Decoupling of Blacklegged Tick Abundance and Lyme Disease Incidence in Southern Maine, USA

Lyme disease is caused by the bacterial spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt, and Brenner (Spirocheatales: Spirochaetaceae) which is transmitted through the bite of an infected blacklegged tick Ixodes scapularis Say (Ixodida: Ixodidae). Maine, USA, is a high Lyme disease incidence state, with rising incidence of Lyme disease and other tick-borne illnesses associated with increasing I. scapularis abundance and northward range expansion. Members of the public submitted ticks to a tick identification program (1990–2013). From these passive surveillance data, we characterized temporal trends in I. scapularis submission rate (an index of abundance), comparing Maine's northern tier (seven counties) versus southern tier (nine counties). In the northern tier, the I. scapularis submission rate increased throughout the duration of the time series, suggesting I. scapularis was emergent but not established. By contrast, in the southern tier, submission rate increased initially but leveled off after 10–14 yr, suggesting I. scapularis was established by the mid-2000s. Active (field) surveillance data from a site in the southern tier—bird tick burdens and questing adult tick collections—corroborated this leveling pattern. Lyme disease incidence and I. scapularis submission rate were temporally correlated in the northern but not southern tier. This suggested a decoupling of reported disease incidence and entomological risk.