Eastern Oregon Counties See Jump in Mosquitoes with West Nile Virus

Posted on August 12, 2023

Growing mosquito populations raise risk for virus infections in humans, OHA says

PORTLAND, Ore.—Health officials are reminding people heading outdoors in eastern Oregon to prevent mosquito bites after a recent jump in the number of mosquito pools – collections of up to 50 insects – testing positive for West Nile virus, according to local vector control districts.

Emilio DeBess, D.V.M., state public health veterinarian at Oregon Health Authority’s (OHA) Public Health Division, said high heat combined with sporadic precipitation has created perfect conditions for mosquito growth in recent weeks. As a result, eastern parts of the state are seeing more mosquitoes and a corresponding increase in traps containing West Nile-positive insects.

The increase in mosquito populations may be behind new human cases of the virus.

“Eastern Oregon has seen a little bit of rain, leading to additional water available for mosquitoes to lay eggs,” said DeBess. “We saw an increase of 13 West Nile-positive mosquito pools in one week, 10 of them in Baker County, and two presumptive human cases.”

So far in 2023, a total of 22 mosquito pools have tested positive for the virus. Eleven have been in Baker County, seven in Malheur County, two in Union County, and one each in Jackson and Umatilla counties.

Last year, there were five human West Nile virus cases in Oregon, and three cases among horses, with 45 positive mosquito pools. There also were five human cases in 2021, along with eight horse cases, two bird cases and 75 positive mosquito pools.

Groups at risk for severe disease include adults 50 and older, immunocompromised people and those living with certain conditions such as diabetes and high blood pressure.

While most infected people show little to no signs of disease, one in five show signs of West Nile fever. Flu-like symptoms can last from a few days to several weeks and may include fever above 100 degrees Fahrenheit, severe headaches, stiff neck, mental confusion, muscle weakness, shaking, paralysis or rash. Anyone experiencing such symptoms should contact their health care provider.

The easiest and best way to avoid mosquito-borne diseases is to prevent mosquito bites:

  • People should mosquito-proof their homes by following these steps:
    • Eliminate standing water in and around the home and business where mosquitoes can breed.
    • At least once or twice a week, empty water from flowerpots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels and cans.
    • Clean out clogged rain gutters.
    • Remove discarded tires and other items that could collect water.
    • Look for containers or trash in places that may be hard to see, such as under bushes or under your home.
  • Take personal precautions to prevent mosquito bites.
    • Apply insect repellent to exposed skin. Generally, the more active ingredients a repellent contains the longer it can protect against mosquito bites. Repellents containing DEET, oil of lemon eucalyptus or Picaridin are recommended; follow directions on the container for applying it to the skin.
    • Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children. When using an insecticide or insect repellent, be sure to read and follow the manufacturer’s DIRECTIONS FOR USE, as printed on the products.
    • When weather permits, wear long-sleeved shirts and long pants whenever you are outdoors.
    • Place mosquito netting over infant carriers when outdoors with infants.
    • Consider staying indoors at dawn and dusk, which are peak mosquito biting times. Install or repair window and door screens so that mosquitoes cannot get indoors.

Additional resources, such as frequently asked questions and information on finding local mosquito control agencies, are available on OHA’s West Nile virus page and CDC website. Current and past virus data can be found on the West Nile Virus Activity page.


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