Ticks Aren’t Multiplying, But The Diseases They Carry Are

HANOVER, N.H. — The blacklegged ticks lurking in northeastern forests aren’t necessarily multiplying, but they’re becoming more dangerous. New research reveals that while tick populations have remained surprisingly stable across much of New England and New York, the percentage of ticks infected with disease-causing pathogens has skyrocketed, meaning each tick bite now carries a substantially higher risk of making you sick.

A multi-state research team led by scientists from Dartmouth College compiled and analyzed over three decades of tick surveillance data from Connecticut, Maine, New Hampshire, New York, and Vermont from 1989 to 2022. Their findings, published in Parasites & Vectors, help explain why reported cases of tick-borne diseases like anaplasmosis and babesiosis have more than doubled nationally in recent years, while Lyme disease cases have remained relatively stable in many areas despite all three being transmitted by the same tick species.

The researchers found that while blacklegged tick (also called a deer tick) numbers have remained consistent in southern New England and New York, the percentage of these ticks carrying disease-causing pathogens has climbed significantly. For example, the prevalence of Borrelia burgdorferi, the bacteria causing Lyme disease, in adult ticks increased from about 31% to nearly 54% in New York between 2007 and 2021.

In northern reaches of the region, including parts of Maine and Vermont, both tick abundance and pathogen prevalence are increasing, a double whammy for residents and visitors to those areas. Previous research has also shown that blacklegged ticks have been expanding their range northward as climate and habitat conditions become more favorable.

The team examined both adult ticks and nymphs, the immature ticks about the size of a poppy seed that are responsible for many human infections because they’re harder to spot during tick checks. For the period 2017-2021, they found the statewide average prevalence of B. burgdorferi ranged from 19% to 25% in nymphs and from 49% to 54% in adult ticks.

Lyme disease remains the most common tick-borne illness, but other pathogens show similar upward trends. Anaplasma phagocytophilum, which causes anaplasmosis (a disease that can cause fever, headache, muscle pain and, in severe cases, respiratory and organ failure), increased in prevalence in both nymphs and adults across the study period. Similarly, Babesia microti, the parasite causing babesiosis (which can produce malaria-like symptoms and be life-threatening for the elderly or those with compromised immune systems), also showed increased prevalence.

However, Borrelia miyamotoi, a relatively newly recognized pathogen that causes a relapsing fever illness, showed no significant change in prevalence over the study period.

Public health experts have warned for years about the expanding threat of tick-borne diseases. This study provides concrete evidence that the threat is intensifying even in areas where tick control efforts might appear successful based solely on tick abundance metrics.

Across states, Connecticut showed the lowest overall increase in tick abundance but still experienced rising pathogen prevalence. Maine and Vermont saw both tick populations and pathogen loads increase, particularly in northernmost counties where ticks were previously rare or absent.

New York had the highest average abundance of both nymphal and adult blacklegged ticks, while Vermont showed a curious pattern of having relatively few nymphs but substantial adult tick populations. Researchers suspect this discrepancy might be due to differences in sampling methods and timing rather than actual ecological patterns.

Standardized tick surveillance protocols could give a more precise look at populations across states. The Centers for Disease Control and Prevention established guidelines for county-level tick surveillance in 2019, which should help future research efforts.

This research supports what many residents of the Northeast have observed anecdotally: tick-borne diseases continue to pose a serious and growing public health threat, even in communities with longstanding tick awareness programs.

It’s not just about how many ticks are in your neighborhood, but what those ticks are carrying. Pathogen prevalence is on an upward climb, and the stakes of each outdoor adventure—and each tick check afterward—have quietly but significantly increased for millions of Americans living in tick country.