Where mosquito season is getting longer

As global temperatures rise due to climate change, mosquito season has lengthened across much of the contiguous United States, a development that scientists warn could heighten the risk of outbreaks of diseases such as dengue and malaria, and the spread of West Nile and Zika viruses.

Over the past four decades, more than two-thirds of the country has seen an increase in “mosquito days” — defined as days with an average humidity of at least 42 percent and temperatures between 50 and 95 degrees Fahrenheit (10 and 35 degrees Celsius).

The Northeast, which has warmed faster than the rest of the country, experienced the biggest increase in mosquito days. In Vermont, for instance, the past five years have averaged 17 more mosquito days compared with the period from 1980 to 2009.

Mosquito season shortened in hotter regions such as Texas, which experienced more frequent temperatures exceeding 95 degrees, pushing mosquitoes past their physiological limits. The growing prevalence of droughts in the southwest has led to fewer days with sufficient humidity for mosquitoes to thrive.

Good mosquito data is hard to find. While some state and local agencies monitor mosquito populations, there is no comprehensive national database. Unlike ornithologists, who can analyze trends from billions of observations contributed by hobbyist birders, mosquito researchers lack a similar community of citizen scientists dedicated to tracking the movements of backyard mosquitoes.

Instead, scientists use weather data to map when and where mosquitoes are likely to thrive. “Since temperature and humidity are good indicators for mosquito survival, and they are widely available, it makes them good proxies that we can use as a starting point,” Rebeca de Jesús Crespo, a mosquito ecologist at Louisiana State University, explained in an email.

Temperature and humidity are less predictive of mosquito abundance in cities, where the environments we create for our comfort also protect mosquitoes from the elements. On hot summer days, mosquitoes shelter indoors or in the shade, which helps explain why there are more mosquitoes in downtown Phoenix than in the surrounding desert. In some cities, mosquitoes have even managed to survive the winter by seeking refuge in sewers.

Other factors — such as rainfall, nutrient availability and the presence of predators — influence mosquito abundance, but “these maps are good indicators of factors that are relevant for mosquito vector survival,” de Jesús Crespo said.

In Austin, the past five years had fewer summer mosquito days compared to 1980-2009 because temperatures more often got too hot. But in most places, the climate grew kinder to mosquitoes. Jacksonville, Fla., suffered a 17-day jump in mosquito days, mostly because of warming spring temperatures. Portland, Ore., and Washington both experienced double-digit jumps in mosquito days because of warming autumns.

For most people in the United States, mosquitoes are little more than an annoyance. But for an unfortunate few, they pose a far more serious threat.

Marie Heilman, a 58-year-old former computer engineer from Vacaville, Calif., had seen the billboards and public service announcements on TV and in magazines that described the symptoms of West Nile virus. But even after spending five days in bed, drifting between sleep and confusion, crawling to the bathroom through waves of nausea, Heilman still did not suspect the true cause of her ailment.

“I just thought, oh, I’ve just got some vertigo,” she said. “You know, women are women. We just kind of tolerate things.”

It wasn’t until her legs failed and her daughter had to wheel her into the hospital that she grasped the seriousness of her condition. The doctors gave her a spinal tap and sent the sample to a lab. By the time her result came back positive for West Nile, she was already in a coma. Her organs began to fail, and the doctors asked her family to sign a do-not-resuscitate form.

“I physically went through the hell with it, but my husband went through the mental part of it,” Heilman said. “They told him his 40-year-old wife was dying, and there was nothing more they can do, and it’s just a matter of time.”

Since West Nile appeared in the United States in 1999, more than 2,000 cases have been reported annually. One in 5 people develop symptoms from the virus, meaning the actual number of infections is probably higher.

West Nile is one of several mosquito-borne illnesses that scientists fear could become more common as the climate warms. Research shows “higher global average temperatures are making wider geographic areas more suitable for transmission” of dengue, malaria and other mosquito-borne diseases, according to the U.N. Intergovernmental Panel on Climate Change. The United States records about 3,200 dengue cases and 2,000 malaria cases per year.

For a mosquito to transmit a pathogen, several steps must occur. First, a larva must develop into a healthy adult capable of reproduction. To lay eggs, an adult female obtains protein by feeding on human blood. If she bites a person who is infected, she may acquire a pathogen, which then replicates inside her and can be transmitted to another human when she bites again.

This entire process takes about two weeks. The longer the weather remains suitable for the mosquito’s survival, the greater her chances of passing on the pathogen before she dies.

“We are likely to see more mosquito-borne disease just because that season is longer,” said Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies. “It’s just timing and probability.”

Because West Nile affects the central nervous system, it can lead to severe conditions such as meningitis, an inflammation of the membrane surrounding the brain and spinal cord, and encephalitis, an inflammation of the brain tissue itself. Heilman developed both, making her one of the unfortunate few — about 1 in 150, according to the Centers for Disease Control and Prevention — who become seriously ill from West Nile.

Heilman remained in a coma for 15 days. Then, somehow, she woke up — as if from a “crazy dream,” she said.

It would be four months before she could walk again, eight months before she could drive, and a year and a half before she could read without being overwhelmed by nausea. She had to quit her job at Lockheed Martin. Eighteen years later, she still struggles with fatigue, dizziness and vision problems. She has undergone three surgeries to repair nerve damage in her right shoulder.

These days, when her grandson plays football, Heilman makes sure he’s wearing insect repellent. She mixes a few drops into sunscreen and lathers it on. She urges people to fix broken sprinklers and eliminate pools of water in their yards and flower pots, as mosquitoes thrive in stagnant water. If you see a dead bird, she advises, report it to your local mosquito-control district.

“I don’t want anybody else to ever go through what I went through. It’s just absolutely horrible,” Heilman said. “I don’t think most people think about getting bit by mosquitoes. I know I sure didn’t.”