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Prepare Your Horse and Farm for the Mosquito Season

Mosquito season is upon us despite a long hard winter for many. Besides being pesky and annoying, mosquitos also transmit diseases to horses. The following article discusses specific diseases, treatment, prevention and mosquito control important for the horse farm.


Protect your horses from mosquito transmitted diseases with timely vaccinations and insect control practices on your farm.

Encephalomyelitis Encephalomyelitis refers to a disease that causes inflammation of the brain and spinal cord, usually as a result of an infectious agent. There are three variants of equine encephalomyelitis: Eastern (EEE), Western (WEE), and Venezuelan (VEE), all of which are caused by viruses transmitted primarily by mosquitoes. All three can be serious, but EEE has a much higher fatality rate than the others. Following are some facts about the three strains.

A horse in lateral recumbency, meaning it is laying on its side.

Eastern Equine Encephalomyelitis Typical symptoms include loss of appetite, fever, and change in behavior. Within 12 to 24 hours of the initial symptoms, the horse will exhibit head pressing, circling, and often become blind. As the disease progresses, the horse may become recumbent and might suffer seizures. In the worst cases, horses die from respiratory arrest two to three days after the onset of clinical signs. The fatality rate for horses with EEE may be as high as 90%. Horses that do survive are frequently left with permanent neurological impairment. Treatment is mainly supportive since there are no anti-viral drugs available for EEE. Fortunately, EEE is relatively rare and is generally restricted to the eastern, southeastern and some southern states.

Western Equine Encephalomyelitis After an incubation period of one to three weeks, horses will develop a fever up to 105 degrees. In many cases, this will be the only symptom-- the horse’s natural immune response will clear the virus from the body within 48 hours after the onset of the fever. If the horse is unable to clear the virus, the situation becomes much more serious. The central nervous system is attacked, resulting in symptoms that include agitation, compulsive walking and circling, and sometimes crashing into walls. This stage is followed by extreme depression, reluctance to move, and drooping posture. The horse may also have tremors, difficulty swallowing, and an uncoordinated gait. If the horse remains standing, it may fully recover, but horses that become recumbent have a poor prognosis. Overall, the fatality rate for WEE is close to 50%. WEE is the most common strain of equine encephalomyelitis, with outbreaks recorded in the western and mid-western states. Variants of WEE have also caused sporadic cases in the northeast and southeast.

Venezuelan Equine Encephalomyelitis This strain occurs in South and Central America but has not been diagnosed in the United States for more than 20 years.

The risk of exposure and the geographic distribution of EEE and WEE vary from year-to-year depending on the distribution of mosquitoes and birds that act as reservoirs for the virus.

The following are the AAEP recommendations for vaccination against EEE and WEE:

Adult Horses: Previously Vaccinated against EEE/WEE

• Annual revaccination must be completed prior to vector season in the spring. In animals of high risk or with limited immunity, more frequent vaccination or appropriately-timed vaccination is recommended in order to induce protective immunity during periods of likely exposure. In areas where mosquitoes are active year-round, many veterinarians elect to vaccinate horses at 6-month intervals to ensure uniform protection throughout the year, although this practice is not specifically recommended by manufacturers of vaccines.

Adult Horses: Previously Unvaccinated against EEE/WEE or of Unknown Vaccinal History

• Administer primary series of 2 doses with a 4-to-6 week interval. Revaccinate prior to onset of next vector season and annually thereafter.

Pregnant Mares: Previously Vaccinated Against EEE/WEE

• Vaccinate 4 to 6 weeks before foaling.

Pregnant Mares: Unvaccinated or Having Unknown Vaccinal History

• Immediately begin 2-dose primary series with a 4-week interval. Booster at 4 to 6 weeks before foaling or prior to the onset of the next vector season—whichever occurs first.

Foals of Mares Vaccinated Against EEE/WEE in the Pre-Partum Period

• Administer a primary 3-dose series beginning at 4 to 6 months of age. A 4-to-6 week interval between the first and second doses is recommended. The third dose should be administered at 10 to12 months of age prior to the onset of the next mosquito season.

• In the southeastern U.S., due to earlier seasonal disease risk, vaccination may be started at 2 to 3 months of age. When initiating vaccinations in younger foals, a series of 4 primary doses should be administered, with a 4-week interval between the first and second doses and a 4-week interval between the second and third doses. The fourth dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

Foals of Unvaccinated Mares or Having Unknown Vaccinal History

• Administer a primary 3-dose series beginning at 3 to 4 months of age. A 4-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age before the onset of the next mosquito season.

Horses Having Been Naturally Infected and Recovered

• Recovered horses likely develop lifelong immunity. Consider revaccination only if the immune status of the animal changes the risk for susceptibility to infection. Examples of these conditions would include the long-term use of corticosteroids and pituitary adenoma.

Pregnant mares have specific vaccination requirements.

West Nile Virus The West Nile virus (WNV) causes serious and potentially fatal neurological disease in horses. It is transmitted by mosquitoes, which bite infected birds and then spread the virus to other species. Since its first occurrence in New York in 1999, migrating birds have spread WNV throughout the United States and Canada. Although the number of equine cases has dropped off markedly, WNV continues to be a threat. The equine mortality rate remains at about 30%. Those at greatest risk are the young, the old, and those with compromised immune systems. On a population basis, most horses bitten by WNV-infected mosquitoes don’t become ill. This provides little consolation, however, to a horse owner whose horse contracts the disease.

Horses that do become infected with WNV are most likely to exhibit clinical signs that are neurological in nature. Muscle fasciculations (muscle twitching) seem to be one of the most common signs. Additional signs that may be seen are incoordination, muscle weakness, fever, somnolence (sleepiness), inability to eat and drink, recumbency, and seizures. Horses clinically affected by WNV appear to have a 60% chance of recovery and return to normal function when treated. However, as there is no specific treatment, horses that are clinically-infected are treated by supportive measures, such as IV fluids, control of fever, and safe, quiet housing. The horses that remain able to stand or are able to stand with assistance usually make a complete recovery over several weeks to months, though up to 40% may have persistent neurological deficits. Horses that become recumbent have a guarded prognosis for survival.

Once a horse has been infected with the WNV and survived, it is protected from development of clinical disease for an extended period of time. Nonetheless, prevention remains the best option. There are now several types of vaccines available; and most veterinarians recommend vaccinating all horses, especially in areas where WNV is known to be present.

In addition to vaccinations, recommendations for reducing the risk of your horses developing WNV include minimizing your horses’ exposure to mosquitoes and mosquito control. To reduce mosquito exposure, stable horses at dusk and dawn. Use fly blankets, masks, and leg wraps; turn barn lights off; use fans to move air; and spray mosquito repellents. Mosquito control programs include reducing standing water, encouraging natural predators (fish, birds, etc.), and using chemicals for larval and adult mosquito control.

Mosquito repellants and fly masks can help reduce mosquito exposure.

Mosquito Control Biting insects are also carriers for blood-transmitted diseases like West Nile Virus and Equine Infectious Anemia. Practicing fly and mosquito control involves the following:

  • Eliminate standing water around the farm.

  • Apply bug repellent to horses.

  • Install automatic repellents in barn alleyways and stalls.

  • Use fly sheets and hoods on horses to protect against biting insects.

  • Keep horses inside during dusk hours when flying insects are at their most active.

  • Consider using complimentary bugs as a natural way to eliminate harmful insects.

  • Hang bug strips in barn alleyways.

Additional Resources

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