2018 West Nile Virus Season Begins

With the arrival of summer in the United States, several state health departments are reporting their first West Nile virus (WNV)-positive mosquito samples of the season, and reminding the public to take precautions to avoid getting bitten by mosquitos.

From summer through fall, the United States has experienced an annual WNV season since the first cases of the mosquito-borne disease were reported in the country in 1999. Although 8 out of 10 individuals who become infected with WNV experience no symptoms, 1 in 5 will experience febrile illness symptoms, including headache, body aches, vomiting, diarrhea, and rash, with fatigue potentially lasting for weeks. For about 1 in 150 individuals with WNV, severe neuroinvasive illness can occur, affecting the central nervous system and leading to encephalitis or meningitis. Symptoms of this serious illness can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and vision loss. About 1 in 10 who develop neuroinvasive WNV die.

In 2017, the US Centers for Disease Control and Prevention (CDC) reported WNV in mosquitos, birds, and humans in 47 states and the District of Columbia. There were 2002 human cases of WNV, 1339 of which were classified as neuroinvasive. California reported the highest number of human cases of any state, with a total of 509 cases—including 376 which were neuroinvasive—and 44 deaths.

This year, the California Department of Public Health (CDPH) reported its first confirmed human cases of WNV on June 15. These included 4 cases in Los Angeles, Kern, and Riverside Counties. In addition, as of June 8, state health officials reported detecting the virus in 14 dead birds from 7 counties and 4 mosquito samples from 3 counties.

"West Nile virus activity in the state is increasing,” said CDPH director and state public health officer Karen Smith, MD, MPH in a recent press release. “I urge Californians to take every possible precaution to protect against mosquito bites."

On June 26, officials from the Massachusetts Department of Public Health reported the state’s first WNV-positive mosquito of the season. The sample was collected in the town of Weymouth in Norfolk County on June 20. Although Massachusetts health officials have not elevated the risk level since making the finding, the announcement serves as a reminder to the public to practice mosquito safety.

“The first WNV positive mosquito sample is often identified in Massachusetts during the last week in June,” said state epidemiologist Catherine Brown, MPH, BSc, DVM. “Risk for human infection generally builds through the season with peak risk occurring in August.”

To prevent mosquito bites and protect against WNV, health experts recommend the use of a bug repellant including DEET, picaridin, oil of lemon eucalyptus or IR3535 in the ingredients. During dusk and dawn, when mosquitos are most active, individuals should wear pants and long sleeves to avoid mosquito bites. Draining standing water from bird baths, buckets, flower pots, and other containers can also remove areas for mosquitoes to lay their eggs.