Dynamics of tick-borne disease systems: minor role of recent climate change

Tick-borne disease systems are very sensitive to climate through the impact of temperature and moisture stress on rates of the demographic processes of ticks. There is no a priori reason, however, to expect tick abundance or seasonal activity patterns to respond to climate change in ways that inevitably increase the risk of infection by the transmitted pathogens. Changing host availability may be more important than climate in determining tick abundance. The credibility of any (inherently untestable) predictions of future system-specific changes will be strengthened if based on satisfactory explanations of the past. Tick-borne encephalitis (TBE) in Europe is presented as a case study: observed patterns of climate change are too similar within and between countries to provide the sole explanation for the extreme spatio-temporal heterogeneity of the marked upsurges in TBE incidence over the past two decades. Instead, a nexus of interacting factors affecting both the risk of infection and exposure of humans to that risk, and each differing in force in space and time, is a more powerful model. Many of these factors are driven by socio-economic changes, and include climate, land cover, wildlife, agricultural practices, industrial activities, (un)employment and income. The same principle may apply to the periodic epidemics of Crimean-Congo haemorrhagic fever.