Veterinarians Help Protect Horses from Deadly Mosquito-Transmitted Viruses

Veterinarians Help Protect Horses from Deadly Mosquito-Transmitted Viruses

By Dr. Julie Ann McGwin, Equine Program Veterinarian
Wisconsin Department of Agriculture, Trade and Consumer Protection

With the Midwest Horse Fair coming up April 12-14, veterinarians will be busy working with horse owners to ensure animals meet health requirements for participating in the event. This is also the time of year to discuss vaccinating horses to protect them from mosquitos carrying the deadly West Nile Virus (WNV) and Eastern Equine Encephalitis (EEE). While Wisconsin had one of the wettest years on record last year, the number of confirmed WNV and EEE cases was significantly lower than in previous years. In 2017, there were 24 confirmed cases each of WNV and EEE. In 2018, there were only three cases of WNV and two cases of EEE. These numbers only account for what is reported, but it is encouraging to see more animals being protected from these viruses with your help.

Vaccination for WNV and EEE can usually prevent horses from getting ill from these viruses. WNV and EEE are fatal in approximately 30 percent and 90 percent of clinical cases, respectively. Both EEE and WNV are considered reportable diseases in Wisconsin. Any horse that tests positive is also reported to the Equine Disease Communication Center (EDCC) which provides national surveillance for these viruses and other infectious diseases. The EDCC can be a useful tool in predicting what mosquito activity will look like in Wisconsin as southern states experience warmer temperatures and provide initial reports of virus activity.

Basics of WNV and EEE

WNV and EEE may cause encephalitis. Symptoms in horses include the following:
• Depression
• Appetite loss
• Drooping eyelids and lower lip
• Fever
• Weakness
• Twitching
• Paralysis or lack of coordination
• Aimless wandering
• Circling
• Blindness

Horses may also go down, be unable to rise, exhibit seizures or become unresponsive, especially with EEE, which can be fatal within 24 hours.

The virus is not contagious between horses. While humans may also be infected by WNV and EEE, the viruses do not pass directly between people and horses. Mosquitoes carry the viruses from infected birds and the only route of transmission is from a mosquito bite.

Vaccination Process

Horses that have never been vaccinated will need two doses of the vaccination initially, and then boosters at least annually. It takes at least two weeks to build up enough antibodies to protect the horse and the vaccine will not protect the horse if given after the horse is infected. Horse owners should check with their veterinarian to ensure their horse’s vaccines are current.

Environmental Factors

Besides vaccination, other steps to limit a horse’s exposure to mosquitoes during warm weather include the following:
• Removing items from the property that could collect stagnant water such as old tires, tin cans and plastic
• Keeping rain gutters clean and draining properly.
• Cleaning and chlorinating swimming pools, outdoor saunas and hot tubs, and draining water from pool covers.
• Turning wading pools and wheelbarrows upside down when not in use.
• Emptying and replacing water in birdbaths at least once a week.
• Keeping horses in the barn from dusk to dawn, when mosquitoes are most active.
• Using equine mosquito repellents.

The viruses require amplification within particular avian species before the concentration of virus particles is sufficient to allow disease transmission. Due to the amplification required and because the viruses follow mosquito populations, the threat varies depending on the weather but normally starts in mid- to late summer and remains until the first killing frost.


For more information about EEE and WNV and other infectious diseases, visit the Wisconsin Department of Agriculture, Trade and Consumer Protection or EDCC.