In the US, the world’s deadliest animal is on the move

The deadliest animal in the world is smaller than a pencil eraser and weighs around two-thousandths of a gram – less than the weight of a single raindrop. Every year, it kills an estimated 700,000 people by partaking in what scientists grimly call a “blood meal.”

It’s the mosquito – and, increasingly, it’s on the move.

These global shifts, which will only accelerate as the planet warms, have sparked concern that the diseases mosquitoes carry will exact an even higher toll in the months and years to come.

In June alone, five cases of locally transmitted malaria were discovered in Texas and Florida: the first cases acquired in the United States in two decades. These cases, experts say, are unlikely to have a connection to warming temperatures – conditions in Florida and Texas are already suitable for malaria-carrying mosquitoes. But as urban heat islands expand and temperatures rise, mosquito-borne diseases are expected to travel outside of their typical regions.

“Climate change allows the creeping edge of mosquito ranges to expand,” said Sadie Ryan, a professor of medical geography at the University of Florida.

Earlier this year, Georgetown University researchers published a paper in Biology Letters demonstrating that malaria mosquitoes’ ranges have already shifted in Africa over the past century, farther from the equator and into higher altitudes.

Malaria cases worldwide declined steadily for nearly two decades. But that progress stalled as cases have flatlined and even ticked up in some countries in the past few years. Cases increased to an estimated 247 million in 2021 from a recent low of 231 million in 2018, according to data from the World Health Organization.

Mosquitoes don’t kill the way a shark or a lion does: Instead, they are “vectors” for many painful and life-threatening diseases, from dengue fever to malaria to chikungunya. When a mosquito “bites” someone – by stabbing a needle-filled proboscis deep into a blood vessel – it both sucks out blood and leaves some of its own saliva behind.

That saliva, when contaminated by virus or parasite, can make people sick, often painfully so. Dengue fever is also known as “breakbone” fever; the name chikungunya comes from an African word meaning “to be contorted,” as patients often bend over from severe joint and muscle pain.

And there is reason to think that those excruciating and sometimes deadly diseases will spread as temperatures warm. Like all insects, mosquitoes are coldblooded and rely on ambient temperatures to sustain their body temperatures. They thrive, particularly, on temperatures between 50 and 95 degrees Fahrenheit – and, unlike most humans, who wilt under high humidity, mosquitoes love damp air.

In the United States, for example, many regions are already seeing an increase in “mosquito days” as temperatures rise. According to a report from the research and communications nonprofit Climate Central, between 1979 and 2022, many areas of the country saw an increase in days when temperature and humidity were a kind of “Goldilocks zone” for mosquitoes.

“We’re definitely seeing this prolonging of seasons” for mosquitoes, Ryan explained.

Different mosquitoes thrive under different temperatures. The Anopheles mosquito carries malaria; the Aedes aegypti and the Aedes albopictus mosquitoes carry diseases like dengue and chikungunya. But the A. aegypti thrives at higher temperatures than the A. albopictus. As different parts of the world warm at different rates, some mosquito-borne diseases will thrive while others will be put under stress.

According to a study published in 2019, both species are expected to spread northward in the United States over the next 30 years. By 2050, the A. aegypti could increase its range in the Mid-Atlantic and the Midwest; the A. albopictus could make it as far north as Michigan and Minnesota.

There are other factors that are also changing the patterns of disease transmission. As urban areas expand and populations increase, mosquitoes like the A. aegypti – which love living around humans – can find more places to live and more people to feed on. The species has made gains in Southern California, for example, infuriating the region’s residents.

In some regions, there may be winners and losers of the mosquito migration. Many mosquito-borne diseases have an upper limit for easy transmission – if it gets too hot, diseases like dengue won’t be as prevalent in broiling tropical regions and will instead continue to shift toward the poles.

But Ryan says this shouldn’t offer much comfort. “Temperatures that are too hot for dengue transmission are going to be no fun for anyone to live in,” she said.