the virus for many months through their hibernation and then provide an active source of the virus for mosquitoes to pick up the next season," she relayed.

Although not the "take-home message" of the study, this research does highlight the need for all horses potentially at risk for EEE to be fully vaccinated. As of Oct. 2, 166 horses in 15 states had been diagnosed with this almost 100% preventable viral disease.

The study, "Detection of eastern encephalomyelitis virus in RNA in North American snakes," published by the American Journal of Tropical Medicine and Hygiene, is available for free through PubMed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.