In much of the Northeast, the risk of being bitten by a blacklegged tick, also known as a deer tick, is heightened during the spring, summer, and fall months. A recent study by Dartmouth, published in Parasites and Vectors, reveals that half of the adult blacklegged ticks carry the Lyme disease-causing bacteria, while 20% to 25% of younger ticks (nymphs) harbor the bacteria as well.
A group of researchers from various universities, health departments, and agricultural agencies in the Northeast carried out a meta-analysis evaluating the population of blacklegged ticks and their capability to transmit pathogens responsible for Lyme disease and three other tick-related illnesses from 1989 to 2021. This study focused on several states, including Connecticut, New York, New Hampshire, Vermont, and Maine.
Data collection in Maine commenced in 1989, while most other states started gathering data around the mid-2000s. Massachusetts and Rhode Island were excluded from the study due to either lack of available data or insufficient data quality.
Lyme disease was initially identified in Lyme, Connecticut in 1975. Its symptoms can vary based on the disease’s progression and severity, potentially including rash, fever, chills, fatigue, muscle and joint pains, and swollen lymph nodes. If not treated, more severe and prolonged symptoms may arise.
The disease is caused by the bacterium Borrelia burgdorferi. Certain small mammals, including some white-footed mice, chipmunks, birds, and squirrels, can carry this bacteria in their bloodstream, which makes them “competent hosts.” Blacklegged ticks aren’t born with Lyme disease bacteria. However, when they feed on an infected host, they can acquire the bacteria and then possibly transmit it to humans via bites. On the other hand, white-tailed deer are considered “incompetent hosts,” as they do not transmit Lyme disease bacteria even though they serve as a food source for blacklegged ticks.
Blacklegged ticks generally take three meals of blood over their two-year lifespan: first, as larvae in the summer of their initial year; second, as nymphs in the following late spring (around May or June); and finally as adults in the autumn, typically from September to November.
Generally, a tick needs to remain attached to a person for at least 24 hours for the Lyme disease bacteria to be transmitted. While adult blacklegged ticks are more likely to carry the bacteria due to their larger size (about the size of a sesame seed), there is significant concern regarding nymphs, which are much smaller (about the size of a poppy seed) and therefore harder to detect.
“Although the transmission of the Lyme disease bacteria involves a complex chain, our findings highlight the abundance of blacklegged ticks and the proportion carrying disease-causing pathogens throughout the Northeast,” explains lead author Lucas Price, who was a postdoctoral fellow in geography at Dartmouth during the study and is now a wildlife biologist with the Bureau of Land Management in the Interior Department.
The team investigated both the abundance of blacklegged ticks and the presence of Lyme bacteria and other pathogens, allowing them to assess how tick populations and the pathogens they carry are evolving over time and in different locations.
“In contrast to the extensively documented spread of blacklegged ticks and Lyme disease over the past three decades, we observed only minimal changes in tick abundance. This might be due to the fact that we often begin sampling in locations only after ticks are already well-established,” remarks senior author Jonathan Winter, an associate geography professor and director of the Applied Hydroclimatology Group at Dartmouth. “However, we did observe an increase in the percentage of blacklegged ticks carrying the Lyme disease bacteria.”
These results reinforce recommendations from the Centers for Disease Control and Prevention (CDC) and health experts, who suggest a variety of measures to prevent tick bites, including thorough tick checks after outdoor activities in areas where ticks that carry pathogens are found. Although much of the data was available publicly before the study, the researchers standardized the surveys across different states, leading to one of the most comprehensive datasets on tick populations and pathogen prevalence in the United States, thereby establishing a benchmark for future studies.
The research team is currently also working on another study exploring how climate change affects the prevalence of blacklegged ticks and Lyme disease in the Northeast.
Joseph Savage, a graduate student in Dartmouth’s Ecology, Evolution, Environment & Society program, also contributed to this research.
