Extreme heat and rain are fueling rising cases of mosquito-borne diseases

When it rains, it pours. And when it pours, mosquitoes pop up. 

A series of storms slammed the northeastern United States on August 18, unleashing torrents of rain and causing flash floods across parts of New York and Connecticut. In Oxford, Conn., a potentially record-breaking rainfall of roughly 38 centimeters (15 inches) fell in 24 hours. The National Weather Service is confirming whether the measurements beat the state’s single-day record, set 69 years ago when Hurricane Diane dumped about 32 centimeters of rain.

Meanwhile, millions of people were under excessive heat warnings as Texas and the Southwest baked under a heat dome last week. Phoenix extended its ongoing, record-long streak of triple-digit Fahrenheit temperatures, and air conditioner use across Texas on August 20 pushed energy demand to a record high (SN: 8/12/24).

Extreme heat and rain are formidable enough on their own, and it’s getting hotter and wetter with climate change. But combined, these conditions can bring on yet another foe: mosquitoes.

Mosquitoes flourish when there’s plenty of water around to lay eggs in, and warm temperatures help those offspring thrive. The insects themselves aren’t a problem — though their bites are annoyingly itchy. But the viruses, bacteria and parasites they carry are.

With that in mind, some readers are wondering whether a hotter, wetter climate will raise the risk of mosquito-borne diseases, or those transmitted by ticks. And if so, what can we do to protect ourselves?  

The short answer to the first question is yes. The longer answer is that we’re already seeing it happen for some diseases.

It’s been a record year for dengue fever, a disease transmitted by mosquitoes such as Aedes aegypti, the yellow fever mosquito. The Americas have been hit hard, amassing more than 10 million documented cases as of August 21. That’s roughly 94 percent of the 11.5 million cases recorded around the world. Cases in the Americas alone also more than double the previous global record of 5.3 million cases that was set just last year.

Climate change, El Niño, urbanization and the number of people susceptible to the virus may have all played a part in the massive outbreak, according to the World Health Organization (SN: 7/13/23). El Niño, for instance, brings heavy rains to some parts of the world and drought to others. Even drought can be risky because A. aegypti is an urban mosquito that has no problem laying its eggs in containers that store water around homes (SN: 10/7/19). And dense cities mean more people around to bite — and possibly infect (SN: 11/2/22).

Rising temperatures may have already boosted dengue transmission by around 18 percent in the Americas and Asia compared with what levels would have been in a world without warming, researchers reported in a preprint earlier this year. Depending on how hot the planet gets, transmission could go up even more by 2050, to 40 to 57 percent higher on average than without climate change.

Researchers expect that other mosquito-borne viruses such as West Nile — in the headlines after former National Institute of Allergy and Infectious Diseases director Anthony Fauci contracted the disease— and chikungunya will also be more common with climate change. And residents in Massachusetts are on high alert for yet another threat: Eastern equine encephalitis, a mosquito-borne disease with a 30 percent fatality rate that rarely pops up in eastern or Gulf Coast states. A man in his 80s became the first human case since 2020, state health officials announced on August 16. The town of Plymouth, Mass., began closing its parks at dusk on August 23 to protect residents after the virus was detected in a local horse.

Other pathogens like the parasitic disease malaria could also affect more people as temperatures warm (SN: 6/30/23). On August 2, the Pan American Health Organization issued a health alert after an increase of confirmed cases of Oropouche fever, which is transmitted from animals to people through midge bites, hit new parts of South America and the Caribbean — likely driven in part by climate change. The virus, which typically causes mild symptoms, became deadly for the first time this year, causing two fatalities.

Ticks, on the other hand, live longer than mosquitoes do, making them less susceptible to short-term weather changes like torrential rains or heat waves. Still, warming winters help more tick larvae survive until spring, meaning larger populations to spread disease, such as Powassan virus, transmitted by ticks in North America and Russia. Deer ticks (Ixodes scapularis), which can carry the bacteria that causes Lyme disease, are slowly expanding their range north, putting more people at risk (SN: 3/19/14).

The best ways to protect yourself? Ultimately it comes down to this: Don’t get bitten. Use repellents and wear protective clothing, do tick checks, and use screens to make sure flying insects can’t come inside. Also, empty standing water where mosquitoes might lay eggs. Fewer eggs means fewer mosquitoes around to go after your blood.   

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Please keep sending in your questions about Earth’s extreme climate records — we’ll look for ones to answer in upcoming Extreme Climate Update columns.