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A review of Kudoa species (myxozoa, multivalvulida) affecting seafood quality, with emphasis on K. thyrsites in atlantic mackerel (Scomber scombrus)

Members of the multivalvulid myxozoan genus Kudoa occur in many marine teleosts worldwide. Several species are of concern to fishery and aquaculture since they cause post-mortem myoliquefaction of the somatic fish muscle, commonly known as ‘soft flesh’. | Journal of Fisheries science and Technology Special issue - 2015 A REVIEW OF KUDOA SPECIES (MYXOZOA, MULTIVALVULIDA) AFFECTING SEAFOOD QUALITY, WITH EMPHASIS ON K. THYRSITES IN ATLANTIC MACKEREL (SCOMBER SCOMBRUS) Arne Levsen1 ABSTRACT Members of the multivalvulid myxozoan genus Kudoa occur in many marine teleosts worldwide. Several species are of concern to fishery and aquaculture since they cause post-mortem myoliquefaction of the somatic fish muscle, commonly known as ‘soft flesh’. Actual Kudoa species may strongly reduce the market value of affected fish, e.g. K. thyrsites in farmed Atlantic salmon (Salmo salar) and several commercially important wild fish species incl. Atlantic mackerel (Scomber scombrus), and K. paniformis in Pacific hake (Merluccius productus). Myoliquefaction is induced by Cathepsin L proteases released by the parasite’s sporogenic stages within the fish muscle, and becomes evident 38-56 h post-catch. As part of a parasite surveillance program of Atlantic mackerel from the North Sea, at least 300 Atlantic mackerel are examined each season for clinical signs of myoliquefaction and the presence of extrasporogonic stages by using a K. thyrsites specific molecular marker. Prevalence of post-mortem myoliquefaction varies 0.6% - 3.1% between years in texture tested fish (n=2500), and differs significantly between medium (400-600g) and large mackerel (>600g). PCR-testing revealed up to 50% prevalence of cryptic K. thyrsites stages in various organs incl. the muscle, heart and foregut. No inflammatory host responses are apparent around plasmodia within host muscle fibres. Currently, no method exists for early and non-destructive detection of fish prone to parasite-induced myoliquefaction. Thus, Kudoa-infected fish may inevitably reach the consumer. Considering the presently observed climate change and extension of the mackerel’s northerly distribution area, K. thyrsites’ may become a serious economic loss factor in NE Atlantic salmon .