Dengue fever is surging worldwide. A hotter planet will make it worse.

The sharp-eyed doctor suspected dengue, a disease that is often missed but is now exploding around the world.

The girl, Genesis Polanco Marte, is among a record 10 million people who have fallen ill with dengue so far this year — an unprecedented surge that scientists say is fueled in part by climate change. Soaring global temperatures have accelerated the life cycles and expanded the ranges of the mosquitoes that carry dengue, helping spread the virus to roughly one in every 800 people on the planet in the past six months alone. An influx of patients has overwhelmed hospitals from Brazil to Bangladesh, recalling the worst days of the coronavirus pandemic. Puerto Rico declared a public health emergency this spring, with more dengue cases reported in the first five months of 2024 than all of last year. Public health officials are bracing for the virus to crop up in more temperate regions, including the southernmost portions of the United States.

“The storm’s comin’, folks,” Grayson Brown, executive director of the nonprofit Puerto Rico Vector Control Unit, advised a group of California officials in a recent webinar. “It’s here in Puerto Rico, but you guys are going to feel it pretty soon.”

Last week, the Centers for Disease Control and Prevention warned of an increased risk of dengue infections in the United States, urging clinicians to stay on alert for the disease when treating feverish patients who have traveled to places with dengue transmission.

But even as human-made warming spurs cases to historic highs, dengue remains one of the world’s most neglected tropical diseases, according to the World Health Organization. Three out of four cases are mild or asymptomatic, making the illness difficult to track. And because the virus comes in four varieties, or serotypes, natural immunity after one illness does not protect against future infections with other types. What makes dengue unusual is that the risk of severe complications may actually increase with sequential infections of a different type.

There is no cure for the virus, which in severe cases can lead to plasma leaking from veins, internal bleeding, organ failure and, in rare instances, death. Unlike other illnesses, vaccination is complicated. Few options are available, and few people know about them. The only vaccine available in the United States is for children 9 to 16 years old who have already been infected with dengue — those most vulnerable to hospitalization. But it won’t be available after 2026.

The crisis in Puerto Rico is a warning sign for the rest of the United States. It shows how quickly an outbreak can metastasize in communities with fragile infrastructure, underfunded health systems and temperatures that get hotter with each passing year.

Without drastic action to control the virus and slow climate change, research suggests some 2 billion additional people across the globe could be at risk for dengue in the next 50 years.

Rising temperatures spur global dengue spread

It has been more than a decade since Puerto Rico saw its last dengue outbreak. Though the virus is endemic in the territory and typically recurs every five to seven years, that cycle was interrupted by the emergence of Zika — a closely related virus that tore through the island in 2016 and gave some cross protection against dengue — and the isolation measures necessitated by the coronavirus.

But the return of global travel — especially Caribbean cruises — brought thousands of tourists who had been exposed to dengue elsewhere, introducing strains that hadn’t been dominant in Puerto Rico. The virus spread swiftly through the population of susceptible people, reaching Genesis in late May.

The girl had been feverish for several days before she arrived at the hospital. Her doctor, Zurisadai Rivera Acosta, pressed on the girl’s fingertip and saw it took longer than normal for the color beneath to return to pink — a sign of dehydration. More concerning, the doctor noted, she had begun vomiting and her count of blood platelets was low. Rivera admitted the girl to the hospital amid signs her condition was deteriorating. Genesis was one of 91 dengue cases reported in Puerto Rico that week, health department data show.

Puerto Rico public health officials are bracing for case counts to soar as the island heads into the hot and rainy season. By mid-June, the territory had reported more than 1,500 cases. At least two people have died.

Sweltering and stormy is the preferred weather for Aedes aegypti, the mosquito that is the main vector for dengue in the Americas. It lays its eggs wherever there is standing water: in vent pipes of septic tanks, water meters, discarded tires and broken flower pots. A single bottle cap filled with rainwater can hold more than 100 eggs, said Sadie Ryan, a medical geographer at the University of Florida who specializes in insect-borne diseases.

“They’re tenacious, they’re pernicious,” Ryan said. “Really, they’re just good at being everywhere.”

Its eating habits further bolster the bug’s ability to wreak havoc. Unlike the mosquitoes that transmit malaria, which require only a single blood meal before laying their eggs, female Aedes aegypti are “sippers,” Ryan said. They behave like tiny vampires at a human buffet, flitting from person to person, potentially spreading disease with each bite.

