How climate change helped Lyme disease invade America

When President Donald Trump announced that he was pulling the US out of the Paris climate accord, Yale epidemiology researcher Katharine Walter felt gutted. “[Global warming] is already happening,” she said, “and the effects are already here.”

Walter studies Lyme disease, the tick-borne illness that’s spreading frighteningly quickly in the Eastern and Midwestern US, due in part to climate change. Lyme cases have more than doubled since the 1990s, and the number of counties that are now deemed high-risk for Lyme has increased by more than 320 percent in the same period.

“These effects of climate change will be felt globally, but also here in the US,” Walter said, “and here in New York, in Trump’s backyard.”

New York state is an epicenter for Lyme. More than 90 percent of cases pop up in the Northeast, upper Midwest, and mid-Atlantic. And it’s why New York Sen. Chuck Schumer has been calling on the federal government to more aggressively tackle Lyme.

But Trump’s policies on climate change, Walter said, will likely do the opposite, and make climate-sensitive infectious diseases like Lyme even more common. Here are four things to know as we enter the season for the disease.

1) Lyme disease spreads to people through tick bites — but the disease can be really hard to diagnose

Lyme is the most common vector-borne disease in the US, more common West Nile or the Zika virus. But unlike Zika, which is transmitted from mosquitoes to humans, Lyme reaches people through tick bites after circulating through a chain of other species.

The bacteria typically live in mice, chipmunks, birds, and deer in wooded areas. And these are all animals that ticks feast on.

Ticks like humans too. They’re attracted to the warmth and carbon dioxide we give off. Though they can’t jump or fly, they typically crawl onto us when we brush against them while outdoors — walking through tall grass, playing in fields. If they’re carrying the Borrelia burgdorferi bacteria, they can infect us when they bite. (In most cases, the tick needs to be attached to the skin for more than 36 hours before the Lyme bacteria can be transmitted.)

According to the Centers for Disease Control and Prevention, common Lyme symptoms include fever, headache, fatigue, and muscle and joint aches. There’s also that telltale skin rash, which manifests in a ring-shape on the body — but not everybody who has Lyme gets a rash or even the bullseye on their body (and Lyme-related rashes can also come in many shapes and sizes).

If Lyme is diagnosed early enough, an antibiotic can treat the illness. But when it’s left untreated, the bacterial infection can spread into the joints, heart, and nervous system — causing more severe and dangerous symptoms like arthritis, heart palpitations, brain inflammation, and nerve pain. In some patients, even those who have been treated for Lyme, symptoms can persist for months and researchers don’t understand why. (This is typically called “chronic Lyme disease” or “post-Lyme syndrome” — and it’s controversial.)

To make matters more complicated, Lyme can be really difficult to diagnose. Many people never get a rash, and can’t recall a tick bite. The FDA-approved blood tests on offer in the US aren’t always accurate. So doctors are often left diagnosing the disease on the basis of clinical symptoms that can be really vague.

“It’s one of these diseases that has a very nonspecific, acute presentation,” said Duane Gubler, an infectious diseases specialist and former director of the division of vector-borne disease at the Centers for Disease Control and Prevention. “There are many people out there with symptoms of chronic fatigue, muscle pain, and Lyme disease has become one of the catchalls for people who have an illness of unknown etiology and there’s simply no clinical or lab test that can diagnose these people.”

One patient, Priscilla Rodriguez, thinks she got bitten by a tick while horse riding in the parks around Washington, DC. It took nearly half a dozen doctors over a year to finally diagnose and treat her. “It was like whac-a-mole,” she said. “The disease pops up at a different system of the body, so you think it’s a different thing, you go to different doctors, and they can’t figure it out.”

2) Lyme disease has become increasingly common

In the US, the incidence of Lyme disease has doubled since 1991, from about four cases per 100,000 people to eight per 100,000 people. About 30,000 people are known to get sick with the disease each year, and the CDC thinks the real number of cases is about 10 times that. It’s also spread to a wider area of the country.

The risk for the disease is concentrated in the Northeast, mid-Atlantic, and upper Midwest — with some pockets of Lyme cases on the West Coast. One of the key drivers in the Northeast is the reforestation of land that used to be farmland, and an increase in deer populations in those regrown forests.

Hunting protections on deer have also helped the animals proliferate, and deer are particularly important in spreading Lyme to new areas because adult ticks often feed and mate on them. (As Walter said, “Deer help determine where ticks can survive and, therefore, the distribution of Lyme disease risk.”)

With suburbanization, more and more people are living near the animals that carry Lyme. “People have moved into these woodlots and built their houses right in the middle of where Lyme is maintained in the mouse and deer populations,” Gubler added. “That’s why we’ve seen such a dramatic increase in Lyme in the US.”

3) A driver of the uptick in Lyme incidence: global warming

The Environmental Protection Agency tracks the number of Lyme cases, along with heat-related deaths and severe weather events, as an indicator of global warming.

That’s because researchers think climate change is another major driver of the trend — and they expect the situation to get much worse during the 21st century.

One of the most important determinants of where ticks can live is temperature, because they can’t survive in very cold climates. Now that we are experiencing warmer winters, ticks are able to live further and further north.

“Tick survival depends on the local climate, and the distribution or geographic range of where ticks occur is increasing — and this is driven in part by climate change” and, in particular, higher winter temperatures, Walter explained.

Warmer temperatures can also increase the developmental rate of ticks — the time it takes for an immature tick to fully develop. In one study, researchers estimated global warming increased tick reproduction by up to two times in the US, and up to five times in Canada.

So climate change is aiding the disease two-fold: helping ticks reproduce, and helping them live in more parts of the US.

4) Beware plentiful acorn seasons

There are other factors that influence how far Lyme can spread: plentiful acorn seasons. It may sound weird, but it’s another fascinating aspect of the ecology that helps Lyme proliferate — and part of why 2017 was a very bad year for the disease.

Tick-carrying mice feast on acorns, and there was a bumper crop for acorns in the US over the past two years. More acorns means there’s more food for mice, helping rodent populations grow and increasing the number of hosts that carry Lyme-infected ticks.

“In the summer following acorn booms, white-footed mouse numbers explode,” according to the Cary Institute of Ecosystem Studies. The heavy mouse populations mean more feeding opportunities for ticks, Cary Institute disease ecologist Rick Ostfeld explained in a statement. “And that means that two years following a good acorn crop we see high abundance of infected ticks, which represents a risk of human exposure to tick borne disease.”

To protect yourself, the CDC suggests avoiding areas with lots of mice and deer, and checking your body for ticks after you’ve been in wooded areas where you might have gotten bitten. If you find a tick on your body, the CDC recommends using tweezers to carefully remove it. Wearing insect repellent and clothing that covers your body can help keep ticks away, too.

At a broader level, policymakers can help drive down the risk of Lyme infection, Walter said, and tackling climate change should be a top priority.