The West Nile virus (WNV) cases in humans and horses have been on the rise in 2012 and, according to the Centers for Disease Control and Prevention, human cases are at their highest levels since WNV was first detected in the United States in 1999. Tracy Norman, VMD, Dipl. ACVIM, clinical assistant professor at the Texas A&M University College of Veterinary Medicine & Biomedical Sciences Large Animal Clinic, suggests protecting horses by vaccinating against the disease and taking measures to prevent mosquito bites.
The virus is transmitted by mosquitoes from avian hosts to humans and horses. Both humans and horses are considered "dead-end" hosts, which means it is not contagious from horse to horse or horse to human. If bitten by an infectious mosquito, the virus can multiply in the blood system, cross the blood brain barrier, and infect the brain. There, it can cause inflammation of the brain, interfering with central nervous system functions.
Most horses infected with the virus do not exhibit signs of the disease. For those that do, however, symptoms are similar to other neurologic diseases and can include impairment of basic motor skills (including loss of coordination or asymmetrical weakness, a change in behavior, or drowsiness. Some horses with WNV can have a fever early in the disease and show symptoms such as sensitivity to touch and sound, and muscle twitching in the face, muzzle, and neck.
"These typical neurologic signs are not always present in infected horses, sometimes infected horses just appear colicky," Norman said. "You should always consult with a veterinarian if you suspect a horse of having West Nile virus. Confirmation of infection is easily diagnosed through a blood test. Then owners and the veterinarian can plan a course of treatment."
Norman explained that the main treatment for WNV is supportive care. Often anti-inflammatory drugs (such as flunixin, steroids, and dimethyl sulfoxide [commonly called DMSO]) and intravenous fluids are used. If the horse is having difficulty balancing, a sling can be used to support the horse as it recovers.
"The idea is to keep the horse healthy so it can fight the virus," Norman said.
"If the horse is down and cannot get back up on its own, the outlook is pretty bad," Norman said. "The mortality rate for West Nile is about 30 to 40%. Many infected horses will survive, but many of those will have residual neurological impairment. Not all horses will regain their previous performance levels."
Norman stressed that WNV vaccines are the best way to prevent infection in horses. While the vaccine is not 100% effective at preventing clinical disease, the vaccine can help reduce the severity of the symptoms (because of some pre-existing immunity from the vaccine). Vaccinated horses that do become sick with WNV are in general less sick, requiring less intensive treatment, are sick for shorter periods of time, and have a better chance at making a full recovery than unvaccinated horses. Vaccination against WNV is recommended by the American Association of Equine Practitioners as one of the core vaccines that all horses should receive.
Additionally, horse owners can take steps to reduce the amount of mosquitoes around their facilities by eliminating standing water; keeping stalls and pens clean; using equine mosquito repellents, fly sheets, and fly masks; and placing fans inside stalls (mosquitoes have difficulty flying in wind).
The vaccines provide year-long coverage, but in some areas like Texas, with long summers and short, mild winters, some veterinarians will give the vaccine twice a year. Norman recommends vaccinating horses that have not yet been vaccinated this year to help protect horses through the fall mosquito months.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.