Horses are not to Blame for this Year’s Spike in Human “Triple E” Cases

Carrie Swanson, Extension Agent, Virginia Cooperative Extension

Unlike the general population, most horse owners are familiar with Eastern Equine Encephalitis (or EEE). At least we know it’s something we should vaccinate our horses for. But, you may not understand this disease in terms of its risk to humans, why it has been in the national news recently and how horses factor into things. So, here’s the scoop…

The eastern equine encephalitis virus (EEEV) is transmitted to humans via the bite of a mosquito that has fed on an infected bird. Mostly the disease cycles between birds and a specific species of mosquito the feeds almost exclusively on avian species. Transmission to humans requires a mosquito to feed on an infected bird and then a human. Horses are also susceptible to the virus, but they are not a significant risk factor for human infection because, like humans, they are a “dead-end” host (the concentration of the virus in their bloodstreams is too low to infect mosquitoes).

Overall, only about 4-5% of human EEEV infections results in EEE. Infection with EEEV is thought to confer life-long immunity against re-infection. Risk of infection is higher in areas where EEEV is prevalent and for people who engage in outdoor activities in those areas. People over the age of 50 and under the age of 15 are at higher risk of developing the disease.

Typically, the United States averages about 7 cases of EEE each year. Florida, Georgia, Massachusetts and New York typically have the largest number of cases. EEEV transmission is most common in and around freshwater hardwood swamps. Although rare, this disease is very serious. So far in 2019, more than 30 people have become ill due to the disease, and 11 of those have died (typical fatality rate is 30%). Scientists believe the increased number of cases this year is due to several factors, including weather patterns along bird migration paths.

The best protection for your horse is a vaccine, but since there is no EEE vaccine for humans, the best protection for us is mosquito repellents.

For more information on EEE and human infection check out the Center for Disease Control and Prevention website.

For more information on EEE in horses, check out the American Association of Equine Practitioners website.

Map of human cases of EEE in the US from 2009-2018

Map of Human EEE cases in U.S. from 2009-2018

Are there poisonous plants in your pasture?

Horses that have access to adequate forage generally avoid poisonous plants, but it's still important to be able to identify dangerous plants in your pastures. Young horses and "easy keepers" kept in dry lots are often willing to try anything green. Some plants become more palatable when sprayed with herbicide; horses may even seek-out these poisonous plants, even when alternative forages are available. To see pictures of some of the most common poisonous plants found in Virginia, click on the link below:

Common Virginia Poisonous Plants (PDF)


Extension Horse Facebook

Information and updates geared specifically towards Virginia horse owners! We're working to build photo albums of local plants poisonous to horses, weed identification, pasture management tips, and more. Like our page and let us know what you'd like to see added!