Mosquitoes: More expected, one disease ‘coming back with a vengeance’ in Valley

Parlier research center works to understand and control mosquitoes

Anton Cornel worked with local mosquito abatement districts in 2018 in an attempt to control mosquitoes including the Aedes aegypti, also known as the yellow fever mosquito.
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Anton Cornel worked with local mosquito abatement districts in 2018 in an attempt to control mosquitoes including the Aedes aegypti, also known as the yellow fever mosquito.

Heavy winter snowpack and more rain this spring likely means more mosquitoes are coming soon to the central San Joaquin Valley.

“We are expecting a fairly significant increase in mosquito activity,” said Michael Cavanagh, district manager of the Kings Mosquito Abatement District based in Hanford.

Mosquitoes need water to lay their eggs, so more water means more places for the insects to breed.

Mosquitoes carrying West Nile virus remain a concern in the Valley, and an increasing number of mosquitoes have been found to be carrying Saint Louis encephalitis. That disease has been “coming back with a vengeance” over the past couple years, said Ryan McNeil, district manager of the Fresno Mosquito and Vector Control District.

“It’s not just a handful of mosquitoes,” McNeil said. “We are talking about hundreds or potentially thousands that are infected with these diseases amongst us throughout the year.”

Mosquito-borne diseases

A major focus and concern for mosquito districts now is another mosquito, the Aedes aegypti, because it’s relatively new to the Valley and has the ability to spread Zika, yellow fever, dengue, and chikungunya viruses if those diseases were brought here.

McNeil said all it would take is one person infected in a different country and bitten by a mosquito upon returning home to spread those new diseases in the area.

There are no reported cases in California of Zika being transmitted by a mosquito, said Steve Mulligan, district manager of Consolidated Mosquito Abatement District.

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Mosquitoes of the Culex genus can pick up West Nile virus and Saint Louis encephalitis from infected birds. However, a person infected with either of these diseases can’t pass it to a mosquito.

Dead birds, which can be tested for West Nile, can be reported online at

Other longtime mosquitoes in the Valley, of the Anopheles genus, are capable of spreading malaria if that disease were here.

“There’s always that potential, and that’s something we’re continually concerned about,” Mulligan said.

The last known local case of malaria transmission by a mosquito was in 1986 in the Skaggs Bridge area of Fresno County, where it appeared two people were infected, according to records found by Conlin Reis, district manager of Fresno Westside Mosquito Abatement District, which covers more than 1,200 more rural square miles.

Samples testing positive for West Nile and Saint Louis encephalitis in mosquitoes increased from 2017 to 2018 in the 283 square miles covered by the Fresno Mosquito and Vector Control District, based on tested trappings of insects. Mosquitoes infected with Saint Louis encephalitis more than doubled in 2018 in that district that includes downtown Fresno, Easton, Malaga, Biola, and Kerman.

That’s consistent with statewide trends of more Saint Louis encephalitis in mosquitoes throughout California, Mulligan said.

The milder and more common Saint Louis encephalitis symptoms include fever, headache, nausea, vomiting and tiredness, and are similar to West Nile symptoms. About 1 out of 150 infected with West Nile develop a serious, sometimes fatal, illness. Many will have no symptoms.

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West Nile infected at least 218 people in California last year, according to California’s website. In the Valley, that included 14 people in Fresno County, 13 in Kern County, eight in Tulare County, four in Madera County, and two in Merced County.

No human cases of West Nile have been reported in 2019.

Experts say despite there likely being more mosquitoes this year because of the extra wet winter, that doesn’t mean more infections are expected. One theory is that’s because mosquitoes will likely be more spread out, utilizing a number of water sources instead of all coming to the same spot, which hopefully means there will be less mingling and transmission.

The new mosquito in town

The concerning Aedes aegypti mosquito was first found in the region in Madera and Clovis in the summer of 2013 and is now well-established throughout the Valley and Southern California. Mulligan said it’s found as far north as Merced County.

