Arboviral Disease, 2015

The primary arboviral encephalitides found in Minnesota have been La Crosse encephalitis, Western equine encephalitis (WEE), and more recently, West Nile virus (WNV). Both WNV and WEE are maintained in mosquito-to-bird transmission cycles involving several different species of each, and regional variation in vectors and reservoirs is likely. WNV is established throughout Minnesota, and will probably be present in the state to some extent every year, whereas human infections of WEE occur more sporadically. Human disease risk will likely continue to be higher in central and western Minnesota where the primary mosquito vector, Culex tarsalis, is most abundant. Interpreting the effect of weather on arboviral transmission is complex, making it extremely difficult to predict the number of people who will become infected in any given year.

Nine WNV disease cases were reported in 2015. There were no deaths attributed to WNV infections, but 3 had neuroinvasive presentations including encephalitis or meningitis. The other 6 cases had West Nile fever. Four of the cases were male, and the median age was 43 years (range, 30 to 81 years). Five cases were hospitalized. Eight cases reported symptom onset in August or September, although 1 case reported a symptom onset in June. Six asymptomatic WNV-positive blood donors were also identified in 2015.

In 2015, 1 case of La Crosse encephalitis was reported. The single case was a 44 year-old male, much older than the median case age from 1985-2015. The disease, which primarily affects children, is transmitted through the bite of infected Aedes triseriatus (Eastern Tree Hole) mosquitoes, and is maintained in a cycle that includes mosquitoes and small mammals. Exposure to infected mosquitoes typically occurs 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. Since 1985, 139 cases have been reported from 22 Minnesota counties, primarily in the southeastern part of the state. Most persons infected with La Crosse encephalitis virus have no apparent symptoms, but severe disease can occur in children. The median case age is 6 years (range, <1 to 49). 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 to date in the United States.

Two cases of Jamestown Canyon virus were reported. This virus is transmitted by Aedes genus mosquitoes, and the maintenance cycle in nature is thought to include deer and other large mammals. Much remains unknown about the clinical spectrum of Jamestown Canyon virus, but the typical presentation includes fever, and in more severe cases, meningitis or encephalitis. The virus is likely widespread in Minnesota. Patients were aged 56 and 77 years, and disease presentations ranged from fever to more severe illness, including meningoencephalitis.

• For up to date information see>> Vector-borne Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2015