Nocardial Infection of Cultured Yellowtail

Abstract

Since 1967 the damage of cultured yellowtails by nocardial infection has been increasing year after year, and it is urgently required to take some effective countermeasures. However, chemotherapy for this infection has not yet been established, partly because tubercles formed in the diseased fish rendered drugs loss effective. Preventive measures by the use of biological characteristics of the organism and by vaccination are briefly discussed in this paper.
1. Preventive and Control Steps from Aspects of Viability of Pathogenic Organisms in Seawater and Mode of Infection:
In the seawater, the pathogenic bacteria could not survived long, especially all organisms being killed in the open seawater within 2 days. However, they survived more than 90 days by the addition of fish extracts at a level of 100 ppm to the open seawater. In the survery of bacterial distribution at culture farms by the fluorescent antibody method, a large number of fluorescent-positive bacteria were detected during the summer when the seawater was heavily polluted. Meanwhile, the challenge test with the water suspended with virulent bacteria revealed that the gill and the digestive system were most vulnerable to the bacteria, and that the higher the bacterial level in the bath, the greater the bacterial count in the challenged fish. From these results it can be assumed that, although the pathogenic bacteria cannot survive long in fresh open seawater, their viability is greatly strengthened in the polluted environment. Nutrients from the feed may participate in this pollution. These conditions would provide a favorable habitat for pathogenic bacteria, and thus, favor the infection if their susceptibility increases. The primary preventive steps will therefore be to minimize the outflow of extracts from the feed during thawing in the seawater, and to avoid overcrowding and overfeeding, thereby keeping the culturing enviromnent clean. Further, as injuries and malnutition of fishes are the greatest factors to increase their susceptibility, it will be essential to avoid injuries and to keep the fishes in nutritionally good condition. As sanitary procedures, accidental introduction of carrier and diseased fishes should be avoided, and dead fish must be incinerated for the prevention of further spread of the disease.
2. Prevention with Vaccination:
When cultured yellow tails were inoculated with killed bacteria, their serum antibody titers rose to 10, 000 in 6 weeks. Further, the effectiveness of vaccination was also demonstrated by the results from challenge tests. Therefore, it is considered that further studies on vaccination will make it possible to prevent this infection.