Our Stories › Explainer

Climate Change Bites

As temperatures rise, mosquitoes and ticks thrive. And so do the diseases they carry.

Frederic Labaune/Getty Images

Weather and infectious disease have long been intertwined—and this has become painfully evident in the past few decades.

In 1999, West Nile virus, which originated in Uganda and is spread by mosquitoes, killed seven in New York City. In 2014, another mosquito-borne virus, called chikungunya (which means “bending over in pain” in the Makonde language of East Africa), afflicted 11 Florida residents, and cases have since been reported in 46 states and Washington, D.C.

In each case, the virus to blame had not previously been reported in the affected area. Extreme or shifting weather patterns helped create more habitat that's inviting to the mosquitoes that can carry those illnesses—and experts say that outbreaks are bound to become more common as the effects of climate change become more prevalent.

“A longer warm-weather season and changing rainfall patterns are allowing the insects that can transmit disease to humans to thrive for longer periods each year—and to simultaneously move into broader areas,” explains Kim Knowlton, senior scientist and deputy director of NRDC's Science Center.

Knowlton and other experts say climate change means Americans need to learn more about diseases that are vector-borne, or transmitted by agents such as insects. Among the biggest threats—because we have neither effective vaccines nor guaranteed cures for them—are West Nile virus and dengue fever, both spread by mosquitoes, and Lyme disease, transmitted by deer ticks.

Mosquito matters

Parts of the United States that once had steady precipitation are now experiencing periods of drought punctuated by heavy rainfall. Sudden deluges leave behind puddles of standing water—moisture that mosquitoes need to hatch their eggs. In addition, hotter weather is shortening these eggs’ incubation times, increasing the overall mosquito population. Female mosquitoes are the ones that bite, and warmer weather makes them more likely to do so.

West Nile, now the most common mosquito-borne illness in the United States, is spread when infected birds are bitten by bugs that then go on to bite humans. Though 70 percent to 80 percent of West Nile–infected people do not develop symptoms, the virus occasionally causes fevers accompanied by body aches, disorientation, diarrhea, neck stiffness, headache, joint pain, and tremors, and it can spread to the brain. About 1 percent of people with West Nile go on to develop potentially fatal encephalitis or meningitis.

Dengue fever has long persisted in Asia and Latin America but recently spread to the United States as well. The strains of the virus that causes it (there are four of them) can bring on high fever, headache, pain behind the eyes, and crippling bone, muscle, or joint aches. “Most cases usually just require bed rest and lots of fluids,” says Knowlton. But contracting dengue also puts you at risk for a later bout with dengue hemorrhagic fever if you are infected a second time. This potentially fatal condition can cause bleeding from the nose or gums, easy bruising, and blood in vomit or stool

Tick transmission

When a deer tick bites a rodent infected by the Borrelia burgdorferi bacterium, that tick can then bite a human and infect him or her with Lyme disease. Longer, hotter summers have expanded the geographical range of the deer tick, also called the blacklegged tick, and Lyme disease has traveled to every state except Hawaii since it was first reported in 1975 in the Lyme, Connecticut, area.

A 2015 study by the Centers for Disease Control and Prevention found that 260 high-risk counties in the Northeast and upper Midwest had double the number of Lyme cases than authorities had expected. Lyme is also spreading to Canada, where thousands of people are believed to be infected.

Dark brown and with larva as small as poppy seeds, deer ticks are difficult to spot. But if an infected tick clings to your skin and feeds on your blood for 36 to 48 hours, you may develop the bull’s-eye rash, flulike symptoms, and joint pain that signal infection. “If you’re infected with Lyme disease and don’t get treated right away, there can be long-term effects such as debilitating and persistent fatigue,” says Knowlton.

Avoiding risk

Over the past 40 years, temperatures in the United States have risen an average of 0.26 to 0.43 degrees Fahrenheit per decade. Because climate change is now a fact of life, it’s essential that Americans protect themselves from rapidly spreading vector-borne diseases that have no definitive cures. If you live in an area where ticks and mosquitoes are prevalent, Knowlton and other experts recommend that you:

  • Install or fix window and door screens so mosquitoes can’t enter your home.
  • Avoid going outdoors at dawn and dusk, when mosquitoes are most active.
  • Drain standing water from birdbaths, kiddie pools, storm-drain catchments, and other places where it can collect.
  • Spray insect repellent containing a low concentration of DEET (30 percent or less) on your clothes and exposed skin—but not your face—before stepping outdoors.
  • Ward off deer ticks during a hike by tucking pant cuffs into long socks.
  • Wear light-colored outdoor clothing so you can better spot dark deer ticks.
  • Check yourself for ticks when you return from outdoors.
  • Remove an attached tick using fine-tipped tweezers, grasping the tick as close to your skin's surface as possible. Then clean your skin and hands with rubbing alcohol, an iodine scrub, or soap and water.
  • Use a washcloth or loofah from head to toe in the shower to brush away any unattached ticks that may be on your body.
  • If you develop suspicious symptoms, seek immediate treatment if you develop suspicious symptoms to safeguard your health—and that of others in your area.
NRDC in Action

Scientist Kim Knowlton monitors the inextricable connections between the planet's fragile health and our own.

Personal Action

As temperatures and carbon levels rise, even breathing has become a challenge. Here’s what you can do to help clear the air.

Join Us