Idaho West Nile Virus Update

Positive mosquito test results and increasing numbers of specific types of mosquitoes that carry West Nile virus (WNV) suggest the risk of catching WNV is on the rise in Idaho.

As of Wednesday, positive mosquito pools have been found in Ada, Canyon, Gem and Washington counties. On Tuesday, Ada County announced plans to conduct limited aerial spraying of pesticide to reduce ever-increasing populations of Culex mosquitoes, the species that transmits West Nile virus to people and animals.  Spraying is expected to occur Thursday or Friday night this week, weather permitting.

In a news release, Ada County Mosquito Abatement District Director Brian Wilbur said that Culex mosquito populations are doubling each week despite mosquito control efforts. “We must take a proactive approach and address this threat with an aerial application so we can significantly reduce the mosquito numbers and slow the progression of West Nile virus in our area before the situation reaches emergency status,” he said.

During the last five years, Idaho WNV infections have averaged less than 20 per year. However, Idaho did reach emergency status in 2006 when we led the nation with almost 1,000 human infections that contributed to 23 deaths. Last year, 19 WNV infections were reported from 15 counties, with no deaths. West Nile virus was detected in the Panhandle for the first time last year in a mosquito pool from Boundary County.

“If you are outdoors and mosquitoes are active, it is critical that you avoid being bitten,” says Dr. Leslie Tengelsen from the Idaho Division of Public Health. “The alarming increase in mosquito numbers coupled with a growing number of positive mosquito pools could result in a large number of illnesses unless we all take precautions.”

To reduce the likelihood of WNV infection, avoid mosquitoes, particularly between dusk and dawn when they are most active. In addition, you should:

  • Cover up exposed skin when outdoors and apply DEET or other EPA-approved insect repellent to exposed skin and clothing. Carefully follow instructions on the product label, especially for children.
  • Insect-proof your home by repairing or replacing screens.
  • Reduce standing water on your property; check and drain toys, trays or pots outdoors which may hold water.
  • Change bird baths and static decorative ponds weekly as they may provide a suitable mosquito habitat.

WNV does not usually affect domestic animals, including dogs and cats, but it can cause severe illness in horses and certain species of birds. Although there is no human vaccine available, there are several vaccines available for horses. People are advised to keep their horses vaccinated annually.

For more information, visit To read the Ada County Mosquito Abatement District news release announcing aerial spraying, click here.

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