Three ways to find a fluke: Evaluating diagnostic methods for Fasciola hepatica surveillance in Swedish sheep

Infections with Fasciola hepatica are most prevalent in south-western Sweden, but climate change may facilitate further spread, as both the parasite and its snail host thrive in humid, mild conditions. Sheep are particularly susceptible, resulting in decreased productivity and economic losses as well as clinical disease and death. Routine field diagnosis in Sweden currently relies on sedimentation of faecal samples, although semi-quantitative PCR and coproantigen ELISA (cELISA) are available alternatives. This study presents the first Swedish comparison of diagnostic performance using composite faecal samples - each representing three ewes - collected from 73 flocks in spring (N = 47) and autumn (N = 58). Sedimentation, quantitative PCR (qPCR), and cELISA (evaluated at 2% and 8% optical density thresholds) were compared for their ability to detect F. hepatica. Farmers' parasite control practices were also surveyed. Concordance among methods was assessed using Cohen's kappa (κ). Sedimentation and qPCR showed substantial agreement (κ = 0.72), qPCR and cELISA 8% cut-off showed considerable agreement (κ = 0.61), sedimentation and cELISA showed moderate agreement (κ = 0.56), while cELISA 2% cut-off naturally identified more positive samples but did not correlate with the other methods. These findings suggest that cELISA has potential as an adjunct diagnostic tool, particularly for detecting early infections, but false-negative results in pooled samples limit its suitability as a replacement for sedimentation or qPCR in routine testing. Survey responses indicated that most farmers were aware of parasite risks and followed general sampling and deworming guidelines, though specific attention to fluke control was limited.