Arbovirus, 2005

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 2005, WNV cases were reported from 43 states and the District of Columbia; nationwide, 3,000 human cases of WNV disease were reported, including 119 fatalities. The largest WNV outbreaks during 2005 occurred in California (880 cases), Illinois (252 cases), and South Dakota (229 cases).

In Minnesota, 45 cases of WNV disease were reported in 2005 (down from 148 cases in 2003). Twenty-seven (60%) case-patients had West Nile (WN) fever; 13 (29%) had encephalitis, and five (11%) had meningitis. The median age of all WN case-patients was 52 years (range, 26 to 82 years); WN encephalitis patients tended to be younger than in recent years (2005 median, 60 years; range 36-82 years, vs. 2003-2004 median, 74 years; range, 38 to 96 years). Three WN encephalitis patients (82, 76, and 36 years old) died from their illness. The 36-year-old patient had pre-existing health problems. Twenty-nine cases (64%) occurred among residents of western and south central Minnesota. The earliest case-patient had onset of symptoms on June 29; the latest on September 26. Similar to previous years, the peak in illness onsets was from July 15 through September 15 (31 [69%] 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 2005, cooler than normal spring weather may have shortened the time period available for WNV to amplify efficiently between birds and mosquitoes, likely contributing to the reduced incidence. 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 remain absent in the northeastern third of Minnesota, which corresponds to the region where Cx. tarsalis is rare or absent.

During 2005, two cases of La Crosse encephalitis were reported; both in members of the same family. 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 2005, 121 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, 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, 2005