Microsporidian encephalomyelitis of the greater amberjack Seriola dumerili is characterized by a spiral swimming movement in diseased fish, and has been reported in fish imported into Japan from China. In 2010, a similar disease was found in cultured yellowtail Seriola quinqueradiata, which were raised in Japan using wild-captured fingerlings. In the present study, a microsporidian was consistently observed in the nervous tissue of all diseased yellowtail specimens, supporting the hypothesis that the microsporidian is the etiologic agent of the disease. Morphological and molecular similarities between the yellowtail and amberjack microsporidia indicate that they belong to the same species. Phylogenetic analysis showed that the present microsporidian was closely related to the genus Spraguea. Histopathological examination of affected yellowtail revealed encephalomyelitis with severe gliosis, degenerate axons and necrotic nerve cells with cytoplasmic vacuoles, together with aggregates of microsporidian spores in the lesions. Field investigations of cultured yellowtail at the same farm where the disease occurred in 2010 showed that disease incidence and infection prevalence decreased in 2011 and 2012. The present microsporidian was probably transferred to Japan from China through importation of juvenile greater amberjack, followed by transmission to cultured yellowtail.
2013 The Japanese Society of Fish Pathology