Arboviral Encephalitis, 2004

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 2004, WNV cases were reported from 47 states and the District reported from 47 states and the District cases of WNV disease were reported, including 98 fatalities. The largest WNV outbreaks during 2004 occurred in Arizona and California.

In Minnesota, 34 cases of WNV disease were reported in 2004 (down from 148 cases in 2003, and 48 in 2002). Twenty-one (62%) case-patients had West Nile (WN) fever; five (15%) had meningitis, and eight (24%) had encephalitis. The median age was 53 years (range, 3 to 83 years), but WN encephalitis patients tended to be older (median, 74 years; range, 51 to 83 years). Two WN encephalitis patients (51 and 66 years old) with preexisting health problems died from their illness. Twenty-three cases (68%) occurred among residents of western and south central Minnesota. The earliest case-patient had onset of symptoms on July 12; the latest on October 8. Similar to previous years, the peak in illness onset was from August 1, 2004 through September 19, 2004 (24 of 34 [71%] cases).

The field ecology of WNV is complex. The virus 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 is likely. In 2004, cooler than normal weather in August kept mosquito numbers relatively low and likely contributed to the reduced incidence. Interpreting the effect of weather on WNV transmission demonstrates the difficulty of predicting how many people will become infected. WNV appears to be established throughout Minnesota; it will probably be present in the state to some extent every year. The disease risk to humans, however, will likely continue to be higher in central and western Minnesota where the primary vector, Culex tarsalis, is most abundant.

During 2004, two 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 2004, 119 cases were reported from 20 southeastern Minnesota counties, with a median of six cases (range, 2 to 13 cases) reported annually. Disease onsets have been reported from June through September; most onsets have occurred from mid-July through mid-September.

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