Late summer to early fall is prime West Nile Virus (WNV) time and this year has been no exception. Spread by mosquitoes and harbored within the native bird population, this disease is set to hit record numbers of human infections, not quite hitting the 1,600 case mark as of this writing, with the Dallas, Texas area being one of the hardest hit. Horse owners may be very well aware of this news, as horses are susceptible to this neurologic and potentially lethal virus, just like humans.
WNV is a member of the Flavivirus genus, which is found around the world. However, WNV in particular was not present in the U.S. until 1999, when an outbreak occurred in New York City. Since then, it has rapidly spread across the U.S. so that it is now endemic in all of the lower 48 states and is also found in Canada and Mexico. The CDC has the best up-to-date information on the disease with excellent graphics for those interested.
Birds are considered the reservoir for this virus, meaning this is where the virus can replicate and remain infective. When a mosquito bites an infected bird, the virus can then be transmitted to whatever the mosquito feeds on next: bird, human, or horse. Sporadic reports of a few other mammals such as dogs, cats, and squirrels have reported testing positive for WNV, but for a reason I don’t understand, the virus is primarily only a trouble-maker for the birds, equines, and us. Mosquitoes therefore are considered the vector for this disease.
It is important to note that humans and horses are considered dead end hosts for WNV. This means that once infected, humans and horses don’t achieve the high levels of virus replication in the blood to become reservoirs themselves. This also means that humans and horses cannot contract WNV directly from another human or horse, with the possible exception being a blood transfusion. In general, for transmission of WNV, a bite from an infected mosquito (created by feeding on an infected bird) is required.
Horses are similar to humans in that young and old individuals are at highest risk of clinical disease from WNV. In horses, clinical signs of infection are seen after about a week’s incubation period after the initial bite. Signs begin as a mild fever and lethargy, and then develop quickly into neurological problems, as the virus travels to the brain and spinal cord, causing inflammation. Depending on the specific location of the inflammation, the horse may exhibit general weakness, muscle tremors, or even total paralysis. Often, a combination of all or any of these is seen.
Treatment for WNV is supportive care — no antivirals on the market are useful for equine therapy. Anti-inflammatories are heavily used to try to reduce the amount of brain and spinal cord swelling and sometimes antioxidants like vitamin E are given to help combat the oxidative damage that occurs within the nervous system. Physical therapy and sometimes even support with a sling may be required if paralysis or severe weakness makes the horse recumbent. Affected horses that recover may have neurological deficits for the rest of their lives. Mortality in horses from WNV is about 30 percent.
Luckily for the horse population, there are USDA approved WNV vaccines on the market. I personally was surprised and impressed at the speed at which these were produced. There still isn’t a vaccine available for humans, due mostly to the differences in approval processes between human and animal vaccines.
Horses should be vaccinated yearly against WNV. This should be done in the spring, ahead of the mosquito season. Horses across the country should receive this vaccine; it’s considered a "core vaccine" by the American Association of Equine Practitioners.
It’s also a good idea to practice mosquito control on your farm. Stalling horses during the evenings and night when mosquitoes are most likely to feed will help decrease the chances of bites, and removing standing water around the stable will help eliminate this insect’s breeding area.
I personally have never seen a horse infected by West Nile. I believe this is because the horse population around my practice is mostly well vaccinated. This represents veterinary preventative medicine at its best.
So here’s my PSA for the week (please read it in an authoritative but friendly voice): WNV is out there and it’s deadly, so protect your horse!
Dr. Anna O’Brien