The first documented epizootic of pasteurellosis in juvenile gilthead seabream (Sparus aurata) cultured in Spain is described. The affected fish showed no apparent surface lesions and only some of them displayed slight haemorrhagic areas in the head and gills. However, fish often exhibited enlarged spleen with typical whitish tubercles. Losses of approximately 26 000 fingerlings (40% of the stock) occurred over a 4-week period (August–September 1990). Microbiological analysis of moribund fish revealed the presence in pure culture in all the organs examined of a bacterium which was characterized biochemically and serologically asPasteurella piscicida. The isolate was sensitive to most antimicrobial agents tested, and the epizootic was effectively controlled by oral administration of chloramphenicol and oxytetracycline. The virulence assays revealed that theP. piscicida isolate was pathogenic for gilthead seabream with a LD50 ranging from 1.6 × 105 to 9.5 × 105 (depending on the fish size), and also for turbot (Scophthalmus maximus) with LD50 ≤ 1.6 × 104. The failure to recover the inoculated strain from the survivor gilthead seabream indicates that the carrier state ofP. piscicida in the infected population cannot be demonstrated. The histopathological changes observed in the internal organs of diseased fish are typical of a bacterial septicaemia showing extensive, acute multifocal necrosis in spleen and kidney with large masses of bacterial cells

This study was supported by grants PB-87-1027 and MAR 9 l- 1133~CO2-01 from the Dirección General de Investigación Científica y Técnica (DGICYT), and XUGA 8030389 from the Xunta de Galicia, Spain

15 pages, 3 figures, 3 tables

Peer reviewed