Impact of microclimate on immature tick-rodent host interactions (Acari : Ixodidae): Implications for parasite transmission

Rodents play a significant role in enzootic cycles of tick-borne pathogens, notably, in the northern hemisphere, tick-borne encephalitis virus and Lym e borreliosis spirochaetes. The relative numbers of nymphal and larval tick s feeding on rodents are crucial variables in determining the probability o f rodent infection and the degree of amplification of infection prevalence in the tick population. Manipulation of the microclimate within quasinatura l experimental arenas revealed that under increasingly dry conditions the n umbers of unfed nymphal Ixodes ricinus L. questing in upper layers of the h erbage decreased, whereas the rate of fat use and the numbers of nymphs fee ding on small rodents, both increased. This is consistent with nymphs desce nding to the moist lower vegetation layers for water replenishment, where t hey would come into contact with small hosts. Very few larvae quested or fe d on rodents under the dry conditions, but many more did so once the humidi ty increased, suggesting that larvae escape desiccation by becoming quiesce nt. The ratio of larvae to nymphs feeding on rodents thus increases with in creasing humidity, contributing to the seasonal and geographical variation in disease transmission dynamics.