West Nile virus has been detected in areas of Sunnyvale and Santa Clara by the Santa Clara County Vector Control District, which monitors human diseases that are transmitted by parasites, viruses and bacteria.
The manager for the vector control district, Nayer Zahiri, said there have never been any fatal cases of West Nile virus in Santa Clara County, and human cases of the virus have been dropping in recent years.
Two dead American crows tested positive for the disease in the last week, and officials were able to trap and test mosquitoes in the area.
County officials said truck-mounted adult mosquito control treatment will begin in the affected areas, which include the 94085, 95051 and 95054 ZIP codes, centered in the area of Central Expressway and Semiconductor Drive in Santa Clara next to Sunnyvale.
The treatment will begin at 11 p.m. Thursday and last about three hours, according to county officials. Residents do not need to leave their homes during the treatment but may wish to close windows and doors to avoid possible chemical sensitivity.
According to Zahiri, there were no human cases in Santa Clara County in 2017, but there was one in 2016, eight in 2015 and 10 at a peak in 2014. She attributed the high numbers in 2014 to very dry weather, saying birds are more likely to contract the disease when sharing limited water sources with mosquitoes.
Zahiri said it's extremely difficult to predict whether the virus will increase or decrease in coming years, but described heat as a large factor in mosquito breeding where water is available.
West Nile virus first arrived to California in 2003 and 292 people have died in the state due to the disease in the last 15 years, according to county officials. The record year for fatalities in the state was 2015, with 53 deaths.
Most people who contract the disease do not experience any symptoms, but some have fevers and headaches while severe cases involve neurological damage or death, according to Zahiri.
County officials said individuals older than 50 with chronic medical conditions like diabetes, cancer or kidney disease are most at risk.
To avoid contracting the disease from mosquitoes, officials recommended dumping standing water where the bugs can breed, using tight-fitting screen doors and properly using bug repellant. Zahiri said even small containers can have enough water for the bugs to breed.
Residents can also request free mosquito removal services by calling the county at (408) 918-4770.