Is Global Warming Driving Outbreaks of Tick-Borne Diseases?

In a brief review of the roles played by various factors that may influence the spread of tick-borne diseases, Sarah Randolph (2010) of the University of Oxford's Department of Zoology in the United Kingdom begins by noting that many vector-borne diseases "have shown marked increases in both distribution and incidence during the past few decades, just as human-induced climate change is thought to have exceeded random fluctuations." And she writes, in this regard, that "this coincidence has led to the general perception that climate change has driven disease emergence," which concept has been promoted by numerous climate-alarmist publications and pronouncements. However, she wisely notes that "climate change is the inevitable backdrop for all recent events," most of which no one would ever even dream of attributing to how the planet's temperature may have behaved concurrently.

After describing some of the outbreaks of tick-borne disease in Europe over the past couple of decades, Randolph states that "the inescapable conclusion is that the observed climate change alone cannot explain the full heterogeneity in the epidemiological change, either within the Baltic States or amongst Central and Eastern European countries," citing the work of Sumilo et al. (2007). Instead, she writes that "a nexus of interrelated causal factors -- abiotic, biotic and human -- has been identified," and that "each factor appears to operate synergistically, but with differential force in space and time, which would inevitably generate the observed epidemiological heterogeneity."

Many of these factors, as she continues, "were the unintended consequences of the fall of Soviet rule and the subsequent socio-economic transition (Sumilo et al., 2008b)," among which she cites "agricultural reforms resulting in changed land cover and land use, and an increased reliance on subsistence farming; reduction in the use of pesticides, and also in the emission of atmospheric pollution as industries collapsed; increased unemployment and poverty, but also wealth and leisure time in other sectors of the population as market forces took hold."

In concluding, Randolph says "there is increasing evidence from detailed analyses that rapid changes in the incidence of tick-borne diseases are driven as much, if not more, by human behavior that determines exposure to infected ticks than by tick population biology that determines the abundance of infected ticks [italics added]," as per the findings of Sumilo et al. (2008a) and Randolph et al. (2008). Hence, she ends her brief analysis by stating that "while nobody would deny the sensitivity of ticks and tick-borne disease systems to climatic factors that largely determine their geographical distributions, the evidence is that climate change has not been the most significant factor driving the recent temporal patterns in the epidemiology of tick-borne diseases."

Additional References
Randolph,S.E. 2001. Tick-borne encephalitis in Europe. The Lancet 358: 1731-1732.

Randolph, S.E., Asokliene, L., Avsic-Zupanc, T., Bormane, A., Burri, C., Golovljova, I., Hubalek, Z., Knap, N., Kondrusik, M., Kupca, A., Pejcoch, M., Vasilenko, V. and Zygutiene, M. 2008. Variable spikes in TBE incidence in 2006 independent of variable tick abundance but related to weather. Parasites and Vectors 1: e44.

Sumilo, D., Asokliene, L., Avsic-Zupanc, T., Bormane, A., Vasilenko, V., Lucenko, I., Golovljova, I. and Randolph, S.E. 2008a. Behavioral responses to perceived risk of tick-borne encephalitis: vaccination and avoidance in the Baltics and Slovenia. Vaccine 26: 2580-2588.

Sumilo, D., Asokliene, L., Bormane, A., Vasilenko, V., Golovljova, I. and Randolph, S.E. 2007. Climate change cannot explain the upsurge of tick-borne encephalitis in the Baltics. PLos ONE 2: e500.

Sumilo, D., Bormane, A., Asokliene, L., Vasilenko, V., Golovljova, I., Avsic-Zupanc, T., Hubalek, Z. and Randolph, S.E. 2008b. Socio-economic factors in the differential upsurge of tick-borne encephalitis in Central and Eastern Europe. Reviews in Medical Virology 18: 81-95.