Eastern and western equine encephalomyelitis are a pair of closely related viral diseases that affect the horse’s central nervous system. Both are caused by alphaviruses of the family Togaviridae, are carried by birds and spread by mosquitoes. Birds that carry the viruses do not become seriously ill. However, a mosquito who feeds on an infected bird can transmit the virus to its next host. Horses are considered dead-end hosts, meaning that they cannot pass the virus on to mosquitoes or other animals once infected. 

Eastern equine encephalomyelitis (EEE) is more deadly than western equine encephalomyelitis (WEE). (A related disease, Venezuelan equine encephalomyelitis [VEE], occurs in Central and South America. The last recorded cases in the United States occurred in southern Texas in 1971.) In fact, EEE easily ranks among the worst diseases a horse could get.

EEE is both fast-acting and highly fatal: After an initial incubation period of five to 10 days, a horse may at first appear listless and lose his appetite. Within 24 hours, he will show neurological signs such as incoordination, head pressing and seizures. A day later, he may be comatose and unresponsive until death. The disease is fatal in 90 percent of cases. The only treatment is supportive care, including intravenous fluids and corticosteroids. Survivors are likely to have lifelong neurological impairment.

Your horse’s risk of exposure to this virus depends in part on where he lives. EEE, as the name implies, is endemic to the eastern United States, especially the South. In 2016, there were 116 cases reported nationwide, according to the U.S. Department of Agriculture. Most (24) were in Florida, but cases were also reported in every coastal state from Virginia to Texas, plus Tennessee and Arkansas. It’s not unusual for cases to appear in northern or north-central states, too. In 2016, there were isolated cases in New Jersey and Michigan, plus an outbreak of 19 in Wisconsin. The total national cases climbed higher than 200 or 300 six times between 2003 and 2015, including 712 in 2003. In that same time frame, the lowest numbers reported were 60 in 2011 and 70 in 2015.

Small birds, especially sparrows and finches, are the primary reservoir species for EEE, meaning that when bitten by infected mosquitoes, they can harbor and amplify the virus without becoming sick themselves. One mosquito species, Culiseta melanura, which bites only birds, is the primary vector for spreading the EEE virus among local bird populations. C. melanura breeds in wet, peaty lowlands where it lays its eggs among the root systems of hardwood trees that thrive in these areas, including red maples, cedars, bay and gum trees. Once local birds are infected, other mosquito species may pick up the virus from them and transmit it to horses, people and other mammals. Horses are considered dead-end hosts, meaning that they cannot pass the virus on to mosquitoes or other animals.

Western equine encephalitis (WEE), as the name implies, is found primarily in the western United States. The WEE virus is very similar to the one that causes EEE, and it produces similar signs, including fever and depression, ataxia, head pressing, paralysis and convulsions. WEE, however, is far less deadly than EEE. Around 50 percent of horses diagnosed with WEE recover from the illness, and the survival rate may be even higher because milder cases may never be identified.

WEE virus is also amplified by small birds and is spread primarily by a different mosquito vector, Culex tarsalis, which breeds in irrigation ditches in agricultural areas as well as in natural ponds and waterways west of the Mississippi River. No outbreaks of WEE have occurred in recent years. However, the virus is still found in surveys of birds and mosquitoes, and an outbreak remains a possibility.

Vaccinate regularly

The first and best defense against EEE and WEE is vaccination. The American Association of Equine Practitioners (AAEP) includes EEE/WEE on its list of “core vaccines,” which are recommended for the majority of horses. A single formula protects horses against both EEE and WEE. The vaccine contains an inactivated adjuvanted whole virus, which means that it contains whole viruses that have been rendered inactive, combined with an adjuvant, a substance that stimulates a greater immune response.

Your veterinarian can advise you on the most appropriate vaccination schedule for your horse. An adult horse who has never been vaccinated needs a two-dose series, four to six weeks apart, which needs to be completed before mosquitoes and other insect vectors are active for the season. An adult horse who has been previously vaccinated needs an annual booster prior to the start of mosquito season. In some cases—for horses with compromised immunity, for example, or those who live in areas where mosquitoes are active year-round—your veterinarian may recommend boosters every four to six months.

Other preventive measures

Any steps you take to reduce your horse’s exposure to mosquitoes will help to reduce the threat of EEE and WEE.

• Reduce or eliminate standing water. Dispose of old tires and other trash or debris that collects rain-water. Keep garbage cans covered, and take unused buckets inside. Overturn wheelbarrows or prop them against walls when not in use. If you place tarps over unused trailers, boats or other vehicles, make sure water is not collecting in their folds. Also keep your gutters and drainage ditches unclogged and flowing freely, and make sure persistent puddles are not forming under downspouts or dripping faucets.

• Change your animals’ drinking water regularly. Empty and provide fresh, clean water in buckets as well as birdbaths and drinking bowls for dogs and cats every few days. Changing water in larger troughs at least once a week will prevent any hatched mosquito larvae from reaching maturity.

• Use larvicidal products. If standing water is difficult to change or remove, consider using larvicidal products. Many contain Bti—Bacillus thuringiensis subspecies israelensis, a bacterium that produces toxins that kill mosquito larvae in water. These products are not toxic to people or other animals, but check with your veterinarian for recommendations on specific products that are safe for use in water your horse will drink.

• Encourage predators. Fish, dragonflies, birds and other predators will feed on both larvae and adult mosquitoes in and around healthy natural ponds and streams. If natural water sources on your property are producing too many mosquitoes, talk to your local extension agent for advice on how to reduce stagnation and boost the numbers of beneficial species.

• Apply repellents and protective garments. Read the label to make sure your fly spray also works against mosquitoes, then be diligent in applying repellents and using other measures, such as fly sheets, to protect your horse, especially if his pasture lies close to prime mosquito habitat. Although some more robust mosquito species may disperse as far as seven miles, most fly no more than two or three miles from their point of hatching. Also apply repellents before trail rides, especially if you’re heading into the woods or near wetlands.

• Keep horses inside at dusk and dawn, when mosquitoes are most active. Install fine screens over barn windows to help keep mosquitoes out, and place fans where they will keep the air moving through stalls. Mosquitoes are weak fliers, and even a mild breeze will keep them grounded.

This article first appeared in EQUUS #370