Summer is when the West Nile virus poses the greatest threat to the health of horses in the United States. It’s important for veterinarians to work with horse owners this time of year to prevent, detect and accurately diagnose a potential infection.
The death rate among U.S. horses ranges from 30 to 40 percent for West Nile disease. Of horses that recover from the disease, up to 40 percent may exhibit neurological signs for six months or more after the initial diagnosis.
TVMDL has documented a recent and significant increase in the percentage of horses that test positive for exposure to the West Nile virus. In 2016, TVMDL confirmed 129 horses positive for West Nile Virus in Texas. The positive horses were located in 71 counties with most cases in Eastern Texas. There were also six confirmed cases of Eastern Equine Encephalitis in Texas.
West Nile virus (WNV) is a flavivirus transmitted primarily by mosquitoes. It occurs worldwide and is most commonly found in Africa, Asia and the Middle East. The virus was first detected in the United States in 1999 in New York and subsequently spread across the country. North America is now considered endemic for West Nile virus.
All horses and other equids appear to be susceptible to WNV; the disease does not attack one age group more than another. The death rate for clinically affected horses in the U.S. is approximately 30-percent. Of horses that recover from the disease, as many as 40-percent may continue to exhibit some neurological affects six months or longer after the initial diagnosis of the disease.
How is the virus transmitted?
Nature maintains WNV through a transmission cycle between wild birds and mosquitoes. Birds serve as a reservoir for the virus. Periodically, mosquitoes will also transmit the virus to horses and to humans. There is no evidence of the virus being transmitted directly from horse to horse or from horses to humans.
Most cases of WNV occur during the summer or early fall.
What are the clinical signs in horses?
The incubation period averages between 3 and 15 days in horses. In the U.S., clinical disease has been reported to develop in anywhere from 10- to 30-percent of infected horses.
Clinical signs include:
- Depression, stupor or changes in behavior
- Stumbling, staggering, wobbling or lameness
- An inability to stand
- Muscle twitching
- Grinding teeth
How do I know if my horse is infected?
If your horse exhibits clinical signs of WNV, you should contact your veterinarian immediately.
Diagnosis of WNV cannot be based solely on clinical signs. Other diseases that cause similar signs in the horse include Rabies, Equine Protozoal Myeloencephalitis, Equine Herpesvirus-1, botulism, bacterial meningitis, wobbler syndrome, and Eastern, Western and Venezuelan Encephalomyelitis.
Several serological tests can be used to diagnose WNV in clinically ill horses. The most reliable is the IgM antibody capture ELISA, which can confirm recent exposure to the virus. The IgM antibody rises quickly after exposure, but it is relatively short lived. A positive IgM antibody capture ELISA indicates infection likely occurred within the previous three months. Data indicates that little IgM is detected in recently vaccinated horses.
What can I do to protect my horses?
You should consult your local veterinarian concerning a vaccination protocol for your horse. Vaccination is a primary method of reducing the risk of infection from WNV. Vaccination is recommended for all horses in geographical areas where WNV is known to occur.
Vaccination involves an initial administration of two doses of the vaccine three to six weeks apart, followed by an annual or semi-annual booster. It is unknown whether a horse that has recovered from WNV infection can become re-infected later in life.
The American Association of Equine Practitioners suggests vaccinating recovered horses one year after the acute illness and then including them in a routine vaccination program.
Mosquito control around your barn, stable and home is also important in reducing the risk of WNV exposure. For more information on mosquito control related to WNV, consult the Texas Department of State Health Services website at www.dshs.state.tx.us/idcu/disease/arboviral/westNile.
For information on diagnostic testing services for WNV, visit tvmdl.tamu.edu.