West Nile virus (WNV) first appeared in the United States in 1999, but it remains a serious horse health threat.

In 2016, there were 377 equine WNV cases across the United States—an increase of 152 cases from what veterinarians diagnosed in 2015. Horses are at the highest risk for contracting WNV during peak mosquito season, which typically occurs July through October in the United States. Fortunately it’s not too late to help protect horses against this devastating disease.

“Vaccination is extremely effective against West Nile virus and remains the most effective way to help protect horses against the disease, in conjunction with mosquito control,” said Kevin Hankins, DVM, MBA, senior equine technical services veterinarian for Zoetis.

When properly vaccinated, research has shown that horses can be 30 times less likely to contract WNV.

West Nile virus is transmitted by mosquitoes—which feed on infected birds—to horses, humans, and other mammals. Hankins said the uptick in 2016 cases is likely due to the drought that occurred in 2015. Droughts can diminish water sources and increase the number of small, stagnant pools of water, presenting ideal breeding grounds for mosquitoes. During a drought, bird populations often decrease. As birds flock to wetter areas of the country, mosquitoes are left to feed on other warm-blooded animals nearby, such as horses.

When considering the 377 equine WNV cases recorded across the United States in 2016, Hankins cautioned that “the numbers are likely much greater. Some states only report West Nile virus cases if the disease is presented in neurological form.”

In conjunction with vaccination, proper barn management techniques also can help reduce WNV risk. Take steps to:

  • Eliminate any mosquito-breeding habitats by removing all potential sources of stagnant water, such as in unused troughs, wheelbarrows, ditches, and tarps;
  • Hang fans throughout the barn where horses are stabled, as mosquitoes avoid moving air;
  • Empty and clean any water-holding containers at least weekly; and
  • Apply insect repellent or bring horses inside from dusk to dawn during peak mosquito feeding hours.

“It’s a multistep protection process,” said Hankins. “Vaccination against West Nile is key because it’s shown to be so effective, but horse owners also need to be aware of, and eliminate, risk of exposure to a potentially infected mosquito population.”

West Nile does not always lead to signs of illness in horses. For horses that do become clinically ill, the virus infects the central nervous system and can cause signs such as loss of appetite and depression. Other clinical signs can include fever, weakness or paralysis of hind limbs, impaired vision, ataxia (incoordination), aimless wandering, walking in circles, hyperexcitability, and coma. If horse owners notice signs of West Nile infection in their horses, they should contact a veterinarian immediately. West Nile virus is fatal in about 30% of horses that exhibit clinical signs of disease.