Arboviral Disease, 2006

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 2006, WNV cases were reported from 43 states and the District of Columbia; nationwide, 4,269 human cases of WNV disease were reported, including 177 fatalities. The largest WNV outbreaks during 2006 occurred in Idaho (996 cases), Texas (354 cases), and Colorado (345 cases).

In Minnesota, 65 cases of WNV disease were reported in 2006 (the highest total since the 148 cases reported in 2003). Thirty-four (52%) case-patients had West Nile (WN) fever; 23 (35%) had meningitis, and eight (12%) had encephalitis. The median age of all WN case-patients was 50 years (range, 3 to 88 years); WN encephalitis patients were older (median, 76 years; range, 44 to 83 years). Two WN encephalitis patients and one WN meningitis patient (72, 76, and 88 years old, respectively) died from their illness. Forty-two cases (65%) occurred among residents of western and south central Minnesota. The earliest case-patient had onset of symptoms on July 2; the latest on September 22. Similar to previous years, the peak in illness onsets was from July 15 through September 15 (53 [82%] 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 2006, warm spring and summer weather lead to early and efficient amplification of WNV between birds and mosquitoes, likely contributing to the increased incidence of human cases. Interpreting the effect of weather on WNV transmission is extremely complex, leading to great difficulty in predicting how many people will become infected in a given year. 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 mosquito vector, Culex tarsalis, is most abundant. Locally acquired cases of WNV disease remain absent in the northeastern third of Minnesota, which corresponds to the region where Cx. tarsalis is rare or absent.

During 2006, only one case of La Crosse encephalitis was reported to MDH. The disease, which primarily affects children, is transmitted through the bite of infected Aedes 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 2006, 122 cases were reported from 21 southeastern Minnesota counties, with a median of five cases (range, 1 to 13 cases) reported annually. The median case-patient age was 6 years. Disease onsets have been reported from June through September, but 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, 2006