First Report of 'Soft Flesh' Induced by the Parasite Kudoa thyrsites (Myxosporea) in Commercial Codfish From Norway

Kudoa thyrsites is a myxosporean parasite that infects the skeletal muscle of various teleost fish species globally. Severe infections lead to 'soft flesh' in fish fillets, resulting in food spoilage and subsequent discard. While K. thyrsites has previously been identified in migratory Atlantic mackerel in the northern Northeast Atlantic Ocean, it has not been observed in resident or farmed fish species in Norwegian waters until now. This study presents the first evidence of K. thyrsites infection and the associated 'soft flesh' condition in resident commercially important gadoid species from Norwegian waters, including Norwegian coastal cod (NCC), Northeast Arctic cod (NEA) and tusk. Molecular analyses confirmed the parasitic infection in 'soft flesh'-affected fish sampled from multiple coastal locations in Norway. The life cycle of Kudoa remains unknown but likely involves an alternating annelid host as in other myxosporeans. These findings in resident hosts suggest that the parasite completes its life cycle also at higher latitudes, in northern Norway. Consequently, there is a risk for the Norwegian fishing industry, as the effect of the parasite on fish fillet texture can occasionally occur and impact both consumer acceptance and industry revenues.