Mosquitoes cause three of the major diseases that affect horses today. Eastern and Western equine encephalomyelitis have been around for many years. These can be transmitted by mosquitoes or arthropods. The West Nile Virus has become a problem in the last few years. Knowing the cause, clinical signs, treatment and prevention of these diseases are important, since letting these diseases go untreated can lead to the demise of a favorite horse.  

Eastern equine encephalomyelitis (EEE) remains a concern in the United States. Western equine encephalomyelitis (WEE) has been recognized in reservoir bird hosts in the Eastern United States, but clinical disease is rarely identified. The Togaviradae virus, found in wild birds, and transmitted to horses and humans via the bite of an infected mosquito, causes EEE and WEE. 

Horses do not develop high enough levels of the EEE or WEE virus in their blood to be contagious to other animals or humans. Horses are considered sentinels for human beings in a given area, because generally, increased numbers of horse cases precede cases in human beings by 2-5 weeks. Clinical signs in humans include fever, headache, confusion, stupor and seizures, with a five to 15% mortality rate.  

The acute (sudden) clinical signs of EEE and WEE in horses are nonspecific and include mild to severe fever (103-106° F) lasting 24 to 28 hours and potentially not detected, poor appetite, and stiffness. Many cases of WEE do not progress beyond this point. With EEE, progression is nearly certain. An incubation period of one to three weeks then occurs. As the disease progresses, clinical signs include propulsive walking, depression, and somnolence, to aggression and excitability. Some horses may become frenzied after any stimulation. Later signs indicate the increased severity of brain dysfunction and include head pressing, propulsive walking, blindness, circling, head tilt and facial and appendicular (limbs) muscle fasciculations (trembling or twitching). Paralysis of the pharynx, larynx and tongue are common at this stage. Defecation and urination become difficult. Complete paralysis and death are often preceded by recumbency (lying down, unable to get up) for one to seven days. This final stage may appear two to four days after the first signs appear. Animals that are comatose rarely survive. If animals do survive, they show gradual improvement of function over weeks to months.   

Call a veterinarian immediately if the horse is suspected to have EEE or WEE. Treatment for EEE or WEE is supportive care. Horses will have immunity from these diseases for up to two years after infection but may have permanent brain damage. The mortality rate for EEE is 75-100%. The mortality rate for WEE is 20-50%.  

A vaccine is available for EEE and WEE, it is a combination vaccine. This combination vaccine can be a “three-way” or “four-way” combination. The “three-way” combination vaccine, or three in one, protects against EEE, WEE, and tetanus. The “four-way” combination vaccine, or four in one, protects against EEE, WEE, tetanus, and influenza. 

According to the American Association of Equine Practitioners (AAEP) (2023), annual vaccinations for EEE/WEE prior to the onset of the mosquito season are recommended for most adult horses. Horses that have never been vaccinated or have an unknown vaccination series will need to complete a two-dose series, 4-6 weeks apart, and then may start on an annual revaccination schedule. Special considerations should be made for broodmares, horses less than 5 years of age, and horses residing in areas of high risk or with prolonged mosquito seasons. All vaccination programs should be made in consultation with a licensed veterinarian.

West Nile Virus (WNV) is a potentially deadly disease that can affect horses and humans. This flavivirus, similar to WEE and EEE, is found in wild birds and transmitted to horses and humans via the bite of an infected mosquito. West Nile Virus began appearing in 1999. There is no evidence suggests that horses can transmit the virus to humans or other horses. Some people get mild flu-like symptoms such as fever, headache, body aches or even a mild rash when infected. In others, West Nile Virus can cause encephalitis (a severe infection of the brain) or meningitis (an infection of the lining of the brain and spinal cord). Signs and symptoms of these serious infections include a sudden and severe headache, high fever, stiff neck, muscle weakness, tremors, convulsions, confusion and loss of consciousness or coma. In a minority of cases, these infections can cause death. There is a four to 47% mortality rate in humans. 

In horses, the virus usually appears in the blood three to eight days after a horse is infected with WNV; clinical signs appear shortly after that. In a small percentage of cases, WNV overwhelms the immune system and penetrates the blood-brain barrier, resulting in inflammation of the brain and spinal cord, called West Nile encephalitis. Clinical signs include loss of appetite and depression, fever (101° F or higher for two or more days), weakness of hind limbs, paralysis of hind limbs, muscle fasciculations (twitching or trembling), impaired vision, ataxia (incoordination), head pressing, head tilt, aimless wandering, convulsions, paralysis of the muzzle or tongue, droopy ear, vertigo, drowsiness, narcolepsy, inability to swallow, circling, hyperexcitability or coma. Horses that are recumbent and reluctant to rise are least likely to survive. Call the veterinarian immediately if WNV is suspected. 

