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. Most cases occur in children under age 16 years. LaCrosse encephalitis virus was first isolated in the United States in 1963. The risk of exposures has been increasing as people move into previously undeveloped areas where the virus lives. There is no vaccine for LaCrosse encephalitis.