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