ANDERSON, Ind. — West Nile virus cases are rising in humans and horses this year. Just as people must take precautions against the virus, horse owners should take steps to protect their animals.
Dr. Tracy Norman recommends vaccinating horses against the disease and taking measures to prevent mosquito bites. Norman is clinical assistant professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences Large Animal Clinic.
Mosquitoes transmit the virus from avian hosts to humans and horses, which are considered “dead-end” hosts of West Nile because the virus is not contagious among them. In those bitten by an infected mosquito, the virus can multiply in the blood system, cross the blood brain barrier and infect the brain. There, it can cause inflammation, interfering with central nervous system functions.
Most infected horses do not show signs of the disease. (Most humans don't either, for that matter.) In horses that do show signs of the virus, symptoms are similar to other neurological diseases. They can include impairment of basic motor skills - including loss of coordination or asymmetrical weakness - a change in behavior or drowsiness. Horses with West Nile may have a fever early in the disease and show sensitivity to touch and sound, as well as twitching in the face, muzzle and neck.
“These typical neurological signs are not always present in infected horses. Sometimes infected horses just appear colicky,” said Norman. “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.”
The main treatment is supportive care. Anti-inflammatory drugs - such as Banamine, steroids and dimethyl sulfoxide - and intravenous fluids are often used. A sling can be used to support a horse that is having trouble balancing 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 percent. 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 vaccines against West Nile are the best way to prevent infection in horses. The vaccine is not completely effective at preventing clinical disease, but it can help reduce the severity of symptoms.
Vaccinated horses that become sick with West Nile generally are less sick, require less intensive treatment, are sick for shorter periods and have a better chance at full recovery. The American Association of Equine Practitioners recommends the West Nile vaccination as one of the core vaccines all horses should receive.
Horse owners can take other steps to reduce the number of mosquitos 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.(Mosquitos have difficulty flying in wind.)
Vaccines provide year-long coverage, but in areas like Texas, with long summers and mild winters, some veterinarians give the vaccine twice a year. Norman recommends vaccinating horses that have not yet been vaccinated this year to help protect them through the fall mosquito months.
Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University.