Jason Farned and his team at the San Gabriel Valley Mosquito and Vector Control District had spent years preparing for the likely arrival of dengue, a dangerous virus typically found in tropical climates outside the mainland United States.

They’d watched nervously as invasive Aedes mosquito species that can carry the virus appeared in Los Angeles about a decade ago and began to spread, likely introduced by international trade and enticed to stay by a warming climate that makes it easier for mosquitoes to thrive.

Then, in October 2023, an email came from the Pasadena Public Health Department: A person in the city had contracted California’s first-known case of dengue from a local mosquito.

“When it happens in real time, real life, you know, it is very different,” Farned said. “There’s no room for error here. We have to be quick and effective in identifying the most at-risk areas and responding.”

Across California, public health and pest control authorities are facing a new reality as the Aedes mosquitoes bring the threat of dengue and potentially other tropical diseases, such as chikungunya, Zika, and yellow fever, that were once of concern only to international travelers.

So far this year, authorities have identified at least 13 cases of locally acquired dengue, up from two in 2023, with 11 in Los Angeles County and two in the San Diego area. The Aedes mosquitoes spread the disease by biting an infected person and then biting a previously uninfected person.

Mosquito-borne viral illnesses, chiefly malaria, have long been a scourge in many tropical regions, and preventive measures focus mainly on controlling the mosquitoes. The Aedes mosquitoes, known for their aggressive, daytime biting, are now present in at least 24 California counties. They breed in water, in as little as a capful.

“When these locally acquired cases occur, … we want to act on them pretty quickly so that it does not become an endemic infection in our region,” said Aiman Halai, director of the Los Angeles County Department of Public Health’s Vector-Borne Disease Unit.

California officials are hoping to beat back dengue by expanding mosquito surveillance, developing detailed response plans for mosquito outbreaks and human infections, and improving data sharing across agencies. They’re also going door to door in neighborhoods to remove standing water sources and apply pesticides. Residents are advised to wear bug repellent and long-sleeved clothing and control mosquitoes around their homes to prevent biting and infection.

Some vector control districts — local agencies charged with managing disease-bearing insects and other animals — are even growing their own sterile mosquitoes to release into the wild to reduce local Aedes populations.

Outside of California, locally acquired dengue cases have occurred in Arizona, Florida, Hawaii, and Texas. In March, Puerto Rico declared a public health emergency after a spike in cases there, where dengue is endemic. Meanwhile, worldwide dengue cases are on track to more than double this year, with 12.3 million documented through August, up from 6.5 million in 2023, according to the World Health Organization.

Most people who get dengue have no symptoms, but about 1 in 4 become ill. A mild case can feel like the flu and usually dissipates within a week, but about 5% of those infected with dengue become very sick, with symptoms that can include internal bleeding, shock, and organ failure, and the most severe cases can be fatal. People infected a second time are at especially high risk.

There is no specific medication to treat dengue. Japanese pharmaceutical company Takeda developed a vaccine that has won approval in Europe and elsewhere, though it withdrew an application to the FDA last year, saying it could not provide data requested by the agency. A vaccine developed in Brazil could soon be approved for use in that country. But the only FDA-approved vaccine is authorized only for children in narrow circumstances and will soon go out of production.

At the San Gabriel Valley Mosquito and Vector Control District, one of five agencies tasked with mosquito control across Los Angeles County, public health workers have put together an Aedes and dengue response plan based on updated guidance from the state.

When they discover a case, they identify all the properties and public spaces within 150 meters — roughly the distance an Aedes mosquito can fly — and then go door to door, removing standing water, where mosquitoes can breed; applying pesticides from backpacks or trucks; and educating residents about the risk of dengue and how to protect themselves. District officials also set traps to catch mosquitoes so they can figure out their prevalence and test them for dengue.

Since local dengue cases began to appear, the district has gotten more efficient in implementing its response plan, district manager Farned said. All full-time and seasonal staff members — about 40 people — have been trained in a variety of tasks, such as door-to-door education and coming in during off-hours to answer phones, Farned said.

While vector control teams respond to cases, separate teams from the Los Angeles County Public Health Department go door to door in the affected neighborhood when they determine that a dengue case was locally acquired, surveying residents and offering free dengue testing to try to identify others who may be infected.

Additionally, the department has been sending alerts to local health providers, advising them to be on the lookout for possible dengue cases and test for it when suspected, even among patients who haven’t traveled to a place where dengue is endemic. This advice follows a national alert put out by the Centers for Disease Control and Prevention in June. Health authorities are also emphasizing that people who travel to locations with dengue should continue to wear mosquito repellent when they get home, to reduce the risk of spreading the disease to local mosquitoes.

As happened during the covid-19 pandemic, mistrust of public health authorities can make outreach challenging for health and pest control teams in some neighborhoods, officials said.

Pest control officers can seek a warrant to enter and treat a property for mosquitoes if a homeowner refuses to give access, said Jeremy Wittie, a former president and the legislative committee chair for the Mosquito and Vector Control Association of California, which represents the more than 70 mosquito and vector control agencies in California. This is easier in districts such as his, the Coachella Valley Mosquito and Vector Control District, that have warrants giving officials standing permission to enter a property after 24 hours without needing to ask a judge.

In counties such as Santa Clara, where Aedes mosquitoes first appeared in 2022 but have yet to establish themselves, officials hope to suppress the threat with stepped-up surveillance, speedy eradication efforts, and more public outreach. Santa Clara County Vector Control District Manager Nayer Zahiri said the aim was to eliminate Aedes but acknowledged the climate conditions that encourage the mosquitoes’ spread are “totally out of our control.”

In some counties with pervasive mosquito problems, including San Diego, San Joaquin, and Stanislaus, officials have sprayed pesticides from planes or helicopters to address spikes in local mosquito populations, Wittie said. These sprayings typically aim to control the larvae of a different type of mosquito, Culex, that can spread West Nile virus and which — unlike the Aedes species, which thrive in urban habitats — are found in harder-to-reach rural environments, Wittie said.

Aerial spraying hasn’t been deployed to address the recent dengue outbreaks, which are in more urban environments where spraying from trucks is a better option, Wittie said. Drones are another option that some vector control authorities are exploring.

Some districts are experimenting with the decades-old sterile insect technique, commonly used for other pests such as fruit and screwworm flies, in which males are sterilized with radiation and then released to mate, resulting in eggs that don’t hatch. (Female mosquitoes are separated from the males before sterilization and not released. Only the females bite).

Ultimately, the public will have to take the mosquito threat more seriously and contribute to prevention efforts, Wittie said. “This mosquito is going to be here to stay, unfortunately. I hope it kind of wakes people up and pushes them to be part of that solution.”

This article was produced by KFF Health News, which publishes California Healthline, an editorially independent service of the California Health Care Foundation.