University of Wisconsin - Extension

MADISON, Wis.-- West Nile Virus, a mosquito-borne disease that causes mild illness in humans but is particularly deadly for horses, has resulted in the death of one of two infected horses in Wisconsin so far this year. An entomologist at the University of Wisconsin-Madison/Extension says the disease started showing up about two weeks ahead of schedule this year.

"The big question now is how bad will West Nile be in Wisconsin this summer," said UW-Extension insect expert Phil Pelletteri. "Last year, Wisconsin was the 'hole in the donut.' Illinois, Minnesota, Michigan and Iowa all had many more cases of both equine and human infection by West Nile." Chicago, Detroit and Cleveland were West Nile hot spots, he added, since the mosquito that carries it likes to breed in storm sewers.

Like St Louis encephalitis, West Nile Virus is a disease that originates with birds and the same Culex pipiens mosquito is most likely to carry it. Pelletteri says the disease rate accelerates during the summer months as the amount of virus in the bird population increases. During the summer of 2002, the office of the state veterinarian confirmed 166 confirmed cases in horses. Thirty-five horses died of the disease.

Symptoms in humans are usually mild, but horses that contract West Nile show stumbling, lack of coordination, limb weakness and partial paralysis. Other symptoms may include depression, appetite loss, drooping eyelids and lower lip, weakness, twitching, aimless wandering, circling and blindness. Fever is not a frequent symptom in horses. The mortality rate is very high.

The virus spreads through the bite of a mosquito infected with the virus. Horses do not spread the virus to humans, other horses or other animals.

Horse owners can protect their animals from West Nile and from equine encephalitis, another mosquito-borne disease with a death rate even higher than West Nile, with vaccines. Veterinarians also recommend protecting animals from mosquitoes by removing standing water and keeping animals inside from dusk to dawn.

The Culex pipiens mosquito is one of 53 varieties of mosquitoes in Wisconsin. Pelletteri calls it a "back yard mosquito, because it breeds in containers where water is allowed to stand — old tires, clogged gutters and bird baths. Throughout the world mosquitoes spread a variety of diseases, malaria, yellow fever, dengue, dog heart worm, encephalitis, and elephantitis to name a few. Many of these diseases are serious health threats.

"Even from an ecological perspective, it's hard to give mosquitoes a good word," Pelletteri said, explaining that while many other animals and birds eat mosquitoes, there are plenty of other kinds of bugs to feed them.

His advice for dealing with the mosquito season to protect both humans and animals is to use vaccines that are available to prevent mosquito-borne illnesses, make sure there are no places where water will accumulate and turn into an attractive breeding area, and avoid bites by wearing protective clothing and repellents and staying indoors after dusk.