Carey A. Williams, Ph.D., Extension Specialist in Equine Management; Wayne Crans, Ph.D., Research Professor in Entomology; and Jennifer Gruener, Graduate Assistant in Entomology.

Q. What is West Nile virus?

A. West Nile virus (WNV) is an old world mosquito-borne pathogen that appeared unexpectantly in the New York metropolitan area in the fall of 1999. The disease was first thought to be St. Louis encephalitis, a closely related mosquito-borne virus that is relatively common in the continental United States. Virus isolations from wild crows, which were dying from the disease throughout the New York-metropolitan area, as well as captive birds from the Bronx Zoo prompted the USDA’s National Veterinarian Services Laboratories to investigate. In late September 1999, CDC officials announced that the virus appeared to be West Nile virus, a pathogen previously unseen in the Western Hemisphere. Avian cases were quickly linked to a series of human ailments in the New York City area that ultimately produced 62 human cases of WNV with 7 deaths. The virus was recognized as an important pathogen of horses as it spread from its eastern focus, affecting large numbers of animals as it moved westward. West Nile virus reached the west coast of the United States in 2002 and was responsible for more than 14,000 diagnosed equine cases that year.

Q. How does West Nile virus cycle in nature?

A. West Nile virus is a disease of wild birds that is maintained in the avian population by mosquitoes that use birds as their preferred blood meal hosts. The Northern House Mosquito, Culex pipiens, is an important bird-feeding species that perpetuates WNV in urban areas as well as farm settings throughout the northeastern United States. WNV is transmitted to humans and horses by mosquitoes that occasionally bite birds but normally use mammals as their preferred blood meal hosts. Culex pipiens may be directly responsible for some human and equine infection, but mammal biters are thought to produce the majority of WNV cases each year.

Q. How do horses contract West Nile virus?

A. Horses contract WNV when a mosquito that has previously fed on an infected bird bites them. Birds circulate high levels of the pathogen in their blood and serve as the sole source of the virus for mosquitoes. Neither horses nor humans circulate enough virus in their blood when they acquire the disease to pass the virus back to mosquitoes. West Nile virus cannot be spread directly from horse to horse or from horse to human. A mosquito that has previously fed on an infected bird is required in all cases.

Q. What is the seasonal progression of West Nile virus?

A. West Nile virus is at its lowest levels in early spring and builds in intensity as the summer season progresses. Infected mosquitoes are rarely found in large numbers until the month of August. Most transmission to horses takes place during late summer into fall, the same time that infection rates peak in mosquitoes and birds. Horses that have not been protected through proper vaccination may contract the infection at this time of year.

Q. Why should horse owners be concerned about West Nile virus?

A. West Nile virus is a serious threat to horses. In New Jersey, even though WNV affected only a small percentage of the equine population, the mortality rate of those infected from 2000 to 2003 was 34%.

Q. How can I tell if my horse is infected with West Nile virus?

A. Infected horses may display one or more of the following symptoms: Lack of coordination and stumbling (most commonly described symptom), depression or apprehension, anorexia (off feed), weakness of the hind limbs, falling down, inability to rise, flaccid paralysis of the lower lip (droopy lip), muscle twitching, grinding teeth, inability to swallow, head pressing, colicky appearance, aimless wandering, hypersensitivity and excitability, excessive sweating, disorientation, convulsions, and possible total paralysis.

Q. What should I do if I see any of these signs?

A. Call your veterinarian immediately. Prompt treatment may be life-saving!

Q. Is euthanasia of West Nile virus infected horses necessary?

A. No. Horses are humanely euthanized only when the viral infection is so severe they will not be able to recover. For those that survive, a full recovery is likely. About two out of every three horses that become ill will survive.

Q. Will horses and/or farms affected by the virus be quarantined?

A. No. Since infected horses are not carriers for the disease, quarantine is not necessary.

Q. How can I reduce the risk of West Nile virus to my horses?

A. You can significantly reduce the risk of West Nile virus to your horses by taking the following steps:

1. Call your veterinarian:

  • Have your horses vaccinated!
  • Ask about proper nutrition and supplements that can be part of an overall wellness plan to keep your horses healthy, and may protect them from diseases such as West Nile virus.

2. Contact your local county mosquito control agency:

  • They are trained professionals that can help you learn to recognize mosquito larvae and their habitats.
  • They can locate and assess mosquito breeding habitats on your farm and in nearby areas.
  • This service is free of charge and available to county residents.

