Arboviral Disease, 2012: DCN - Minnesota Dept. of Health

Arboviral Disease, 2012

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, 535 human cases (including 16 fatalities) were reported from 2002 to 2012. In 2012, WNV cases were reported from 48 states and the District of Columbia; nationwide, 5,674 human cases of WNV disease were reported, including 286 fatalities. The largest WNV case counts
during 2012 occurred in Texas (1,868 cases), California (479), and Louisiana (335).

In Minnesota, 70 cases of WNV disease were reported in 2012 (the third highest annual case total to date and well above the 2002-2012 median of 45 cases [range, 2-148]). Thirty-four (49%), including 1 fatal case, had encephalitis or meningitis. The other 36 (51%) cases had West Nile (WN) fever. Median age was 55 years (range,
7 to 87 years). As in past years, most cases occurred among residents of western and central Minnesota (Table 2) with illness onsets peaking in mid to late summer (median onset August 11, range May 29 to September 17; only 5 [7%] with onsets prior to July 15, and a peak of 37 cases [53%] in August). Thirty-five WNV-positive blood donors
were identified during 2012. While 33 remained asymptomatic, one donor developed WN encephalitis and another had WN fever.

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 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 2012, 4 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 (e.g., waste tires, buckets, or cans) that provide mosquito breeding habitats are abundant. From 1985 through 2012, 130 cases were reported from 22 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. A 2012 Stearns County case represented the farthest north and west that La Crosse encephalitis has been reported in the United States.

Tick-borne Arbovirus

Powassan virus (POW) is a tickborne 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 half of patients. Approximately 10-15% of cases are fatal. Since 2008, 21 cases (1 fatal) of POW disease have been reported in Minnesota residents. Most had neuroinvasive disease (11 encephalitis and 8 meningitis) but 2 were non-neuroinvasive POW fever cases.
Sixteen (76%) cases were male. Median age was 49 years (range, 3 mos. to 74 years) and 6 (29%) were immunocompromised. Eighteen (86%) had onset of illness between May through August and 3 (14%) had October or November onsets. Eleven
of 21 cases were reported in 2011 vs. 4 cases in 2012. Cases were exposed to ticks in several north-central Minnesota counties. MDH has also identified POW virus-positive ticks at sites in four of five counties that have been investigated to date (Clearwater, Cass, Pine, and Houston but not in Anoka). 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 PHL is available to test cerebrospinal fluid and serum specimens from suspect cases (i.e., patients with viral encephalitis or meningitis of unknown etiology).

Updated Tuesday, 30-Jan-2018 15:47:04 CST