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Tiny Terrors: Equine Mosquito-Borne Diseases

Tiny Terrors: Equine Mosquito-Borne Diseases

By Nancy S. Loving

A look at current trends in mosquito-borne diseases affecting horses

Recent news reports describe accelerated wildlife species loss because of significant changes in habitat and food sources. But one creature that seems to be thriving is one of the world’s most dangerous: the mosquito. Mosquito-borne disease affects all mammals, with some of the most virulent infecting horse and human similarly: Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), and West Nile virus (WNV).

The good news is horses can’t spread EEE, WEE, or WNV to other horses or humans, and vice versa. Female mosquitoes are the intermediate or “bridge” vectors that cause infection. Birds (and sometimes rodents) carry the viruses but don’t always show clinical signs. Mosquitoes bite the birds and pick up the virus; then those carrier mosquitoes pass the virus on to horses, humans, or other birds when they take their next blood meal. People and horses are considered dead-end hosts because they don’t have enough infective virus in their blood to be transferred through mosquitoes or body fluids to other humans or animals.

Let’s examine recent trends with these three encephalitides—diseases causing brain inflammation—so you can understand how to best protect your horse.

Eastern Equine Encephalomyelitis

Culiseta melanura mosquitoes transmit EEE, causing serious neurologic disease in horses, with a 90% fatality rate. These mosquitoes reside east of the Mississippi, localizing EEE to states in and around that region. Besides having a fever, infected horses develop an uncoordinated gait (ataxia) and often experience involuntary muscle twitching. Progressive encephalitic signs develop, such as headpressing, aimless wandering, seizures, hyperexcitability, and coma. Once a horse goes down, he’s unable to rise.

Through 2019, most equine cases of EEE were seen in Michigan (29), Florida (28), Louisiana (18), and near the Michigan border in Indiana (11). The earliest cases appeared in March and continued throughout the year.

Wendy Vaala, VMD, Dipl. ACVIM, director of life-cycle management equine and companion animal at Merck Animal Health and a member of the American Association of Equine Practitioners’ vaccination guideline review group, says while officials reported far more human EEE cases in 2019 than in any previous year, they did not see a similar spike in equine cases. As of mid-December, an unprecedented number of 38 human cases with 15 fatalities had been reported in nine states, and 182 equine cases of EEE had been reported in 24 states. (For comparison, EEE reportedly infected 712 horses in the high-level year of 2003.)

“Increased spikes in EEE cases have been associated with a variety of factors,” says Vaala, including:

  • Warm, wet weather early in the year and/or extending longer into the fall favors a robust mosquito population capable of transmitting the virus among birds and from birds to humans and horses. The northern Midwest experienced a very wet 2019 beginning in early spring and extending into the fall.
  • A new variant of the EEE virus might be present. In Florida the virus circulates year-round, providing opportunities for it to undergo genetic changes that could impact susceptible bird populations. More birds become infected and might carry the new variant north during normal migration. For example, in Massachusetts two of the last outbreak cycles of EEE involved new variants of the virus.
  • Health officials often document EEE cyclical trends. After a year with particularly high levels of viral activity, surviving bird populations develop longlived immunity to that strain, resulting in a transient decrease in EEE cases for a number of years. Once that bird population dies off, another period of increased susceptibility to EEE infection and carrier status in younger birds contributes to further EEE activity.

Usually, the number of WNV cases in horses is double the number of EEE cases. But in 2019 the number of EEE cases far outnumbered the 88 WNV cases reported by early December. Most were horses that were unvaccinated, undervaccinated, or had an unknown vaccination history, says Vaala.


Western Equine Encephalomyelitis

The Culex tarsalis mosquito, which lives primarily in the western part of the United States, transmits WEE, a neurologic disease similar to EEE. Fatality rate among infected horses is 40-50%. Recent years have seen a dramatic drop in equine cases, with none reported in the western U.S. since 2004. However, birds and mosquitoes in this area still harbor the virus, so vets recommend vaccinating horses annually as a safeguard.

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