3. Reduce local mosquito populations:

  • Empty and clean watering troughs at least every four days.
  • Consider purchasing watering troughs with bottom drainage plugs.
  • Check automated watering systems at least once a week to make sure they are working properly and not sustaining mosquito larvae.
  • Turn over all unused containers such as buckets, feed pans, and wheelbarrows.
  • Drill holes in the bottom of tires and other containers that cannot be discarded or readily emptied (call your county mosquito agency for guidance).
  • Check all indoor and outdoor drainage systems to make sure they are free of debris and operating properly.
  • Clean gutters on all outbuildings to make sure they are not holding water.
  • Grade pastures when possible to prevent rain water from collecting in temporary pools.
  • If you discover standing water on your property and are unsure how to eliminate it, Contact Your County Mosquito Control Agency at: 

Concentrate on the recommendations above instead of using minimally effective mosquito control measures: e.g., fly sheets and masks; repellents for long term control; fly misters; bug zappers.

Q. Do mosquitoes only bite at dusk and dawn?

A. No. Most species of mosquitoes seek blood meals at dusk and dawn, but many species are day biters and some are nocturnal. Recommendations for turnout cannot be made until the species that transmit WNV to horses have been identified.

Q. Is there a vaccine for West Nile virus?

A. Yes, there are two intramuscular vaccines available for horses. The original vaccine and the most common is West Nile-INNOVATORTM vaccine, manufactured by Fort Dodge Animal Health, a division of Wyeth. More recently Merial, a division of Merck and Aventis Companies, has developed RECOMBITEK® Equine West Nile Virus Vaccine, a vaccine using recombinant DNA.

Vaccination of all healthy horses is strongly recommended. The West Nile-INNOVATORTM vaccine requires two injections, spaced three to six weeks apart. This part is critical in order for the vaccine to take its full effect. Immunity may not develop for four to six weeks after the second injection. A booster is recommended every six months to continue protection. In New Jersey 44% of the infected horses were not vaccinated, 21% had an unknown vaccination record and 27% were vaccinated improperly (including missing the three to six week window). The vaccination has been shown to be 93 to 95% effective.

Some early reports in 2002 suggested that the Fort Dodge West Nile virus vaccine, approved by the U.S. Department of Agriculture (USDA), may cause pregnant mares to abort or give birth to deformed foals. The misleading information in those articles has sparked many anxious phone calls from horse owners, veterinarians, and others involved with horses. As a result, some horse owners choose not to use the effective preventive measure against West Nile virus available to them.

The USDA continues to recommend use of the approved vaccine as protection against West Nile virus. Millions of doses of the Fort Dodge vaccine have been used since the USDA’s Center for Veterinary Biologics approved its use in 2001.

The Center for Veterinary Biologics, within the USDA’s Animal and Plant Health Inspection Service, maintains a toll-free telephone hotline (800-752-6255) and a mailbox on its web site,, and actively encourages veterinarians and other vaccine consumers to report problems with vaccines. The Center and the vaccine manufacturer will continue to collect, monitor, and track the performance of this vaccine.

Q. What is the proper method of vaccinating horses for West Nile virus?

A. Previously Unvaccinated Horses

  • Vaccinate all previously unvaccinated adult horses in March/April with a two-dose primary series, three to six weeks apart.
  • Talk to your veterinarian about vaccinating pregnant mares during the first trimester.
  • Vaccinate all young horses (less than one year of age) in March/April with a three-dose primary series. Allow three to four weeks between doses one and two, and allow six to eight weeks between doses two and three.
  • There is no minimum age recommendation for the vaccine; however, Fort Dodge Animal Health suggests that if a mare has been vaccinated for WNV, the foal can be vaccinated when you would normally vaccinate for eastern and western encephalomyelitis. If the foal is from a mare that has not been vaccinated for WNV, the foal should be vaccinated at six to eight weeks of age and receive a series of three injections spaced three to six weeks apart (recommended by the NJ Department of Agriculture).

Previously Vaccinated Horses

  • Should maintain their spring booster when encephalitis shot is given (preferably before the start of mosquito season).
  • Should be followed up with another booster six months later (aim for before the peak mosquito season).


Q. Where can I get more information about West Nile virus?

A. Local mosquito surveillance and control information is available from your county mosquito control agency, and specific questions regarding WNV may be addressed to your local health department or for any animal health related issues contact the NJ Department of Agriculture.

References and additional sites of West Nile virus information:

The Animal and Plant Health inspection Service (APHIS):

New Jersey Department of Agriculture: or call (609) 292-3965.

Centers for Disease Control:

Department of Health and Senior Services:

New Jersey Department of Environmental Protection:

New Jersey Mosquito Biology and Control (also links to County Mosquito Control information):

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© 2004 by Rutgers Cooperative Extension, New Jersey Agricultural Experiment Station, Rutgers, The State University of New Jersey. This material may be copied for educational purposes only by not-for-profit accredited educational institutions.





Distributed in cooperation with U.S. Department of Agriculture in furtherance of the Acts of Congress on May 8 and June 30, 1914. Rutgers Cooperative Extension works in agriculture, family and consumer sciences, and 4-H.  Dr. Karyn Malinowski, Director of Extension. Rutgers Cooperative Extension provides information and educational services to all people without regard to race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Rutgers Cooperative Extension is an Equal Opportunity Program Provider and Employer.