West Nile virus has been found in mosquitoes in Baker County for the first time this year.
The mosquitoes were trapped July 28 in the Keating Valley about 15 miles east of Baker City, said Matt Hutchinson, manager of the Baker Valley Vector Control District.
It takes about a week for test results to arrive, and Hutchinson announced the positive test on Thursday, Aug. 4.
Hutchinson is responsible for controlling mosquitoes in a 200,000-acre area that includes most of Baker, Keating and Bowen valleys.
The virus, which mosquitoes can spread to people, has been detected in mosquitoes in Baker County in all but two years (2018 and 2020) over the past decade.
Typically the virus is confined to insects and birds.
Baker County is the second Oregon county to have infected mosquitoes in 2022. Two batches of mosquitoes tested positive earlier in Malheur County.
Hutchinson sends dozens of “pools” of dead mosquitoes — each pool consists of around 10 to 50 insects — to a lab at Oregon State University where the bugs are tested for West Nile virus and other mosquito-borne diseases.
So far this summer, Hutchinson said 160 pools of mosquitoes from Baker County have been tested, with only the one positive for West Nile virus.
He sent 38 more pools for testing on Thursday, Aug. 4. Results usually arrive within five or six days, he said.
Hutchinson said he has sent about twice as many mosquito pools for testing as he did at this time a year ago, a difference he attributes to the severe 2021 drought.
With much less water around, which mosquitoes need for rearing their eggs, populations were unusually low in the county last year, he said.
This year is more typical, Hutchinson said.
The infected mosquitoes are the culex tarsalis species, a permanent water mosquito that is by far the most common carrier of West Nile virus locally.
Hutchinson said culex tarsalis and a related species that’s also a common vector for the virus, culex pipiens, are more common in the county later in the summer.
The floodwater mosquitoes that predominate during spring and earlier summer rarely are infected with the virus, he said.
In 2021 the first confirmed infection in Baker County was in mosquitoes trapped on July 19, also in the Keating Valley.
Overall in 2021 the virus was detected in 19 mosquito pools, one person and one horse in Baker County, according to the Oregon Health Authority (OHA).
The person recovered, as most people do who contract the virus.
Over the past several years, the Keating Valley area, including where the infected mosquitoes were trapped in 2021 and 2022, has been a “hot spot for our West Nile activity,” Hutchinson said last year.
Most people infected with West Nile virus will show little or no signs of disease. 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. Most people with febrile illness due to West Nile virus recover completely, but fatigue and weakness can last for weeks or months. It is important that you contact your health care provider if you experience any of these symptoms.
The incubation period is usually two to 14 days. Rarely, infected individuals may develop neuro-invasive disease (infection of the brain or spinal cord) that can be severe or may cause death. This is especially of concern to people 50 and older, people with immune-compromising conditions, and people with diabetes or high blood pressure.
Hutchinson and Dr. Emilio DeBess, of the OHA, recommend residents take the following steps to protect against mosquito bites:
• Eliminate sources of standing water that are a breeding ground for mosquitoes, including watering troughs, bird baths, ornamental ponds, buckets, wading and swimming pools not in use, and old tires.
• When engaged in outdoor activities at dusk and dawn when mosquitoes are most active, protect yourself by using mosquito repellants 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.