Climate crisis is helping spread deadly mosquito viruses in the US

• Mosquito-borne illnesses are rising in the US, including triple E, West Nile, and Oropouche.
• Climate change and international travel are spreading these diseases into new places.
• Mosquito season is lasting longer due to rising temperatures and changing rainfall patterns.

Mosquito season is in full swing, and the pesky insects are carrying diseases into unexpected places.

Certain communities in Massachusetts, New Hampshire, and Vermont are on high alert for a rare virus called eastern equine encephalitis, or triple E. Health officials have reported at least six cases, including one death, and are encouraging residents to stay inside from dusk until dawn, when mosquitoes are most active. Some towns are spraying pesticides to reduce the risks of exposure.

The outbreak is small but serious because about 30% of people who contract severe triple E die, and many survivors develop neurological problems, according to the Centers for Disease Control and Prevention . There's no vaccine or cure.

Over the past two decades there have been sporadic outbreaks in the Northeast — and in 2019, there were a record 38 cases in the US. Researchers are still trying to understand why, and there are many environmental factors, including the bird populations the mosquito typically picks up the virus from. Shannon LaDeau, a senior scientist at the Cary Institute of Ecosystem Studies, said the climate crisis is certainly playing a role. As spring and fall temperatures warm, mosquitoes can emerge earlier and survive later into the year. They also can squeeze in more reproductive cycles.

"Everything about mosquito development speeds up with temperature," LaDeau said. "How fast they go from egg to adult shortens when it's warmer. Viruses replicate in the mosquito faster when it's warmer. So you get more mosquitoes faster, and then their chances of interacting with a human is higher."

Precipitation patterns also matter, LaDeau said. In the Northeast, a warm spring was followed by a lot of rain in early summer, which is conducive to larger mosquito populations by August.

The changes aren't playing out just in the Northeast. Health officials have described an uptick in other mosquito-borne illnesses in the US and globally because of climate change and international travel and trade.

Dr. Peter Hotez, the dean of the National School of Tropical Medicine at Baylor, told "PBS NewsHour" that he was concerned about a rising number of vector-borne illnesses — those spread by mosquitoes, ticks, and fleas — across the Western Hemisphere, including triple E, West Nile, and dengue.

The Oropouche virus, which normally circulates in parts of South America, Central America, and the Caribbean, was reported in the US and Europe for the first time this year. Yellow fever is expanding into the fringes of the Amazon in Brazil.

"We're seeing a general surge in vector-borne diseases across the hemisphere," Hotez said. "Brazil and parts of the Caribbean are ground zero, but it's starting to filter in particularly in Texas and the Gulf Coast. This could be a new normal for us during this time of climate change together with urbanization and other factors."