Arboviral Encephalitis, 2003

La Crosse encephalitis and Western equine encephalitis historically have been the primary arboviral encephalitides found in Minnesota. During July 2002, West Nile virus (WNV) was identified in Minnesota for the first time. In 2003, Minnesota was on the eastern edge of a large WNV outbreak in the Great Plains states.

WNV is maintained in a mosquito-to-bird transmission cycle. Several mosquito and bird species may be involved in this cycle, and regional variation in vector and reservoir species occurs. During 2003, WNV was found in 46 states and the District of Columbia. In 2003, 9,858 human cases of WNV disease were reported in the United States, including 262 fatalities. In Minnesota, 148 cases (4 fatal) of WNV disease were reported. Ninety-nine (67%) Minnesota case patients had West Nile fever; 25 (17%) had meningitis, and 24 (16%) had encephalitis. The median case-patient age was 47 years (range, 2 to 96 years), but WNV encephalitis patients tended to be older (median, 74 years; range, 38 to 96 years).

For a more complete update of WNV activity in Minnesota see “West Nile Virus in Minnesota: 2003 Update” in the May/June 2004 issue (vol. 32, no. 3) of the Disease Control Newsletter.

During 2003, three cases of La Crosse encephalitis were reported. 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 species, especially in areas where water-holding containers (e.g., waste tires, buckets, or cans) that provide mosquito breeding habitats are abundant. From 1985 through 2003, 117 cases were reported from 19 southeastern Minnesota counties, with a median of five cases (range, three to 13 cases) reported annually. Disease onsets have been reported from June through September; most onsets have occurred from mid-July through mid- September.

• For up to date information see>> Vector-borne Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2003