Snakes and Eastern Equine Encephalomyelitis

How often are cottonmouth snakes, copperhead snakes, and horses mentioned in the same discussion? Until recently, the connection wasn’t very frequent, but a new discovery about Eastern equine encephalomyelitis (EEE) shows that snakes may play an important role in overwintering the virus that causes this disease.

The virus that causes EEE was known to be carried by birds and was spread to horses, humans, and other animals by mosquitoes. When a mosquito fed on an infected bird’s blood and then bit another creature, the virus was transferred to a new host. The disease was found year-around in Florida and other southern states, but it was less common in northern regions where birds typically leave for warmer climates during the winter. However, a number of cases have been reported in Massachusetts during 2012.

A study by researchers at the University of Florida found the virus in the blood of copperheads and cottonmouths, snakes that are relatives of another pit viper, the rattlesnake. Cottonmouth snakes are rarely found north of North Carolina, while rattlesnakes and copperheads have a range that stretches from Florida to Maine. Tests of some infected mosquitoes showed they had recently fed on snake blood, showing that snakes can provide a reservoir for EEE virus before birds arrive in the spring and after they leave northern areas in the fall. Snakes and birds aren’t harmed by the virus, unlike humans and some other animals that may die from the infection. A preventive vaccine is available for equines, but immunization is not available for humans.

The scientists suggested that spraying marshes and swampy areas earlier in the spring when snakes and mosquitoes become active might help to break the cycle of infection. Attempting to control mosquitoes after bird populations return in the spring might be missing an opportunity for controlling the problem at an earlier stage.