From middle July to late August every year, whirling disease caused by the metacercaria of a fluke occurrs in natural anchovies (Engraulis japonica) and cultured yellowtails (Seriola quinqueradiata) in the Tsushima Islands. The only symptom is circular swimming, or whirling, near the sea surface. Fifty diseased anchovies, 100 diseased yellowtails and 10 normal yellowtails were examined parasitologically and histopathologically.
All the diseased fishes examined had one or at times two meta cercaria within the interbrain, but it was never detected in the normal fishes. Neither pathogenic bacteria nor protozoa were found in any of the diseased fishes.
The metacercaria cyst was white and had the diameter of about 0.8 mm. According to KAMEGAYA et al. (unpublished) this fluke belongs to the genus Galactosmum. Neurons around the metacercaria in the interbrain were found to fall into degeneration or necrosis affected by pressure of the cyst.
Because of the abov-mentioned facts, the present authors consider that whirling movement of the infected fishes caused by the metacercaria.
The Japanese Society of Fish Pathology