West Nile Virus Cases Are On The Rise

Along with the usual seasonal suspects of summer sores and pigeon fever, summer and fall are also prime time for increases in disease caused by West Nile virus (WNV).

West Nile virus is found across the U.S. It is transmitted by mosquitoes and causes inflammation in the brain. Although many species are susceptible to infection, horses account for more than 95% of reported non-human cases. General treatment includes supportive care and anti-inflammatory therapy. Many horses make a full recovery in time, while others make a partial recovery with varying degrees of residual deficits. Depending on disease severity and availability of supportive care, the disease can be fatal in up to 33% of horses.

“Horses affected with WNV often have muscle tremors, facial twitching, abnormal responses to stimulation like noise or touch, and varying degrees of weakness,” said Dr. Mallory Lehman, an Equine Internal Medicine resident at the UC Davis veterinary hospital who has been managing recent cases with WNV.  “In severe cases, horses can progress to becoming unable to stand. Intensive supportive care is required for days to weeks and prognosis is guarded. Transporting affected horses to the hospital while they are still able to stand is important for both the safety of the horse and the safety of the owner. Some referral hospitals are able to provide sling support to help horses stay standing while they are recovering.“

Mosquitoes that have fed on infected birds transmit WNV. Therefore, the most effective methods to prevent and control infection in horses include appropriate vaccination and proper insect control in the stable area. Current recommendations include administering a WNV vaccine as an annual core vaccine. Mosquito control includes eliminating breeding sites by removing standing water, cleaning water containers, and stocking water troughs with fish that feed on mosquito larvae. It is also important to minimize horses’ exposure to mosquitoes through use of repellents and bringing horses in from pastures at peak mosquito feeding periods (dawn and dusk).

Horses are considered “dead end” hosts for the virus and are not able to transmit it to other animals or people once they are infected. This means that barns that housed affected horses do not need to be quarantined. However, the vaccination status of all other horses on the premises should be confirmed and boosters may be recommended after consultation with a veterinarian. Insect control protocols should be evaluated if cases are showing up on a farm.

“Vaccination timing is extremely important to take into consideration for vector-borne diseases such as WNV. Be aware of the peak times of year for mosquito activity and vaccinate with the goal of boosting immunity during that time”, said Lehman. “In addition, vaccination protocol recommendations, including boosters, should be followed closely. A single WNV vaccine is not considered effective in preventing and minimizing infection, and we have seen horses affected with significant disease that did not receive the appropriate booster protocol.”

In general, the WNV vaccines that are available for horses are excellent at either preventing disease or greatly reducing the severity of clinical signs of infection. Simply including the WNV vaccine in your vaccination protocol is an excellent way of minimizing the impact of a significantly life-altering disease in your horse. Similarly, it is critical to find out if, and when, a newly purchased or imported horse has been vaccinated. If that information is not available, it is better to err on the side of caution and assume that the horse is not well protected.

Follow updates on the status of WNV in the state from the California Department of Food and Agriculture here: https://www.cdfa.ca.gov/ahfss/Animal_Health/WNV_Info.html. Mosquitoes that carry WNV are most active in summer and early fall, so now is an ideal time to discuss your horses’ vaccination status with your veterinarian.