Ticks in a changing climate

My first venture into the world of blogging, nearly two years ago now, discussed tickborne diseases in the UK, and mentioned that as ticks flourish in warm wet climates, climate change might increase the risk of such diseases. A colleague recently drew my attention to a review article1 about the effects of climate change on ticks and the diseases that they carry in Europe. This looked at changes that have happened already and those that might happen if, as predicted by the IPCC, temperatures in northern Europe rise by 1.5 to 2.5ºC over the next few decades (if they rise more than that, we will probably have more serious problems to worry about).

Studies in Sweden show that Ixodes ricinus, the common sheep tick that carries Lyme disease, and in some areas tickborne encephalitis (TBE), has already spread northwards by several degrees, and this is expected to continue as the winters become shorter and less cold. An increased incidence of TBE has already been observed in the Stockholm area. However, in the mild climate of the British Isles this tick is already active for much of the year, so it may be less affected here and we may not notice so much difference.

Among other important tick species, Dermacentor reticulatus (vector of a number of different animal and human diseases) benefits from warm summers, has already expanded its range in countries such as Germany, and is expected to do so further; Hyalomma marginatum (vector of Crimean-Congo haemorrhagic fever in the Balkans) appears to require warm autumns and could also spread north; and despite its habit of living in buildings, the brown dog tick Rhipicephalus sanguineus is also limited to southern Europe by summer temperatures and could spread north in a warmer climate.

Few studies from the British Isles are quoted, but data collected in Ireland in the hot summer of 1976 suggest changes in the times of year at which different developmental stages are active; hot dry weather may not reduce I. ricinus activity if there is sufficient vegetation cover.

Not surprisingly, the relationship between climate, the distribution and activity of ticks, and the transmission of tickborne diseases is not simple. As well as temperature and humidity, other factors such as vegetation and the abundance and behaviour of hosts are also important, and these may be affected both by climate and by other factors. For example, in a warm climate people may be more frequently bitten by ticks simply because they spend more time out of doors. On the other hand a study2 of the increase in TBE in the Baltic states in the early 1990s showed too much variation from place to place for climate to be the cause (the authors suggest post-Communist social changes as a more likely explanation). Data may be inadequate — for example Lyme disease is difficult to diagnose and not always reported when it is diagnosed. Mathematical models that do not include all variables can only give an indication of what might happen.

A search of CAB Abstracts and Global Health for relevant articles revealed, among other items, a symposium3 apparently on the subject (one of the editors is also one of the authors of the review I first came across), although many of the articles are about relevant baseline studies rather than directly about climate change. Some of the articles that I found suggested links between climate and tickborne diseases, while others suggested that things were not so clear; one of the symposium papers4 argued that there was 'a plethora of models based on empirical data of variable quality and a disturbing lack of empirical long-term studies'. There still did not appear to be much from the UK, so I remain unsure exactly how my risks of catching Lyme disease in Scotland will be affected.

Overall it seems that climate change will affect tickborne diseases, and in many cases it is likely to increase the risk, but on the whole there is insufficient evidence to make detailed predictions. Of course ticks are not the only disease vectors to be affected by climate change — see for example previous entries on this blog about bluetongue and chikungunya virus.

1: Gray, J. S. et al.:'Effects of climate change in ticks and tickborne diseases in Europe.' Interdisciplinary perspectives on infectious diseases (2009) Article ID 593232. DOI: doi:10.1155/2009/593232

2: Sumilo, D. et al.:'Climate change cannot explain the upsurge of tick-borne encephalitis in the Baltics'. PLoS One (2007) 2 (6) e500. DOI: doi:10.1371/journal.pone.0000500

3: Süss, J. and Kahl, O. (editors): 'Proceedings of the IXth International Jena Symposium on Tick-borne Diseases (formerly IPS), Climate change and tick-borne diseases, Jena, Germany, 15-17 March, 2007'. International Journal of Medical Microbiology (2008) 298 (Supplement 1) pp. 1-378. DOI: doi:10.1016/j.ijmm.2008.05.008

4: Eisen, L.: 'Climate change and tick-borne diseases: a research field in need of long-term empirical field studies'. In: 'Proceedings of the IXth International Jena Symposium on Tick-borne Diseases (formerly IPS), Climate change and tick-borne diseases, Jena, Germany, 15-17 March, 2007'. International Journal of Medical Microbiology (2008) 298 (Supplement 1) pp. 12-18. DOI: doi:10.1016/j.ijmm.2007.10.004

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