Altitudinal patterns of tick and host abundance: a potential role for climate change in regulating tick-borne diseases?
The impact of climate change on vector-borne infectious diseases is currently controversial. In Europe the primary arthropod vectors of zoonotic diseases are ticks, which transmit Borrelia burgdorferi sensu lato (the agent of Lyme disease), tick-borne encephalitis virus and louping ill virus between humans, livestock and wildlife. Ixodes ricinus ticks and reported tick-borne disease cases are currently increasing in the UK. Theories for this include climate change and increasing host abundance. This study aimed to test how I. ricinus tick abundance might be influenced by climate change in Scotland by using altitudinal gradients as a proxy, while also taking into account the effects of hosts, vegetation and weather effects. It was predicted that tick abundance would be higher at lower altitudes (i.e. warmer climates) and increase with host abundance. Surveys were conducted on nine hills in Scotland, all of open moorland habitat. Tick abundance was positively associated with deer abundance, but even after taking this into account, there was a strong negative association of ticks with altitude. This was probably a real climate effect, with temperature (and humidity, i.e. saturation deficit) most likely playing an important role. It could be inferred that ticks may become more abundant at higher altitudes in response to climate warming. This has potential implications for pathogen prevalence such as louping ill virus if tick numbers increase at elevations where competent transmission hosts (red grouse Lagopus lagopus scoticus and mountain hares Lepus timidus) occur in higher numbers.
KeywordsIxodes ricinus Louping ill virus Lyme disease Deer Elevation
I am extremely grateful to Scot Ramsay for helping organise and conduct field work, Antonio Rodriguez and Julien Barlet for fieldwork assistance, and the land owners and managers for permission to access field sites. I thank Jim McLeod for assimilating spatial environmental data, Jackie Potts, Mark Brewer and David Elston for invaluable statistical advice, Colin Beale and David Johnson for help with graphics and Jack Lennon and three anonymous referees for comments on the manuscript. This research was funded by the Scottish Government, Rural and Environment Research and Analysis Directorate.
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