Treatment for WNV is supportive therapy including the administration of anti-inflammatory medications (sometimes including corticosteroids) and intravenous fluids. There is a 30-40% mortality rate in horses.  

A fully licensed vaccine for WNV is available for horses. The vaccination program is similar to that for EEE and WEE (AAEP, 2023). These vaccines are thought to protect for six to eight months (Fabus & Skylis, 2017). All vaccination programs should be made in consultation with a licensed veterinarian.

It can be difficult to tell whether it is EEE or WNV. Typically, horses with WNV have lower fevers than horses with EEE (101° F for WNV vs. 103- 106° F for EEE). EEE is also more progressive in clinical signs. EEE cases generally have more severe neurological symptoms related to brain and spinal cord damage than horses with WNV. Horses that have EEE typically do not respond to Banamine or steroids; these will help in cases of WNV.  

Prevention of all three of these diseases should be aimed at both vaccinating and reducing the mosquito population: 

• Keep up to date on the horse’s vaccinations and vaccinate against EEE, WEE, and WNV annually in the spring. Vaccinate once a year in northern or central Connecticut; twice per year near the coast or if planning to go South with the horse in the fall or winter.  

• Use insect repellents to prevent bites.  

• Avoid outdoor activities at dawn and dusk or turning out horses at dawn and dusk when mosquitoes are most active. When outdoors at dawn and dusk, wear long sleeves and long pants in addition to using insect repellent.  

• Use fans in buildings (mosquitoes are poor fliers in wind).  

• Cover horses with fly sheets.  

• Feed animals away from stagnant water.  

• Remove standing water such as water in old tires, pet bowls, kiddie pools, birdbaths, flowerpots, buckets, etc. Overturn wheelbarrows and muck buckets not in use. When possible, drill holes in containers that must be left outside. Drain water tubs and turn them over when paddocks or fields will be unoccupied more than a week. During summer months, empty and scrub out water troughs once a week to dump out breeding larvae or contact the local health department, mosquito-control program or pest-control operator for information on larvicides approved by the Environmental Protection Agency. Fill in ditches, depressions, and other areas that tend to collect stagnant, dirty water.  

• Keep gutters clear of leaves and debris along barns and outbuildings.  

• Avoid turning lights on inside or near stables after dusk and place incandescent bulbs around the perimeter of the barn to lure mosquitoes away from the horses. 

Additional Resources

American Association of Equine Practitioners. (2023). Vaccinations for adult horses. Available at: https://aaep.org/wp-content/uploads/2024/02/Adult_Horse_Vaccine_Chart_2023_SUB.pdf

Animal and Plant Health Inspection Services – United States Department of Agriculture. (2003, March). West Nile Virus Fact Sheet. www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/fs_ahwnv.html

Animal and Plant Health Inspection Services – United States Department of Agriculture. (2003, March). Questions and Answers about West Nile Virus. www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/faq_ahwnv.html

Beckett, Stewart, DVM. (2006, April). Beckett and Associates Veterinary Services. Personal interview.

Bertone, JJ. (Spring 1999). Eastern, Western and Venezuelan Encephalomyelitis in Horses. Horse Industry Handbook 625-1-625-6.

Centers for Disease Control and Prevention, Department of Health and Human Services. Undated. West Nile Virus fact sheet.

Centers for Disease Control and Prevention, Department of Health and Human Services. Undated. West Nile Virus Questions and Answers. www.cdc.gov/ncidod/dvbid/westnile/qa/wnv_horses.htm

Comerford P, Diehl N, Gripp S. (Spring 2003). West Nile Encephalitis in Horses. Horse Industry Handbook 665-1-665-2. Meszoly J. Virus Update. June 2004. Equus 308: 41-50.

Fabus, T., & Skylis, L. (2017, April 11). Vaccinating your horse for eastern equine encephalitis. Michigan State University Extension. https://msu-prod.dotcmscloud.com/news/vaccinating_your_horse_for_eastern_equine_encephalomyelitis

Porter MB, Long MT, Gelman LM et al. (2003). West Nile Virus Encephalomyelitis in horses: 46 cases (2001). Journal of the American Veterinary Medical Association 222(9): 1241-1247.

Texas Department of Health. (2003, August). West Nile Virus Fact Sheet for Veterinarians. www.tdh.state.tx.us/zoonosis/diseases/Arboviral/westNile/information/wnFactvt.asp

Texas Department of Health. (2003, March). What you need to know about West Nile Virus.

United States Department of Agriculture. (2001, August). Prevention and Control of West Nile Virus Infection in Equine and Other Livestock or Poultry. www.aphis.usda.gov/oa/wnv/prv.htm