The seasonal occurrence of early Lyme borreliosis cases (Figure 1) is linked closely to that of the feeding activities of the vector tick, Ixodes ricinus (Figure 2). Thus the seasonality of EM is determined primarily by the activity pattern of nymphal and adult ticks and is further influenced by the propensity of people to engage in outdoor pursuits in early summer. The seasonality of slightly later manifestations such as acute neuroborreliosis is less related to tick activity and that of late stage Lyme borreliosis (Lyme arthritis, ACA and late neuroborreliosis) is independent of tick activity patterns.

Nonetheless, Goren et al. demonstrated a shift towards an earlier seasonal timing (4-6 weeks) over the 25-year study period, which appeared unrelated to changes in population demographics.