‘A warmer, sicker world’: Mosquitoes carrying deadly diseases are on an unstoppable march across the US

According to infectious disease experts, unusual patterns with mosquito-borne pathogens are becoming the norm. Earlier in September 2024, a man in New Hampshire was reported by his family to have been hospitalised with not one but three mosquito-borne illnesses at the same time: West Nile fever, EEE and St Louis encephalitis.

“What we’re seeing more and more is people infected with multiple mosquito-borne diseases at the same time,” says Chloé Lahondère, a biologist who studies blood-sucking insects at Virginia Tech.

Within the US, and around the globe, mosquito-borne diseases are popping up in increasingly atypical places. “Last year, we had locally acquired cases of malaria in the US for the first time in 20 years,” says Desiree LaBeaud, a professor of paediatrics and infectious disease at Stanford University.

Locally acquired dengue fever has also been detected in California and Arizona. Meanwhile, record numbers of dengue cases have been reported across Europe while unprecedented deadly outbreaks have occurred in Peru.

The climate connection

Much of this is related to shifting climate patterns, which are allowing various mosquito populations that carry these viruses, such as various sub-species of the Aedes genus of mosquitoes and the species Culex coronator, to spread into new areas. As temperatures rise, mosquitoes can invade and thrive in habitats which once represented hostile environments. Since it was first detected in Louisiana in 2004, C. coronator has spread to almost of the south-eastern states, bringing diseases such as West Nile and St Louis encephalitis, while other mosquito species are finding that they can comfortably survive even further north.

“People have been surprised by Aedes-borne diseases like dengue popping up in California,” says Sadie Ryan, a professor in medical geography at the University of Florida. “But once they examined all the mosquito surveillance data, they were finding these species in traps even quite [far] north in the state, which hadn’t been seen before. They’re not very well established there yet, but it’s a warning.”

With even mountainous regions becoming warmer and more mosquito-friendly, diseases like malaria and dengue are even beginning to reach some of the most remote places on the planet. For example, while malaria is endemic to Nepal, the disease incidence is increasing in the hills and mountains of the country, locations previously considered malaria-free. “Malaria is very much thought of as a lowland disease,” says Paul Tambyah, president of the International Society for Infectious Diseases. “But now the definition of lowland is creeping upwards as the temperatures get warmer, so mosquitoes are able to survive at altitudes.”

This is predicted to have drastic consequences for public health. Mosquito-borne diseases already afflict around 700 million people every year, resulting in one million deaths. Over the next century, climate projections suggest that around one billion people, mainly in Europe and subtropical regions, will be infected by a mosquito-borne virus for the first time, while disease transmission within parts of Asia and sub-Saharan Africa is expected to transition from being seasonal to year-round.

Some researchers have described this as a steady move to “a warmer, sicker world”. But to understand why this is happening, we need to understand a little more about the biology of mosquitoes and their life cycles.