The number of dengue cases in the Americas has surpassed three million this year, as climate change makes people more vulnerable to the disease and the world more hospitable to the mosquitoes that carry it. 

Rising temperatures and shifting rainfall patterns are providing a boon to the Aedis aegypti mosquito, the main carrier of dengue. Warmer year-round temperatures are allowing the mosquitoes to thrive for longer periods of the year, and extending their mating season, allowing them to reproduce in greater numbers. 

Climate change is also extending the geographical range where the mosquito can survive, as warmer winters and milder autumns lead to fewer cold-weather deaths, meaning that more mosquitoes survive to adulthood.

“The mosquito in particular is a vector that has continually spread across the world,” Dr Raman Velayudhan, an expert at the WHO’s neglected tropical disease unit, told reporters on Wednesday. “It is a silent expansion … and right now a population of nearly 4 billion are at risk of this disease.”

The effects of global warming on the spread of Aedis mosquitoes are most visible in Brazil, which has already reported 2.4 million cases this year as mosquitoes invade its southern states, which are now hot enough to support them.

The last time #dengue cases soared like this in Latin America it presaged the emergence of #Zika — another deadly mosquito-spread virus.
Halfway through the year, it's already eclipsed the rates in 2015-16 and is likely to surpass the record rate in 2019. pic.twitter.com/ZuvuuagcdZ

— Laurie Garrett (@Laurie_Garrett) July 31, 2023

The incidence of dengue in Peru, Argentina and Bolivia has also risen sharply as a result of climate conditions. Peru has already recorded a caseload over four times higher than the average of the last five years – the country’s worst-ever outbreak of the disease. The outbreak in Argentina is also one of the largest in the country’s history. 

The World Health Organization (WHO) warned in July that the world is on track for over four million dengue cases in 2023, and that climate change is playing a significant role in its spread. The record for global dengue cases was set in 2019 when 5.3 million cases were reported. Global case numbers have risen eightfold since 2000, with 4.2 million reported in 2022.

Global dengue case numbers are vast underestimations, Velayudhan said, citing the 80% asymptomatic case rate and pandemic-disrupted reporting systems.

“We need to convey this message that dengue is a silent disease,” said Velayudhan. “We need to treat it as an endemic disease with epidemic potential.” 

Dengue can cause a range of symptoms, from mild fever to severe illness. There is no specific treatment for dengue, but early diagnosis and supportive care can help to reduce the risk of death. It is fatal in less than 1% of cases. 

A window into the future of dengue

Computational epidemiologists at the University of Michigan have projected that Brazil’s epidemic potential for dengue could increase by 10-20% by 2040, as temperatures rise and rainy seasons get longer. This could lead to longer dengue seasons in Brazil, and could also be mirrored in other countries around the world.

The ongoing El Niño event, which is characterized by higher temperatures and extreme weather patterns, may provide a window into the future of dengue. Its effects on rainfall in the Americas and during monsoon season in Asia will likely play a large role in how close the world gets to the 5.3 million case record set in 2019.

In Bangladesh, monsoon rains have already led to a deadly dengue outbreak. Health experts in the country, including the Bangladesh Medical Association, have urged the government to declare a “public health emergency.” Some have compared the outbreak to the country’s 2019 dengue epidemic, which was so severe that many in Bangladesh refer to it as the “year of dengue.”

In July, meanwhile, changing climate patterns led the European Center for Disease Control to sound the alarm over the rising risk of mosquito-borne diseases across the continent, from which Europe has historically been protected.

“If this continues, we can expect to see more cases and possibly deaths from diseases such as dengue, chikungunya and West Nile fever,” Andrea Ammon, director of ECDC said at a press conference last month.

And the risks are not limited to dengue: Research shows the spread of over half of infectious diseases — including arboviruses like Zika, chikungunya and dengue — are aggravated by climate change.

Disease-carrying ticks, bacteria, algae and fungi and mosquitoes are all on the move. As the world warms, the diseases they carry will follow them as they expand their geographical reach to adapt to climate change.

Image Credits: Patrick de Noirmont/Sanofi Pasteur, Sanofi Pasteur/Flickr.