Ticks capable of carrying and transmitting more than one potentially fatal disease at the same time are becoming increasingly common in the northeastern U.S., according to a new long-term analysis that raises fresh public health concerns for the region.
The research found that a growing share of blacklegged ticks—also known as deer ticks—are infected with multiple disease-causing pathogens. The study was led by Cary Institute of Ecosystem Studies disease ecologist Shannon LaDeau and conducted in partnership with the SUNY Center for Vector-Borne Diseases at Upstate Medical University.
The findings come from nearly a decade of tick surveillance and point to a more complex and potentially dangerous tick-borne disease landscape, particularly because different infections require different treatments.
“Healthcare workers should be on the lookout for rising co-infection risks," LaDeau said in a statement. "And for people spending time outdoors in the Northeast, as a general rule, if the ground is not freezing, it’s a good idea to take precautions to avoid tick bites. Prevention is key.”
In their study, the researchers screened more than 2,000 blacklegged ticks for a suite of 16 pathogens at the Upstate Tick Testing Laboratory. The ticks were collected between 2014 and 2022 from forested research plots on the Cary Institute’s campus in Dutchess County, New York, an area that hosts a long-running project focused on the ecology of tick-borne disease.
The study focused on nymphal ticks, the immature life stage most likely to transmit disease to humans because they are small, difficult to detect and commonly active during spring and early summer, when people spend more time outdoors.
A central finding was the rise in co-infections. Roughly one in 10 nymphal ticks tested positive for at least two disease-causing pathogens. By the end of the study period, co-infections were approaching 11 percent of sampled ticks, higher than researchers expected based on previous work.
“Overall, it’s a concerning co-infection rate that has been increasing over time,” LaDeau said.
The most common pairing was Borrelia burgdorferi, the bacterium that causes Lyme disease, and Babesia microti, a parasite responsible for babesiosis, a malaria-like illness. Co-infection with these two pathogens occurred more frequently than would be expected by chance.
This overlap has important medical implications. Lyme disease is typically treated with antibiotics such as doxycycline, while babesiosis requires antiparasitic medications. If one infection is missed, patients may not respond fully to treatment.
Overall, more than 38 percent of the nymphal ticks tested were found to be capable of transmitting at least one disease-causing pathogen to people.
B. microti was the most common pathogen detected, found in more than 21 percent of ticks. Babesiosis can cause fever, chills, sweating, fatigue, muscle pain and anemia. While many cases are mild, the disease can be severe or even fatal, particularly in older adults or people with compromised immune systems. The prevalence of Babesia was notably higher than past estimates, which had placed it below 10 percent.
B. burgdorferi was found in 19.3 percent of nymphal ticks and was the only pathogen detected at every study site and in every year of sampling. The researchers also found that Lyme disease risk is increasing: the odds of a nymphal tick carrying B. burgdorferi rose by an average of 5.7 percent each year over the study period.
Several other pathogens were detected at lower levels. Anaplasma phagocytophilum, which causes anaplasmosis and can lead to severe complications if untreated, was found in about 5.8 percent of ticks per year. Borrelia miyamotoi, which causes Hard Tick Relapsing Fever, appeared in roughly 2 percent of ticks annually.
“The probability of being exposed to a pathogen by a single bite from a nymphal tick is approaching 40 percent," coauthor Richard Ostfeld, a disease ecologist at Cary Institute, added in a statement. “That seems uncomfortably high.
"These ticks are so small, most people don’t even realize when one is feeding on them.”
Reference
LaDeau, S. L., Oggenfuss, K., Schmidt, A., Thangamani, S., & Ostfeld, R. S. (2025). Ecological dynamics of blacklegged ticks, vertebrate hosts, and associated zoonotic pathogens in northeastern forests. Ecosphere, 16(12). https://doi.org/10.1002/ecs2.70508
