Across most of the Northeast, getting bitten by a blacklegged tick—also called a deer tick—is a risk during spring, summer, and fall. A new Dartmouth study, published in Parasites & Vectors, finds that 50% of adult blacklegged ticks carry the bacteria that causes Lyme disease while up to 25% of the younger (nymph) blacklegged ticks carry the bacteria.

A team of researchers from universities, health departments, and agricultural agencies from across the Northeast conducted a meta-analysis of data from 1989 to 2021 on how many blacklegged ticks there are and how many of them have the potential to pass pathogens responsible for Lyme disease and three other tick-borne diseases in the Northeast. The states were Connecticut, New York, New Hampshire, Vermont, and Maine.

Data was collected in Maine starting in 1989, while most of the other states began data collection in the mid- 2000s. Massachusetts and Rhode Island were not represented in the study due to either unavailable or insufficient data.

Lyme disease was first discovered in Lyme, Conn., in 1975. Its symptoms can vary depending on the stage and severity of the disease but can include a rash, fever, chills, fatigue, muscle or joint aches, and swollen lymph nodes. If left untreated, prolonged and more severe symptoms may develop.

Lyme disease is caused by a bacteria called Borrelia burgdorferi. Some but not all, white-footed mice, chipmunks, birds, squirrels, and other small animals carry the bacteria in their blood, making them “competent” hosts. 

Blacklegged ticks are not born infected with the Lyme disease bacteria. But when a blacklegged tick feeds on an infected host, the tick can get the bacteria that causes Lyme disease and then potentially spread it to humans through its bite. Other animals, like white-tailed deer, are “incompetent hosts,” so while they are a food source for blacklegged ticks, they do not transmit the Lyme disease bacteria. 

Blacklegged ticks typically consume three blood meals over the course of a two-year life cycle: after they hatch into larvae in the midsummer of their first year; as nymphs during the following late spring, often in May or June; and as adults that fall, most likely between September and November.

In general, ticks must be attached to a person for at least 24 hours to transmit the Lyme disease bacteria. So even though adult blacklegged ticks, which are about the size of a sesame seed, are more likely to carry the bacteria, there is particular concern about the younger ticks or nymphs, which are only about the size of a poppy seed, making them difficult to spot.

“While the bacteria responsible for Lyme disease has a complicated chain of transmission, our results show the relative abundance of blacklegged ticks, and just how many of them are carrying disease-causing pathogens throughout the Northeast,” says lead author Lucas Price, who was a postdoctoral fellow in geography at Dartmouth at the time of the study and is now a wildlife biologist at the Interior Department’s Bureau of Land Management. 

The researchers analyzed the abundance of blacklegged ticks and the presence of Lyme disease bacteria and other pathogens so that they could determine how blacklegged ticks and the pathogens they carry are changing in time and space.

“Contrary to the well-documented spread of blacklegged ticks and Lyme disease over the past 30 years, we found very small changes in the abundance of blacklegged ticks, but think this is likely because we usually don’t start sampling a location for blacklegged ticks until they’re already established,” says senior author Jonathan Winter, an associate professor of geography and director of the Applied Hydroclimatology Group at Dartmouth. “However, we did find an increase in the percentage of blacklegged ticks that carry the Lyme disease bacteria.” 

These findings underscore advice from the Centers for Disease Control and Prevention and health professionals, who recommend a range of tick bite prevention measures, including conducting full-body tick checks after spending time outdoors in regions where pathogen-carrying ticks are present. While much of the data were already available publicly prior to the study, the team made the surveys consistent across states, creating one of the most comprehensive tick abundance and pathogen prevalence datasets in the United States, and establishing a baseline that can be used in the future.

 The researchers have another study underway examining the relationship between climate change and the prevalence of blacklegged ticks and Lyme disease in the Northeast.

Joseph Savage, a graduate student in the Ecology, Evolution, Environment & Society program, also contributed to the study.