West Nile Virus

Understand the nasty truth about this mosquito-borne disease.

From AQHA Corporate Partner Pfizer

According to the American Association of Equine Practitioners, West Nile virus is the leading cause of arbovirus encephalitis in horses and humans in the United States¹. Since 1999, more than 24,000 cases of WNV have been reported in U.S. horses, with 1,069 cases reported in 2006¹. As of October 2007, 250 equine cases were reported. The decline is said by health experts to reflect both vaccination and naturally acquired immunity¹. In 2010, 146 WNV cases were reported across the country, with the greatest number of cases in California and Florida².

“It is a good sign that the number of cases  has declined over the last decade,” says Dr. Rocky Bigbie, senior area veterinarian at Pfizer Animal Health. “However, horses still represent 96.9 percent of all non-human mammalian cases of WNV disease, which makes vaccinating against WNV important to any preventative health care program.”

What Is West Nile Encephalitis?
West Nile encephalitis is inflammation of the central nervous system, which is caused by an infection with West Nile virus³. Prior to 1999, WNV was found only in Africa, eastern Europe and West Asia³. Today, WNV has been identified in all of the continental United States, as well as in most parts of Canada and Mexico¹.

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How Is West Nile Virus Transmitted?
West Nile virus is transmitted by mosquitoes to horses, humans and a number of other mammals¹. The virus, located in the mosquito’s salivary glands, is transmitted by many different mosquito species and varies geographically. When mosquitoes feed on the horse, the virus is injected into its blood stream². The virus then replicates and may cause illness. Mosquitoes become infected with WNV when they feed on infected birds or other animals². Horses and humans are considered dead-end hosts and will not transmit the virus from horse to horse or horse to human¹.

Does a West Nile Virus Infection Always Lead to Illness?
Infection with WNV does not always lead to signs of illness. However, horses appear to be a species that is susceptible to infection with this virus³. In horses that become clinically ill, the virus infects the central nervous system and may cause symptoms such as loss of appetite and depression. Other signs may include fever, weakness or paralysis of hind limbs, impaired vision, ataxia, aimless wandering, walking in circles, hyper-excitability or coma³. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33 percent¹. Data has supported that 40 percent of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavior abnormalities, six months post-diagnosis¹.

How to Help Protect Horses Against West Nile Virus?
Vaccinations are the most effective way to help protect horses against WNV and other encephalic or mosquito-borne diseases, such as eastern equine encephalitis and western equine encephalitis. According to the AAEP vaccination guidelines, West Nile is considered a core vaccination along with EEE, WEE, tetanus and rabies¹. For adult horses previously vaccinated against WNV, an annual booster should be administered in the spring prior to mosquito season. It is best to consult a veterinarian on the horse’s vaccination program, as more frequent vaccination may be required to meet the vaccination needs of the individual horse.

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Pfizer Animal Health offers the trusted Fort Dodge line of vaccines, including WEST NILE-INNOVATOR® and the Mosquito Shot ™ (WEST NILE-INNOVATOR® + EWT), which helps protect against eastern equine encephalitis virus, western equine encephalitis virus and West Nile virus in a single vaccine. The WEST NILE-INNOVATOR vaccines, along with other Pfizer Animal Health equine vaccine combinations, are included in the Equine Immunization Support Guarantee (ISG).

The Pfizer Animal Health Equine Immunization Support Guarantee program provides up to $5,000 for reasonable diagnostic and treatment costs if a horse properly vaccinated by a veterinarian contracts the corresponding equine disease. Disease protection backed by the Equine Immunization Support Guarantee includes infection from West Nile virus, equine influenza virus, tetanus, eastern equine encephalitis virus, western equine encephalitis virus and Venezuelan equine encephalitis virus. Vaccinations must be performed by a licensed veterinarian with an established client-patient relationship to be eligible.

Is West Nile Virus Still a Threat Today?
West Nile virus is absolutely a threat today. As of April 8, California has already reported two counties with a dead bird and mosquito samples that have tested positive for WNV4. Now is the time to begin working with a veterinarian to get horses vaccinated against West Nile virus. A vaccination program should be used in conjunction with good mosquito reduction/avoidance measures. The most effective method is to destroy any mosquito breeding habitat by removing all potential sources of stagnant water. Water buckets, water troughs, plastic containers or any water holding container should be cleaned or emptied on a weekly basis5. For a complete list of preventative measures, visit Horse owners are encouraged to contact a veterinarian immediately should they notice any signs or symptom of WNV infection in their horses, especially if they are exhibiting neurological signs.


1. Core Vaccination Guidelines. American Association of Equine Practitioners. 2008. Available at: Accessed April 11, 2011.

2. Disease Maps 2010. U.S. Department of the Interior. U.S. Geological Survey. Updated January 11, 2011. Available at: Accessed February 15, 2011.

3. What Horse Owners Should Know About West Nile Virus. Pennsylvania’s West Nile Virus Control Program. 2000. Available at: Accessed on April 11, 2011.

4. Latest West Nile Virus Activity in California. California West Nile Virus Website. Updated April 8, 2011. Available at: Accessed on April 14, 2011.

5. West Nile Virus. The Florida Department of Agriculture and Consumer Services: The Division of Animal Industry. Updated March 11, 2011. Available at Accessed April 14, 2011.