Harris Farms Health Statement – West Nile Virus

Coalinga, CA (August 4, 2020) – Harris Farms is deeply concerned with the health and well being of all equines. Among the many good practices we embrace, here is our farm’s vaccination policy For West Nile Virus.

West Nile virus is included in the list of core vaccines that the American Association of Equine Practitioners recommends for all horses regardless of where they live or whether they travel. Most cases of West Nile virus could have been prevented with proper vaccination. A recent case seen in Stanislaus County in California was in an unvaccinated two year old Quarter Horse. Measures that can be utilized to prevent this disease include proper vaccination and reducing mosquito exposure.

West Nile virus is a member of the Flaviviridae family in the genus Flavivirus. It was first introduced into the U.S. in 1999 and rapidly spread to the 48 contiguous states in the following years. Since 1999 more than twenty-five thousand cases of West Nile encephalitis has been reported in the United States. Many wild bird species can carry high levels of the virus and serve as reservoirs for it. Mosquitoes transmit the virus by feeding on the blood of infected birds and then on susceptible species like horses and humans. Both horses and humans are dead end hosts meaning they cannot transmit the disease even if an infected animal is bitten by a mosquito and that mosquito bites another horse or human.

Clinically the virus damages the brain and spinal cord so infected animals’ exhibit fever and neurological signs such as limb incoordination and stumbling. Additionally this can be accompanied by fever and face, neck or trunk muscle twitching. Behavioral signs can also be seen in horses such as hyperreactivity or depression. In severe cases horses might be unable to rise develop paralysis, coma and eventual death. The case fatality rate in horses in around 33%.

A vaccine was developed in 2001 and widespread vaccination efforts resulted in a dramatic drop in infection rates across the country. The vaccine promotes excellent immunity when administered according to recommendations. The vaccine should have an initial dose given and then be boostered with a second dose 4-6 weeks later. The second vaccine is crucial to elicit the protective immunity. Horses that primarily only receive one vaccine may not be fully protected. After the initial series the horse can receive annual boosters and be fully protected unless in a very high risk area (southern states where mosquitoes remain active all year long). In high risk areas vaccinating twice a year may be

At Harris Farms our foals receive an initial three dose series of vaccines given prior to their yearling year. This ensures that there is no maternal antibody interference with their own immune system. Core vaccines (Rabies, Eastern and Western Encephalitis and West Nile) are then boostered annually. Pregnant mares are boostered with these vaccines one month prior to foaling and colostrum levels are checked at foaling to insure that a high immunoglobulin level is transferred to the foal.

Jeanne Bowers-Lepore, D.V.M.
Veterinarian Harris Farms Horse Division