Drought conditions in the United States in 2015 likely contributed to the increase in the number of West Nile cases the following year.
New data reveals that there were 377 cases of West Nile infection horses across the US in 2016 — an increase of 152 cases from 2015.
Horses are at the highest risk for the disease during peak mosquito season, from July through October in the United States.
West Nile virus is transmitted by mosquitoes — which feed on infected birds — to horses, humans and other mammals.
“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,” says Kevin Hankins, a senior veterinarian in Equine Technical Services with animal health company Zoetis, which markets the vaccine West Nile-Innovator.
Hankins explains that the uptick in 2016 cases is likely because of the 2015 drought. 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.
Hankins cautions that the number of cases was probably greater than the 377 reported last year. Some states report West Nile virus cases only if the disease is presented in neurological form.
Properly vaccinated horses have shown to be 30 times less likely to contract West Nile. In conjunction with vaccination, proper barn management techniques also can help prevent West Nile, such as:
- Eliminating any mosquito-breeding habitats by removing all potential sources of stagnant water, such as in unused troughs, wheelbarrows, ditches and tarps.
- Hanging fans throughout the barn where horses are stabled, as mosquitoes avoid moving air.
- Cleaning and emptying any water-holding containers on a weekly basis.
- Applying insect repellent or bring horses inside from dusk to dawn during the peak mosquito feeding hours.
“It’s a multistep protection process,” says 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 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% of horses that exhibit clinical signs of disease.