West Nile virus activity is on the rise in some northern California counties, according to the Sacramento-Yolo Mosquito and Vector Control District. Three dead birds and 13 mosquito samples tested positive for the virus on Aug. 13, “the highest number of positive results reported in a single day this year,” the agency said in a news release.

Northern California’s first human West Nile virus case of the year has been confirmed in Butte County, according to local health officials.

BUTTE COUNTY, Calif. — Health officials in Butte County have confirmed the first human case of West Nile Virus (WNV) this year. Butte County Department of Public Health (BCPH) said the patient, who is between 30 and 40 years old, is severely ill. So far, 9 cases have been reported in California this year, BCPH said.