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, andPeru 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."

WHO scientists ringalarms

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 professorin 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.

In Pakistan, there has been an ongoing dengue epidemic since 2011, said Dr. Imran Hassan Khan, chair of the country's Dengue Expert Advisory Group. The mosquitoes appear to be adapting to a longer rainy season, and now likely live throughout homes where they can infect people with this virus at any time of day, he explained.

"We're unable to eradicate it," he added. "It's impossible to eradicate it."

Rates of the disease have risen eight-fold around the world in the last two decades, WHO figures suggest. Scientists also suspect far more cases are going unreported, given the wide range of potential symptoms, from internal bleeding, organ failure and death on the severe end, all the way to mild ailments or even no symptoms at all.

WATCH | Insect-borne infections on the rise in Canada:

Insect-borne infections on the rise thanks to climate change

1 year ago

Duration 1:56

Diseases carried by insects that infect humans, like Lyme disease and West Nile virus, are on the rise in Canada. The shorter, less severe winters due to climate change have allowed those insects to expand their range.

2nd infections can be worse than the 1st

One of the most alarming aspects of dengue's rapid rise is that one exposure to the virus doesn't protect you from infection with a different serotype and can actually mean your second round is worse.

There are four distinct serotypes of dengue, explainedThais dos Santos, advisor for surveillance and control of arboviral diseases at the Pan American Health Organization (PAHO).

"In terms of immunity, they act as four distinct viruses," she said, adding that one dengue infection is thought to provide life-long immunity to that specific serotype, but not the others. "Once you get that secondary infection it has been well-documented that you're more likely to have severe symptoms."

The mechanism at play is known as "antibody-dependent enhancement," in which the antibodies against dengue produced by someone's immune system fall to a low range. Research suggests that leads to a domino effect where the few antibodies still left are able to bind to the virus, pull it into cells, and give it space to replicate but their level remains too low to actually kill those invaders.

"Your body starts fighting it, but not really properly," explained Heendeniya, who said that haywire immune response leads to an increased risk of internal bleeding and hemorrhagic fever.

Why that reaction happens with dengue to a degree not seen in many other infections is still not fully understood, but researchers note what's clear is that it complicates the use of vaccines. Two commercially available dengue vaccines are being used by various countries, but most global guidance recommends only vaccinating children in high-risk areas who have had a prior confirmed infection, given there's also a risk for severe disease if someone catches dengue after immunization.

"I think we're still trying to understand a lot of those mechanisms, and everyone's immune system is going to be different there's so much we still don't know," said Barr.

Half of world's population now at risk

Questions about how dengue operates are now more pressing as climate change and widespread human migration are expected to fuel its continued spread, with potentially dire consequences. Already, roughly half of the world's population is now at risk, with an estimated 100 to 400 million infections occurring every year, the WHO has said.

"For the longest time, dengue was considered an 'emerging' infectious disease," Heendeniya said. "I would say it has already emerged."

Modelling studies also project wider expansion of the virus within specific countries. One paper in the Lancet Planetary Health, for instance, projected a higher risk of dengue through much of mainland China by the year 2100.

As for Canada, the dengue virus isn't found in mosquitos here at least not yet. But Barr warns that could change. Local transmission in countries such as France and Croatia was only reported for the first time in 2010, WHO data shows.

Canada's historically colder climate hasn't provided the conditions for these mosquitoes to thrive year-round, Barr said. However, an adult Aedes aegypti was found in a trap in Ontario for the first time in 2017, a discovery health officials believe signaled the species is now becoming established.

Rising temperatures, Barr warned, could make this country more hospitable, and if more of these mosquitoes hitch a ride here, locally-acquired dengue infections in the years ahead could become a real possibility.

"Dengue is always a plane ride away," he said.