Understanding Climatic Controls of Blacklegged Ticks and Lyme Disease: Experiments and Models to Quantify Risk in a Changing Climate

Dr. Richard Ostfeld | Cary Institute of Ecoystems Studies

RC-2637

Objective

Anthropogenic climate change is increasing global temperatures, changing weather patterns, and, according to many, increasing the extent of vector-borne disease and impact on human health. This last assertion, however, rests heavily on the assumption that a vector’s current distribution is restricted by climatic conditions. The blacklegged tick (Ixodes scapularis) vectors the agents of several emerging diseases in North America, including Lyme disease (LD), anaplasmosis, and babesiosis. Military personnel are particularly hard hit by Lyme and other tick-borne diseases. It has long been hypothesized that the distribution and abundance of I. scapularis and the pathogens it transmits are limited by climatic extremes, but this hypothesis has never been directly tested.

This project proposes to determine the direct and indirect effects of naturally varying climatic conditions, using experimental warming, on the survival and development of all life stages of I. scapularis, their host-seeking behavior, and pathogen replication and transmissibility. It will then integrate these many effects with a series of models to more fully understand how current and (projected) future climatic conditions shape the distribution, dynamics, and risk of vector-borne disease. The primary research objectives are 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:
    1. Identify the most vulnerable transitions/stages to target in control interventions and
    2. Characterize the phenology and climatic conditions that lead to increased human risk
     
  3. Determine the effects of climate change 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.

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Technical Approach

Proposed research is innovative in design, technique, and in synthesis of field and experimental data to evaluate how climate change may affect tick populations and tick borne diseases. First, no prior studies have attempted to understand the abiotic determinants of survival at all life stages (egg, larva, nymph, adult) of any vector under natural conditions. A broad-scale field study of climate-driven survival of I. scapularis is planned using a spatially distributed design that includes military sites within an endemic zone, sparsely occupied but potentially suitable sites, and sites predicted to be unsuitable for large tick populations. Second, we take advantage of vetted experimental enclosures, which allow ticks the freedom to move into and out of soil layers naturally, facilitating demographic studies in nature. Third, a hazard-based approach to survival and development is used that enables exploration of the separate and combined influences of average and extreme conditions. Fourth, the project will conduct the first experiment that addresses the impact of experimental warming on the persistence and replication rates of tick-borne zoonotic pathogens within the tick vector. Fifth, the influence of climate change on encounter rates between ticks and (surrogate) hosts will be examined, as well as the probability of transmission given such encounters. Sixth, these empirically-derived estimates of climate-dependent tick demography and pathogen transmission will be used to parameterize robust models of tick population growth and R  for tick-borne disease spread. This will facilitate an understanding of the importance of each life stage to tick population growth and R , and to project spread in future climates. Lastly, conditional (Bayesian) modeling framework provides an honest accounting of parameter uncertainty.

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Benefits

These efforts will provide evidence-based predictions concerning the rate of expansion (or contraction) of tick populations and LD risk in the eastern USA, focusing on military lands, under climate change. Accurately projecting the locations and timing of the spread of tick populations will facilitate advance warning of encroaching risk, enabling early deployment of educational materials or targeted interventions. This research will identify key vulnerabilities in the life cycle of the tick vector, which can be targeted in tick-control efforts. These results will facilitate mitigation, prevention, and educational efforts to reduce the burden of tick-borne disease in military personnel and civilians.

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Points of Contact

Principal Investigator

Mr. Richard Ostfeld

Cary Institute of Ecoystem Studies

Phone: 845-677-7600 x136

Program Manager

Resource Conservation and Resiliency

SERDP and ESTCP

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