Photo: Graham Hickling
Lyme disease is one of the most devastating tick-borne infections in the United States, affecting more than 300,000 people each year. It’s also one of the most mysterious: The creature that spreads it—the black-legged tick—lives throughout the country. Yet the northeastern United States is home to far more cases than anywhere else. Now, researchers have identified an unexpected reason: lizards.
Black-legged ticks (Ixodes scapularis), also known as deer ticks, carry corkscrew-shaped bacteria that cause Lyme disease. The ticks pick up the pathogens—spirochetes that belong to the genus Borrelia—when they suck the blood of animals like mice, deer, and lizards. In the next stage of their life cycle, the ticks may latch onto an unlucky human. But every host transmits the microbes differently. Reptiles are worse transmitters than mammals, so ticks that have lived on reptiles are less likely to make people sick.
The north-south divide in Lyme cases is a fairly sharp line right along the border of Virginia and North Carolina. Researchers have hypothesized that disparity in cases stems from ticks feeding on different hosts in the two regions.
To test the idea, Jean Tsao, a disease ecologist at Michigan State University, and colleagues conducted an extensive study of eastern ticks—their abundance, behavior, and hosts—over 2 years at eight field sites across the United States. They found a clear divide in ticks’ preferred hosts and behavior south of Virginia, matching the pattern of both tick infections and Lyme disease.
The stark difference seems to be tied to one host in particular. In the northeast, black-legged ticks latch onto small mammals like the white-footed mouse, which are notorious for transmitting the Lyme disease bacteria to the bugs. But in the south, the ticks prefer to feed on lizards, particularly skinks. These sleek, smooth-scaled reptiles often live in leaves and twigs that have fallen on the ground—so-called leaf litter—and are particularly poor transmitters of the Lyme pathogens. So fewer southern ticks are infected and fewer people get sick—the team reported last week in PLOS Biology.
The researchers took the right approach to solving the mystery, says Andrea Swei, a disease ecologist at San Francisco State University who was not involved with the study. “They’re comparing apples to apples here, and this allows them to say a lot about host association patterns across a large geographic area.”
In a previous study, Tsao and her colleagues observed that northeastern and southeastern ticks also seek hosts differently. In the south, the bugs stayed under the forest litter to avoid dehydration from the heat. Northern ticks were more outgoing, climbing onto leaves and twigs, where they were much more likely to encounter and bite humans. That, combined with fewer lizards, makes ticks “a double whammy” in the northeast, Tsao says.
“The quirks of tick ecology have consequences for human health,” says study co-author Howard Ginsberg, an ecologist at the U.S. Geological Survey’s Patuxent Wildlife Research Center. He hopes the work will inform efforts to both track and reduce the spread of Lyme disease.
Climate change may alter these patterns, Swei notes. Observations show northeastern ticks have already been expanding their ranges. At the same time, the researchers speculate that a warming climate could alter tick behaviors and the presence of particular hosts, upending the patterns of Lyme disease incidence. It is important to keep an eye on the regions around the north-south divide, Swei says. “As that zone shifts,” she says, “it would really change the disease risk for people that live right on that border.”