The susceptibility of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) to the monogenean Discocotyle sagittata in the United Kingdom was assessed by experimental infection of naive fish. One month postinfection with 100 oncomiracidia/host, brown trout harbored significantly lower burdens (27.7 worms/host ± 4.13 SE) than rainbow trout (47.8 worms/host ± 3.90; P = 0.002). This indicates that the consistently lower prevalence and intensity of D. sagittata recorded in naturally infected farmed fishes reflects differences in susceptibility to the parasite. The outcome may be related to the comparatively short-term association of this parasite with rainbow trout (introduced to Britain in the 1880s) compared with the established native host–parasite association.

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