A life-threatening emaciation disease of unknown cause(s) is affecting the farming of olive flounders (Paralichthys olivaceus) and turbots (Scophthalmus maximus) on Jeju Island, Korea. As this is one of the major industries in the region, it is of great concern to local farmers trying to develop successful and sustainable aquaculture. We examined 16 olive flounders and one turbot cultured at three farms located in the southern part of Jeju Island, which manifested moderate to severe emaciation such as thinning of the body with notable appearance of bony ridges of the skull on heads. Fresh mucosal scrapings of the intestinal mucosa contained many myxosporean vegetative stages at various developments but not fully grown spores. Histological examination of gastrointestinal and other visceral organs revealed striking changes in the intestinal mucosa such as detachment and loss of the epithelium due to intensive parasitism of the myxosporean vegetative stages, accompanied by considerable leukocyte infiltration in the lamina propria, and at the final stage villus atrophy with no epithelial lining. Specific polymerase chain reaction using a pair of primers targeting a fragment of the 18S ribosomal RNA gene (rDNA) of Enteromyxum leei, a known pathogen causing myxosporean emaciation disease in a variety of cultured fish in Mediterranean countries and Japan, amplified 433-bp products in almost all diseased fish samples, particularly the gastrointestinal tract. Nearly the whole length of the 18S rDNA, 1672-bp long excluding primer-aligning sequences, of the present Korean isolate was comparable to those of E. leei isolates from Japan and Europe, particularly those from the former region. Taking the heavy load of various developmental stages of E. leei in the gastrointestinal mucosa into account, we ascribe the emaciation disease of the fish examined in the present study to this well-known myxosporean species and not to another unknown pathogen(s).

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