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Tick-Borne Diseases: Jumping to Climate Change Conclusions
Volume 9, Number 25: 21 June 2006

Sarah Randolph of the University of Oxford's Department of Zoology writes that "over the past two decades, tick-borne diseases have increased and now constitute a major health problem in many parts of Europe," and that "because climate has changed globally over the same decades, the common assumption is that climate change is the cause of increased incidence of these, and many insect-borne, infections, but this has rarely been tested by appropriate retrospective analyses." Hence, she proceeds to do just that in her mini-review of the subject (Randolph, 2004).

For starters, Randolph notes that ticks "seasonal abundance and the pace and rate of pathogen transmission, can only be predicted with fully functional tick population and pathogen transmission models, which we do not yet have [our italics]," and that "a further complication arises from host responses to climate, especially rodent population dynamics that are differentially susceptible to climate variables at different times of the year." On the basis of biological process-based models, therefore, she concludes "it is not yet possible to predict whether the incidence of any tick-borne disease will increase or decrease at actual levels of climate change in any one place."

Looking at what has already happened in the real world also complicates matters. Randolph reports, for example, that tick-borne encephalitis (TBE) incidence "showed a 3-fold step increase from 1983 to 1986 in Sweden, doubled in 1993 in the Czech Republic, increased even more dramatically in the same year in Lithuania and Poland, but declined markedly in 1997 in Hungary, Croatia and Slovenia," and that "within each country, TBE incidence has changed to different degrees in different regions." In addition, she says that "climate data do not reveal any obvious differences between sites where TBE did or did not 'emerge', and in Sweden increases in TBE pre-dated the onset of warmer springs and winters."

Changes in certain biotic factors, however, do provide some tantalizing inferences, a case in point being the significant increase in the density of roe deer (which are the principal host for adult I. ricinus ticks, and are therefore crucial to tick population maintenance) across most of Europe, including Scandinavia. "Data from Denmark," in Randolph's words, "offer the best documented evidence for the impact of increasing densities of deer on both temporal and spatial variation in the rise of a tick-borne disease in Europe, this time Lyme borreliosis [LB] (Jensen and Frandsen, 2000; Jensen et al., 2000)." From 1984 to 1998, for example, Randolph notes that "an increase in LB paralleled an increase in deer density," and that "spatial variation in tick density across 35 sites in 1996, and in LB cases across 12 countries in 1993-95, was also correlated with deer density," which "accords with the seminal role attributed to white-tailed deer in the emergence of LB in the USA (Spielman et al., 1985; Wilson et al., 1985)."

In yet another vein, Randolph writes that "it is impossible not to be impressed by the striking coincidence between the extreme increases in tick-borne diseases and the end of communist rule in eastern Europe," noting that "a number of epidemiologically relevant sociological changes resulted from this transition, including changes in agricultural practices and in public health services, and the increase in both poverty and wealth with their impact on work and leisure activities." Nevertheless, she cautions that "before we can safely conclude this [i.e., that "the fall of communist rule at the end of the 20th century may have stimulated a dramatic rise in infectious disease in eastern Europe"], we need good data on sociological factors at resolutions to match our epidemiological observations."

Clearly, levels of tick-borne diseases the world over are dependent on so vast an array of pertinent factors that it is the height of simple-minded folly to attribute their ups and downs to a singular overriding factor such as global climate change. In fact, it is likely counter-productive, encouraging the implementation of ineffective measures to combat climate change that result in a siphoning of funds from more promising programs that without a doubt could do a much better job of both identifying the true agents of change in the levels of various infectious diseases and ameliorating their negative consequences.

Sherwood, Keith and Craig Idso

Jensen, P.M. and Frandsen, F. 2000. Temporal risk assessment for Lyme borreliosis in Denmark. Scandinavian Journal of Infectious Diseases 35: 539-544.

Jensen, P.M., Hansen, H. and Frandsen, F. 2000. Spatial risk assessment for Lyme borreliosis in Denmark. Scandinavian Journal of Infectious Diseases 35: 545-550.

Randolph, S. 2004. Evidence that climate change has caused "emergence" of tick-borne diseases in Europe? International Journal of Medical Microbiology 293, Supplement 37: 5-15.

Spielman, A., Wilson, M.L., Levine, J.F. and Piesman, J. 1985. Ecology of Ixodes dammini-borne human babesiosis and Lyme disease. Annual Review of Entomology 30: 439-460.

Wilson, M.L., Adler, G.H. and Spielman, A. 1985. Correlation between abundance of deer and that of the deer tick, Ixodes dammini (Acari: Ixodidae). Annals of the Entomological Society of America 78: 172-176.