Professor describes mosquito-borne virus threat to horses; stresses need for vaccination

A female Ochlerotatus notoscriptus feeds on a human arm in Tasmania, Australia. Photo: JJ Harrison ( (Own work) CC BY-SA 3.0 via Wikimedia Commons
A female  mosquito feeds on a human arm. Photo: JJ Harrison ( (Own work) CC BY-SA 3.0 via Wikimedia Commons

An ongoing effort is needed to alert North American owners to the risks that mosquito-borne diseases pose to their horses and the need for vaccination, an equine veterinary professor says.

Peter Timoney, who is chair in equine veterinary science at the Gluck Equine Research Center, part of the University of Kentucky, says all avenues of communication, including social media, should be used to reach owners.

Timoney says the two most common causes of brain inflammation in North America are Eastern Equine Encephalomyelitis (EEE) and West Nile Encephalitis (WNE) viruses.

The two are not restricted to equids in terms of their host range; each can be transmitted to humans and certain other mammalian and avian species.

“Both diseases are a source of concern for the equine industry, not only from the potentially life-threatening consequences of either infection, but also from the economic losses involved,” Timoney writes in the latest issue of Equine Disease Quarterly.

Eastern equine encephalomyelitis poses an annual threat to equids in the Gulf and Atlantic coastal states and the Great Lakes region, extending in certain years as far north as eastern Canada.

It is occasionally recorded in some inland states such as Arkansas, Oklahoma, Tennessee, Kentucky and Iowa. Evidence of EEE activity is most often reported in Florida, in which it has been confirmed as early as January, as recorded in 2018.

Over the past 15 years, the yearly incidence of EEE cases in equids has ranged from 60 in 2011 to 712 in 2003, with an annual average of 206 cases.

Last year, 86 cases of the disease were reported in 13 states.

In temperate regions, transmission of EEE virus is seasonal, occurring in the summer and the fall.

In sub-tropical regions such as Florida, there is a year-round risk of EEE, with virus transmission peaking in summer.

Timoney, who has a research interest in infectious diseases, says horses and humans are dead-end hosts of EEE virus and neither plays a role in the natural life-cycle of the virus.

Infections in horses, mules, and donkeys are frequently life-threatening, with case fatality rates as high as 90 percent.

West Nile encephalitis (WNE) is also a cause of concern to vets and the equine industry.

Within four years of the introduction of the virus in New York State in 1999, it had spread to 48 states and several provinces in Canada. Since 1999, the yearly incidence of WNE cases in equids has ranged from 60 in 2000 to 15,257 in 2002.

The annual average number of cases over the past 10 years was 272. In 2017, 307 equine cases were reported in 39 states.

Similar to EEE, transmission of WNE virus is seasonal, occurring in the summer and extending well into the fall. “Unlike EEE, only about 10% of WNE virus-exposed horses will develop clinical infections,” Timoney notes.

Reported case-fatality rates in affected horses can reach 30-40 percent, less than half that encountered in cases of EEE.

He continues: “The American Association of Equine Practitioners, in accordance with criteria defined by the American Veterinary Medical Association with respect to ‘core vaccines’ — namely those that protect against diseases that are endemic, of potential public health significance, and represent a risk of causing severe disease — strongly recommends that horses be immunized against EEE and WNE.”

Available inactivated whole-virus vaccines against EEE (including Western Equine Encephalomyelitis) have been shown to be safe and effective in protecting against this disease, he says.

“Two inactivated whole-virus vaccines, a live canary pox vector vaccine and an inactivated flavivirus chimera vaccine are available against WNE. All have been confirmed safe and effective in preventing the disease.

“Despite the AAEP recommendations to horse owners to vaccinate their horses against EEE and WNE, regrettably many fail to do so.

“The vast majority of equine cases of EEE and WNE either have no history of vaccination against the particular virus or else the vaccination history is incomplete.

“There is need for an ongoing concerted effort, utilizing all avenues of communication including social media, to alert horse owners of the dangers of these two vector-borne diseases and of the importance of vaccination as an effective means of prevention and averting the losses that continue to occur every year in unprotected horses.”

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