Arboviral Encephalitis, 2001
La Crosse encephalitis and Western equine encephalitis (WEE) have historically been the primary arboviral encephalitides found in Minnesota. Confirmed cases are defined as those which are clinically and epidemiologically compatible with arboviral encephalitis and meet one or more of the following laboratory criteria: a four-fold or greater rise in antibody titer to the virus; isolation of virus from, or detection of viral antigen in, tissues or body fluids; or detection of specific IgM antibody in cerebrospinal fluid. Probable cases are defined as clinically compatible cases occurring during a period when arboviral transmission is likely, with an elevated and stable (i.e., two-fold change or less) antibody titer to the virus.
La Crosse encephalitis is the most commonly reported arboviral infection in Minnesota. The disease, which primarily affects children, is transmitted through the bite of infected Ochlerotatus triseriatus (Eastern Tree Hole) mosquitoes. Persons are exposed to infected mosquitoes in wooded or shaded areas inhabited by this mosquito, especially in areas where water-holding containers (e.g., waste tires, buckets, or cans) are abundant and may be utilized as mosquito breeding habitat. During 2001, twelve cases (three confirmed, nine probable) of La Crosse encephalitis were reported to MDH. From 1985 through 2001, 101 cases were reported from 17 southeastern Minnesota counties, with a median of five cases (range, three to 12 cases) reported yearly. Disease onsets have been reported from June through September; most cases have onset from mid-July through mid-September. For a more detailed discussion of arboviral encephalitis in Minnesota, including West Nile virus, please refer to the July 2002 issue (vol. 30, no. 4) of the Disease Control Newsletter.