Arboviral Disease, 2010
LaCrosse 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, 463 human cases (including 14 fatalities) were reported from 2002 to 2010. In 2010, WNV cases were reported from 40 states and the District of Columbia; nationwide, 1,021 human cases of WNV disease were reported, including 57 fatalities. The largest WNV case counts during 2010 occurred in Arizona (167 cases), New York (128), and California (111).
In Minnesota, 8 cases of WNV disease were reported in 2010 (the second lowest annual case total to date). Three cases had West Nile fever, and 5 had neuroinvasive disease (encephalitis or meningitis). The median age of all WNV cases was 40 years (range, 12 to 63 years). All cases occurred among residents of western and central Minnesota. Similar to previous years, onset of symptoms for the cases occurred in mid to late summer (median August 25; range, July 21 to September 25).
WNV 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. Interpreting the effect of weather on WNV transmission is also 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 2010, 1 case of LaCrosse 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 2010, 125 cases were reported from 21 southeastern Minnesota counties, with a median of 4 cases (range, 0 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.
Powassan virus (POW) is a tick-borne flavivirus that includes a strain (lineage II or “deer tick virus”) that is transmitted by Ixodes scapularis. The virus can cause encephalitis or meningitis, and long-term sequelae occur in approximately 50% of patients. Approximately 10-15% of cases are fatal. From 2008-2010, 6 cases of POW disease were reported in Minnesota residents. Three of those cases (Itasca, Kanabec, and Carlton County residents) were reported in 2010. MDH has identified POW virus-positive ticks at sites in all four counties that have been investigated to date (Clearwater, Cass, Pine, and Houston). Thus, the virus appears to be widely distributed in the same wooded parts of the state that are endemic to other tick-borne diseases transmitted by I. scapularis such as Lyme disease.
POW virus testing is not widely available; however, the PHL can test cerebrospinal fluid and serum specimens from suspect cases (i.e., patients with viral encephalitis or meningitis of unknown etiology).
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