The West Nile virus has now been detected in mosquitoes or dead birds across eight areas of San Joaquin County as health officials continue to monitor the virus during peak mosquito season.
As of Aug. 1,274 of 1,535 mosquito samples tested in San Joaquin County this year were positive for West Nile virus, according to the county’s Mosquito and Vector Control District. Five of 53 dead birds tested by the district also were positive.
The district’s latest surveillance map shows West Nile virus activity in Stockton, Manteca, Tracy Clements, the Delta islands, Escalon, Lockeford and Thornton. Positive samples include two species known to spread the virus — the northern house mosquito and eastern encephalitis mosquito —- as well as infected dead birds.
The new numbers mark a continued increase in virus activity this summer.
San Joaquin County’s first human West Nile virus case was reported on Aug. 6. State surveillance data last updated July 31 showed 51 positive mosquito samples, three infected dead birds and one infected horse in the county. The human case involved a woman in her 60s; officials did not disclose whether she lived, whether she had been hospitalized or her condition.
Since then, the number of identified positive mosquito samples has risen by 23 since the July 31 snapshot, while two additional dead birds have tested positive.
West Nile virus was first detected in San Joaquin County this year in June, when mosquitoes collected at Upper Jones Tract in the Delta islands west of Stockton tested positive. At the time, California had not yet reported a human infection.
The mosquito district said it continues to trap and test mosquitoes and dead birds each week, using those results to identify areas where targeted mosquito-control treatments may be needed. The district has conducted ground and aerial spraying in several parts of the county this summer, including areas of Stockton, Manteca, Tracy and the Delta.
West Nile virus is most commonly transmitted to people through the bite of an infected mosquito. About 80% of infected people develop no symptoms, while roughly 20% may experience fever, headache, body aches and other flu-like symptoms. About 1% develop serious neurological illness.
Health officials recommend using insect repellent, wearing long sleeves and pants when mosquitoes are active and regularly dumping standing water around homes where mosquitoes can breed.