水産養殖研究開発ジャーナル

概要

Recently Emerged Myxosporean Encephalomyelitis of Cultured Yellowtail Seriola quinqueradiata in Japan

Hiroshi Yokoyama, Fei Meng, Makiko Hirai, Shusaku Takagi, Takayuki Katagiri, Makoto Endo and Kazuo Ogawa

Encephalomyelitis associated with developmental stages of a myxosporean parasite has recently occurred in cultured yellowtail Seriola quinqueradiata from western Japan. Although mature spore stages of the parasite were not found in the spinal cord, the myxosporean was molecularly identified as Myxobolus spirosulcatus by the 18S rDNA analysis and in situ hybridization and has been previously reported as a non-pathogenic parasite infecting the bile duct of yellowtail. However, the causal relationship between the encephalomyelitis and M. spirosulcatus is still controversial. Typical signs of the disease were reduced feeding, erratic and abnormal swimming, skin ulceration from the jaw to the abdomen, and redness of the brain. Histopathologically, this disease was characterized by 1) gliosis and multi-focal glial nodules, 2) nerve cell necrosis with neurophagia, 3) blood congestion and hemorrhages in the central nervous tissue, 4) swollen eosinophilic nerve fibers and degenerative axons. In the present study, we conducted further histopathological observations and tried to determine the disease mechanisms in relation to water temperature and feeding regimes. It was indicated that developing plasmodia directly caused the gliosis and glial nodules in the nervous system. Fluctuating water temperature was suggested to be partly responsible for disease outbreak. Detection rates of M. spirosulcatus spores in the bile were significantly higher in the constant temperature groups than those in the fluctuating temperature groups. Further studies on the biology of M. spirosulcatus are required to implement a management strategy to the myxosporean encephalomyelitis of yellowtail.