West Nile Virus Facts: Preventing WNV in Horses



West Nile Virus (WNV) is a viral disease that can cause encephalitis or meningitis, infection of the brain and the spinal cord or their protective covering. Prior to 1999, the disease was found only in Africa, Asia, and southern Europe. This virus has been identified in all of the continental United States, most of Canada and Mexico. The virus is transmitted from avian reservoir hosts by mosquitoes (and infrequently by other bloodsucking insects) to horses, humans and a number of other mammals.

West Nile virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States. Since 1999, over 25,000 cases of WNV encephalitis have been reported in U.S. horses. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease.

The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%.  Data have supported that 40% of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, 6-months post-diagnosis. Thus vaccination for West Nile virus is recommended as a core vaccine and is an essential standard of care for all horses in North America.

Vaccinate annually in the spring, prior to the onset of the insect vector season. For animals at high risk or with limited immunity, more frequent vaccination or appropriately timed revaccination is recommended in order to induce protective immunity during periods of likely exposure. For instance, juvenile horses (<5 years of age) appear to be more susceptible than adult horses that have likely been vaccinated and/or had subclinical exposure. Geriatric horses (>15 years of age) have been demonstrated to have enhanced susceptibility to WNV disease. Therefore, more frequent vaccination may be recommended to meet the vaccination needs of these horses.

You can reduce the number of mosquitoes around your home and neighborhood by reducing the amount of standing water available for mosquito breeding. Here are some simple steps you can take.

• Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers on your property. Pay special attention to discarded tires. That’s where lots of mosquitoes breed.

• Clean clogged roof gutters every year, particularly if the leaves from surrounding trees have a tendency to plug up the drains. Millions of mosquitoes can breed in roof gutters each season.

• Turn over plastic wading pools when not in use. A wading pool becomes a place for mosquitoes to breed.

• Turn over wheelbarrows and don’t let water stagnate in birdbaths. Both provide breeding habitats for domestic mosquitoes.

• Aerate ornamental pools or stock them with fish. Water gardens can become major mosquito producers if they are allowed to stagnate.

• Clean and chlorinate swimming pools when not in use. A swimming pool left untended by a family on vacation for a month can produce enough mosquitoes to infest an entire neighborhood. Mosquitoes may even breed in the water that collects on pool covers.

• Use landscaping to eliminate standing water that collects on your property. Mosquitoes may breed in any puddle that lasts for more than four days.


There are also some easy steps you can take to prevent mosquitoes from affecting your horses.

• House horses indoors during peak periods of mosquito activity (dusk and dawn).

• Avoid turning on lights inside the stable during the evening and overnight. Mosquitoes are attracted to lights.

• Place incandescent bulbs around the perimeter of the stable to attract mosquitoes away from the horses. Black lights don’t attract mosquitoes well.

• Remove all birds (including chickens) that are in or close to the stable.

• Look around the property periodically for dead birds, such as crows. Any dead birds should be reported to the local health department. Use rubber gloves to handle dead birds or use an implement, such as a shovel.

• Topical preparations containing mosquito repellents are available for horses. Read the product label before using and follow all instructions.

• Use fans on the horses while in the stable to help deter mosquitoes.

• Fog stable premises with a pesticide in the evening to reduce mosquitoes. Read directions carefully before using.


For help in assessing mosquito exposure risks on your property and for suggested control practices, please contact your county Extension office, county Department of Environmental Protection, local Department of Health, local veterinarian, or mosquito and pest control company.









West Nile Virus and Horses FAQs

Q. Has West Nile virus caused severe illness or death in horses?

A. Yes, while data suggest that most horses infected with West Nile virus recover, results of investigations indicate that West Nile virus has caused deaths in horses in the United States.


Q. How do the horses become infected with West Nile virus?

A. The same way humans become infected—by the bite of infectious mosquitoes. The virus is located in the mosquito's salivary glands. When mosquitoes bite or "feed" on the horse, the virus is injected into its blood system. The virus then multiplies and may cause illness. The mosquitoes become infected when they feed on infected birds or other animals.


Q. How does the virus cause severe illness or death in horses?

A. Following transmission by an infected mosquito, West Nile virus multiplies in the horse's blood system, crosses the blood brain barrier, and infects the brain. The virus interferes with normal central nervous system functioning and causes inflammation of the brain.


Q. Can I get infected with West Nile virus by caring for an infected horse?

A. West Nile virus is transmitted by infectious mosquitoes. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.


Q. Can a horse infected with West Nile virus infect horses in neighboring stalls?

A. No. There is no documented evidence that West Nile virus is transmitted between horses. However, horses with suspected West Nile virus should be isolated from mosquito bites, if at all possible.


Q. My horse is vaccinated against eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE). Will these vaccines protect my horse against West Nile virus infection?

A. No. EEE, WEE, and VEE belong to another family of viruses for which there is no cross-protection.


Q. Can I vaccinate my horse against West Nile virus infection?

A. Yes, West Nile virus vaccines for horses are available through veterinarians. Horse owners throughout the US should strongly consider vaccinating their equines. Consult your veterinarian for more details on timing of vaccination.


Q. How long will a horse infected with West Nile virus be infectious?

A. We do not know if an infected horse can be infectious (i.e., cause mosquitoes feeding on it to become infected). However, previously published data suggest that the virus is detectable in the blood for only a few days.


Q. What is the treatment for a horse infected with West Nile virus?  Should it be destroyed?

A. There is no reason to destroy a horse just because it has been infected with West Nile virus. Data suggest that most horses recover from the infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.






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