Arboviral Disease, 2013

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 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.

In Minnesota, 80 cases of WNV disease were reported in 2013 (the third highest annual case total to date [range, 2 to 148]). Three (4%) patients died from complications of their WNV infection. Thirty-one (38%) cases, including the 3 fatalities, had encephalitis or meningitis. The other 49 (61%) cases had West Nile (WN) fever. Sixty-nine percent (55) of the cases were male, and the median age was 57 years (range, 7 to 92 years). In 2013, 41 (51%) of WNV cases were hospitalized. All but 1 case reported symptom onset in July, August, or September (median onset August 15, range June 16 to September 22), and as in past years, most cases occurred among residents of western and central Minnesota (Table 2). Twenty-seven WNV-positive blood donors were identified during 2013; 25 remained asymptomatic, and 2 developed WN fever.

Nationwide, 2,469 human cases of WNV disease were reported from 47 states and the District of Columbia. There were 119 fatalities. A total of 431 viremic donors were reported from 36 states, 60 (14%) of whom developed clinical disease. The largest WNV case counts during 2013 occurred in California (379 cases), Colorado (322 cases), and Nebraska (226).

In 2013, 5 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, 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, 135 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 patient 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.

Minnesota reported its first case of Jamestown Canyon virus in 2013, 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.

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 Ixodes 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, or 82%) reported illness onsets between May and August. Four patients (18%) had onset dates in October or November. 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 five counties that have been investigated to date (Clearwater, Cass, Pine, Anoka 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 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, 2013