MADISON – An unvaccinated yearling Standardbred cross gelding from Clark County is the first reported Wisconsin horse to have become infected with West Nile Virus (WNV) and euthanized this year.

"It’s been a very wet summer so far this year, which contributes to a growing mosquito problem,” says Dr. Julie McGwin, equine program veterinarian for the Wisconsin Department of Agriculture, Trade and Consumer Protection.

West Nile Virus is caused by a virus transmitted by mosquitoes, and may cause encephalitis, or inflammation of the brain, and is fatal to horses in 90 percent of cases. Symptoms in horses include depression, appetite loss, drooping eyelids and lower lip, fever, weakness, twitching, paralysis or lack of coordination, aimless wandering, circling and blindness.

The virus is not contagious between horses but can be carried by mosquitos from an avian, or bird, host to horses and humans. While humans may also be infected by WNV, the virus does not pass directly between people and horses. Mosquitoes biting warm-blooded animals is the only route of transmission.

Horses that have not already been vaccinated this year for WNV or other mosquito-borne diseases are at greater risk.

“Those horse owners who have vaccinated should check with their veterinarians to see whether a booster is appropriate,” McGwin said.

Horses that have never been vaccinated will need two doses two to four weeks apart, and the vaccine will take at least two weeks to build up enough antibodies to protect them. Vaccines will not protect horses that have already been infected when they receive the injections. Vaccines are available that protect against WNV and another common mosquito-borne disease, Eastern Equine Encephalitis (EEE).

Besides vaccination, McGwin recommends taking other steps to limit horses' exposure to mosquitoes during warm weather:

• Remove items from surrounding property that could collect stagnant water such as old tires, tin cans, plastic containers.
• Keep rain gutters clean and draining properly.
• Clean and chlorinate swimming pools, outdoor saunas, and hot tubs, and drain water from pool covers.
• Turn wading pools and wheelbarrows upside down when not in use.
• Empty and replace water in birdbaths at least once a week.
• Consider keeping horses in the barn from dusk to dawn, when mosquitoes are most active.
• Discuss the use of equine mosquito repellents with your veterinarian.

Last year Wisconsin had seven confirmed cases of WNV in horses, but there could have been many more unconfirmed cases. Because WNV follows mosquito populations, it normally occurs beginning in mid- to late summer and remains a threat until the first killing frost.