During the first half of the summer, Culex mosquitoes normally feed on birds, from which they can pick up the virus. In mid to late summer, the mosquitoes change their feeding habits from birds to other animals and humans. The Culex mosquito bites from dusk to dawn. Symptoms of St. Louis encephalitis, which is most common among persons older than 55 years of age, usually begin five to 15 days following the bite of an infected mosquito. Symptoms range from a slight fever or headache to rapid onset of severe headache, high fever, muscle aches, stiffness in the back of the neck and disorientation.
Most patients recover fully, although the disease can cause permanent neurological damage or death.
Symptoms of LaCrosse encephalitis, which mainly affects children, are similar to those of St. Louis encephalitis. The Department conducts a surveillance program beginning each spring that monitors birds and mosquitoes for evidence of St. Louis encephalitis. Birds are trapped and their blood tested in about 50 counties. So far, more than 2,000 birds and hundreds of pools of mosquitoes have been tested, but all have been negative for St. Louis encephalitis.