Arboviral Disease, 2019

Endemic Mosquito-borne Arboviral Diseases

Historically, the primary arboviral encephalitides found in Minnesota have been La Crosse encephalitis, Western equine encephalitis (WEE), and West Nile virus (WNV) encephalitis, but in recent years other viruses like Jamestown Canyon have emerged as significant causes of disease. While WNV and WEE are maintained in mosquito-to-bird transmission cycles involving several different species of each, La Crosse and Jamestown Canyon viruses use mammals instead of birds as part of their transmission cycles. WNV is established throughout Minnesota, and will probably be present in the state to some extent every year, whereas human cases of other diseases may occur more sporadically. Interpreting the effect of weather on arboviral transmission is complex, making it difficult to predict the number of people who will become infected in any given year.

In Minnesota, 5 WNV disease cases were reported in 2019, a total that is much lower than the median number of cases per year from the previous 5 years (30 from 2013 to 2018). Two had neuroinvasive presentations including encephalitis or meningitis. The other cases had West Nile fever. None of the cases died. Four cases were male, and the median age was 51 years (range, 16 to 70). Two cases were hospitalized. Four cases reported symptom onset in July, August, or September. One asymptomatic WNV-positive blood donor was also identified in 2019. Risks for human WNV infection continue to be higher in central and western Minnesota where the primary mosquito vector, Culex tarsalis, is most abundant.

In 2019, 1 case of La Crosse encephalitis was reported in a Minnesota resident. The case was a 7-year-old male with an unknown exposure. 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 species, especially in areas where water-holding containers (e.g., waste tires, buckets, or cans) that provide breeding habitats are abundant. Since 1985, 145 cases have been reported from 22 Minnesota counties, primarily in the southeastern part of the state. Many people who are infected have no apparent symptoms, but severe disease is more common in children. Most people report an illness onset during the typical arboviral season from mid-July through mid- September.

In 2019, 21 cases of Jamestown Canyon virus disease, a California group virus related to La Crosse, were reported. The virus is transmitted by Aedes 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. Cases were aged 1 month to 86 years, with a median of 66 years, and 66% were male. Eleven (52%) presented with neuroinvasive disease, including meningitis, encephalitis and acute flaccid paralysis, and most were residents of counties in north central and northeastern Minnesota. Due to the mosquito vectors involved in the transmission cycle for this virus, disease onsets can occur from late spring through the early part of the fall.

Imported Mosquito-borne Arboviral Diseases

Dengue

Dengue fever is one of the most frequently occurring mosquitoborne diseases worldwide, with an estimated 390 million infections, with nearly 100 million people experiencing symptomatic disease each year. Four serotypes of dengue virus are transmitted to humans through the bite of Aedes aegypti and Ae. albopictus mosquitoes. Dengue is considered endemic in more than 100 countries in tropical or subtropical regions around the world, and risk is widespread, especially where water- holding containers (e.g., waste tires, buckets, or cans) provide abundant mosquito breeding habitat.

In 2019, 20 cases were reported in Minnesota residents. The median case age was 40 years (range, 7 to 69 years) and onset of symptoms occurred throughout the year from February through November. Seventeen resided in the metropolitan area, and all infections were acquired abroad. Cases reported travel to many areas of the world, including to India (8), Ethiopia (3), Mexico (2), the Caribbean (2), South America (2), Southeast Asia (2), and Central America (1).

Chikungunya

Chikungunya virus is a mosquitoborne alphavirus found in Africa, Asia, and Europe. In late 2013, locally acquired cases appeared for the first time in the Americas on the Caribbean island of St. Martin, and the virus subsequently has spread throughout Central and South America. The virus is transmitted by the same Aedes spp. mosquitoes (Ae. aegypti and Ae. albopictus) that also transmit dengue and Zika viruses.

Unlike many other mosquitoborne viruses, most people who are infected with chikungunya develop symptoms. The most common symptoms are fever and joint pain, but patients may also experience headache, muscle aches, or rash. Symptoms usually begin 3-7 days after a person is bitten by an infected mosquito, and most recover within a week. Joint pain may persist for weeks to years after the initial illness.

In 2019, 18 cases were reported in Minnesota residents. The median case age was 41 (range, 2 to 71 years). Fifteen resided in the metropolitan area and symptom onsets occurred all year, from January through December. All represented imported infections acquired abroad, and travel occurred to many areas of the world. Ten traveled to Asia, six went to Africa, and one each visited the Caribbean and South America.

Zika Virus

Zika virus is a mosquitoborne flavivirus that was initially discovered in 1947 in Uganda, and the first human cases were identified in 1952. Historically this virus occurred only sporadically in Africa and Asia, but it gained attention after it resulted in outbreaks in Micronesia in 2007 and French Polynesia in 2013-2014. In spring 2015, cases were reported from Brazil, representing the first time the virus had been found in the Americas. Since then, the virus has spread to most countries and territories in the Western Hemisphere, and infections during pregnancy have been associated with adverse fetal outcomes, including microcephaly. Zika has been shown to be transmitted perinatally as well as through sexual contact, a route of transmission that has never before been associated with a mosquitoborne virus. The mosquito vectors for humans are the same Aedes spp. mosquitoes (Ae. aegypti and Ae. albopictus) that transmit dengue virus and Chikungunya virus.

Although the outbreak in the Americas peaked in 2016, cases are still reported from around the region. The risk for infection persists throughout many areas of the world, but the ability to detect a new outbreak varies by country, and reporting of new outbreaks may be delayed several weeks to months. Since most people (up to 80%) that are infected with Zika do not develop symptoms, it is possible that many infections, and even small outbreaks, may go undetected.

In 2019, 4 cases of Zika virus disease was reported; 3 cases exhibited febrile illness and one was asymptomatic. Three of the 4 cases were female and 2 of these were pregnant women. One case each was linked to Belize, Guyana, the Philippines and Cuba. 

Endemic Tick-borne Arboviral Disease

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 those patients. Approximately 10-15% of cases are fatal. Since the first case in 2008, there have been cases every year except for 2014 and 2015, with a peak of 11 cases in 2011 (range, 1 to 11). Seven cases of POW were reported in 2019. Six cases were male, and ages ranged from 58 to 76 years. All but 1 case in 2019 presented with meningitis or encephalitis, and 2 cases died. Similar to other tickborne diseases, the majority of patients report being exposed to ticks in north central Minnesota. Most illness onset fell in June or July but one case had an onset date at the beginning of November. Based on findings from routine tick surveillance activities, 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.

• Find up to date information at>> Vector-borne Diseases
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2019