Dengue fever spiked to record levels in 2024: Climate change will make it even worse

Dengue fever, a potentially fatal virus spread by mosquitoes, is sweeping across the Americas, breaking records with a skyrocketing rate of infections.

Cases have spiked in large part due to increasing global temperatures wrought by greenhouse gas emissions, new research shows.

Nearly a fifth of dengue infections in the Americas and Southeast Asia were propelled by climate change, according to a study that researchers from the University of Maryland, Harvard University and Stanford University presented Saturday at the annual American Society for Tropical Medicine and Hygiene meeting in New Orleans.

The latest data from the Pan-American Health Organization shows that in the first 10-plus months of 2024, there were nearly 7,500 deaths and more than 12.3 million infections – three times the number of cases in 2023, which was record-setting at the time.

“Dengue is really having its biggest year in history,” said Dr. Gabriela Paz-Bailey, chief of the U.S. Centers for Disease Control and Prevention’s dengue branch, based in Puerto Rico. “Around the world, dengue cases have been rising at an alarming rate.”

The sharp uptick has caused concern in the U.S., especially in Puerto Rico, where officials estimate up to 50% of infections have resulted in hospitalization, Paz-Bailey said. The U.S. has recorded nearly 7,300 infections this year, compared to 1,462 in 2023. Most of these cases were in Puerto Rico, which declared a public health emergency this spring and has remained under emergency status.

The continental U.S. hasn’t had a high rate of infections compared with the early 2010s. But officials worry about projections in a warming climate.

Most people who get dengue don’t show symptoms, but those who do sometimes develop a high fever, body aches, nausea and rashes. Some infections result in hospitalization or death.

In severe cases – usually about 1 in 20 infected individuals, people bleed internally or from their nose or mouth and go into shock. Infants, elderly people and pregnant women are most at risk for serious forms of disease.

There are four dengue viruses. A person infected with one of them could have limited immunity to some of these viruses. Populations are sometimes exposed to various strains, reigniting outbreaks. Repeated infections can trigger severe dengue.

Climate change already responsible for record dengue increases

The new study by Maryland, Harvard and Stanford didn’t include the benchmark 2024 spike. But it found that climate change had propelled dengue’s spread as temperatures increased.

Climate change caused 19% of dengue infections across the Americas and parts of Southeast Asia, the study found. The infection rates were significantly higher in regions previously thought too cool to support the Aedes egypti mosquito, which can carry dengue and other diseases.

By 2050, climate change could cause cases to increase by 40% to 57% from today’s levels. In cooler urban areas in Bolivia, Peru, Mexico, Brazil and Colombia where the climate is expected to warm temperatures, researchers determined dengue would affect upwards of 257 million people.

“This suggests that we need to be thinking about pandemic preparedness, especially in those parts of the world that are vulnerable to increases in dengue driven by climate change,” said Mallory Jessica Harris, a study co-author who is a postdoctoral researcher at the University of Maryland Institute for Health Computing. “Even in the past few years, we’ve seen these really large dengue epidemics repeatedly.”

Researchers also shared future projections, based on analyses of 21 countries using an average of 11 years of records for each. The study did not include sub-Saharan Africa or South Asia, two areas also affected by dengue transmission. It also did not include data from the continental U.S.

The study found that temperatures around 82 degrees Fahrenheit are best suited to the mosquitos that can carry dengue. Some regions are projected to warm beyond that range, making it harder for infected mosquitos, or other creatures, to survive. Unfortunately, the research shows that large swaths of the Americas will, instead, reach optimal temperatures in the coming decades. Infections could rise 150 to 200% in previously cooler areas where temperatures are expected to warm to temperatures ideal for dengue transmission.

This includes several cities in the Americas, such as Lima, Peru, which has had unprecedented increases in dengue infections in 2024. This year the U.S. State Department issued a health alert about dengue transmission in Lima. Cases tend to rise during the summer months in the Southern Hemisphere when it is winter in North America.

During warmer winters in the southern region, infected mosquitoes can endure through seasons that typically would kill their populations.

Urbanization, poverty also factors in rise of dengue

Climate change isn’t the only factor that explains the continued rise in dengue, said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

Urbanization, deforestation and poverty also contribute to creating environments suitable for infected mosquitoes, Hotez said. This phenomenon could affect the U.S., including fast-growing areas across the Sun Belt.

“We shouldn’t discount the likelihood that arboviruses, especially dengue, could become a regular occurrence in the southern part of the U.S.,” Hotez said, citing Texas, Florida and other states along the Gulf Coast.

People living in poverty often lack air conditioning and window or door screens to cool people indoors and prevent mosquitos from entering. They also lack efficient sanitation to remove standing water where mosquitoes can grow and infect people nearby.

Paz-Bailey, of the CDC, said the return of travel after the COVID-19 pandemic also contributed to a record increase in dengue transmissions in the U.S. But that could worsen in the future. After an infected person enters the U.S., a local mosquito could bite that person, contract the virus, and then spread it to other people nearby.

Warming conditions in much of the U.S. would then sustain a continued local spread, which has already happened in California and Florida.

Extreme weather, such as hurricanes, can help sustain mosquitoes’ habitats. A storm’s wake allows standing water to form, creating thriving environments for mosquitoes.

Vaccines lacking, but there’s promise in modifying mosquitoes

There are steps we can take to reduce transmission. One that the study highlights is reducing greenhouse gas emissions that drive up temperatures.

Vaccines can also help, but none are readily available. The U.S. has limited access, and the last remaining doses of the vaccine Dengvaxia, approved by the U.S. Food and Drug Administration in 2019, are expected to expire in 2026. Sanofi, the company that makes it, plans to discontinue vaccine production in 2025.

Greater detection of disease-carrying mosquitoes can help prevent outbreaks before they occur. However, reducing the environments where mosquitoes reproduce is especially tricky, since they need as little as a bottle cap of standing water to hatch their eggs.

Researchers have, instead, looked to engineer mosquitoes to prevent the spread of dengue. Infecting mosquitoes with Wolbachia bacteria has dramatically reduced dengue, according to a study recently shared by researchers from the World Mosquito Program on results from the Brazilian city of Niterói, near Rio de Janeiro. But researchers acknowledged that prevention programs like this take years to be effective at sustaining safer mosquito populations.

Kristie Ebi, a professor at the University of Washington’s Center for Health and the Global Environment, said historical context is useful for helping us prepare for current and projected spread. The CDC was founded nearly 80 years ago to prevent mosquito-borne diseases and the agency proved successful at addressing malaria and yellow fever.

Increased preparation is imperative, Ebi said, because, “Climate change is causing people to suffer and die today.”

People can prevent the illness by protecting against mosquito bites. This includes covering your arms and legs with protective, loose-fitting clothing and spraying yourself with insect repellant.

This article originally appeared on USA TODAY: Climate change is triggering a record number of dengue fever cases

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