Arboviral Disease, 2014

Mosquito-borne Arboviruses

Historically, 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 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.

In Minnesota, 21 cases of WNV disease were reported in 2014. There were no deaths, but 6 (29%) had neuroinvasive presentations including encephalitis or meningitis. The other 15 (71%) cases had West Nile (WN) fever. Seventy- one percent (15) of the cases in 2014 were male, and the median age was 48 years (range, 9 to 73 years). Seven (33%) cases were hospitalized. The majority (86%) reported symptom onset in July, August, or September, although onsets ranged from June 1 to October 8. Similar to past years, most cases occurred among residents of western and central Minnesota (Table 2 [PDF]). Five asymptomatic WNV-positive blood donors were also identified in 2014.

In 2014, 4 cases of La Crosse encephalitis were reported. Cases ranged in age from 6 to 11 years, and all exhibited neuroinvasive symptoms like encephalitis. 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. Many people who are infected with La Crosse encephalitis have no apparent symptoms, but severe disease can occur in children. The median case age for La Crosse encephalitis patients 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.

Four cases of Jamestown Canyon virus were reported in 2014, a California group virus related to La Crosse. The 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 11 to 62 years, and disease presentations ranged from fever to more severe illness, including acute flaccid paralysis and encephalitis.

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 long-term sequelae occur in approximately half of those patients. Approximately 10-15% of cases are fatal. Since 2008, 22 cases (1 fatal) of POW disease have been reported in Minnesota residents. Most of these patients had neuroinvasive disease (12 encephalitis and 8 meningitis) but 2 were non-neuroinvasive POW fever cases. Seventeen (77%) cases have been male, and the median age is 52 years (range, 3 mos. to 75 years). Seven patients (32%) were
immunocompromised. Similar to other tick-borne diseases, the majority of patients (18, 82%) reported illness onsets between May and August.
Four cases (18%) had onset dates in October or November. With the exception of 2014, cases have been reported every year since 2008, with a peak of 11 in 2011 (range, 1 to 11). Cases were exposed to ticks in several north-central Minnesota counties. MDH has also identified POW virus-positive ticks at sites in the six counties that have been investigated to date (Clearwater, Cass, Pine, Anoka, Morrison, and Houston). Thus, the virus appears to be widely distributed in the same wooded parts of the state that are endemic to other pathogens transmitted by I. scapularis.

POW virus testing is not widely available; however, the PHL will  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, 2014