Arboviral Encephalitis, 2000
La Crosse encephalitis and Western equine encephalitis (WEE) are 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 shady 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 2000, eight probable cases of La Crosse encephalitis were reported to MDH. From 1985 to 1999, 81 cases were reported, with a median of five cases (range, three to 12 cases) reported yearly. The disease has been reported in 16 southeastern Minnesota counties (Figure 1). The highest incidence rates statewide occur in Houston County (mean annual incidence of 21 cases per 100,000 persons <19 years of age; range, 0 to 68 per 100,000). During 2000, six of eight cases were residents of the western Twin Cities metropolitan area (i.e., Carver, Hennepin, and Wright Counties). Disease onsets have been reported from June through September; most cases have onset from mid-July through mid-September.
WEE occurs infrequently in Minnesota, often as part of a regional epidemic or epizootic in midwestern states and southern Canada. There were no reported cases during 2000. The most recent reports were single cases during 1999 and 1983 and 15 cases in 1975. The virus is transmitted to humans and horses through the bite of Culex tarsalis mosquitoes in years when virus-infected vector populations are relatively high. The mosquitoes usually feed on birds and maintain WEE virus in a mosquito-bird cycle of transmission. However, in mid-summer when vector populations rise, a significant part of the mosquito's feeding may switch to mammalian hosts such as humans and horses (both considered to be dead-end hosts for WEE virus).