Since 1980, a disease called “Jaundice”, which is characterized by a yellow coloration of the skin and muscle, has been known to cause varying mortalities of 5-20 percent in cultured yellowtail, Seriola quinqueradiata, in Japan. Hematological and bacteriological studies were carried out to clarify the cause of the disease.
The diseased fish exhibited low hematocrit values and high plasma bilirubin concentrations. Thin rod-shaped organisms stained with Giemsa were found on the blood smear preparations of all the diseased fish examined. Among 8 culture media examined, only L-15 medium supplemented with 10% fetal calf serum and nutrient broth gave successful result to culture the organisms, the former yielding better growth. On electron microscopy the organism is a single cell, having a cell wall and an inner membrane but no nuclear membrane. Thus, the organism is considered as a bacterium. Morphological and culture characteristics together with its pathogenicity indicate that the bacterium is different from any known species of fish pathogenic bacteria.
Inoculation of the isolated bacteria to yellowtail produced high mortalities and symptoms similar to those observed in spontaneous jaundice yellowtail, and the same bacteria were reisolated from all the dead fish in the infection experiment. From these results, it is concluded that the jaundice of yellowtail is an infection caused by an unknown pathogenic bacteria.
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