Tick-borne diseases are on the rise in the northeastern US, with many ticks carrying more than one pathogen. So reports a recent analysis published in Ecosphere by researchers at Cary Institute of Ecosystem Studies and the SUNY Center for Vector-Borne Diseases at Upstate Medical University.
As part of the study, led by Cary Institute disease ecologist Shannon LaDeau , more than 2,000 blacklegged ticks were screened for a suite of 16 pathogens at the Upstate Tick Testing Laboratory. Ticks were collected over nine years from forested plots at the Cary Institute’s campus in Dutchess County, NY. The site is home to a long-term research project on the ecology of tick-borne disease.
The study focused on nymphal ticks, the life stage most likely to spread tick-borne diseases to people. “Rates of infection tend to be even higher in adult ticks because they have more opportunities to feed and acquire pathogens,” LaDeau notes. “But they are also bigger and so easier for people to detect and remove before they spread disease.’
A key finding is that more nymphal ticks are carrying multiple pathogens. Of the thousands of ticks that were collected between 2014 and 2022, roughly 1 in 10 tested positive for at least two disease-causing pathogens.
“Overall, it’s a concerning co-infection rate that has been increasing over time,” LaDeau noted.
Most commonly found together were Borrelia burgdorferi, the bacterium that causes Lyme disease , and Babesia microti, a parasite that causes a malaria-like disease known as babesiosis . Co-infection with these two pathogens was more frequent than expected, approaching 11% by the end of the study.
These results have important public health implications because the two pathogens require different treatments. Lyme disease is treated with antibiotics such as doxycycline, while Babesia infections are treated with antiparasitic medications.
“In the Northeast, when someone gets sick after a tick bite, medical professionals should be testing for both pathogens to ensure proper diagnosis and treatment,” said LaDeau.
More than 38% of the ticks tested were capable of spreading at least one disease-causing pathogen to people.
“The probability of being exposed to a pathogen by a single bite from a nymphal tick is approaching 40%," said coauthor Richard Ostfeld , a disease ecologist at Cary Institute who has been studying tick-borne diseases for decades. “That seems uncomfortably high. These ticks are so small, most people don’t even realize when one is feeding on them.”
Babesia microti, the parasite behind babesiosis, was the most common, found in more than 21% of the nymphal ticks that were tested. Babesiosis is characterized by fever, chills, sweating, muscle pain, fatigue, and anemia. Most cases are mild, but occasionally, babesiosis can be fatal. Babesia was more widespread than the researchers expected; past research had placed this parasite’s prevalence at less than 10%.
Borrelia burgdorferi, the bacterium that causes Lyme disease, was found in 19.3% of nymphal ticks and was the only pathogen detected in all the study sites and in every year. It is also becoming more common: The odds of a nymphal tick carrying Borrelia burgdorferi increased by an average of 5.7% each year during the study period.
“Babesia infection rates in ticks bounce up and down from year to year, but our results show it is quite common. In the final two years of the study, it was more prevalent than Borrelia,” said LaDeau. “The rise in Babesia infection in our study is consistent with rising numbers of human babesiosis cases in the Northeast.”
Ostfeld was surprised by the yearly variation in infections. “These results further support our contention that the risk of tick-borne disease varies considerably between years,” he said. “Long-term monitoring is the best current method for predicting which years will be really bad and which years not as bad.”
Other pathogens found in the tested ticks included:
Anaplasma phagocytophilum, the bacterium that causes anaplasmosis , an illness characterized by flu-like symptoms such as fever, headache, and muscle aches. Without treatment, more serious symptoms such as seizures, respiratory failure, and internal hemorrhaging can develop. The scientists detected this bacterium in 5.8% of sampled ticks per year.
Borrelia miyamotoi, the cause of Hard Tick Relapsing Fever , was rarer — only 2% of ticks per year carried this pathogen.
Rickettsia rickettsii and Rickettsia parkeri were detected in 10 ticks. This finding is important because these bacteria, which can cause Rocky Mountain Spotted Fever and other types of spotted fever in people, are usually only found in other species of ticks.
Powassan virus was detected in one tick. This rare virus can cause inflammation of the brain (encephalitis) or the membranes around the brain and spinal cord (meningitis).
The researchers note that Bartonella bacteria were not detected in the tested ticks. Bartonella bacteria can cause infections such as cat scratch disease, but it remains unclear whether ticks can spread the bacteria to people. Until now, ticks on the Cary campus had never been tested for Bartonella.
“Some health practitioners in our area and elsewhere suggest that Bartonella is an emerging tick-borne pathogen,” said Ostfeld, “but we see no evidence of that at the Cary Institute sites.”
Co-author Saravanan Thangamani explains, "The pathogens found in ticks collected from Cary's long-term monitoring sites closely match those found in ticks that people encounter and submit to the Upstate Tick Testing Laboratory at SUNY Upstate Medical University. This strong agreement between ecological sampling and public tick submissions underscores that the risks identified in natural environments are reflected in ticks encountered by people.” Thangamani adds, “These data highlight the importance of ongoing monitoring to detect and manage emerging public health risks.”
LaDeau concludes, “Healthcare workers should be on the lookout for rising co-infection risks. 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.”
Supported in part by the U.S. National Science Foundation’s Long Term Research in Environmental Biology (LTREB) program (grants DEB-1947756, DEB-1456527, and DEB-0949702), as well as EPA STAR grant (83489701) and grants from the John Drulle, MD Memorial Lyme Fund, Inc. Pathogen screening was partially funded by Upstate Foundation and SUNY Empire Innovation Professorship funds.
Shannon L. LaDeau - Cary Institute of Ecosystem Studies
Kelly Oggenfuss - Cary Institute of Ecosystem Studies
Alexander Schmidt - Upstate Medical University
Saravanan Thangamani - Upstate Medical University
Richard S. Ostfeld - Cary Institute of Ecosystem Studies
Cary Institute of Ecosystem Studies is an independent nonprofit center for environmental research. Since 1983, our scientists have been investigating the complex interactions that govern the natural world and the impacts of climate change on these systems. Our findings lead to more effective resource management, policy actions, and environmental literacy. Staff are global experts in the ecology of: forests, freshwater, soils, cities, and disease.
Ecosphere
Ecological dynamics of blacklegged ticks, vertebrate hosts, and associated zoonotic pathogens in northeastern forests
17-Dec-2025