West Nile Virus, Arborviruses Still Source of Severe Illness

Veronica Hackethal, MD

June 26, 2014

West Nile virus and other arboviruses continued to cause severe disease in the United States in 2013, with West Nile virus being the most common cause of neuroinvasive arboviral infection, according to surveillance results published by researchers at the Centers for Disease Control and Prevention (CDC) on June 20 in the Morbidity and Mortality Weekly Report.

Infected mosquitoes and ticks pass arboviral diseases to humans through bites. Person-to-person transmission rarely happens through blood transfusion or organ transplantation, Nicole P. Lindsey, from the Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC, Atlanta, Georgia, and colleagues write. Although most infections in humans do not cause symptoms, a sign of infection is a high fever. Less commonly, arboviruses can enter the nervous system.

The authors collected data reported to the CDC via ArboNET in 2013. Data included information from blood donors, infected animals, and mosquitoes, as well as dead birds and dead animals. The researchers note that testing and reporting to ArboNET may be incomplete, which could result in underestimation of true disease.

In 2013, 2605 cases of nationally notifiable arbovirus infections were reported to the CDC. Alaska and Hawaii did not report any cases. West Nile virus remained the most commonly reported arbovirus (2469 cases), followed by La Crosse virus (85), Jamestown Canyon virus (22), Powassan virus (15), eastern equine encephalitis (8), unspecified California serogroup virus (5, infectious agent unknown), and St. Louis encephalitis virus (1).

More than 90% of arboviral infections occurred in April through September.

West Nile virus remained the most common cause of neuroinvasive arboviral infections, but the incidence of West Nile virus neuroinvasive disease was significantly lower in 2013 compared with in 2012, when a large outbreak occurred. Fifty-one percent (n = 1267) of reported West Nile virus cases were neuroinvasive, with the highest incidence among people aged 70 years or older. Nine percent (n = 111) of patients with neuroinvasive disease died.

West Nile virus cases peaked in early September, with the highest incidences in North Dakota, South Dakota, Nebraska, and Wyoming. Five percent (n=119) of patients died; their median age was 78 years.

La Crosse virus caused the highest number of neuroinvasive arboviral infections in children, with a median age of 7 years. Ninety-one percent of La Crosse Virus infections (n = 77) were neuroinvasive. Two patients (2%) died.

In 2013, reported cases of Jamestown Canyon virus were higher than in any other year so far. The authors attributed this jump to the beginning of routine testing for Jamestown Canyon virus at the CDC. They note that in previous years, the number of cases of Jamestown Canyon virus infection may have been underestimated. Sixty-eight percent (n = 15) of Jamestown Canyon virus cases were neuroinvasive, but no patients died.

Eighty percent (n = 12) of Powassan virus infections were neuroinvasive, and 2 patients (13%) died.

Eastern equine encephalitis, although rare, remained the most serious arboviral infection. All 8 cases of eastern equine encephalitis were neuroinvasive, and 4 patients (50%) died. The median age of those who died was 62 years.

"WNV and other arboviruses continue to be a source of severe illness each year for substantial numbers of persons in the United States," the authors emphasize. "Maintaining surveillance remains important to identify outbreaks and guide prevention efforts. Prevention efforts depend upon applying insecticides, reducing mosquito breeding grounds, use of repellents, and wearing protective clothing."

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2014;63:521-528. Full text

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