The control of emerging parasites requires a fundamental knowledge of where and when rates of transmission are high. Data on spatiotemporal patterns of infection are challenging to obtain, particularly for complex life cycle parasites that involve transmission into multiple obligate hosts. The lancet liver fluke, Dicrocoelium dendriticum, has a long history of colonization outside its native host and geographical range in continental Europe. Infection patterns involving adult and metacercarial stages have been characterized for this trematode in a region of emergence in western Canada within co-grazing herbivores and ants, but infection patterns in snail intermediate hosts in this region are unknown. We combined spatiotemporal prevalence surveys with sequence analyses of the cytochrome c oxidase subunit 1 (COI) barcoding gene from samples of sporocyst tissue in infected snails to confirm that D. dendriticum utilizes 3 sympatric species of Oreohelid land snail (Oreohelix subrudis, Oreohelix sp., and Oreohelix cooperi) as first intermediate host. Mean prevalence within a total sample of 900 adult snails collected over 1 field season from 6 sites was 9.9 ± 2.4%. For each species of snail, prevalence ranged between 5–30% within monthly samples, with peaks in mid-summer followed by declines in fall. Between-site variation in prevalence was low and non-significant, implying that rates of transmission of D. dendriticum miracidia from domestic stock and wildlife into snails are similar within localized sites, despite high variation in local habitat characteristics and in the structure of the definitive host community.

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