This story was originally published by Grist and appears here as part of the Climate Desk collaboration.

This year, nearly 100,000 people in Bangladesh have contracted dengue fever, a mosquito-borne viral disease common in tropical and subtropical regions of the world. The number of infected patients is overwhelming the fragile hospital system there. More than 450 people have died so far, the deadliest dengue outbreak in the nation of approximately 170 million since record-keeping began in 2000. Sri Lanka, nearby, is also experiencing a sharp spike — more than 40,000 cases of dengue this year alone.

Similar dengue-driven crises are unfolding in other parts of the globe. The Americas are in a “public health emergency,” according to the World Health Organization, or WHO: Peru experienced its largest dengue outbreak ever this summer; Brazil, Bolivia, and Argentina are also reporting alarmingly high numbers of cases.

In the United States, five cases of locally acquired dengue have been reported in Florida this month alone, prompting local health officials to put Miami-Dade and Broward counties on alert. The state has reported a total of 11 cases of locally transmitted dengue so far in 2023.

Workers from the Florida Keys’ mosquito control department load a drone to spread BTI larvicide in an effort to eradicate dengue-carrying mosquitoes on July 8, 2020, in Key Largo. Photo by Getty Images/Grist

These outbreaks are concerning, but they’re not particularly surprising to experts who have been tracking dengue for the past several decades. Cases of dengue — which can cause fever, rashes, vomiting, and, in severe instances, internal bleeding, organ failure, and death — have been rising for years.

Since the beginning of the century, global cases of the disease, carried by the Aedes genus of mosquitoes, have skyrocketed, from roughly 500,000 in 2000 to more than five million in 2019. In the first seven months of 2023, worldwide cases spiked to more than three million, and over 1,500 deaths have been reported — numbers that are expected to rise as the summer continues.

There are likely hundreds of millions more unreported incidents each year, as dengue produces mild or no symptoms in most people. But as more people get infected, the percentage who end up developing the severe form of the disease will increase, too. Experts say a tangled web of factors is driving the surge, but one culprit stands out: climate change.

Dengue patients, protected under mosquito nets, receive treatment at the Dengue Corner of Sylhet MAG Osmani Medical College & Hospital in Bangladesh. Photo by Getty Images/Grist

In the 1970s, global cases of dengue fever, or break-bone fever as it’s also commonly known, were low. Dengue had been more prevalent 20 years prior, but an aggressive campaign to eradicate Aedes aegypti mosquitoes using the now-banned insecticide dichlorodiphenyltrichloroethane, or DDT, lowered rates. The campaign was particularly successful in the Americas, where dengue and yellow fever, both carried by A. aegypti, were an omnipresent threat.

#Dengue fever cases surge as temperatures rise. #ClimateChange #GlobalWarming #Mosquitoes #HumanHealth

But spraying DDT, a known carcinogen, into the environment quickly became an unsustainable mosquito-control measure. By the 1980s, as DDT was being phased out and a century of rampant fossil fuel use began to significantly influence the global climate, the disease began to spread again, and fast. In the next couple of decades, dengue would be found in 100 countries, up from just a handful of countries in the 1960s. Today, it’s been detected in more than 140 nations.

Photo by Getty Images/Grist

“This is the [mosquito-borne] disease that has grown most substantially in the past 10 years,” Felipe J Colón-González, a climate and health researcher who works at the global charitable foundation the Wellcome Trust, told Grist. “There are many factors that are related to climate.”

In order to gauge the influence of global warming on the spread of dengue, researchers look at three interconnected clues: where mosquitoes move, how quickly they develop, and how often they reproduce.

Like any creature on Earth, mosquitoes thrive within a specific temperature range. The insects can’t withstand temperatures that are too dry or cold. Anywhere below 57 F, particularly when there’s low humidity, is unlivable. But most mosquitoes can’t withstand temperatures that are too wet or hot, either — large rainstorms wash them out and they tend to die off at 90 F and above.

Human industrial activity has warmed the planet by about 2 F, on average, a seemingly small change that has had enormous implications for the spread of infectious disease — and life on Earth writ large.

Nepal, a mountainous country in South Asia, is a perfect example of how even a slight temperature change can open up a Pandora’s box of disease. Dengue wasn’t present in Nepal until 2004 when the first case was recorded. Less than two decades later, in 2022, the country, which is warming more than 1 F every decade, experienced its largest outbreak ever — 54,232 cases and 67 deaths. Researchers in Nepal noted that the nation’s mountains are undergoing “unusually large” fluctuations in temperature. Snow cover on those mountains is melting away as climate change accelerates, inviting pests into new, higher territories. Afghanistan, also long considered too mountainous for Aedes mosquitoes, is witnessing a similar trend.

Climate change isn’t just inspiring mosquitoes to move to higher elevations — it’s prompting the bugs to mature more quickly and produce more generations of offspring in a single season.

Warmer temperatures increase both mosquitoes’ rate of survival and development, and the rate at which they feed. Female mosquitoes, the ones that bite humans, digest blood more quickly when it’s warm and humid out. That leads to more disease. “Because the metabolism is faster, they have to feed many more times in a life cycle so there’s more probability of an infection,” said Colón-González.

Even temperatures that should be too hot for mosquitoes don’t always kill them off. The insects hide in cool corners and under couch cushions to escape the heat, seeking shade much like humans do. “Mosquitoes are annoyingly intelligent creatures,” Colón-González said.

It’s clear that climate change is helping mosquitoes, and the diseases they carry, extend their reach across much of the planet. Roughly half the globe is now at risk for dengue, Raman Velayudhan, who leads the WHO’s program for the control of neglected tropical diseases, said recently. But mosquitoes are not invincible. Researchers have had success artificially infecting Aedes mosquitoes with a bacterium that prevents the transmission of dengue from mosquitoes to humans. Pilot studies in South America and Southeast Asia have shown that the bacterium, called Wolbachia, can be incredibly effective: Cases of dengue in Yogyakarta, Indonesia, went down 77 per cent following the release of Wolbachia mosquitoes.

And other, more dependable and scalable methods of curbing dengue transmission exist. As is the case with many climate-driven illnesses, keeping communities safe from dengue ultimately comes down to resources and access.

In the U.S., climate projections indicate that the atmospheric conditions for dengue will be ideal throughout much of the country by the end of the century. But it’s unlikely that dengue will become as widespread an issue as it is in underdeveloped countries. That’s because most American homes have window screens that keep bugs out, and a large portion of the population has access to air conditioners that keep humidity low inside. Houses in the U.S. are spaced further apart than elsewhere in the world, which means a mosquito that breeds in one house won’t necessarily bite people in the house next door.

Americans also have widespread access to mosquito repellant. And in most areas, drinking water containers and sanitation systems are stored underground, which means mosquitoes can’t breed in them. That’s why in Texas, dengue is a rare disease while as many as 20 per cent of all dengue deaths in the Americas occur in Mexico. Two places that share a border and the same environmental conditions can have two completely different health outcomes.

“It’s true that the climate is going to become more suitable for dengue,” Colón-González said, pointing to rising temperatures and cases all over the globe. But the built environment, human behaviour, and the quality of public health systems also play important roles — and point at potential silver linings that could help mitigate the dengue burden in countries with fewer resources.

“It’s not just the climate,” he said.

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