Observational and experimental studies on the acquisition of Anisakis sp. larvae (nematoda: Ascaridida) by trout in fresh water

In the enclosed fresh-water environmsnt of Hanningfield Reservoir, Essex, England, Anisakis sp. larvae (parasites of marine fish) were found in 55 per cent of 40 brown trout and in 26·2 per cent of 61 rainbow trout. Experimental infection by intubating larvae into the stomach was more successful in brown trout (50·6 per cent recovery rate) than in rainbow trout (27 per cent recovery rate). Some larvae reached the body-cavity as early as 2 h after infection. They penetrated the region between the oesophagus and intestine immediately posterior to the caecal openings. Fewer larvae successfully penetrated the gut wall of brown trout within 24 h at 8°C than at 15 ± 1°C. It appears that the reservoir trout acquired Anisakis by being fed as juveniles on untreated marine fish offal containing live larvae.