Photo Credit: Jim Gathany/Centers for Disease Control and Prevention
In the early 1970s, a group of families living in Connecticut were suffering from an odd range of symptoms: skin rashes, headaches, swollen knees, paralysis and severe chronic fatigue. Looking for answers, two of the mothers started doing their own research; soon after, scientists joined their quest and called their condition Lyme disease, after the town where they lived.
A decade later, entomologist Willy Burgdorfer discovered the families’ symptoms were caused by a bacterium transmitted by the deer tick, or black-legged tick (Ixodes scapularis). Although this gave patients antibiotics as a treatment option, it was only the first step on what is proving to be a long road to understanding the disease.
Lyme disease has since become a big problem in northern latitudes, and as we see more and more cases moving further north, there are signs that it could be the first epidemic of climate change.
CDC map showing the risk of Lyme disease in the United States, particularly its concentration in the Northeast Megalopolis and western Wisconsin. In 2016, the CDC noted that the “development and survival of ticks, their animal hosts (such as deer), and the bacterium that causes Lyme disease are all strongly influenced by climatic factors, especially temperature, precipitation, and humidity.” (image: The Center for Disease Control and Prevention/National Park Service)
Lyme disease is the most commonly reported vector-borne disease in the United States, and the health care system spends up to $1.3 billion a year on treating it—about $3,000 a patient in the year following their diagnosis.
When a person is infected, they go through stages of symptoms, the most recognizable being the “bull’s eye rash” (erythema migrans) found at the bite site in about half of all cases. Other symptoms are flu-like—sore throat, fever, headache and fatigue—making it difficult to diagnose without a clear indication of a tick bite. Currently, lab tests detect antibodies rather than the bacteria, and they’re not very good: Average test accuracy ranges from 30.6-86.2 percent.
The bacterium that causes Lyme disease, Borrelia burgdorferi, is tiny but complex. Its elegant spiral shape can shift into something rounder or granular, depending on its environment. Cells can join together to form slimy biofilms—protective layers that help it survive the onslaught of antibiotics. The bacteria can live inside animal cells, making it even harder to catch.
But what makes it more complex is how the bacterium survives and gets transmitted. B.burgdorferi circulates between the tick and an animal host because it can’t pass from tick to offspring directly. When a tick larva feeds on an infected animal, like a deer or a mouse, the bacteria settle in its midgut, where they stay as the larva molts into a nymph and hunkers down for winter. When the nymph feeds on a new animal in the spring, it passes on the infection, completing the cycle.
“The boldest writer of science fiction could not invent a creature so ingenious, whose existence is entwined with that of so many other species,” wrote Arno Karlen, author of Biography of a Germ. It’s this intertwined life cycle that is linking the spread of Lyme disease to climate change: The ticks and host animals are moving with the changing climate, taking the disease with them.
Reaching New Habitats
“Lyme disease is moving, breaking out, spreading like an epidemic,” writes Mary Beth Pfeiffer, author of Lyme: The First Epidemic of Climate Change, in the digital magazine Aeon, and it seems it’s partly down to climate change. According to the report “Climate Change Indicators in the United States, 2014: Third Edition,” published by the U.S. Environmental Protection Agency, “The incidence of Lyme disease in the United States has approximately doubled since 1991, from 3.74 reported cases per 100,000 people to 7.01 reported cases per 100,000 people in 2012.”
The report notes that deer ticks are mostly active when temperatures reach 45˚F or higher and humidity is at or above 85 percent, concluding that “warming temperatures associated with climate change could increase the range of suitable tick habitat, and are therefore one of multiple factors driving the observed spread of Lyme disease.” As such, in 2014 the EPA named Lyme disease one of four new indicators to track the impact of climate change.
It’s not just an American problem; Lyme disease appears to be spreading globally. In western Europe, the official number of cases is about 85,000 a year, but 2016 estimates put the number at more like 232,000. The number of cases is rising on the whole, from fewer than 5,000 in 1990 to 35,000 in 2010, according to the World Health Organization.
In the U.S., there’s an increase in cases every year, from 10,000 new cases reported in 1995 to 30,000 new cases in 2014. But these official numbers are dwarfed by the Centers for Disease Control and Prevention’s estimate for cases diagnosed by doctors (up to 376,000) and confirmed by lab evidence (up to 444,000). Just like in Europe, the disease is spreading with its vector: In 1996, the CDC reported deer ticks in 396 U.S. counties; by 2015, they had reached a total of 842 counties.