In Puerto Rico’s crowded urban areas, most families cannot afford air conditioning so they keep cool by opening windows and doors, which lack screens to keep mosquitoes out.

Meanwhile, human-caused warming is spawning an explosion of mosquitoes here. Greenhouse gas emissions, mostly from burning fossil fuels, have raised average temperatures in the commonwealth by about 2 degrees Fahrenheit since 1950, according to the National Centers for Climate Information.

The change has been a boon to Aedes aegypti, which is able to transmit diseases at higher temperatures than other mosquito species. In laboratory experiments, researchers have found that warmer conditions can make the insect grow faster, bite more people and lay more eggs. Heat also makes the dengue virus more infectious and allows it to replicate faster inside its hosts.

Models and real-world data show that these mosquitoes can transmit dengue at temperatures ranging from 64 to 94.1 degrees Fahrenheit — conditions that are found in Puerto Rico every month of the year.

Though this species is found in several states, including Texas, Florida and even California, the mosquito’s predilection for heat has historically limited dengue’s reach. Even when the virus hitches a ride via travelers from tropical regions, low nighttime and winter temperatures prevent it from spreading very far.

But officials are increasingly concerned that rising temperatures could set the stage for more outbreaks in the United States. Florida has already reported eight cases from local spread this year, health department data show — and the state’s warmest months are yet to come.

“Even one case in an area that doesn’t usually see dengue can consume a large number of resources, as well as create considerable public concern,” said Gabriela Paz-Bailey, chief of the CDC’s dengue branch in Puerto Rico. “It means the mosquito has acquired the virus, and you have the potential for additional transmission happening.”

In tropical regions across Latin America, Africa and Asia, where dengue once circulated primarily during summer months, a lengthening warm season is turning the disease into a year-round phenomenon. Meanwhile, the shifting climate is allowing dengue to infiltrate temperate regions and high-altitude communities where it has never been found before.

Nepal, which hadn’t seen a dengue case before 2004, recorded more than 50,000 cases in each of the last two years. Mauritius and Chad have experienced their first-ever significant outbreaks in the past 12 months. Meanwhile, Italy, France and Spain documented dozens of instances of local transmission of the virus in 2023 — suggesting the disease may be gaining a foothold in spots where winter cold once kept it at bay.

But it’s not just rising temperatures that contribute to disease spread, researchers say. Climate-induced droughts can prompt people to stockpile water, creating more mosquito habitat. Escalating hurricanes and floods also produce standing water while simultaneously forcing people from their homes and increasing their exposure to mosquitoes, said Mallory Harris, a disease ecologist at Stanford University.

By combining climate models with simulations of disease spread, Harris is developing techniques that can help link cases to climate disasters — and project how future storms and droughts could trigger new outbreaks. In an analysis of Cyclone Yaku, which ravaged Peru’s northern coast in March 2023, she found that the storm was responsible for 33,000 dengue cases. Nearly 400 people died of the virus.

Only U.S. dengue vaccine runs out in 2026

The fact that the dengue virus comes in multiple serotypes and has an unusual mechanism for causing severe illness in people makes it especially tricky to fight. An infection with one type generates disease-fighting antibodies that protect a person from future infection with that variety. But those same antibodies can bind to viruses of a different serotype, facilitating their entry and causing more severe illness.

Dengvaxia, developed by the French-based manufacturer Sanofi, is the only vaccine approved for use in the United States. It protects against all four dengue types and is approved for children 9 to 16 years old living in high-risk areas such as Puerto Rico. The shots are covered by most health insurance plans. But the three-dose regimen — administered six months apart over the course of a year to be fully protected — requires patients to have a laboratory-confirmed previous dengue infection. It’s the only vaccine with such a requirement, complicating rollout efforts in vulnerable communities.

In May, WHO expanded the use of a second vaccine, Qdenga, which is already approved in several hard-hit countries, including Argentina, Brazil, Colombia and Indonesia and throughout Europe. The vaccine, developed by the Japanese company Takeda and is recommended by WHO for children ages 6 to 16, requires only two shots and can be used regardless of prior infection. But the company withdrew its application from the Food and Drug Administration in July 2023 because of data collection issues.

A third vaccine being developed by the National Institutes of Health is still in clinical trials and won’t be available in the United States for at least a few years.

