Arboviral Disease, 2007

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; subsequently, 441 human cases (including 14 fatalities) were reported from 2002 to 2007. In 2007, WNV cases were reported from 43 states; nationwide, 3,623 human cases of WNV disease were reported, including 124 fatalities. The largest WNV case counts during 2007 occurred in Colorado (576 cases), California (380 cases), and North Dakota (369 cases). Most of the states with large case totals were Great Plains states, and WNV transmission to humans was especially intense in the northern Great Plains and adjacent areas (2,318 [98.5%] of 2,353 western Canada’s WNV cases in 2007 were residents of Manitoba, Saskatchewan, or Alberta).

In Minnesota, 101 cases of WNV disease were reported in 2007 (the highest total since 148 cases were reported in 2003). Fifty-eight (57%) case-patients had West Nile (WN) fever; 23 (23%) had meningitis, and 20 (20%) had encephalitis. The median age of all WN case-patients was 52 years (range, 3 to 86 years); WN encephalitis patients were older (median, 70 years; range, 18 to 86 years). Two WN encephalitis patients (62 and 72 years old) died from their illness. Seventy-six cases (75%) occurred among residents of western and central Minnesota. Statewide WNV incidence was 1.9 cases per 100,000, but ranged up to 148.6, 104.7, and 66.6 cases per 100,000 in Big Stone, Norman, and Kittson counties, respectively. The 2007 WNV transmission season was the longest in Minnesota to date; the earliest case-patient had onset of symptoms on June 25; the latest on October 12. Similar to previous years, the peak in illness onsets was from July 15 through September 15 (85 [84%] cases).

The field ecology of WNV is complex. The virus is maintained in a mosquito-to-bird transmission cycle. Several mosquito and bird species are involved in this cycle, and regional variation in vector and reservoir species is likely. In 2007, warm spring and early summer weather lead to early and efficient amplification of WNV between birds and mosquitoes and an unusually large early season peak in vector numbers, 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. Until 2007, locally acquired cases of WNV disease were absent in the northeastern third of Minnesota, which corresponds to the region where Cx. tarsalis is rare or absent. However three cases with possible local exposure (St. Louis County [n=2] and Koochiching County) were reported during 2007.

During 2007, 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 2007, 123 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>> Mosquito-Transmitted Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2007