Arboviral Disease, 2011

Mosquito-borne Arboviruses

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, 465 human cases (including 15 fatalities) were reported from 2002 to 2011. In 2011, WNV cases were reported from 43 states and the District of Columbia; nationwide, 712 human cases of WNV disease were reported, including 43 fatalities. The largest WNV case counts during 2011 occurred in California (158 cases), Arizona (69), and Mississippi (52).

In Minnesota, 2 cases of WNV disease were reported in 2011 (the lowest annual case total to date). One was a fatal encephalitis case, and the other case had West Nile (WN) fever. Both cases were elderly (≤ 65 years). While most past WNV disease cases occurred among residents of western and central Minnesota, both 2011 cases were Hennepin County residents. Similar to previous years, onset of symptoms for both cases occurred within the typical high risk period of mid to late summer (August for both cases).

WNV is maintained in a mosquitoto-bird transmission cycle. Several mosquito and bird species are involved in this cycle, and regional variation in vector and reservoir species exists. 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.

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 2011, 1 case of La Crosse encephalitis was reported. 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 2011, 126 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.

Tick-borne Arbovirus

Powassan virus (POW) is a tick-borne flavivirus that includes a strain (lineage II or “deer tick virus”) that is transmitted by I. scapularis. The virus can cause encephalitis or meningitis, and longterm sequelae occur in approximately half of patients. Approximately 10-15% of cases are fatal. Since 2008, 17 cases (1 fatal) of POW disease have been reported in Minnesota residents. Most had neuroinvasive disease (10 encephalitis and 5 meningitis) but 2 were non-neuroinvasive POW fever cases. Fourteen (82%) cases were male. Median age was 49 years (range, 3 mos. to 70 years) and 6 (35%) were immunocompromised. Fourteen (82%) had onset of illness between May through August and 3 (18%) had October or November onsets. Eleven of the 17 cases were reported in 2011. Cases were exposed to ticks in several north-central Minnesota counties. MDH has also identified POW viruspositive 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.

POW virus testing is not widely available; however, the MDH PHL is available to test cerebrospinal fluid and serum specimens from suspect cases (i.e., patients with viral encephalitis or meningitis of unknown etiology).

• For up to date information see>> Vector-borne Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2011