Arboviral Disease, 2009

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, 455 human cases (including 14 fatalities) were reported from 2002 to 2009. In 2009, WNV cases were reported from 37 states and the District of Columbia; nationwide, 720 human cases of WNV disease were reported, including 32 fatalities. The largest WNV case counts during 2009 occurred in Texas (115 cases), California (112), and Colorado (103).

In Minnesota, 4 cases of WNV disease were reported in 2009 (the lowest annual case total to date). Three cases had West Nile (WN) fever, and 1 had neuroinvasive disease (encephalitis). The median age of all WN cases was 47 years (range, 28 to 70 years). All cases occurred among residents of western and central Minnesota. Similar to previous years, onset of symptoms for 3 cases occurred in mid to late summer (August 24 to October 4). However, a single case had a much earlier onset (June 24).

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 2009, cool spring and early summer weather likely lead to delayed amplification of WNV between birds and mosquitoes, likely contributing to the decreased 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.

During 2008, there was a nationwide recall of a commercial WNV IgM test kit after many false-positive test results were identified in several states. All of the WNV test kits currently available are labeled for use on serum to aid in a presumptive diagnosis of WNV infection in patients with clinical symptoms of neuroinvasive disease. Positive results from these tests should be confirmed at the PHL or CDC.

During 2009, no cases of La Crosse encephalitis were 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 (eg, waste tires, buckets, or cans) that provide mosquito breeding habitats are abundant. From 1985 through 2009, 124 cases were reported from 21 southeastern Minnesota counties, with a median of 5 cases (range, 1 to 13 cases) reported annually. The median case 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, 2009