California Flooding May Create Breeding Grounds for West Nile Virus

Standing water left by flooding rivers in parts of California is raising state health officials’ concerns over mosquito breeding and West Nile virus, as more states continue to detect the virus in mosquito populations and report human cases.

As an arbovirus, or mosquito-borne disease, West Nile virus season in the United States occurs mostly in the summer months when mosquito activity is at its highest. Of those who become infected, about 1 in 5 will develop a fever, along with headache, body aches, vomiting, diarrhea, or rash. Symptoms such as fatigue or weakness can be prolonged, while fewer than 1% of West Nile cases result in severe illness, such as encephalitis or meningitis. Individuals living in West Nile virus-affected areas can take preventive measures against the virus by using insect repellants that ingredients such as DEET, picaridin, IR3535, and some oil of lemon eucalyptus, by staying covered, or by avoiding outdoor activities during mosquitoes’ most active hours: from dusk to dawn. On a community-wide scale, public health agencies rely on the use of insecticides and other mosquito control methods to limit the breeding and activity of the Culex and other mosquito species that transmit West Nile.

Heavy rains, and the flooding waterways and standing water they create, can result in fresh havens for mosquito breeding, as was the case in Louisiana last September, when 10 new human cases of West Nile virus were reported following heavy flooding in the state. As new reports emerge of West Nile-positive mosquitoes in California’s Fresno County, the recent melting of a large snowpack left behind from winter has filled reservoirs, and is causing flooding in the Fresno area along the Kings River. Now, health officials from the Fresno County Department of Public Health are worried that new pools of standing water will soon become larval habitats. In one local news report, Fresno County officials expressed concerns that the area will soon see its first human case of West Nile this season as a result of the recent flooding, and are reminding local residents to help control the mosquito population by removing standing water.

With West Nile-positive mosquitoes as the main precursor to human cases, additional states now reporting infected mosquitoes are warning the public to be on alert. According to a new report from the South Carolina Department of Health and Environmental Control (DHEC), surveillance efforts have identified West Nile-positive mosquitoes in the state’s Beaufort County. The report emphasizes that there have been no human cases in South Carolina this season, and that mosquito control efforts are targeting affected areas with insecticide.

"Identifying mosquitoes carrying West Nile virus in our state is not uncommon," said DHEC staff entomologist Chris Evans, PhD, in a recent statement. "A positive identification should serve as a reminder of the importance of preventing mosquito bites. It's the most important step you can take to prevent the spread of illness from mosquitoes to humans."