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