West Nile Virus Activity Rising Across the State
Marisa Lloyd

West Nile Virus Activity Rising Across the State

Idaho Department of Health

Human West Nile virus (WNV) infections as well as positive mosquito pools and infections in horses are on the rise in Idaho. Detection of the virus has been reported in 13 counties, primarily in south and southwestern Idaho, but also in the Salmon area of Lemhi County and Franklin County, areas that don’t often report WNV activity.

Six human WNV infections, all severe neurologic diseases, including one death, have been reported in Idaho this year, as of Sept. 1. Only one human infection had been reported in Idaho last year by Sept. 1.

“The conditions are right for mosquitoes to transmit the virus now until a killing frost eliminates the mosquito populations,” said Dr. Leslie Tengelsen, Idaho state public health veterinarian in the Division of Public Health, Department of Health and Welfare “Idahoans should assume that WNV is a risk anytime and anywhere mosquitoes are active.“

“We strongly encourage Idahoans to ‘Fight the Bite’ of mosquitoes to protect themselves and their families, no matter where they live in the state,” Tengelsen said. “This includes wearing insect repellent and protective clothing in addition to reducing standing water around our gardens and homes where mosquitoes can lay their eggs.”

WNV activity typically occurs from August through September, but it can be difficult to predict from year to year. In 2006, Idaho had more WNV illnesses than any other state, with almost 1,000 infections and 23 deaths. During more recent seasons, WNV activity has been extremely low.

WNV is usually contracted from the bite of an infected mosquito and is not spread from person to person through casual contact. Symptoms of infection often include fever, headache, body aches, nausea, and sometimes swollen lymph glands or a skin rash. Infection can result in severe illness, especially in people 50 years old and older. Talk to your healthcare provider about testing for WNV if you experience symptoms after being bitten by mosquitoes.


To protect against WNV infection, people should avoid mosquitoes, particularly between dusk and dawn when mosquitoes are most active. In addition, everyone should:

§  Cover up exposed skin when outdoors and apply DEET or other EPA-approved insect repellents 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 property; check and drain toys, trays, or pots outdoors that can hold water.

§  Change birdbaths, static decorative ponds, and animal water tanks weekly to reduce suitable mosquito habitats.


WNV does not usually affect domestic animals but can cause severe illness in horses and some species of birds. Although there is no vaccine for people, there are several annual vaccines for horses.

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