July through October coincides with peak mosquito activity, which can place horses at the highest risk of contracting West Nile virus during this time of year, says vaccine maker Zoetis.
WNV is transmitted by mosquitoes — which feed on infected birds — to horses, humans and other mammals. Last year, the United States reported 395 West Nile virus cases in horses. Texas and Oklahoma topped the charts with 69 and 41 cases, respectively.
The number of reported WNV veterinary cases fell from 1,121 in 2006 to 157 in 2010, and the decline is said by health experts to reflect both vaccination and naturally acquired immunity.
"It is a good sign that the number of cases has declined over the last decade," said Kevin G. Hankins, DVM, senior veterinarian, Equine Veterinary Operations, Zoetis. "However, recent news reports of both human and equine cases indicate this disease is still a risk — especially during this time of year."
Vaccination remains the most effective way to help protect horses against West Nile and other mosquito-borne diseases, such as Eastern equine encephalomyelitis and Western equine encephalomyelitis, Zoetis says.
West Nile is considered a core vaccination requirement, along with vaccinations for EEE, WEE, tetanus and rabies, according to the American Association of Equine Practitioners guidelines.
"We have a disease that is here to stay, an effective vaccine but no treatment in the case of infection," Dr. Hankins said. "That makes vaccination a cheap insurance policy."
In conjunction with vaccination (Zoetis recommends its product, Innovator), use good techniques for managing mosquitoes and avoiding peak mosquito time. This includes:
• Destroying any mosquito breeding habitats by removing all potential sources of stagnant water
• Cleaning and emptying any water-holding container, such as water buckets, water troughs and plastic containers, on a weekly basis
• Applying insect repellents or bring horses inside during the peak mosquito feeding hours between dusk and dawn
WNV does not always lead to signs of illness. In horses that do become clinically ill, the virus infects the central nervous system and might cause symptoms such as loss of appetite and depression, Zoetis says.
Other clinical signs can include fever, weakness or paralysis of hind limbs, impaired vision, ataxia, aimless wandering, walking in circles, hyper-excitability or coma.
Horse owners should contact a veterinarian immediately if they notice signs or symptoms of WNV infection in their horses, especially if they are exhibiting neurological signs. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%.