Phenology of Ixodes ricinus and Lyme borreliosis risk

The phenology of Ixodes ricinus and its consequence for Lyme borreliosis risk are important factors to consider in the prevention of disease. Here, determinants that affect the phenology of I. ricinus are reviewed, and it is discussed to what extent they contribute to the risk of transmission. Ixodes ricinus expresses a strongly seasonal phenology, driven mostly by an endogenous rhythm that is regulated by temperature. Ticks feed on vertebrates, with larvae, nymphs and adults each feeding on specific groups of animals, with rodents the preferred hosts of larvae. Borrelia burgdorferi s.l. parasites, the agents of Lyme borreliosis, use rodents as their main reservoir host, and hence larvae are the tick stage in which infection of ticks occurs. Nymphs, emerging from the dormant larvae the following year, are the infectious stage for humans, and the density of infected nymphs is considered an indicator of Lyme borreliosis risk. Various environmental factors affect development time, feeding success and survival of ticks, where ambient temperature and saturation deficit regulate the rate and probability of successful development. These factors combined, contribute to the variation in Lyme borreliosis risk as reported in western Europe. Although climate change will impact on abiotic and biotic factors regulating the ecology of I. ricinus, this is unlikely to have a large impact on its phenology. Lyme borreliosis risk, however, may be affected due to changes in the diversity and population density of vertebrates on which the ticks depend, causing shifts in tick population density and hence risk of exposure to infection.