Spatiotemporal variation in metacercariae prevalence and intensity for the host manipulator <i>Dicrocoelium dendriticum</i> in two species of wood ant from southern Alberta, Canada

Exposure to certain parasites can occur when hosts encounter clumps of infective larvae during ingestion. Compared to more typical transmission pathways involving the cumulative penetration or ingestion of single larvae, clumped transmission can be expected to lead to complex and variable epidemiological patterns of infection within exposed hosts. We explored this idea in a spatiotemporal survey of wood ants (<i>Formica</i> spp.) infected with metacercariae of the host-manipulating trematode, <i>Dicrocoelium dendriticum.</i> Ant second intermediate hosts are exposed to clumps of cercariae during ingestion of 'slimeballs' that are released onto pasture by land snail first intermediate hosts. In a total sample of >650 ants collected during the host-manipulation phase when ants were attached to plants, metacercariae prevalence was 97% and mean intensity was 27 ± 25 (range in intensity = 1-168; variance:mean ratio = 23.1). None of the observed variation in mean metacercariae intensity could be attributed to year (2021, 2022), month (May to August), or nest. Although the recruitment of metacercariae into ants occurred within a narrow window of transmission each summer, the mixed-age structure of our ant samples likely limited our ability to detect seasonal patterns of mean intensity. The absence of significant spatiotemporal patterns in metacercariae intensity in samples of ants likely also reflects infrequent and variable rates of encounter with cercariae-containing slimeballs by individual ants.