Cooperative Extension Fact Sheet FS526

Carey Williams, Extension Specialist in Equine Management

Dina M. Fonseca, Professor and Director, Center for Vector Biology


After the introduction of West Nile virus (WNV) into the U.S., Carey Williams, Wayne Crans now a Professor Emeritus of Entomology, and Jennifer Gruener then a Rutgers graduate student and now the superintendent of the Warren County Mosquito Commission, created NJAES FS526 “West Nile Virus in Horses: Frequently Asked Questions” in 2004. In the current version we summarize the latest scientific information and statistics including up-to-date information to owners on how to minimize mosquitoes that transmit WNV in their properties and protect their horses.

What Is West Nile Virus?

West Nile Virus (WNV) was first described in 1937 from the blood of an inhabitant of the West Nile district in Uganda. Through the 1950s and into the 1990s WNV recurred in epidemic bursts of mild febrile human disease in Africa, the Middle East, and Europe. In the late 1990s, however, outbreaks in southern Russia, Romania, Israel, and ultimately the New York metropolitan area in the fall of 1999 included cases of more severe neuroinvasive disease and even death. After its first detection in the New World in 1999, the virus was quickly recognized as an important pathogen of horses as it spread south and west from its urban focus. According to the American Association of Equine Practitioners (AAEP) there have been more than 25,000 diagnosed cases of WNV in U.S. horses.

How Does West Nile Virus Cycle in Nature?

The primary amplifier of WNV are bird species such as American robins and house sparrows that are mostly unaffected by the virus but are infectious to mosquitoes. Bird-biting mosquitoes such as the native white dot mosquito, Culex restuans and the exotic northern house mosquito, Culex pipiens transmit the disease among birds. WNV transmission to humans and horses occurs when infected bird-biting mosquitoes, in the NE US primarily Culex pipiens, bite them.


Figure 1.

Figure 1. An adult Culex mosquito. Photo credit: Jim Occi.


How Do Horses Contract West Nile Virus?

Horses contract WNV when an infected mosquito, i.e. one that has previously fed on an infected bird, bites them. 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 WNV infected mosquito is required in all cases.

What Is the Seasonal Progression of West Nile Virus?

West Nile Virus is at its lowest levels in early spring and builds in intensity as the summer season progresses. In the NE US, infected mosquitoes are rarely found in large numbers until August. Furthermore, Culex pipiens increase their likelihood of biting mammals in the fall. Since most transmission to horses takes place during late summer into fall, horses should be protected through proper vaccination in the early spring.


Figure 2.
Figure 2. A female Culex engorged with blood; the route of transmission for WNV. Photo credit: Jim Occi.


Why Should Horse Owners Be Concerned about West Nile Virus?

West Nile Virus is a serious threat to horses. Across the US approximately 20% of infected horses develop neurological signs. In 2018, 493 WNV equine cases occurred in 42 states: 1 case in NJ (APHIS (PDF)). Between 1999 and 2017, more than 27,600 U.S. horses have been confirmed with WNV neuroinvasive disease with an estimated average case fatality rate of 30–40% (AAEP (PDF)) inclusive of both natural death and elective euthanasia.

How Can I Tell If My Horse Is Infected with West Nile Virus?

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.

What Should I Do If I See Any of These Signs?

Call your veterinarian immediately. Prompt treatment may be lifesaving!

Is Euthanasia of West Nile Virus Infected Horses Necessary?

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, however, up to 40% of those recovered have residual clinical signs.

Will Horses and/or Farms Affected by the Virus Be Quarantined?

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


Figure 3.
Figure 3. Culex larvae feeding. Photo credit: Jim Occi.

Figure 4.
Figure 4. A horse drinking out of an automated waterer. Photo credit: C. Williams.


How Can I Reduce the Risk of West Nile Virus to My Horses?

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

Do Mosquitoes Only Bite at Dusk and Dawn?

No. Many species of mosquitoes seek blood meals at dusk and dawn, but some species are day biters, some are nocturnal, and some will bite whenever the opportunity arises all day/night long. Culex pipiens mosquitoes, which are the species most likely to transmit WNV, are night biters that start biting at dusk. They will readily bite indoors.

How Effective Are the Vaccines for West Nile Virus?

Vaccination of all healthy horses is strongly recommended. The vaccines require two injections, spaced three to six weeks apart if it is the first time a horse has been vaccinated for WNV. The full course is critical 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. There were no WNV horse cases in NJ in 2019.


Figure 5.

Figure 5. Vaccinating a horse for WNV. Photo credit: Nancy Frenick, Stable Graphics LLC.


What Is the Proper Method of Vaccinating Horses for West Nile Virus?

It is very important to follow instructions from the manufacturer of the vaccines and/or consult with your veterinarian. More details can be found on the AAEP website.

Previously Unvaccinated Horses

Previously Vaccinated Horses

Where Can I Get More Information about West Nile Virus?

Local mosquito surveillance and control information is available from your county mosquito control agency. Specific questions about WNV in horses can be addressed to your local health department or for any animal health-related issues, contact the NJ Department of Agriculture. It is also advised to consult with your local equine veterinarian for more details about your area and your horse specifically.


Thank you to Sandy Strilec, DVM, New Jersey Department of Agriculture, Division of Animal Health for the review of this fact sheet.

References and Additional Information

October 2020