Quantifying the temperature-dependency of tick seasonality in France and Lyme disease risk in Connecticut

Ticks are the most important arthropod vectors of many human and veterinary diseases in temperate regions. Ticks are highly sensitive to desiccation and questing behaviour in search for animal hosts is driven by ambient temperature. This study assessed the association of meteorological factors with the monthly abundance of nymph Ixodes ricinus in three differing botanically classified homogenous study sites over a two-year period in France and then compared predicted abundance with that observed in the third year. We then examined the meteorological associations with Lyme Disease (LD) cases that are notifiable in Connecticut, the USA, over a six-year period and then compared predictions with the observed number for the subsequent four years. There were predictable negative associations between temperature and nymph activity patterns at four months lag time across all vegetation site types and with LD cases at five months lag time. This corresponded to a decrease in Relative Risk of 8-10% for every 1°C increase. Despite the different tick species in Europe and in northern USA, I. ricinus and Ixodes scapularis respectively, the two species share the same ecological niche and seemingly have a similar temperature-dependency. In the context of global warming, we can expect to observe a shift in the temporal pattern of tick activity and risk of Lyme disease. It remains to be assessed as to whether earlier higher activity peaks may lead to shorter tick seasons with the temperature-dependent depletion of the putatively finite annual nymph stock.