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West Nile watch: Snohomish Health District checking for virus

SNOHOMISH COUNTY --- Local health officials are keeping a sharp eye on the possibility of West Nile virus (WNV) in Snohomish County through a surveillance program at Snohomish Health District. So far this year, all of the 94 dead birds submitted for laboratory testing have tested negative for West Nile virus. Traps to catch adult mosquitoes for species identification have indicated the presence of several mosquito types that could carry WNV.

The countywide mosquito survey shows that approximately 44 percent of the mosquitoes caught were WNV-vector species including Culex pipiens, Culex tarsalis, and Ochlerotatus japonicus . “We have identified 15 different types of mosquitoes this summer in Snohomish County and a number of them are known to transmit West Nile, ” said Amanda Zych, Environmental Health Specialist at the Health District. Zych noted that Culex pipiens (known as the common house mosquito) were collected at several sites in the county. “Culex is a concern because their larvae can grow in water-filled containers found in urban or rural areas. They are also very efficient vectors of West Nile virus,” said Zych. Zych said the O. japonicus is new to Snohomish County and had been collected by the local extension office of Washington State University.

As of the end of July, the Health District had set 171 mosquito traps. They caught 75 mosquitoes in May, 240 mosquitoes in June, and approximately 1,220 mosquitoes in July.

When bitten by an infected mosquito, most people fight off the infection easily and experience only mild symptoms or none at all. People who have WNV cannot transmit it to other people. The risk of developing life-threatening illness is low. Less than 1 percent of the people bitten by a mosquito infected with WNV will become seriously ill. Last October the virus was detected in a dead bird near Snohomish, but to date there has been no human case of WNV in Snohomish County.

The Health District encourages all residents, especially those 50 years of age and older, to protect themselves against mosquitoes, and to reduce mosquito breeding grounds near their homes. Healthy wetlands may deter mosquitoes because they are food for native fish, frogs, toads, newts, salamanders and other mosquito predators. But artificial water collectors such as tires, birdbaths and clogged gutters provide places for mosquitoes to breed and keep safe from predators.

“We want to get through the season without any illness from West Nile virus,” said Randal Darst, the Health District’s Assistant Director of Environmental Health. “The precautions sound simple and they are. If we follow them, we can help protect ourselves from being bitten by the mosquitoes that carry this disease,” he said.

Prevention measures include:

  • Avoid outdoor activities, such as gardening, at dusk and dawn when mosquitoes are most active.
  • Cover up by wearing long-sleeved shirts, long pants, shoes and socks when in mosquito-active areas.
  • Use mosquito repellents containing DEET; follow the directions on the label; for children, use products with 10 percent or less DEET.
  • Ensure screens for doors and windows are tight-fitting and in good repair.
  • Eliminate standing water in flowerpots, clogged rain gutters, tires or other water-holding containers that could be mosquito breeding sites. Change water in birdbaths at least weekly.

For more information about WNV, visit the Snohomish Health District Web site at, or call the Health District’s WNV Hotline at 425.339.8720.

Established in 1959, the Snohomish Health District works for a safer and healthier community through disease prevention, health promotion, and protection from environmental threats.