Cleanup efforts following the devastating Eaton and Palisades fires are underway, but an ongoing concern is swimming pools in the two burn zones, many of which contain stagnant water that has become a breeding ground for mosquitoes that can carry diseases.

In the region affected by the Eaton fire, officials responsible for mosquito control say they do not have the funds needed to provide sufficient treatment for all the pools that are now possible mosquito hothouses. That has sparked public health concerns in an area that has recently seen spikes in locally acquired cases of dengue fever, a potentially fatal mosquito-borne disease.

In mid-May, the San Gabriel Valley Mosquito and Vector Control District, which serves Altadena, Pasadena and Sierra Madre, identified some 1,475 pools in the burn zone as “nonfunctional” — meaning they contained stagnant water due to ash and debris, damaged equipment, or the homeowner’s inability to maintain the pool at the moment. Most of the pools are located in Altadena, and the district has so far been able to treat about half of them with pesticides.

The agency says it should be able to treat the other half with resources it will have available to it, but lacks the funding to provide the two follow-up applications experts say are needed to continue to stave off mosquito breeding throughout the year.

Indeed, officials found mosquitoes breeding in roughly 80% of the approximately 700 pools in the burn area that they treated with pesticides to date. A single swimming pool can become a breeding ground for as many as 3 million mosquitoes in one month.

“This is a public health concern. It will be a risk if it doesn’t get addressed,” said Anais Medina Diaz, spokesperson for the San Gabriel Valley vector control district, which spans 26 cities and unincorporated areas. Vector control districts are local agencies tasked with managing disease-spreading critters like mosquitoes.

Mosquitoes lay their eggs on or near stagnant water. When they hatch, the young develops in the water before emerging as a buzzing adult.

District officials have spent $307,000 to apply a pesticide treatment to about half of the 1,475 affected pools, mostly on the perimeter of the burn scar, Diaz said.

District officials had planned to use that money to respond to anticipated cases of dengue fever, a viral infection spread by invasive mosquitoes. Last year the district saw 11 locally acquired cases of dengue, more than anywhere else in the state.

The emergency reserves aren’t completely tapped, but the district needs to keep funds in its coffers in case disease does strike, Diaz said.

So district officials said they sent a request to the L.A. County Office of Emergency Management and the California Governor’s Office of Emergency Services for an additional $302,000 to cover the costs of an initial pesticide treatment for the more than 700 yet-untreated pools, and to pay for two follow-up treatments for all 1,475 pools throughout the coming year.

On Thursday, the state agency approved a portion of the request, said Pablo Cabrera, a spokesperson for the district. Details and dollar amounts were still being finalized, but district officials said it would be enough to apply pesticides to the pools that have yet to be treated. It would not cover the two subsequent treatments.

Vector control officials began treating the first round of pools in mid-March. Each pesticide treatment controls mosquito larvae for up to 90 days — so the treatment on those initial pools will wear off around mid-June, often the height of mosquito season. The plan is to finish up the remaining pools by the end of June.

Current resources will allow for “full control” of mosquito breeding through August, according to Cabrera. What comes next will depend on what sort of financial support the district will get moving forward.

“We would love to have this kind of funding to be able to carry out these additional treatments. That is what we want to do,” Diaz said. “But we were not set up in a way where we can cover this area and then continue to do our vector control work throughout the San Gabriel Valley.”

In a statement, Ed Chapuis, a spokesperson for the California Governor’s Office of Emergency Services, said that the state is providing resources for urgent treatments and will continue to work with the district. State and/or federal disaster assistance funding could potentially be tapped, he said.

The agency “will continue supporting the district to ensure no delay in their efforts to address public safety,” he said.

Officials with the county Office of Emergency Management said their role was only to connect the state and local agencies.

San Gabriel Valley district officials have requested that $500,000 be set aside in the state budget for mosquito-control efforts in 2026 and beyond. They’re also seeking reimbursement from the Federal Emergency Management Agency for expenses incurred while tackling the first batch of pools.

The district has enhanced surveillance of the burn area and will be as proactive in its approach as possible with the means it has, officials said.

Mosquitoes in the Palisades fire burn scar

Coastal communities that were in the path of January’s destructive Palisades fire are similarly contending with the issue of unmaintained pools.

A survey of the burn area revealed more than 1,700 parcels with a pool or spa where mosquitoes could breed, according to the Los Angeles County West Vector Control District. The district provides services to the western portion of the city of L.A., including Pacific Palisades, as well as the city of Malibu.

The pools need to be “drained and dry or clean and filtered” to prevent breeding, and some preliminary inspections have gone forward, said Aaron Arugay, executive director of the district, in an email. Some mosquito breeding was seen in the problem pools, and was treated, he said.

“Due to the number of parcels, this is going to be an ongoing project all season,” Arugay said.

It’s an unexpected task, but Arugay said he doesn’t anticipate issues with drawing from the district’s budget and reserves to cover the costs for the season. The plan is to bring on temporary seasonal staff to help out in the summer and fall.

What’s at stake

The region’s invasive mosquitoes, Aedes aegypti, were behind what public health officials deemed “unprecedented” local spread of dengue last year in Los Angeles County.

There were 14 local dengue cases in the county last year, and 11 of them were within the footprint of the San Gabriel Valley vector control district. Just two years ago, the state’s first known case of locally acquired dengue — in a Pasadena resident — occurred in the district.

Symptoms of dengue can include joint and muscle pain, severe headaches and bleeding under the skin. Last year, more than a third of L.A. County residents infected with the disease needed to be hospitalized, according to the L.A. County Department of Public Health.

Of particular concern right now are native Culex mosquitoes, which can transmit deadly West Nile virus, Diaz said. This species makes up the majority of the region’s mosquito population, and its season typically peaks earlier than Aedes mosquitoes.

West Nile can cause severe and potentially fatal brain inflammation, among other serious issues. There were 35 confirmed cases of West Nile virus in L.A. County in 2024, including 27 hospitalizations and two deaths, according to data from the L.A. County Department of Public Health.

Culex mosquitoes typically target birds rather than humans, and tend to be satisfied with one slurp of blood, making them comparatively less annoying than Aedes mosquitoes. The Aedes mosquito is known for biting ankles during the day, often striking multiple times in succession.

The Aedes aegypti species of mosquito arrived in California a little over a decade ago, and has since spread to more than a third of state’s counties. Vector control officials have sought to beat back the scourge, galvanized by the unhappy residents and fear of spiking disease that’s starting to be realized.

In recent years, a couple of local districts have rolled out pilot programs entailing the release of sterile male mosquitoes to drive the population down, a relatively new approach to tackling these mosquitoes.

Early data have shown promise, with a mosquito population reduction of nearly 82% in one study area in L.A County last year. (Males don’t bite, so they aren’t contributing to itchy welts or disease spread.)

The method appears to be catching on, and the San Gabriel Valley vector control district plans to launch a similar program next year, Diaz said.

The Greater L.A. County Vector Control District, which covers the largest portion of the county, is currently gauging whether its homeowners will pay up to $20 a year to expand its sterile male release program. Diaz said the San Gabriel Valley district will probably be seeking additional funding from its property owners in 2027.

Such efforts have taken on heightened urgency amid the rise in dengue. And as mosquito season has lengthened in recent years amid warming temperatures, some of the bloodsuckers now stick around until December in parts of the Southland.

On the plus side, recent lower temperatures have slowed the insects’ life cycle in the Eaton fire burn area, and mosquito abundance levels have been below average. But the mercury is poised to rise.