Mosquito-Borne Equine Diseases Prevalent on Both Coasts

As of Aug. 25, Florida led the United States in the number of documented equine cases of Eastern equine encephalitis (EEE), a mosquito-borne disease with mortality rates of around 90% in horses. Washington state officials had confirmed the most equine cases of West Nile virus (WNV) this year.

Thus far in 2009 cases of EEE are largely clustered in the Southeast. According to the USDA's latest report at press time, Florida had 63 reported cases. Other Southern states with double-digit case counts included Georgia (36), Mississippi (26), Alabama (15), and North Carolina (12).

In 2008 there were 185 equine cases of EEE reported in 15 states. Florida also topped that year-end tally with 89 cases.

On the WNV front, as of Aug. 25 Washington state animal health authorities had reported 21 equine cases. Other states reporting multiple cases included Montana (6), California (4), Alabama (3), Kentucky (2), Louisiana (2), and Texas (2).

West Nile virus started its westward march across the United States in 1999, and Western states now lead in the number of confirmed equine cases of the mosquito-borne virus. Washington state also led in equine WNV cases in 2008, topping the reports with 41 cases, followed by California with 32.

Both EEE and WNV are transmitted through the bite of an infected mosquito that has picked up the virus by feeding on an infected bird. Horses are dead-end hosts of these viruses, meaning a mosquito that bites an infected horse cannot transmit the virus to other animals or people. However, horses can serve as sentinels that the virus is active in an area.

Animal health officials recommend owners have their veterinarians vaccinate horses to protect them against mosquito-borne diseases such as WNV, EEE, and Western equine encephalitis as part of their routine health care program. There is no human vaccine for these three viruses.

The Centers for Disease Control (CDC) provides weekly reports of arbovirus case information from its ArboNET reporting system. Access these reports through the USDA Animal Health Monitoring and Surveillance Web site.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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