As temperatures rise, dengue fever infections keep surging around the world

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In Bangladesh, roughly 300,000 people have been infected with dengue this year during the country's worst-ever outbreak of the mosquito-transmitted disease. By mid-November, the death toll hit close to 1,500, as hospitals in the densely populated South Asian country struggled to cope with the surge in patients.

Neighbouring India is also experiencing more and more outbreaks, along with Sri Lanka to the south, where 60,000 cases of dengue have been reported just this year. In Mexico, cases rose more than 330 per cent in 2023 compared to 2022, and Argentina, Bolivia, Brazil, and Peru are also reporting high rates of infections.

The disease — known as "breakbone fever," due to the severe muscle and joint pains it can cause — is also appearing far beyond its usual range in tropical and subtropical climates.

Dozens of dengue cases not tied to travel abroad have been reported across several European countries, including Italy, France, and Spain. Chad, a landlocked country at the crossroads of North and Central Africa, experienced its first known outbreak this year. Meanwhile several U.S. states announced locally acquired cases in recent months, including the country's first known infections in California.

The explosive spread of dengue, through the mosquitos known for carrying the virus, offers a case study in how climate change, human movement, and rising temperatures are all coaligning to fuel the expansion of potentially deadly threats to human health. And, scientists warn, even countries like Canada that have avoided dengue's wrath could experience local transmission of the virus in the decades ahead.

"The frequency of outbreaks is ever increasing," Himmat Singh, a scientist at the National Institute of Malaria Research in New Delhi, told the British Medical Journal. "Mosquitoes are evolving as humans have pushed them to adapt."

While eye-catching climate impacts such as extreme weather events and heat waves will be front and centre at the first dedicated Health Day being held on Sunday at COP28, the World Health Organization (WHO) recently warned our changing climate is also "catalyzing a surge in infectious diseases like dengue" and is calling for health-focused climate action from global governments.

The organization's chief scientist, Dr. Jeremy Farrar, told Reuters in October that he expects dengue will become a major threat in the southern U.S, southern Europe, and new parts of Africa this decade — as warmer temperatures create the conditions for the mosquitoes carrying the infection to spread.

Dengue virus is transmitted by the Aedes aegypti mosquito, a tropical species which also spreads the viruses behind diseases such as Zika, chikungunya, and yellow fever. 

"They harbour a lot of these nasty viruses," said virologist Stephen Barr, an associate professor in Western University's department of microbiology and immunology. "What researchers know is that the traits these mosquitoes have, that are favourable for spreading these viruses, occur in the range of temperatures from about 24 to 29 C."

The species can survive year-round when temperatures are warm enough, and females lay their eggs in areas of shallow, stagnant water, which can mean spaces as small as household containers, potted plants, or even a bottle cap. 

"Once the mosquito habitat is established, it only takes one or two people to bring the virus into that habitat for the mosquito to [spread it]," said Dr. Amila Heendeniya, a clinical infectious diseases physician at the Winnipeg Regional Health Authority and an assistant professor at the University of Manitoba. 

Researchers say warming temperatures and shifts in rainfall patterns due to climate change are creating ideal conditions for these mosquitoes to breed, particularly in areas such as Bangladesh where monsoon-level rain is being reported earlier in the season.