An outbreak of a disease characterized by a peculiar spiral movement in farmed greater amberjack, Seriola dumerili (Risso), occurred in Kagoshima Prefecture, Japan, in May 2008, immediately after importing the fish from China. Although neither bacteria nor viruses were detected in routine diagnostic tests, histopathological observations of the affected fish revealed severe inflammation in the tegmentum of the brain including the medulla oblongata and the anterior part of the spinal cord. In addition, a microsporidian parasite was observed in the nerve cell bodies or axons in the inflamed tissues. We identified a microsporidian small subunit rRNA gene (SSU rDNA) from the lesion, and the sequence showed 96.1% identity with that of Spraguea lophii. Subsequent in situ hybridization using probes presumably specific to the SSU rRNA confirmed that the parasite observed in histopathology harboured the identified SSU rRNA. Apparently degenerated microsporidian cells or spores were also frequently observed in tissue sections. Thus, the disease was most probably caused by the infection of a hitherto unknown microsporidian parasite that has a genetic affinity to the genus Spraguea, in the central nervous system of the amberjack.
ABSTRACT-Mycobacterium marinum (NJB 0419, MY 0401) isolated from yellowtail Seriola quinqueradiata showed drug susceptibility to rifampin, streptomycin (SM), ethambutol, kanamycin, sulfamethoxazole, amikacin, ciprofloxacin and ofloxacin by the broth dilution method. Furthermore, SM, kanamycin and sulfamonomethoxine activities were tested using the proportion method, and M. marinum was most susceptible to SM. Therefore, SM was used for the treatment of yellowtail (mean body weight 236 g) intramuscularly injected with NJB 0419 at a dose of 5.6 ¥ 10 4 CFU/fish. The diet medicated with SM was orally administered at doses of 25 and 50 mg/kg body weight/day for 7 consecutive days after 4 h or 10 days following injection. Fish receiving 50 mg SM had lower mortality than the controls when medication started 4 h after injection. Therefore, SM may be an effective drug to control M. marinum infection. The timing for initiating oral medication with SM requires further study.
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