Early Mosquito Surge Raises West Nile, Disease Concerns Across Bay Area

Warm weather and abundant rain have created ideal conditions for mosquitos across California, experts say. That means more mosquitos, earlier in the year, and greater risk of harmful diseases.

“So far this season, we’re seeing a typical uptick in mosquito activity as the weather warms and random rains continue. Our current surveillance data shows high mosquito counts in several areas, especially following recent rain events,” said Beverly Perez, a vector control community resource specialist with the Santa Clara County Mosquito and Vector Control District.

Judith Pierce, a public education officer with the Alameda County Mosquito Abatement District, said that this March, the county received the third highest number of service requests ever recorded for that month. “We typically do not experience these conditions so early in the year in Alameda County,” Pierce said. “Alameda County has experienced periods of heavy rain followed by warm, sunny weather, which creates ideal breeding conditions for mosquitoes.”

This trend is likely to increase, according to a 2024 analysis of mosquito trends by Climate Central. Over the past 40 years, the number of days of the year highly conducive to mosquito growth has trended upwards across many parts of the United States. Mosquito season used

to last from roughly May to September, but now can start as early as February, Peter Bonkrude, district manager of the Marin/Sonoma Mosquito and Vector Control District, told the San Francisco Chronicle.

The increase in mosquitos can mean more than just itchy bites. In parts of Contra Costa County in February and March, officials found an Aedes aegypti specimen, an aggressive species of mosquito that can transmit diseases like dengue, Zika, yellow fever, and chikungunya. The aedes aegypti first reappeared in Southern California 15 years ago, and has since spread to 28 counties across the state, according to The Chronicle. It was first detected in the Bay Area in 2022, and has been detected across the region in the past season. 10 Aedes aegypti mosquitos have been detected this year in Santa Clara, Perez said. In Alameda County, Aedes aegypti were found in Livermore, Pleasanton, Dublin, and Fremont, according to Pierce.

On May 1, the Alameda County Mosquito Abatement District confirmed the county’s first positive detection of West Nile Virus of the year after a dead bird was found in Newark. About 80% of humans who contract West Nile show no symptoms, but the remaining 20% can develop a flu-like illness three to 14 days after a bite. About 1 in 150 people develop severe symptoms like high fever, neck stiffness, stupor, tremors, or paralysis, according to the Centers for Disease Control and Prevention.

Local mosquito abatement districts are employing a number of strategies to try to fend off the surge. In Santa Clara County, field technicians visit different zones every two weeks to trap mosquitos, and to try to identify emerging mosquito issues. Alameda County said it is increasing trapping efforts across the county, and conducting “extensive” preventative treatments in locations where the mosquitos were detected last year. Contra Costa County is considering a new strategy that involves releasing thousands of sterile male mosquitos that don’t bite to mate with females in order to produce non-viable eggs, according to the Chronicle.

Residents are also encouraged to take a number of preventative measures. Residents are urged to inspect their properties for standing water, a breeding habitat for mosquitos. The mosquito can grow from an egg to an adult in areas as small as a bottle cap of water. An adult mosquito is slightly less than a quarter-inch long, with black and white stripes on its back and legs. They are known to be highly aggressive and active during daylight hours.

To protect yourself from bites:

• Apply insect repellents containing EPA-registred ingredients like DEET, picaridin, IR3535, or oil of lemon eucalyptus
• Wear long-sleeve shirts, pants, socks, and shoes when mosquitos are most active
• Ensure window and door screens are in good condition.
• Report mosquitos biting during the day and any dead birds to your local mosquito abatement district.
• Pump and drain standing water sources at least once a month

SEE ALSO:

• Mosquitoes Are Biting Early In CA. Here's Why it Could Make For 'A Really Bad Summer'
• CA's First Dengue-Positive Mosquito Found In LA County
• Aggressive, Disease-Carrying Mosquitos Found In Dublin
• Aggressive, Disease-Causing Mosquitos Found In Alameda County
• West Nile Virus Detected In Alameda County