Arbovirus is a short form of ‘arthropod-borne virus.’ Arthropods include biting flies, mosquitoes, and ticks. Peak arbovirus season can span from July through October. 

Infected mosquitoes spread Eastern, Western, and Venezuelan equine encephalitis (EEE, WEE, VEE) and West Nile virus (WNV). They can cause severe encephalitis (brain inflammation) in equids and people. These viruses are widespread in birds and rodents, which are reservoirs for disease. 

Last year, the Equine Disease Communication Center (EDCC) received case submissions for 126 EEE cases and 153 WNV cases from North America. Of those numbers, 96 horses died from EEE and 39 died from WNV.

(See Figures 1 and 2 for location of EEE and WNV cases submitted to the EDCC by state and province in 2024. Not that not all states or provinces submit infectious disease cases to the EDCC. Darker colors indicate states/provinces where cases were reported.)

Figure 1: Distribution of EEE cases by state and province submitted to the EDCC in 2024.

Figure 2: Distribution of WNV cases by state and province submitted to the EDCC in 2024.

Vaccination

Unvaccinated horses are particularly susceptible to the effects of arborvirus diseases. Keeping horses up-to-date on vaccinations is the most effective way to prevent life-threatening infection. 

Veterinarians should give boosters to horses four to six weeks after their initial vaccination. You should re-vaccinate yearly at minimum. Veterinarians might recommend more frequent boosters (i.e., twice yearly) in areas with year-round mosquito season in endemic areas.

Find the American Association of Equine Practitioners adult horse vaccination chart here.

With no disease-specific treatment options available for equine arboviruses, prevention is key to avoid serious illness or death. Vaccination is the most effective way of protecting horses against these viruses. Mosquito control can also help prevent disease exposure. 

Vaccinating horses against EEE and WNV reduces the risk of horses contracting a severe disease after being bitten by infected mosquitos. Vaccination lowers the risk of death in horses exposed to EEE and WNV.

In the event that a vaccinated horse becomes infected with either EEE or WNV, the clinical signs tend to be less severe with a quicker recovery time when compared to unvaccinated horses.  Preventing EEE or WNV is more cost-effective than treating a horse with encephalitis, which might require intensive veterinary care or hospitalization with costly supportive treatments. 

Good mosquito management on horse properties can reduce exposure. Provide shelter with fans at dawn and dusk. Eliminate standing water. Use insect repellents. 

Eastern Equine Encephalitis (EEE)

EEE, also known as sleeping sickness, is a viral disease that causes inflammation of the brain and spinal cord. There is no cure for EEE. Horses with clinical signs should receive supportive care. Insect vectors transmit the virus to horses. Infected horses cannot transmit the disease to other horses or humans. An infected mosquito transmits the virus to a horse through biting. The incubation period of EEE is 5 to 14 days after a horse is bitten. 

Clinical signs of the disease include depression and anorexia (not eating), initially without a fever when first infected. Other clinical signs include moderate to high fever, lack of appetite, and lethargy/drowsiness.

The onset of the neurologic disease is frequently sudden and progressive. It can include periods of hyperexcitability, apprehension and/or drowsiness, fine tremors and fasciculations of the face and neck muscles, cranial nerve paralysis, head tilt, droopy lip, weakness, complete paralysis of one or more limbs, circling, convulsions, recumbency, and death. Owners might confuse EEE clinical signs with colic pain.

Horses infected with EEE rarely survive. The mortality rate is 75-95%, and death usually occurs within two to three days of onset of signs. 

West Nile Virus (WNV)

Like EEE, WNV causes inflammation of the nervous system for which there is no cure. Supportive care is administered to horses that show clinical signs.

Some infected horses never show clinical signs of the disease. However, clinical disease develops in up to 39% of horses that are infected.

Horses that survive usually make a full recovery, although some horses have lingering or recurrent neurologic deficits. Equids that become recumbent and are unable to rise have a poorer prognosis than those that remain standing. The approximate mortality rate is up to 40%. 

Clinical signs include fever, lack of appetite, lethargy, and neurologic signs. Hallmark clinical signs include fine muscle fasciculations of the muzzle and face and episodes of somnolence (abnormal drowsiness). Other clinical signs include periods of hyperexcitability, apprehension, cranial nerve paralysis, head tilt, droopy lip, muzzle deviation, complete paralysis of one or more limbs, recumbency, and death. Clinical signs of WNV can be confused with colic pain. 

The incubation period of WNV is seven to 10 days. Similar to EEE-infected horses, WNV-infected horses are not contagious and cannot transmit the disease. 

More WNV cases are expected this year in unvaccinated horses. This is due to abundant rainfall in many areas of the country. Check with a veterinarian to better understand the recommended guidelines for EEE and WNV vaccinations.

About EDCC

The EDCC is an industry-driven information center which works to protect horses and the horse industry from the threat of infectious diseases in North America. The Center is designed to seek and report real-time information about diseases similar to how the Centers for Disease Control and Prevention Center (CDC) alerts the human population about diseases in people.

The EDCC is based in Lexington, Kentucky, at the American Association of Equine Practitioners headquarters, with a website hosted by US Equestrian. The EDCC is funded entirely through the generosity of organizations, industry stake holders, and horse owners. To learn more visit www.equinediseasecc.org.

Contributors to this article were Krista Estell, DVM, DACVIM, Clinical Associate Professor
Marion duPont Scott Equine Medical Center, VMCVM-Virginia Tech, and Leslie C. McLaughlin, VMD, MPH, Adjunct Instructor, College of Veterinary Medicine, Western University of Health Sciences.

Further Reading