Arbovirus Season is Upon Us: Vaccinate Your Horses 

The dog days of summer are here and that also means arbovirus season is in full swing. Peak arbovirus season can span from July through October. 

Equine encephalitis viruses-Eastern, Western, and Venezuelan equine encephalitis (EEE, WEE, VEE) and West Nile Virus (WNV) are spread by infected mosquitos and can cause severe encephalitis (brain inflammation) in equids and people. These viruses are widespread in birds and rodents, making them reservoirs for disease. 

Last year, the Equine Disease Communication Center 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 (note: not all states or provinces submit infectious disease cases to the EDCC).

Figure 1: Distribution of EEE cases by state and province submitted to the EDCC in 2024. The darker red shades indicates where positive cases were located. 

Figure 2: Distribution of WNV cases by state and province submitted to the EDCC in 2024. The darker red shades indicates where positive cases were located.

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. 

The initial vaccination is followed in four-to-six weeks with a booster; yearly revaccination is recommended at minimum. Veterinarians may recommend more frequent boosters (i.e. twice yearly) in areas with year-round mosquito season in endemic areas. Recommendations from the American Association of Equine Practitioners adult horse vaccination chart can be found HERE.

With no disease-specific treatment options available for the 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 infection after being bitten by 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 symptoms 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 may require intensive veterinary care or even hospitalization, alongside costly supportive treatments. 

Another way to lower infection risk is to practice good mosquito management on all properties where horses are kept by providing shelter with fans at dawn and dusk, eliminating standing water and using insect repellents. 

EASTERN EQUINE ENCEPHALITIS (EEE)

EEE, also known as the sleeping sickness, is a viral disease that causes inflammation of the brain and spinal cord. There is no cure for EEE. Supportive care is administered in horses which display clinical signs. The virus can only be transmitted to a horse via an insect vector and infected horses cannot transmit the disease to other horses. The incubation period of EEE after a horse is bitten by mosquitoes is 5 to 14 days. 

Clinical signs of the disease include depression and anorexia, initially without a fever when first infected. Other clinical signs include moderate to high fever, lack of appetite, lethargy/drowsiness. The onset of the neurologic disease is frequently sudden and progressive including 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. Clinical signs may be confused 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 in cases which show clinical signs. Although some infected horses never show clinical signs of the disease, clinical disease develops in up to 39% of horses which are infected. Horses who survive usually make a full recovery, though some horses have lingering or recurrent neurologic deficits. Horses 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. 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 may be confused with colic pain. 

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

The following map shows the regions in north America affected by EEE and WNV in 2024. With the amount  of rain across the country this year more cases can be expected in unvaccinated horses. Check with a veterinarian to better understand the recommended guidelines for EEE and WNV vaccinations.

To learn more about equine encephalitis go to:  https://www.equinediseasecc.org/eastern-equine-encephalitis and West Nile Virus visit https://equinediseasecc.org/west-nile-virus

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.

By Leslie Barlow
EDCC Communication Manager

Dr. Krista Estell, DVM, DACVIM
Clinical Associate Professor
Marion duPont Scott Equine Medical Center 
VMCVM-Virginia Tech

Dr. Leslie C. McLaughlin, VMD, MPH
Adjunct Instructor
College of Veterinary Medicine
Western University of Health Sciences