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First Mosquito With West Nile Virus Found In San Diego County

San Diego County vector control technician Sean Simmons inspects a home for mosquitoes, May 23, 2018.
Claire Trageser
San Diego County vector control technician Sean Simmons inspects a home for mosquitoes, May 23, 2018.

A mosquito caught in routine trapping in the Black Mountain Ranch area has tested positive for West Nile virus, the first appearance of the virus in San Diego County in 2020.

County environmental health officials remind people to protect themselves from mosquitoes, which can potentially transmit the disease, by following the county's "Prevent, Protect, Report" guidelines — including finding and dumping out standing water in yards and around homes to keep mosquitoes from breeding.

Incidences of West Nile virus were mild in San Diego County in 2019. Three people tested positive, but there were no fatalities and all three people were suspected of having contracted the virus while outside the county.


However, West Nile virus remains a potentially deadly threat in San Diego County and the state, officials said. In 2015, 44 San Diego County residents tested positive for West Nile virus and six died. In 2019, 225 California residents became sick from West Nile virus and six died, according to the California Department of Public Health.

West Nile virus is mainly a bird disease, but it can be transmitted to humans by a number of species of mosquitoes — including Culex mosquitoes native to San Diego and, less effectively, by invasive Aedes aegypti mosquitoes also found in the county — if they feed off an infected animal, mainly birds, and then bite people.

Eight out of 10 people who get infected never suffer any symptoms. The 20% who do generally suffer mild symptoms including headache, fever, nausea, fatigue, skin rash or swollen glands. But in rare cases, West Nile virus can make people extremely ill and even kill them.

Protecting against mosquitoes has become even more important for San Diego County residents in recent years. Since 2014, three types of day-biting, invasive Aedes mosquitoes have been found in the county. All of these mosquitoes tend to live and breed near people, in yards and even inside homes.

Two of these species can potentially transmit diseases not naturally found here. Those include chikungunya, dengue and Zika — but only if mosquitoes first bite an infected person. In general, that means local invasive Aedes mosquitoes could only transmit those diseases if they found and bit San Diego County residents who picked up the virus while traveling and returned home still infected.


San Diego County residents may be more vulnerable to being bitten by mosquitoes around their homes now because many have been spending more time at home because of the coronavirus pandemic. Environmental health officials remind people to dump out or remove any item inside or outside of homes that can hold water, such as plant saucers, rain gutters, buckets, garbage cans, toys, old tires and wheelbarrows.

Mosquito fish, available for free by contacting the Vector Control Program, may be used to control mosquito breeding in backyard water sources such as stagnant swimming pools, ponds, fountains and horse troughs.

Wear long sleeves and pants or use insect repellent when outdoors. Use insect repellent that contains DEET, picaridin, oil of lemon eucalyptus or IR3535. Make sure screens on windows and doors are in good condition and secured to keep insects out.

Report increased mosquito activity, or stagnant, green swimming pools and other mosquito-breeding sources, as well as dead birds — crows, ravens, jays, hawks and owls — to environmental health's Vector Control Program by calling 858-694-2888 or emailing Also report if you have been by mosquitoes during daylight hours, or if you find mosquitoes that look like invasive Aedes mosquitoes — small, black with white stripes on legs and backs — by contacting the Vector Control Program.