Climate change and Ixodes tick-borne diseases of humans

Abstract

The evidence that climate warming is changing the distribution of Ixodes ticks and the pathogens they transmit is reviewed and evaluated. The primary approaches are either phenomenological, which typically assume that climate alone limits current and future distributions, or mechanistic, asking which tick-demographic parameters are affected by specific abiotic conditions. Both approaches have promise but are severely limited when applied separately. For instance, phenomenological approaches (e.g. climate envelope models) often select abiotic variables arbitrarily and produce results that can be hard to interpret biologically. On the other hand, although laboratory studies demonstrate strict temperature and humidity thresholds for tick survival, these limits rarely apply to field situations. Similarly, no studies address the influence of abiotic conditions on more than a few life stages, transitions or demographic processes, preventing comprehensive assessments. Nevertheless, despite their divergent approaches, both mechanistic and phenomenological models suggest dramatic range expansions of Ixodes ticks and tick-borne disease as the climate warms. The predicted distributions, however, vary strongly with the models' assumptions, which are rarely tested against reasonable alternatives. These inconsistencies, limited data about key tick-demographic and climatic processes and only limited incorporation of non-climatic processes have weakened the application of this rich area of research to public health policy or actions. We urge further investigation of the influence of climate on vertebrate hosts and tick-borne pathogen dynamics. In addition, testing model assumptions and mechanisms in a range of natural contexts and comparing their relative importance as competing models in a rigorous statistical framework will significantly advance our understanding of how climate change will alter the distribution, dynamics and risk of tick-borne disease.

Keywords: Lyme disease; climate warming; disease ecology; global climate change; tick-borne disease.

Figure 1.

Conceptual model linking climate change to tick-borne disease in humans. Each arrow linking boxes represents an opportunity to fill in knowledge gaps. For example, determining how climate change at global and regional scales influences the key abiotic variables for ticks at local scales is critical. A mechanistic understanding of how specific abiotic variables interactively determine demographic, developmental and behavioural processes in tick vectors and dynamics of tick-borne pathogens requires further development. How climate-driven dynamics of ticks and pathogens affect risk of human exposure, how non-climatic factors influence risk, and how risk is translated into disease incidence are crucial variables for further study. (Online version in colour.)

Figure 2.

Key processes in the life cycle of ixodid ticks controlling their population dynamics and effects on risk of exposure to tick-borne pathogens. Arrows indicate the potential for temperature and humidity variables to affect each process. (Online version in colour.)