Mosquitoes trapped Aug. 25 in Keating Valley tested positive for West Nile virus.
It's the eighth "pool" of mosquitoes in Baker County to be infected with the virus since July 21 (a pool of mosquitoes consists of 10 to 50 insects).
All eight pools were trapped in Keating Valley, said Matt Hutchinson, manager of the Baker Valley Vector Control District.
Hutchinson is responsible for controlling mosquitoes in the 200,000-acre district, which includes most of Baker, Bowen and Keating valleys.
Three Richland-area residents have also tested positive for either West Nile virus or St. Louis encephalitis, according to the Oregon Health Authority.
State officials are waiting for test results from the Centers for Disease Control and Prevention to determine which of the two mosquito-borne illnesses the Baker County residents contracted, although West Nile virus is much more likely.
This summer West Nile virus has been confirmed in mosquitoes in four other counties besides Baker: Union (1 pool); Morrow (9 pools); Klamath (1 pool); and Jackson (13 pools).
Two horses have been infected with the virus, one in Baker and one in Union.
There are four "presumptive" human cases - the three in Baker County and one in Malheur County.