Rare but Deadly Mosquito-Borne Virus Reported in Eight New York Counties

After more than a dozen horses across the state tested positive for the eastern equine encephalitis (EEE) virus — a brain infection spread by the bite of the black-tailed mosquito (Culiseta melanura) — the New York State Department of Health warned residents to take measures to avoid mosquito bites. By Aug. 30, eight New York counties — Cayuga, Madison, Oneida, Orange, Saint Lawrence, Ulster, Washington and Wayne — confirmed that 15 of 16 diseased horses had died; according to the Equine Disease Communication Center, none had been properly vaccinated against the virus.

EEE is an extremely rare but potentially devastating mosquito-borne illness that affects mammals and birds, primarily in the Atlantic states. The disease kills up to 90% of infected horses. Because the animals’ first symptoms include staggering and lethargy, EEE has been dubbed “sleeping sickness.”

EEE isn’t contagious like the flu. Humans can’t catch it from horses, and horses can’t catch it from one another. Mosquitoes transmit the virus by feeding on infected wetland birds, then biting horses and humans. Mammals are “dead-end” hosts, with viral blood levels too low to pass the disease to other hungry mosquitoes.

Since 2003 when the disease became reportable, an average of 11 Americans have been infected by EEE annually, with an estimated four total deaths. But climate change experts and health officials are worried. The mosquito season — lasting an extra 21 days in recent years — still has two months to go. That’s ample time for the disease to spread as black-tails continue to thrive in the Northeast’s warmer, wetter weather.

A majority of humans who get EEE experience mild flu-like symptoms, but 5% develop severe neurologic disease. One-third of those die; half of the survivors are left with permanent brain damage, which can cause disorientation, seizures and paralysis.

Last month, four people were sickened with EEE in Massachusetts, New Jersey, Vermont and Wisconsin. On Aug. 27, the New Hampshire Department of Health announced that one person had died from the disease. One day later Vermont health officials issued a warning recommending people in at-risk areas to stay indoors from sunset to sunrise. So far, there are no reported cases in humans in New York.

Fever, chills and body aches develop within 10 days after being bitten by a black-tail carrying the disease. There is no treatment for EEE: mild symptoms are managed with rest, fluids and anti-inflammatories like ibuprofen. Those who are seriously ill require hospitalization for close monitoring and supportive measures, such as oxygen and intravenous fluids.

Immunization against EEE for horses has been widely available for years. The American Association of Equine Practitioners considers it a “core vaccine” to be given every spring, before the mosquito season. It lowers the animal’s chances of getting sick and lessens the severity of the disease if infected.

Because human vaccinations are not commercially available, the only way to prevent EEE, in humans and in horses, is by minimizing mosquito exposure. 

Consider taking the following precautions: Because the insects lay eggs in standing water — even tiny puddles — police your yard for potential hazards like upright wheelbarrows and discarded tires. Turn over flowerpots and wading pools; drill holes in the bottoms of recycling bins. Make sure gutters are draining properly and change birdbath water twice weekly. Keep black-tails out of your house by closing the doors and repairing any damaged window screens.

Protect yourself from mosquito bites by wearing long sleeves, pants, shoes and socks when outdoors for extended periods. Stay indoors at sunrise and sunset when the insects are the most active. Consider mosquito repellent with DEET, like Cutter Backwoods.

These precautions will also help prevent West Nile virus — the most common mosquito-borne illness in the continental United States, with more than 2,200 cases annually. A much less serious disease, 80% of West Nile victims have no symptoms, and fewer than 1% are disabled or die.