Not only that, but the cases are also moving north. In Europe, ticks were previously not found further north than 60.5˚ latitude; by 2008 they had moved 300 miles further in Sweden to 66˚—and the same in Norway. Researchers estimate that the ticks’ range—and therefore the spread of Lyme disease—could reach as far as 70˚ latitude, covering most of Scandinavia.
It’s the move into Canada that’s worrying researchers like Dr. Nicholas Ogden, a senior research scientist at the Public Health Agency of Canada. “There are approximate estimates of over 1,500 cases last year, so it’s starting to become a real public health issue now,” he said. That’s up from 992 reported cases in 2016 and just 144 in 2009, when the disease became notifiable in Canada.
The deer tick first moved into eastern and central Canada in around 2004, linked to an increase in temperatures. Climate change has an effect on the length of the tick’s life cycle: it’s not about how cold the winters are, but how warm it gets in the spring, summer and autumn that makes a difference. In the freezing Canadian winter, the ticks can survive in the litter layer under the snow. But the warmer it gets in the milder seasons in Canada, the more likely the ticks are to be spotted—and the faster they’re moving northward.
Host species like deer and white-footed mice are also moving further north in Canada the warmer it gets, taking the ticks with them. Writing in the Encyclopedia of the Anthropocene, Dr. Dominick DellaSala, president and chief scientist of the Geos Institute in the U.S., and his colleagues explained: “I. scapularis and its host, white-footed mouse (Peromyscus leucopus), are projected to expand northward in Canada by the 2050s at a rate of 3.5–11 km per year.” Projected temperature increases, they say, could double tick populations and push them 200 kilometers northward by the 2020s, and 1,000 kilometers by the 2080s, by which time their habitat is expected to increase northward by “a whopping 2,013 percent.”
“We see evidence that’s suggestive that if we don’t start to monitor this and do something about it early on, there could be a global pandemic down the road,” said Dr. DellaSala.
Yet even stopping where we currently stand, under the Paris Agreement’s goal to keep global warming below 2˚C, the ticks will continue to move further into Canada, according to Dr. Ogden. “Essentially, the ticks will continue to move into southern Canada because there’s warming already built in over the next few decades,” he said. “And because most of the Canadian population live within a few hundred kilometers of the U.S. border, for a lot of people it’s a done deal. For those further north, we need to do everything we can to protect them. If we do control climate change, then there are populations which we can protect from emerging Lyme disease.”
Climate Change: A Public Health Problem
The geographic spread is causing problems for the health care industry. Although there are tests available, they are relatively unreliable, so doctors tend to diagnose Lyme disease on the basis of the presence of a rash and the exposure to infected ticks. But as Lyme disease spreads, it’s reaching areas where doctors have no experience and little knowledge of the infection.
Dr. DellaSala has a vested interest in addressing this problem, having been personally affected by the U.S. Lyme endemic. “My daughter contracted Lyme disease from a tick bite. I work on climate change for a living and my daughter was bitten was bitten by a species that transmits a disease caused by climate change, so it really hit home to me.”
As a parent and a concerned scientist, Dr. DellaSala has been working to raise awareness of Lyme disease in areas where it has not traditionally been found. “When I first started inquiring, most doctors in southwest Oregon were not even aware Lyme disease was in the region,” he said. “There’s an education hurdle in getting the word out there.”
He says we also need better detection, better treatments and better ways to address global warming if we’re going to tackle Lyme disease. “Climate change is not an environmental problem,” he said. “It’s human health, economic impact, social dislocation and social injustice, with environment thrown in there too. This is the multiplicity of impacts we’re at the beginning phase of seeing, even before we hit the 2˚C change, which is on the horizon. We all need to be working on climate change. We need to be getting off of fossil fuels yesterday.”
For Dr. Ogden, it’s important that people recognize the potential disease-related impacts of climate change. “There’s been a lot of dismissal of the possible impact of climate on vector-borne diseases, on the basis that some people have said we’ll all get malaria and dengue because of climate change,” he said. “But I think the emergence of this vector-borne disease in north America should give people pause for thought about dismissing the effects of climate change on vector-borne diseases.”