Spring and summer months are the prime time for the risk for West Nile virus in horses. In 2016, 377 equine West Nile virus cases were reported across the United States – an increase of 152 cases from 2015.

West Nile virus cycle in horses

Horses are at the highest risk for contracting West Nile virus during peak mosquito season, which occurs July through October in the United States.

Horses are at the highest risk for contracting West Nile virus during peak mosquito season, which occurs July through October in the United States. 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,” said Kevin Hankins, DVM, senior veterinarian, Equine Technical Services, Zoetis.

When properly vaccinated, horses have shown to be 30 times less likely to contract West Nile.

West Nile virus is transmitted by mosquitoes — which feed on infected birds — to horses, humans and other mammals. Dr. Hankins explains that the uptick in 2016 cases is likely due to the drought that occurred in 2015.

Droughts can increase numbers 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 West Nile cases recorded across the United States in 2016, Dr. Hankins cautioned, “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, horse owners can help prevent West Nile by implementing proper horse management techniques, such as:

• Eliminate any mosquito-breeding habitats by removing all potential sources of stagnant water, such as unused troughs, wheelbarrows, ditches and tarps.
• Hang fans throughout the barn where horses are stabled, as mosquitoes avoid moving air.
• Clean and empty any water-holding containers on a weekly basis.
• Apply insect repellent or bring horses inside from dusk to dawn, which is peak mosquito feeding hours.

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 may cause symptoms such as loss of appetite and depression.

Other clinical signs may include fever, weakness or paralysis of hind limbs, impaired vision, ataxia, aimless wandering, walking in circles, hyperexcitability or coma.

If horse owners notice signs or symptoms of West Nile infection in their horses, they should contact a veterinarian immediately. West Nile virus is fatal in 33 percent of horses that exhibit clinical signs of disease.

Treatment is vital for any horse with WNV. At the present time there is no specific "medicine" for a horse that develops WNV, although there are some promising advances being made in that area.

Working with a veterinarian who is able to provide supportive therapy can save the horse’s life.

For advanced cases, horses usually have to be hospitalized. For mild cases, home care may be adequate. Proper nursing care and treatment is important to the recovery process and each animal is assessed according to it's age and health.

Over the last few years, it has been discovered that of those horses that recover, some will relapse within a few months or a year and some will die.

The main focus of veterinarian aided therapy is to decrease brain inflammation, treat the fever, if any, and provide supportive care, which may require 1-4 weeks of intermittent therapy.

Sometimes fluid therapy is necessary for equines that are not able to drink to prevent dehydration that can worsen the horse's condition.