With dengue cases at an extreme high, research points to climate change's role

It has been an exceptionally bad year for dengue fever: Nearly 12 million cases were recorded in the Americas through October, close to triple last year’s total of 4.6 million.

Research being presented at the annual meeting of the American Society of Tropical Medicine and Hygiene on Saturday quantifies the role that climate change has played in the trend, attributing nearly a fifth of the existing dengue burden to rising temperatures.

By 2050, the research predicts, climate change could be responsible for a 60% increase in the incidence of dengue if emissions continue at pace, with some places — like parts of Peru, Mexico, Bolivia and Brazil — seeing spikes of up to 200%.

Public health experts have long warned that global warming enables mosquito-borne diseases to spread to new places because it expands the geographic range where the insects that serve as vectors live and thrive. Mallory Harris, a co-author of the new research and a postdoctoral associate in the University of Maryland’s biology department, said her team’s findings build evidence for the significant role climate change has played in dengue’s spread. More broadly, she said, the research highlights the connection between greenhouse gas emissions and specific health consequences.

“Dengue is a growing health threat that can cause really severe consequences. It’s something that we need to be preparing for,” Harris said. “We should be expecting these sorts of large epidemics in the future and thinking about ways to then respond to them.”

In the United States and its territories, more than 7,200 dengue cases have been tallied so far this year — more than double last year’s total and the highest since 2013.

In June, the Centers for Disease Control and Prevention issued a health advisory warning of an increased risk of dengue infections. After two locally acquired cases were detected in the Florida Keys, the Monroe County Department of Health issued an alert of its own the next month. To date, 53 locally acquired cases — meaning they were not associated with travel to a country where dengue is prevalent — have been recorded in Florida, according to the CDC. California has recorded 15 such cases.

But the vast majority of U.S. cases this year have been in Puerto Rico, which declared a public health emergency in March amid an alarming rise in dengue infections. More than 4,500 locally acquired dengue cases have been reported in Puerto Rico, compared to less than 1,300 last year and even fewer the year before.

The new research, which has not yet been published or peer-reviewed, analyzed temperature records and incidence data on dengue across 21 countries in Asia and the Americas over an average of 11 years. The researchers compared the data to a simulation of what would have happened during that time period without human-caused climate change.

The resulting estimates are likely at the low end of the spectrum, according to a news release about the findings, because of a lack of dengue data from some areas, such as sub-Saharan Africa and South Asia. Plus, predictions for the southern U.S. are difficult to make, given that the virus has only recently emerged as a local threat.

The researchers found that the mosquitoes that carry dengue — called Aedes aegypti — transmit the virus most efficiently when temperatures are between 68 and 82 degrees Fahrenheit. They concluded that even if global greenhouse gas emissions are significantly reduced, the majority of the countries analyzed would still see climate-driven increases in dengue.

“The risks of this are going to increase regardless, so we need to be thinking about mitigation,” Harris said, suggesting a focus on vaccines and efforts to curb mosquito populations.

The maker of the only Food and Drug Administration-approved vaccine for dengue discontinued it earlier this year, according to the CDC.

More than half of people who contract dengue, a viral fever, are asymptomatic. Most of the rest experience mild cases, with symptoms that can include a fever, headache, joint pain, nausea and vomiting. Severe infections — about 2% of cases in U.S. territories from 2010 to 2020, according to the CDC — can cause bleeding under the skin, in the nose or in urine or stool, as well as a sudden drop in blood pressure or even death.

The disease is most common in places with humid, tropical climates across Latin America.

Derek Cummings, a professor of epidemiology at Johns Hopkins Bloomberg School of Public Health who was not involved in the new research, said it takes “something that we all knew — that the temperature is important — but quantifies how much and how important it is to the changes we’re seeing.”

Cummings, who has published his own research on dengue, added that on a global scale, he foresees a need to prioritize vaccinates, better manage mosquito populations and ensure hospitals and health care providers prepare for a rise in dengue cases.

Harris emphasized that the new research shows how climate change’s consequences affect various parts of the world differently.

“The greatest impacts of historic climate change on dengue burden were in places like Peru, Bolivia, Mexico, which are not necessarily the places that are the overall hottest. They’re not necessarily the places that are having the greatest increases in heat deaths,” she said. “There are effects that you might experience in places that are marginally cooler that also need to be taken into account.”