Photograph by Joel Sartore, National Geographic Photo Ark
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A Yellow Fever mosquito, Aedes aegypti, at the Urban Entomology Lab at the University of Florida at Gainesville.

Photograph by Joel Sartore, National Geographic Photo Ark
EnvironmentExplainer

Summertime means disease-carrying insects. Here's what you should know.

Illnesses spread by ticks, mosquitoes, and kissing bugs are on the rise—a trend experts say will continue as the climate warms.

Insects that transmit disease will spread farther and transmit illness more quickly as the planet warms, according to experts at the Centers for Disease Control who monitor vector-borne diseases.

“[Mosquitoes] are really sensitive to climate change,” says vector biologist Lyric Bartholomay from the University of Wisconsin-Madison. “I don’t think there’s any doubt that as temperatures shift, their ranges and places they can go will also change.”

Last week, investment bank Morgan Stanley predicted that drug makers could see their profits soar in the coming decades as climate change exposes more than a billion people around the world to infectious diseases like yellow fever and dengue often spread by insects.

Here, experts explain how these little disease-carrying varmints are adapting to warming climates.

Ticks

Already, the CDC is seeing a sharp increase in reported diseases spread from ticks like Lyme disease and Rocky Mountain spotted fever. In 2017, the most recent year for which data is available, tick-borne diseases were up by about 20 percent, and cases have more than doubled over the past 10 years.

“We’ve seen those populations of ticks expanding northward,” says Ben Beard, the deputy director of the CDC’s Division of Vector-Borne Diseases. “In warmer temperatures they also have greater reproductive capacity, so they reproduce more offspring and they also come through their lifecycle more quickly.”

Ticks attach themselves to people and animals by laying in wait. With their back legs grasping a leaf or branch, they use their front legs to latch on to anything that brushes past. Once on a body, they burrow into the skin and insert a barbed feeding tube. It’s through their saliva that diseases like Lyme are transferred, and the feeding process can take anywhere from 10 minutes to two hours.

Many, but not all, species of ticks remain dormant during the winter and only emerge when temperatures rise and days become longer in the spring. Beard said the CDC is seeing cases of Lyme begin earlier and earlier in the year.

Altering the landscape has also helped tick populations grow. Reforestation of former farmland and expanding suburbs have created more conditions in which humans and ticks come into contact. With the loss of their predators, tick-carrying deer populations have rapidly increased.

“It’s resulted in an increase in the habitat around homes where ticks thrive and people can be exposed,” says Beard. He notes that most people are bitten by ticks in their own backyards.

Mosquitoes

Before 1999 the United States had no known cases of West Nile virus. Since then more than 41,000 cases have been reported in the U.S.

Bartholomay says the peak time for West Nile transmission is currently underway in the U.S.

Once temperatures warm, Asian Tiger mosquitoes, which carry the virus, start rapidly feeding and reproducing.

“When a virus gets into a mosquito it must first replicate in her gut,” says Bartholomay. “Then it moves to the salivary glands. When she feeds, she releases saliva, and the virus goes out with the saliva. When it’s warmer, all of those processes happen faster.”

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Not all environmental changes give mosquitoes a boost. In Iowa, draining fields pocked with standing water to create flat fields for crops robbed mosquitoes of essential breeding grounds. But as temperatures warm, the CDC expects mosquito range to expand. With more trade and travel, scientists worry that diseases not previously seen in the U.S. like Rift Valley fever and chikungunya could spread into the U.S. for the first time. The latter, a viral disease found in the tropics, is a matter of when it arrives, not if, says Bartholomay.

“It’s the kind of thing that keeps us up at night because that would be extraordinarily devastating,” says Bartholomay of Rift Valley fever. The disease can easily kill large swaths of livestock and people.

Kissing bugs

Like ticks and mosquitoes, kissing bugs may increase their range northward as temperatures warm. The penny-sized black bugs can live in common suburban spaces like small shrubs or cracked concrete.

They earned their name for the way they infect humans by biting them around the mouth or eyes. A study by Texas A&M University found that about half the kissing bugs in Texas harbored a parasite called Trypanosoma cruzi in their digestive system. About 30 percent of people who become infected with the parasite will develop Chagas disease, an illness that can lead to heart failure.

“Occasionally you can get symptoms when you’re first infected, but most of the time you don’t even feel anything. The challenge of this disease is it takes 30 to 50 years before you see Chagas disease (develop),” says Maria Elena Bottazzi, a scientist who studies the disease at Baylor University.

“It’s not just climate change,” she says of the bugs’ growing range, “but deforestation, and destabilization through conflict.”

Cases of Chagas are typically found only in Central and South America, and without adequate healthcare, those carrying the disease from Latin America may unwittingly bring the disease north and help spread the disease after mosquitoes transmit their infected blood. From 2013 to 2016, Texas health centers reported 91 cases of Chagas, 20 of which stemmed from people who were infected domestically. While some studies have claimed that the warming climate could push the disease north, Bottazzi says it’s unclear if bug populations are actually growing or if doctors are becoming better at detection.

Infectious future

The CDC provides information on what type of clothing and bug repellent can be worn to prevent bites, as well as ways to avoid outbreaks in a yard.

Bottazzi notes that who gets infected can vary by socioeconomic standing and calls Chagas a “disease of poverty.”

“It’s different in the U.S. because we often have housing with screened windows and air conditioning,” adds Bartholomay. “One of the things we have in our favor is that we have some suitable barriers and ready access to repellants.”

Of the rising rate of vector-borne diseases, Beard says the CDC expects that “this trend is going to continue.”

“There’s no vaccine for any of the vector-borne diseases in the United States. Protecting yourself against ticks and mosquitoes relies on personal protective measures,” he says.