Variable spikes in tick-borne encephalitis incidence in 2006 independent of variable tick abundance but related to weather

Background: The incidence of tick-borne encephalitis showed a
dramatic spike in several countries in Europe in 2006, a year that
was unusually cold in winter but unusually warm and dry in summer
and autumn. In this study we examine the possible causes of the
sudden increase in disease: more abundant infected ticks and/or
increased exposure due to human behaviour, both in response to the
weather. Methods: For eight countries across Europe, field data on
tick abundance for 2005-2007, collected monthly from a total of 41
sites, were analysed in relation to total annual and seasonal TBE
incidence and temperature and rainfall conditions. Results: The
weather in 2006-2007 was exceptional compared with the previous two
decades, but neither the very cold start to 2006, nor the very hot
period from summer 2006 to late spring 2007 had any consistent
impact on tick abundance. Nor was the TBE spike in 2006 related to
changes in tick abundance. Countries varied in the degree of TBE
spike despite similar weather patterns, and also in the degree to
which seasonal variation in TBE incidence matched seasonal tick
activity. Conclusion: The data suggest that the TBE spike was not
due to weather-induced variation in tick population dynamics. An
alternative explanation, supported by qualitative reports and some
data, involves human behavioural responses to weather favourable
for outdoor recreational activities, including wild mushroom and
berry harvest, differentially influenced by national cultural
practices and economic constraints.