Identification of Kudoa ⁠thyrsites in Atlantic mackerel (Scomber ⁠scombrus) displaying post-mortem myoliquefaction: a case study in Scotland

Atlantic mackerel (Scomber scombrus) is a very important and valuable pelagic fish in Scotland. This species is known to be one of the hosts for Kudoa thyrsites, the myxosporean agent of post-mortem myoliquefaction, known as ‘soft tissue’. To date, there have been no official reports of the parasite in Scottish waters, however, the soft tissue condition is known to be found in Scottish landed mackerel. Its impact on this fishery is therefore unknown. The present study aims to report the presence of K. thyrsites-induced post-mortem myoliquefaction in mackerel caught from Scottish waters. Four commercial-sized mackerel displaying features of soft tissue were sampled. The histopathological examination revealed areas of muscular fibre necrosis and the presence of several multivalvulid myxospores in the interstitial space between the muscular fibres and among the liquefied necrotic areas. The presence of the K. thrysites was confirmed by polymerase chain reaction (PCR) in three of the four mackerel muscle tested and showed 100% identity with K. thyrsites isolate MH899081.1.