Lyme disease (LD) is by far the most frequently reported vector-borne disease in the United States, with >300,000 new cases occurring annually (Kuehn et al. 2013). LD is the most frequently reported vector-borne disease, with notable increases in recent decades (Anna et al. 2012, Anonymous 2012). Although many factors contribute to the geographic spread and increased incidence of tick-borne zoonoses among military personnel and civilians, climate change could be an important driver (reviewed by Ostfeld and Brunner 2015). The sensitivities of the pathogens, vectors, and reservoir hosts to changing climatic conditions are poorly understood. The project team used carefully controlled field and lab studies, including experimental warming, to generate data and models on responses by all blacklegged tick (BLT) life stages to climatic conditions.
The specific research objectives of this project were to: 1) Determine how each stage in the tick life cycle is affected by current and future (warmer) climate, 2) Integrate empirical results with a demographic model to: a. Identify the most vulnerable transitions/stages to target in control interventions and b. Characterize the phenology and climatic conditions that lead to increased human risk, 3) Determine the effects of climate on key aspects of transmission of the LD agent, 4) Incorporate these results into an epidemiological model (of R0) to understand the conditions in which LD can and cannot become established, and lastly; 5) Determine experimentally whether climate warming will increase the probability of tick population growth and LD emergence.
Three Department of Defense installations, Fort Drum (FD) in northern New York, West Point (WP) in southeastern New York, and Camp Lejeune (CL) in coastal North Carolina, were selected to investigate the impact of current climatic conditions on the survival, development, and reproduction (oviposition) of all life stages of the BLT. BLTs were placed in tick containers (TC) which were embedded in the ground, flush to the soil surface, at all three sites, with temperature and relative humidity dataloggers placed in TCs. TCs were deployed at stage-appropriate seasons for four years and destructively sampled to monitor tick disposition as a function of abiotic conditions experienced over specified time periods. Hazard of mortality was modeled for each stage, site, and year, using Bayesian statistical approaches. Additional field and laboratory studies were conducted in which temperatures experienced by ticks were manipulated to mimic warming conditions.
Overall, survivorship was high for overwintering nymphs and flat adults and low for flat nymphs and flat larvae over each lifestage’s respective deployment span. There were numerous TCs with zero ticks recovered for flat larva and flat nymph deployments. Conversely, overwintering nymph deployments had none and flat adult deployments had relatively few. Patterns of BLT mortality were highly variable between TCs within sites, between sites and between years. Tick mortality was often unassociated with identifiable abiotic conditions, and ticks showed notable resilience to climatic conditions at all sites. Periods of unusually high temperature and vapor pressure deficit increased tick mortality, but such conditions were infrequently observed. Higher temperatures, observed naturally and experimentally imposed, accelerated tick development and could promote population growth. Results from both the laboratory manipulation and field warming experiment suggest that climate warming will increase immature BLT molting success. The higher molting success in the warmed treatments relative to the controls suggests that the impact of climate warming will vary latitudinally, and that for northern latitudes molting success will increase with increased climate warming.
As climatic conditions change with anthropogenic warming trends, the data suggest that forces in installations experiencing warmer, drier conditions will be under decreasing probabilities of encountering BLTs and hence acquiring tick-borne pathogens. Such locations may include CL. In contrast, forces present at WP, FD, and other installations outside this warmer, drier zone are likely to experience continued high risk of tick-borne diseases, perhaps increasing as a function of accelerated transition rates of BLTs under warming conditions.
Anna, M. M., J. D. Escobar, and A. S. Chapman. 2012. Reported vectorborne and zoonotic diseases, U.S. Air Force, 2000-2011. Msmr 19:11-12; discussion 12-14.
Anonymous 2012. Reported vectorborne and zoonotic diseases, U.S. Army and U.S. Navy, 2000-2011. Msmr 19:15-16.
Kuehn, B. M. 2013. CDC estimates 300,000 US cases of Lyme disease annually. Journal of the American Medical Association 310:1110.
Ostfeld, R., and J. Brunner. 2015. Climate change and Ixodes tick-borne diseases of humans. Philosophical Transactions of the Royal Society B 370:20140051. DOI:10.1098/rstb.2014.0051.