Neuropathology of brain myxosporean disease of fish

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1.Pathological analysis of skeletal deformity of cultured fish : Diseased Japanese mackerel (Scomber japonicus) exhibited dorso-ventral curvature externally. Histopathological examination revealed that the spinal curvature was caused by a brain myxosporean, Myxobolus acanthogobii. Myxosporean cysts were found in the olfactory lobe, optic lobe, the cavity of optic tectum and the fourth ventricle. However, the anatomical location of cysts responsible for the skeletal abnormalities could not be determined.2.Pathological analysis of naturally infected but subclinical wild fishes : Brown-lined puffer (Canthigaster rivulata) and yellowfin goby (Acanthogobius flavimanus) were found to be naturally infected with M.acanthogobii, but did not show any skeletal abnormalities. Histopathological observation showed that numerous cysts were found in the optic lobe, medulla oblongata and cerebellum.3.Development of PCR-based detection method for M.acanthogobii-A PCR method targeting SSU rDNA of M.acanthogobii was developed. Nested-PCR analysis was applied to epizootiological survey of the parasite in feral fishes (37 species). As a result, forksnout searobin (Lepidotrigla alata) and crimson seabream (Evynnis japonica) were found to be the hosts for M.acanthogobii. Further, surveys on possible alternate annelid hosts for M.acanthogobii were conducted, but actinosporeans and PCR-positive reactions were not obtained.4.Experimentally induced abnormal behavior of fish caused by implant by brain surgery : Glass beads (ca.1 mm in diameter) were implanted in the 4th ventricle of goldfish, and swimming behavior was observed. As a result, the experimental fish showed a notable body curvature and whirling behavior. This suggests that spinal curvature is caused by mechanical pressure of foreign particles localized in the 4th ventricle of fish brain.