Most likely maintained in chipmunks and squirrels, and transmitted by Aedes triseriatus, a mosquito usually developing in tree holes, but also detected in abandoned tires, buckets, and cans.
Most infections are mild, but severe illnesses can result in seizures and coma.
Children under the age of 16 seem to be most vulnerable. Milder symptoms include high fever, headache, stiff neck, and nausea. Death occurs in fewer than 1 percent of known cases. The illness was first identified in LaCrosse, Wisconsin in 1963. Unlike some of the other arboviruses, the LaCrosse virus can be passed from female mosquitoes to their offspring.