In rare cases, however, people who live in or visit an area where the virus lives can be infected by the bite of an infected mosquito. After infection, the virus invades the central nervous system, including the spinal cord and brain. LaCrosse encephalitis is usually a mild illness, with fever, headache, nausea, vomiting, and tiredness. People with severe disease, usually children, can have seizures, coma, paralysis, and lasting brain damage. It takes from 5 to 15 days after the bite of an infected mosquito to develop symptoms of LaCrosse encephalitis.
Diagnosis is based on tests of blood or spinal fluid.
There is no specific treatment for LaCrosse encephalitis. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Care of patients centers on treatment of symptoms and complications. Some children have ongoing seizures and swelling of the brain, but deaths are rare. During an average year, about 75 cases of LaCrosse encephalitis are reported.