Nearly one fifth of dengue cases in Latin America and the Caribbean, or about 45 million infections a year, are attributable to climate change, in the past decade, according to a new study by researchers at Harvard and Stanford Universities. 

Rising temperatures combined with mosquito species uniquely suited to sprawling urbanization and deforestation are fueling the staggering increase in dengue cases, with a proportion of cases measurably attributable to climate change, according to the first ever study to “meaningfully” quantify that number. 

Over the past year, both Latin America and the Caribbean saw a record-breaking number of cases – and fatalities – of the so-called “break-bone” fever. Even with the recent regulatory approval of two new vaccines, Takeda Pharma’s Qdenga® and Sanofi’s Dengvaxia, slow rollout and continued challenges in production scale-up have meant the jabs made little impact so far. 

The researchers compiled data from 21 countries in the Americas and in Asia to identify the causal effect of climate change-related temperature increases and dengue cases. The study was presented at the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) 13-17 November, and is available in pre-print form.

The researchers also estimated how different climate change scenarios will impact dengue burden. By 2050 under a high CO2 emissions scenario, the authors predicted  a 61% increase in dengue incidence. Active climate mitigation would cut that figure to roughly 40%. 

At least 257 million people now live in countries where warming temperatures could cause dengue to double by mid-century in either scenario. 

“We can see the direct policy implications of reducing emissions at the global scale,” Dr Kelsey Lyberger, an ecologist at Arizona State University and one of the study authors, told Health Policy Watch. The magnitude of dengue cases will not increase equally, either, Lyberger noted. “Some of the highest relative increases are in places where they don’t necessarily have a high burden currently.”

But already now, increasing temperatures and extreme weather events are already changing the burden of infectious diseases – particularly vector-borne diseases. These temperature increases are already responsible for 45 million excess dengue cases, which the studied identified.  

The study, whose release coincided with the UN Climate Conference (COP29) in Baku, provides further evidence that climate action is “health action,” as termed in a recent WHO COP29 special report.

Cooler regions will see largest change

Dengue responds non-linearly to average daily temperature, with cases peaking at average temperatures of 28℃, and then decreasing. This creates “optimal” conditions for the two primary dengue mosquito vectors, Aedes aegypti and Aedes albopictus, where both species breed and feed faster. But in places where average temperatures are cooler – 15-20℃ on average – climate-change related warming has the largest effect on dengue caseload. 

As a result, projected warming will fuel dengue cases unequally across the globe. “It’s not across the board that we see this increase,” said Lyberger.  

Some cooler regions of Mexico, Peru, Bolivia, and Brazil, for instance, are predicted to see over 150% increases in dengue incidence due to climate change under any emission scenario, the study notes. 

Lack of surveillance data excludes Africa, India, Western Pacific from analysis

While dengue cases continue to rise globally, including in much of the African continent, the lack of high quality surveillance systems means experts are left to guess the impact of climate change on dengue in these regions. 

Countries in the America, Brazil in particular, have felt the burden of dengue cases most acutely, but they often have surveillance methods in place to track infection rate fluctuations.

“We needed a long enough time series of cases to estimate a baseline level of dengue,” said Lyberger. Many African countries face surveillance hurdles in detection, reporting and management, the lack of local laboratory capacity, and the misclassification of cases, noted researchers at Africa CDC. Without a solid surveillance system, countries are at a critical disadvantage–including little early warning systems for dengue surges.

In India, the country’s “poor” surveillance network leads to “huge” under-reporting and a lack of public health response to outbreaks, as a Lancet editorial comments in India’s 2015 dengue season. A 2020 assessment of the country’s surveillance efforts found that dengue surveillance needs to be strengthened, and integrated into existing government initiatives. In that time frame alone, from 2015-2019, cases in the South Asian region increased by 46% according to WHO estimates. The organization attributes climate change, in addition to “high rates of population growth, inadequate water supply and poor waste management systems” coupled with the absence of effective treatment and suitable vector control to the high burden of dengue in the region. 

Preparing for the future

With these new estimates of how climate change will increase dengue cases, researchers hope that such high-burden countries will be motivated to do more tracking and reporting, while those that historically have had few cases can prepare with funding for surveillance and integrated vector management. 

“Since we’ve established this relationship between temperature and cases, the next step is to see how warming, both in the past and in the future, is going to affect the number of cases that we have or will see,” explained Lyberger.

This led researchers to compile data from 21 countries where dengue is endemic – including Brazil, Cambodia, Colombia, and Vietnam – and to examine other factors that could affect dengue infection rates like rainfall, seasonal changes, viral strains, economic shocks, and population density, to isolate the distinct effect of temperature. 

“We were able to pin down that historical warming has already increased dengue cases by about 20%,” said Lyberger. 

“It’s evidence that climate change already has become a significant threat to human health and, for dengue in particular, our data suggests the impact could get much worse,” said Erin Mordecai, PhD, an infectious disease ecologist at Stanford’s Woods Institute for the Environment and the study’s senior author in a press release. 

See Health Policy Watch’s ongoing coverage of dengue here.

Image Credits: Marissa L. Childs, Kelsey Lyberger, Mallory Harris, Marshall Burke, Erin A. Mordecai, WHO.