Arboviral Disease, 2008

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, 451 human cases (including 14 fatalities) were reported from 2002 to 2008. In 2008, WNV cases were reported from 45 states; nationwide, 1,356 human cases of WNV disease were reported, including 44 fatalities. The largest WNV case counts during 2008 occurred in California (445 cases) and Arizona (114 cases).

In Minnesota, 10 cases of WNV disease were reported in 2008 (the lowest annual case total to date). Eight case-patients had West Nile (WN) fever, and two had neuroinvasive disease (meningitis or encephalitis). The median age of all WN case-patients was 47 years (range, 2 to 86 years). Seven cases occurred among residents of western and central Minnesota. Similar to previous years, onset of symptoms occurred in mid to late summer (July 18 to September 1).

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 2008, 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 MDH PHL or CDC.

During 2008, only 1 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 (eg, waste tires, buckets, or cans) that provide mosquito breeding habitats are abundant. From 1985 through 2008, 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-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.

Aedes japonicus, an exotic Asian mosquito that has been moving across the United States since it was first found in New Jersey in 1998, has now been found in five southeastern Minnesota counties. This potential vector of La Crosse encephalitis and WNV to humans uses the same water-holding container breeding habitats as Ae. triseriatus. The Metropolitan Mosquito Control District has confirmed that Ae. japonicus eggs successfully survived the winter of 2008-2009 in Minnesota. We anticipate that this mosquito could eventually become established across most of the state.

• For up to date information see>> Mosquito-Transmitted Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2008