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Mosquito-Borne West Nile Virus Cases Expected To Increase With Spring’s Arrival

Mosquitos are sorted at the Dallas County mosquito lab in Dallas, Aug. 16, 20...

Credit: Associated Press

Above: Mosquitos are sorted at the Dallas County mosquito lab in Dallas, Aug. 16, 2012.

San Diego County officials say residents need to protect themselves from West Nile virus after the mosquito-borne disease was confirmed last week in a dead hawk found in the East County.

“It’s our first finding of the season and that’s sort of an indicator that things are about to start,” said Chris Conlan, supervising vector ecologist for the county’s Vector Control Program.

Conlan said the region’s cool winter temperatures have kept the blood-sucking insects at bay, but as the weather warms up, mosquito population growth will accelerate, he said. That’s because the incubation period for the two dozen mosquito species that live in the county shortens during higher temperatures.

“The warmer it is, the faster they can complete a generation,” Conlan said. “So those mosquitoes that are taking maybe several weeks right now to complete a generation will take maybe as little as seven to 10 days.”

Conlan said San Diego’s abundance of rainfall is also a contributing factor of mosquito activity. More standing water across the region means more breeding areas for the small, flying insects that are drawn to the stagnant pools to lay their eggs, he said.

“But by the same token, some areas that might have been stagnant water are now fresh flowing water and that won’t breed mosquitoes,” Conlan added.

He said it’s too early to determine the severity of the West Nile virus season ahead, which usually lasts through November.

"Until I see more data, I'm not going to be able to make an accurate prediction yet," he said.

West Nile virus is generally transmitted from bird to bird via mosquito bites,” Conlan explained.

Photo by Susan Murphy

Chris Conlan, supervising vector ecologist for San Diego County’s Vector Control Program, looks at mosquitoes under a microscope, March 15, 2019.

“But on occasion, those mosquitoes sometimes will bite a bird, pick up the infection and then at some point later down the line, bite maybe a human or another animal,” Conlan said.

Last year, 215 people across the state were diagnosed with the disease and 10 people died. In 2017, mosquitoes infected more than 500 people and killed nearly four dozen.

Most people infected with West Nile virus don't show symptoms. For the 20 percent who do have symptoms, they include headache, fever and in severe cases, paralysis or death.

While the prime concern for disease carried by mosquitoes in San Diego County is the West Nile virus, mosquitoes in the region can carry other serious illnesses, Conlan warned.

Aedes Aegypti and Aedes Albopictus mosquitoes, first discovered in San Diego in 2014, can carry diseases like Zika, dengue and chikungunya. Over the past five years, the two invasive species have tripled in population and expanded their reach. Fortunately, Conlan said, there is currently no active virus in the region.

Still, with the arrival of spring, people are being urged to take precautions to avoid mosquito bites, including wearing repellent and eliminating standing water at their homes.

"Make sure you’re not breeding mosquitoes in your own backyard," Conlan said. “First and foremost, we encourage the removal of standing water. Also, you can do things like putting good screens on your home and on your doors to keep mosquitoes from entering the house.”

He is urging people to report dead birds and green pools of water by emailing vector@sdcounty.ca.gov, calling (858) 694-2888 or downloading the county’s “Fight the Bite” app.

San Diego County's cool winter temperatures have kept mosquitoes at bay, but as the weather warms up, mosquito growth will accelerate, said Chris Conlan, supervising vector ecologist for the county’s Vector Control Program.

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