Molecular identification of Kudoa thyrsites parasite isolated from Atlantic mackerel (Scomber scombrus) landed in Ireland.

Post-mortem myoliquefaction, commonly referred to as 'jelly flesh', is associated with infection by myxozoan parasites of the genus Kudoa, particularly Kudoa thyrsites; the effects of infections can render fish fillets unusable in food preparation. Although the parasite has been documented in Atlantic mackerel (Scomber scombrus) across parts of the Northeast Atlantic, its presence in mackerel landed in Ireland has not been previously confirmed. In this study, mackerel obtained from commercial landings were screened for signs of post-mortem myoliquefaction. Three individuals displaying characteristics of softening of the musculature were examined using light microscopy and molecular analysis. Microscopic examination revealed numerous spores consistent with Kudoa-like morphology. Sequencing of the amplified region confirmed the presence of K. thyrsites in all three samples. The sequences showed a 100% match with K. thyrsites isolates previously reported from Atlantic mackerel and other host species in the Northeast Atlantic. Moreover, phylogenetic analysis placed the Irish isolate within the expected Atlantic sub-clade. As molecular testing was limited to visibly affected fish, the prevalence of K. thyrsites infection in the mackerel population could not be estimated, highlighting the need for continued monitoring of the parasite in Irish waters to better understand its distribution and potential economic implications for the Northeast Atlantic fishery.