Emily Braswell, who works for the Baker Valley Vector Control District, checks a dipper of water for mosquito larvae on June 9, 2025, near Sam-O Swim Center in Baker City. (Jayson Jacoby/Baker City Herald)

Lab tests of several hundred mosquitoes trapped this summer in Baker County have been free of West Nile virus, but the disease, which the bugs can pass to people through bites, was found recently in neighboring Malheur County.

The mosquito-borne virus typically shows up in Malheur County before spreading to other counties, including Baker, said Matt Hutchinson, manager of the Baker Valley Vector Control District.

The virus was confirmed in three “pools” of mosquitoes captured in Malheur County earlier this month, according to the Oregon Health Authority.

A pool of mosquitoes typically includes 10 to 50 bugs.

Hutchinson said he has sent 100 pools of mosquitoes from his district to a testing lab at Oregon State University in Corvallis. All were negative for West Nile virus.

He is sending another 40 or so pools for testing this week.

With rare exceptions, Hutchinson tests only culex mosquitoes, the variety almost exclusively shown to carry the virus in the county.

He said culex populations started rising a bit earlier than usual in the district, which includes Baker City as well as much of Baker, Bowen and Keating valleys. Typically he has collected 75 to 100 pools of culex mosquitoes by the third week of July.

Culex mosquitoes, which breed in permanent water sources (unlike the floodwater mosquitoes that proliferate in flood-irrigated fields) tend to be more common in Keating Valley as well as near the Powder River, Hutchinson said.

Culex mosquitoes infected with West Nile virus have been trapped in the district in most summers over the past two decades. In most years the virus is first confirmed in late July.

Most people infected with West Nile virus will show little or no signs of disease. But about one in five people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash.

Rarely, the virus can be fatal.

Precautions in the wake of record rainfall

With a record-setting rainstorm in Baker County on Monday, July 21, Hutchinson reminded residents to check their properties for objects, such as old tires, that can hold water and serve as breeding ground for mosquitoes.

He said he’s more concerned about properties in Baker City, where rain is one of the main sources of standing water.

(In the rural parts of the district, mosquito breeding sites are mostly in permanent water sources or flood-irrigated fields and pastures, places that district workers have already mapped.)

Hutchinson recommends residents take the following steps to protect against mosquito bites:

• When engaged in outdoor activities at dusk and dawn when mosquitoes are most active, protect yourself by using mosquito repellents containing DEET, oil of lemon eucalyptus or Picardin, and follow the directions on the container.

• Wear long-sleeved shirts and long pants in mosquito-infested areas.

• Make sure screen doors and windows are in good repair and fit tightly.