By Alejandra Borunda, NPR
Manuel Lara is hunting for mosquitos.
Lara is a technician with the San Gabriel Valley Vector Control District, an agency in Los Angeles County that works to control the spread of insect- and pest-borne diseases. He’s on the hunt for a particularly aggressive mosquito called Aedes aegypti.
An invasive species, the Aedes mosquito is relatively new to Southern California — and it can spread a disease that historically could not be caught within the region: dengue fever.
Lara is poking around a tidy backyard in a Los Angeles suburb when he finds his quarry — or rather, it finds him.
A swarm of tiny, black-and-white striped mosquitoes zoom out of a trash can lid filled with stagnant water.
He startles and immediately pulls out his bug spray. “That’s like, 50 mosquitoes!” he says.
This autumn, Los Angeles County public health officials reported 8 cases of locally acquired dengue fever. It’s a small but significant signal that the region is becoming increasingly hospitable to the disease and the mosquitoes that carry it. Last year was the first time such locally spread cases were ever seen in the region.
Dengue fever is endemic to tropical and sub-tropical regions of the world. It can result in a painful fever and, in extreme cases, even death. Only one in four infected people develop symptoms, but anyone infected can spread the disease if bitten by a mosquito species that can carry it. There is no vaccine yet available for adults, and there is little medical intervention available for those who develop the disease.
Until last year, Southern California had only seen cases of dengue fever in travelers who were infected while abroad. It had not historically spread within the region. But in 2023, for the first time ever, the disease popped up in patients who had not recently traveled.
“It really was a reality check that this can happen in our area,” says Aiman Halai, director of the Vector Borne Disease Unit at the Los Angeles County Department of Public Health.
In recent years, climate change has softened winter temperatures, lengthening the season mosquitoes can thrive. And residents have unwittingly created plenty of Aedes mosquito-friendly habitat.
The combination has resulted in this year’s unprecedented number of cases of dengue fever acquired locally within Southern California, Halai says.
There is little risk of a major outbreak right now, experts say. But, Halai emphasizes that the new reality means everyone in the region—from public health officials to local residents—needs to understand the new risks, and act quickly to control the spread of the disease once cases emerge.
Dengue worldwide is on the rise
Dengue fever is common in places like Central America and Southeast Asia, where temperatures are warm year-round. The disease was largely controlled by the middle of the 20th century. But in recent years, the virus has surged in many parts of the world.
“We're seeing case numbers that we've never seen before,” says Sadie Ryan, a climate and infectious disease expert at the University of Florida.
More than 10 million cases were reported globally this year as of July. Overall, there has been a 10-fold increase in cases since the early 2000s, and as much as 50 times higher than in the 1960s. This year’s surge is particularly prominent in the Americas.
The spread has been driven in part by increasing rates of global travel, along with the dissolution of effective mosquito- and disease- management campaigns in heavily impacted regions like Central America, Ryan says.
An Aedes aegypti mosquito takes a blood meal. The mosquitoes don't eat a whole meal from one person, and often bite several people in quick succession, increasing the chances of spreading disease.Joao Paolo Burini/Getty Images
Climate change also plays a role. Like in California, mosquito season is expanding in many parts of the world, as the chillier temperatures that quelled their populations get milder. The mosquitoes' biting and breeding behavior also shifts with temperature — as does the transmissibility of the disease.
The global surge in cases means more opportunities for transmission, Ryan says, including to travelers from the United States. Once those travelers return home, the virus can spread locally – if they are bitten by one of the Aedes mosquitoes.
Over the past decade, dengue fever outbreaks have become more common in hot, humid corners of the U.S. Puerto Rico, Florida, south Texas, and Hawaii have all experienced an uptick in locally acquired cases.
But until last year, California was not included in that list.
Changing California, changing disease
In contrast with Aedes, a bite from California’s native mosquitoes, in the Culex genus, will not pass along dengue fever, though those mosquitoes can transmit other concerning diseases, like West Nile virus.
But the new mosquitoes have gained a foothold in the region since the early 2010s. They were likely introduced from the Southeastern United States, where they are also invasive but have had a presence since the 17th century.
“Since then, they've really spread and are now present all across [Los Angeles] County,” says Anais Molina Diaz, a spokesperson for the San Gabriel Valley Vector Control District.
California’s weather is warming because of human-caused climate change, lengthening the season in which mosquitoes can survive and breed. Now, the species can theoretically survive nearly as far north as the Oregon border, according to potential range maps from the Centers for Disease Control and Prevention.
The invasive mosquitoes have also exploited friendly conditions unrelated to the climate. They can breed in tiny pockets of standing water — in water pooling underneath a houseplant, in a trash can lid, or even in a bottle cap.
“They love it when we leave little bits of water around for them to breed in. This stuff that everybody leaves about — little bits of trash, even,” Ryan says. A single mosquito, she says, can lay 200 eggs in a bottle cap.
Potential mosquito habitat forms after every rain event, or when people water their gardens.
“We're doing this domestication of the mosquitoes,” Ryan says —providing them with friendly habitat and plenty of food so they can thrive.
Invasive mosquitoes also differ behaviorally from native mosquitoes.
“They’re more aggressive,” says Lara, from the Vector Control District. The native Culex mosquitoes will "bite to get its meal and then just leave,” he says, sating themselves with a single blood meal from a single person. But Aedes? They flit from person to person, snacking. Lara says it almost seems like “they just bite for fun.”
That aggressive behavior likely contributed to the outbreaks seen this autumn, Diaz says.
Small outbreaks ring alarm bells
In September, public health officials learned of a small cluster of locally-acquired dengue fever cases in Baldwin Park, a city east of Los Angeles. A few weeks later, another case popped up in Panorama City; then, a few more in El Monte; and then two more in Baldwin Park, for a total of 8 in Los Angeles County. Another case in the San Diego area was also recently reported.
This season’s cases are far from a major outbreak, says Kim Shriner, medical director of Infection Prevention and Control at Huntington Health.
“What we don't have here in Southern California is another pandemic of dengue,” she says. “What we have is a proliferation of Aedes mosquitoes.”
But, Shriner stresses, “there's probably more dengue out there than we know.”
Cases are likely missed, she says, either because they are asymptomatic or simply are not diagnosed at all by clinicians unused to looking for it.
Shriner and other experts say the key to avoiding a bigger outbreak is controlling the mosquitoes that spread it.
That’s the job of the local vector control agencies — taxpayer-funded agencies that work to control the spread of insect and other pest-borne diseases. Technicians like Lara canvassed backyards and public spaces near the patients’ homes this fall, tipping over containers filled with stagnant water and treating still water sources with selective insecticides to prevent the Aedes larvae from developing.
Public health officials also warn residents to take care to avoid mosquito bites by wearing bug spray, long sleeves, and pants.
Late in the season, the risk of spreading the disease is falling. Southern California winters still get cold enough to halt the spread of mosquitoes, limiting the disease’s persistence.
But as cold seasons continue to warm, and the mosquito population increases, public health officials like Aiman Halai are preparing for more cases in the future.
“It could become a significant problem in years to come,” she says.
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