Marin’s rainy season could bring more mosquitoes

Historic rains could mean Marin is in for a terrific mosquito season.

The cause is standing water, which is a powerful breeding ground for mosquito larvae, said Nizza Sequeira, spokeswoman for the Marin/Sonoma Mosquito and Vector Control District.

Mosquitoes and the disease they bring with their blood-sucking bites could be more prevalent this year, although fears have yet to be confirmed, she said. Tests are underway to estimate current and future mosquito numbers, she said.

“We don’t know for sure if this is going to be a great mosquito season,” Sequeira said. “But we are starting to see some mosquito activity, which is expected for this kind of year.”

This rainy season has been one of the wettest in recent memory, said Matt Mehle, meteorologist with the National Weather Service.

Every site in Marin County was above normal this water year, which measures precipitation since Oct. 1, he said.

“We just had a very active Pacific which opened our proverbial storm door. It opened in December and didn’t close until the end of March,” he said. “We haven’t really had a winter dry spell.”

Corte Madera and Mill Valley saw 119% of normal with 39.1 inches of precipitation. The Lagunitas and Woodacre area received 60 inches of precipitation, Mehle said.

Rainfall at Lake Lagunitas was measured at 74.22 inches, compared to the average rainfall of 48.14 inches, according to the Marin Municipal Water District. The same time last year had 45.49 inches of rain.

The problem, Sequiera noted, is that water can collect in certain places and stay there. Water that does not move can be found in unexpected places such as buckets, water troughs, neglected swimming pools, abandoned children’s toys or loose tarps.

During this time, the agency receives reports from residents about the presence of mosquitoes. Vector Control District programs and services are funded through property taxes and benefit assessments and are provided to all residents of Marin and Sonoma counties, the organization said.

The agency’s mosquito surveillance program is ongoing. It involves using a dipper to test standing water for mosquito larvae and setting traps to assess the current abundance of adult mosquitoes, species and whether any are infected with West Nile virus.

The virus is the biggest mosquito concern, said Marin County Public Health Officer Dr. Matt Willis.

“Most cases are mild, but it can be debilitating in its worst forms,” ​​he said.

The virus spreads when a mosquito bites an infected bird and then bites a human. In Marin, there have been four human cases in the past 10 years, he said.

“The risk varies from year to year, and when we see more standing water, we worry about mosquito breeding grounds,” Willis said. “With wet weather, it is especially important to drain standing water from areas where it collects around our property. We can prevent bites when we are outdoors by covering exposed skin and using an effective repellent.

Sequeira called West Nile “the most prevalent and severe mosquito-borne disease in California.” It is transmitted by Culex mosquitoes. There is no vaccine available for humans to protect against the disease, which can lead to meningitis, encephalitis and death, she said.

West Nile virus is endemic or occurs regularly in Marin and Sonoma counties, she said.

In 2022, 209 people were diagnosed with West Nile virus disease in 28 counties in California, resulting in 13 deaths.

Dead birds are used as an early warning indicator of West Nile virus activity. One dead bird tested positive for West Nile virus in Marin in 2020 and one bird in 2017. Five birds tested positive in 2016.

One human and three dead birds tested positive in 2015. In 2014, three mosquito pools — or groups of mosquitoes — and six dead birds tested positive, health officials said.

No cases have been found in humans, dead birds, sentinel chickens or horses so far this year, according to the California Department of Public Health. Two cases were found in mosquito samples in Alameda and Los Angeles counties.

Sequeira also warned of a certain species of mosquito that lays its eggs in the holes of oaks and laurels. Mosquitoes, which emerge in the spring, pose a risk of spreading canine heartworm.

The agency recommends keeping gutters clean and free of debris; check for standing water in flower pots, birdbaths and outdoor toys; keep swimming pools and spas clean and properly treated; seal rainwater collection containers; use a sump pump to evacuate stagnant water near dwellings; and use screens on windows and doors to keep mosquitoes out of your home.

Residents should also wear long-sleeved shirts and pants and use repellent, she said.

“We want to remind residents that mosquito-borne diseases are a serious concern and that it is essential to take steps to prevent mosquito breeding and protect themselves from bites,” Sequeira said. “By working together to eliminate standing water and protect ourselves from mosquito bites, we can help keep our community healthy and safe.”