Lyme disease was first identified in 1976 in Lyme, Conn. Since then, the disease has become more common in North America, spreading throughout the northeastern and midwestern United States, creeping north into Canada and south into Virginia. About 30,000 cases are reported each year, although it’s estimated that the true prevalence of the disease may be as high as 10 times that number.
Symptoms of Lyme disease include fever, headache, fatigue, and a characteristic target-shaped skin rash. If left untreated, infection can spread to the joints, heart, and nervous system, although most cases of Lyme disease can be treated with antibiotics.
Lyme disease is spread through the bite of a black-legged tick (sometimes called a deer tick, for one of their favored hosts) infected with the bacteria Borrelia burgdorferi or Borrelia mayonii. The main species of bacteria that cause Lyme disease—Borrelia burgdorferi—isn’t anything new: Researchers think this species may have been in North America for more than 20,000 years. So why are human cases of Lyme disease much more recent?
Although there are likely many contributors to the emergence of the disease (including climate change), researchers think changes in land use have played a major role.
That’s why scientists like Pyrros Telionis, a postdoctoral researcher at the University of Virginia’s Biocomplexity Institute, are studying how land cover affects Lyme disease risk.
Telionis said Lyme disease is becoming a major problem in southwestern Virginia, where he is located. “We didn’t have any Lyme at all until about 2000,” he said. “And after 2007, it just shot up to about 5 times the rate it was at in the early aughts. Now [southwestern Virginia has] been a persistent hot spot for more than 10 years.”
In a recent study, Telionis and colleagues used the Virginia Geographic Information Network’s open 1-meter land cover data set to determine how the habitats surrounding people’s homes affected their risk of Lyme disease.
The researchers found that having a home within 100 meters of a high density of forest–herbaceous edge (where a forest meets a field, e.g.) was strongly associated with increased risk of Lyme disease. These results will be presented 9 December at AGU’s Fall Meeting 2020.
What Makes the Forest Edge So Dangerous?
Forest edge habitat, Telionis explained, is an ideal spot for the white-footed mouse, a species on which ticks often feed.
“The more you fragment a forest, the lower the species diversity will be,” said Telionis. “And it turns out that’s actually quite good for the mouse.” Telionis said reduced biodiversity limits the number of species that compete with the mouse for resources, as well as limiting predator species like coyotes and foxes that prey on the mice. “The more you fragment an area by making [e.g.] a subdivision that has a lot of trees mixed with greens, the more you reduce almost all of those [species], except for the mice.”
Maria Diuk-Wasser, a disease ecologist at Columbia University who was not involved in Telionis’s study, said that high-resolution data sets like the one used in his research are very important for understanding the impact of landscape on Lyme disease transmission, especially in urban settings.
Hosts Interact with Landscapes, and with Each Other
While forest fragmentation is definitely an important factor, Diuk-Wasser said there’s still a lot that we don’t know about how the landscape—and the creatures moving throughout the landscape—affects Lyme disease transmission.
In part, this is because the black-legged tick, which transmits Lyme disease, has a fairly long and complicated life cycle. The tick, which lives for about 2 years, goes through three different life stages after hatching and needs to find a new host at each stage. A variety of different animals can serve as hosts—birds, reptiles, rodents, cats, and dogs, as well as large mammals like humans and deer.
Different hosts may prefer different habitats, interact with the landscape in different ways, and have different effects on the spread of Lyme disease. Ticks generally aren’t born with the bacteria that cause Lyme disease; they have to pick it up from a host. Some hosts, like the white-footed mouse, often have very high infection rates, while other hosts, like deer, are not infected at all.
Telionis said that most people get infected with Lyme disease near their homes. Therefore, understanding how the structure of a landscape affects the transmission of Lyme disease could not only help people assess their personal risk but also help developers, urban planners, and policymakers improve the design and management of these landscapes to protect populations from disease.
Diuk-Wasser agreed, and also said it’s important for researchers and urban planners to better understand how people interact with their landscapes—the yards, greenbelts, and parks in a city or neighborhood—and the potential Lyme disease hosts that inhabit them. For example, further research may investigate how structures like paved trails and fences could prevent people from coming into contact with ticks and their hosts in parks and backyards.
—Hannah Thomasy (@HannahThomasy), Science Writer