Adverse moisture events predict seasonal abundance of Lyme disease vector ticks (Ixodes scapularis).

Howard S Ginsberg,Kathryn A Berger,Lutz H Hamel,Thomas N Mather,Katherine D Dugas

doi:10.1186/1756-3305-7-181

Howard S Ginsberg, Kathryn A Berger + Show 3 more

Open Access

https://doi.org/10.1186/1756-3305-7-181

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Abstract

BackgroundLyme 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.MethodsWe 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.ResultsThe 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.ConclusionsOur 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.

Highlights

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
We examined the relationship between extended periods of sub-threshold atmospheric moisture and tick populations using 14 years of nymphal I. scapularis surveillance data collected in Rhode Island (RI)
Total number of Tick adverse moisture event (TAME) recorded during June of those years averaged 11.6 events (Table 1)

Summary

Introduction

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. In the case of the blacklegged tick (Ixodes scapularis), the primary vector of Lyme disease, babesiosis, and human anaplasmosis in North America, authors have suggested several factors that influence vector survival and, abundance. An entomological index has previously been developed for Rhode Island (RI), USA, providing a measure of infected nymphal I. scapularis encountered per unit of time sampled [10] This index was strongly predictive of LB risk for the state, supporting the hypothesis that case distribution was largely a function of peridomestic risk. Mather et al [10] observed extreme variability of tick abundance in similar habitats, suggesting that additional environmental factors may regulate tick population dynamics Given their diminutive size, I. scapularis nymphs can display sensitivity to conditions of low environmental moisture, with laboratory studies confirming greater nymphal mortality under low humidity conditions [11,12]

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