Mosquito-borne illness in horses has been a hot topic this summer with several states reporting a record number of positive test results for Eastern equine encephalitis (EEE). According to the USDA's National Animal Health Surveillance System (NAHSS), this year a total of 228 confirmed EEE cases were reported nationwide as of Nov. 12. But as the mosquito season comes to an end with the arrival of cooler temperatures, fewer cases are being reported.

Eastern equine encephalitis is a potentially fatal viral disease that affects the central nervous system; the virus circulates in bird and mosquito populations, and it is transmitted to horses by infected mosquitoes. The horse is a dead-end host for the virus, meaning they do not pose a risk of transmitting it to other susceptible animals.

Overall, there has been a 25% decrease in the number of EEE cases reported nationwide as compared to 2009. Several states saw drastic increases, however, in the number of horses that tested positive for the disease.

Most notably, Michigan reported 56 confirmed cases of EEE in 2010 and another 81 probable but untested cases; Michigan state law requires that veterinarians report knowledge or suspicion of an EEE case, but does not require that the owner or veterinarian confirm the case by a laboratory test. The state did not see any cases in 2009 and prior to this year had only seen 15 cases since 2003, according to the NAHSS.

All but four of the EEE-positive or suspect cases died or were euthanized as a result of the disease.

"The economy played a role in that many horse owners cut back on their horse-related expenditures, including vaccination against EEE ... leaving their animals vulnerable," said Michigan state veterinarian Steven Halstead, DVM, MS. "The weather (in Michigan)--early, warm spring followed by heavy and ongoing spring and summer rains and prolonged high summer temperatures--was highly favorable to mosquitoes and led to dense mosquito populations."

Florida also saw a large number of horses contract the disease. More than 90 horses tested positive for EEE in 2010, the highest number since 2005 when 150 horses tested positive. Because of its mosquito-friendly climate, Florida has consistently been in the top five states for numbers of confirmed EEE cases in recent years.

As with Michigan, the hike in positive EEE tests in Florida is believed to be weather- and economy-based.

"Since 1982, Florida averages about 75 confirmed cases of EEE each year," says Michael Short, DVM, manager of the Florida State Veterinarian's Equine Programs. "This year is slightly above normal, most likely due to a combination of factors including amount and timing of rainfall, less vaccination by owners due to economy, and potentially other viral factors."

Alternatively, Maine, which saw 15 positive tests in 2009, did not report any cases in 2010. Georgia also showed a drastic decrease in the number of confirmed cases, dropping from 44 in 2009 to just 11 in 2010.

As 2010 comes to an end, Halstead reminds horse owners to take all the necessary precautions to keep their horses safe from EEE in the upcoming year.

"Keep horses ... vaccinated against EEE, as well as other diseases such as West Nile virus, rabies, and tetanus," he said. "It is less expensive to prevent diseases than to have to deal with a sick horse, and the vaccines are fairly inexpensive and are highly effective."

"I think this is sound advice as it is extremely rarely to have a confirmed case of EEE in a horse that has been vaccinated (for EEE) in the past six months, and fairly rare to have a confirmed case with a horse well-vaccinated in the past year ," Short added. "Greater than 90% of the confirmed cases in Florida are in horses that have not been vaccinated in over a year."

Halstead also suggests taking steps to minimize the presence of mosquitoes by removing standing water (which can serve as mosquito breeding grounds) and spraying for mosquitoes, and limiting the horse's exposure to mosquitoes by using mosquito repellent, housing the horse indoors during peak mosquito hours, and using fans in barns.

Halstead stresses that EEE is a reportable disease in most states, meaning that any suspected case must be reported to authorities.

Officials have recorded a decrease in confirmed cases of another mosquito-borne virus--West Nile virus (WNV)-- in 2010. The NAHSS has reported 108 positive WNV tests this year, while 276 were reported in 2009.

Finally, Western equine encephalitis was the least prevalent of the three major mosquito-borne illnesses in 2010; there were no confirmed cases in the United States.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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