People may be noticing more mosquito bites because of this mosquito. It bites throughout the day, not just at dawn and dusk like the Culex mosquitoes usually do.

Experts say Culex mosquitoes usually fill up on blood until full, while the Aedes aegypti won’t, instead biting multiple times or multiple people.

While the Aedes aegypti is a risk for transmitting new diseases in the Valley, it cannot transmit West Nile virus or Saint Louis encephalitis.

The Aedes aegypti are also more aggressive and more likely to follow people indoors.

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Danger: Standing water

The Aedes aegypti utilizes small water sources – like a container in a backyard, or the inside of a corrugated drainage pipe – to lay eggs.

“The good and bad news is most of the sources are in people’s yards,” Mulligan said.

To keep mosquitoes from multiplying, experts say it’s imperative to get rid of standing water in yards and not over-water plants. That way, the bugs have fewer places to breed.

“Get rid of standing water,” McNeil said. “It only takes one bad neighbor to ruin the entire neighborhood. … If people don’t do what they need to do, it’s a perfect recipe for a lot of mosquitoes.”

Mulligan said even in drier years mosquitoes can be just as problematic because people over-irrigate yards and crops.

Mulligan said there are over a thousand problematic unmaintained swimming pools in his district – which covers more than 1,000 square miles, including parts of Fresno and many cities on the east side of the Valley – that were identified during an aerial flyover.

Climate change may also be increasing the mosquito population, Mulligan said.

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Conlin Reis, district manager of Fresno Westside Mosquito Abatement District, said more flowing water isn’t an issue, but things become problematic if fluctuating water levels lead to pockets of trapped standing water or flooding. He said that hasn’t been an issue in his district so far this year, but it could pose more of a problem later as more snow melts.

“Aedes aegypti are able to exploit very tiny water sources including remnant water in almost any container-like object – buckets, plant saucers, toys, even bunched up water-impermeable fabrics – and particularly cryptic water-holding areas such as yard drains connected by corrugated tubing,” Reis said. “Because of this, late season rains can create an abundance of mosquito sources in a single residence.”

Get prepared, help available

People are encouraged to find where water is pooling, and discard unnecessary containers. Reis said any water left standing for five days or more is a problem.

“Yard drains can be screened with window screen material to prevent entry by mosquitoes,” Reis said. “Doing this early can prevent mosquito problems now and in the hotter part of the year when mosquitoes are more active and backyard irrigation takes the place of rain as the source of standing water.”

Mosquitoes typically become noticeable in early spring, peaking in numbers by mid-summer, and remaining a nuisance through late fall, although the bugs are present year-round. Mulligan said only a handful of more than 25 species of mosquitoes in the region are really a biting nuisance to humans.

Mosquito district staffers make home visits to inspect yards for mosquito-breeding sources and can offer treatments, including pesticides approved by the Environmental Protection Agency and planting fish that eat mosquitoes.

People can visit to see what mosquito district has jurisdiction in their neighborhood.

Mosquito districts are primarily focused on urban areas where more people live, but they also inspect more rural and agricultural sources.

Other mosquito sources that district staffers treat include utility vaults, gutters and storm drains. Pooling in orchards or crops, along with cemeteries, where flowers vases are often left out with standing water, can also pose problems. Drones can now be utilized to find and treat problematic areas.

People can avoid mosquito bites by using EPA-approved bug repellants, wearing long-sleeved clothes, and avoiding exposure during dusk and dawn – the primary biting time for Culex mosquitoes.

“Mosquito abatement districts are often spread more thin on years like these,” Reis said. “It is vital that residents take action in their own homes and neighborhood to prevent mosquito production.”

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Carmen George is a features and news reporter for The Fresno Bee. Her stories have been recognized with Best of the West, George F. Gruner, and McClatchy President’s awards, and nine first or second place awards from the California News Publishers Association. She has a passion for sharing people’s stories to highlight issues and promote greater understanding.