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Updated: May 1, 2026
Author Spotlight: Controlled Human Exposure Model for Tick Research and Lyme Disease Studies
Published on: December 1, 2023
Adverse moisture events predict seasonal abundance of Lyme disease vector ticks (Ixodes scapularis)
Kathryn A Berger1, Howard S Ginsberg, Katherine D Dugas
1Center for Vector-Borne Disease, University of Rhode Island, 02881 Kingston, RI, USA. kaberger@ucalgary.ca.
Background:
Lyme borreliosis (LB) is the most commonly reported vector-borne disease in north temperate regions worldwide, affecting an estimated 300,000 people annually in the United States alone. The incidence of LB is correlated with human exposure to its vector, the blacklegged tick (Ixodes scapularis). To date, attempts to model tick encounter risk based on environmental parameters have been equivocal. Previous studies have not considered (1) the differences between relative humidity (RH) in leaf litter and at weather stations, (2) the RH threshold that affects nymphal blacklegged tick survival, and (3) the time required below the threshold to induce mortality. We clarify the association between environmental moisture and tick survival by presenting a significant relationship between the total number of tick adverse moisture events (TAMEs - calculated as microclimatic periods below a RH threshold) and tick abundance each year.
Methods:
We used a 14-year continuous statewide tick surveillance database and corresponding weather data from Rhode Island (RI), USA, to assess the effects of TAMEs on nymphal populations of I. scapularis. These TAMEs were defined as extended periods of time (>8 h below 82% RH in leaf litter). We fit a sigmoid curve comparing weather station data to those collected by loggers placed in tick habitats to estimate RH experienced by nymphal ticks, and compiled the number of historical TAMEs during the 14-year record.
Results:
The total number of TAMEs in June of each year was negatively related to total seasonal nymphal tick densities, suggesting that sub-threshold humidity episodes >8 h in duration naturally lowered nymphal blacklegged tick abundance. Furthermore, TAMEs were positively related to the ratio of tick abundance early in the season when compared to late season, suggesting that lower than average tick abundance for a given year resulted from tick mortality and not from other factors.
Conclusions:
Our results clarify the mechanism by which environmental moisture affects blacklegged tick populations, and offers the possibility to more accurately predict tick abundance and human LB incidence. We describe a method to forecast LB risk in endemic regions and identify the predictive role of microclimatic moisture conditions on tick encounter risk.
Insights
Tick adverse moisture events (TAMEs) significantly impact blacklegged tick populations. Understanding microclimatic humidity thresholds and durations is key to predicting tick abundance and Lyme disease risk.
Area of Science:
- Environmental Science
- Entomology
- Epidemiology
Background:
- Lyme borreliosis (LB) is a prevalent tick-borne illness in temperate regions, with incidence linked to blacklegged tick (Ixodes scapularis) exposure.
- Previous models of tick encounter risk have been limited by not accounting for microclimatic humidity differences or thresholds affecting tick survival.
- This study addresses these gaps by examining the relationship between environmental moisture and tick survival.
Purpose of the Study:
- To clarify the association between environmental moisture and blacklegged tick survival.
- To introduce the concept of Tick Adverse Moisture Events (TAMEs) as a predictor of tick abundance.
- To improve the prediction of human Lyme disease risk.
Main Methods:
- Utilized a 14-year tick surveillance database and weather data from Rhode Island, USA.
- Defined TAMEs as periods exceeding 8 hours below 82% relative humidity in leaf litter.
- Employed a sigmoid curve to estimate microclimatic RH experienced by ticks and calculated historical TAMEs.
Main Results:
- A negative relationship was found between the number of TAMEs in June and total seasonal nymphal tick densities.
- Sub-threshold humidity episodes significantly reduced nymphal tick abundance.
- TAMEs positively correlated with the early-to-late season tick abundance ratio, indicating mortality as the cause of lower abundance.
Conclusions:
- Environmental moisture, specifically TAMEs, directly influences blacklegged tick population dynamics.
- This research provides a method for more accurate prediction of tick abundance and Lyme disease incidence.
- Microclimatic moisture conditions are identified as crucial predictors of tick encounter risk.