Meanwhile, health-care workers in Puerto Rico lament that few residents know about Dengvaxia.

At HealthProMed, Hector Villanueva, the community clinic’s senior adviser for dengue, urged Mayra Rivera to vaccinate her teenage nephews, who Rivera took in this year after their parents died. The boys had been hospitalized with fever, vomiting and diarrhea from the virus in January. Villanueva warned they could become even more severely ill if they were to be infected again.

Rivera eagerly signed them up for shots. The 13-year-old, whose diabetes can make dengue more lethal, received his first dose in April. His older brother is scheduled to receive his shot in July.

But uptake among other children in Puerto Rico has been slow. Many parents aren’t aware of dengue’s dangers and after the pandemic, are tired of hearing about getting more vaccines, Villanueva said.

“Most of the cases, they didn’t know they have dengue or they may have mild to moderate symptoms, so there is low perception of risk,” Villanueva said. “Parents are saying, ‘What are you talking about? Dengue, does that still exist?’”

Only 145 children in Puerto Rico have started the vaccine series since it became available in 2022, according to CDC — a tiny fraction of the roughly 140,000 eligible.

And now access to the vaccine is closing. A few months before Puerto Rico declared its public health emergency in March, Sanofi informed U.S. officials that it has stopped producing Dengvaxia because of low demand. The last doses will expire in August 2026.

Adam Gluck, who leads Sanofi’s U.S. corporate affairs, said the company tried making the vaccine easy to access but the complexity of screening for a prior infection before administering the required three doses kept demand low. The decision to discontinue the vaccine “is not driven by quality, safety or efficacy concerns,” he said in a statement.

Rivera said she is grateful her nephews qualify to receive the shots but is dismayed other children will no longer have the chance to protect themselves against dengue. “If they stop making these vaccines,” she said, “a lot of people will die.”

Combating dengue in Puerto Rico

On a recent steamy morning, a mosquito-control technician from Puerto Rico’s Vector Control Unit peered into a trap outside a home, a plastic bucket filled with water and hay whose odor was designed to attract egg-laying females. Sure enough, when he opened the trap, a mosquito with white markings was stuck on the special adhesive paper.

With schools out for the summer, another group of technicians went from classroom to classroom at a nearby elementary school, trapping mosquitoes to identify locations that could have been super spreaders. Workers thrust vacuum-like machines along walls, in corners, under piles of papers to flush out the insects, then caught their quarry in butterfly nets.

Unlike many mosquitoes, Aedes aegypti tend to bite during the day and are resistant to the most commonly used insecticide. So officials must focus on identifying and destroying mosquito habitats in high-transmission areas to reduce spread.

Teams rely on low-tech traps placed outside homes to collect mosquitoes, then test them to determine what percentage carry the virus. In areas with high rates of dengue-carrying mosquitoes, field teams apply larvicide and go door-to-door urging residents to use repellent and get rid of breeding grounds, authorities said.

The ongoing explosion of cases presages a future in which dengue becomes one of the dominant mosquito-borne threats to humanity, experts said, in some countries even eclipsing malaria. As temperatures in tropical regions get too hot for other mosquito species, Aedes aegypti is poised to take over.

Singapore, Brazil and Colombia have programs to infect mosquitoes with a bacteria called Wolbachia, which blocks offspring released into the wild from transmitting the dengue virus. But that expensive and labor-intensive strategy has not been approved in the United States.

In Puerto Rico, one big challenge remains awareness among clinicians, who seldom treat the disease. The CDC and Puerto Rico health department are training doctors to monitor for warning signs of severe dengue, including abdominal pain, persistent vomiting and bleeding from the gums or nose. Unlike other diseases, where fever reduction is often a sign someone is getting better, the reverse is true for dengue.

Rivera, the emergency room pediatrician who recently treated 9-year-old Genesis, said she recognized the dengue symptoms in the young patient only because her own aunt and cousin had contracted dengue during the coronavirus pandemic. When Rivera rushed them to a hospital, doctors insisted the two had covid, not dengue. Her aunt almost died, Rivera said.

“There’s no rapid test for dengue,” Rivera said. “We have to diagnose it clinically.”

Genesis received intravenous fluids in the hospital, and her platelet count gradually trended up. Three days later, she was allowed to go home. Despite her recovery, the girl remains vulnerable to a second infection.

The mosquitoes are out here, waiting to bite.

Laura M. Quintero contributed to this report.