DIFFERENTIAL SUSCEPTIBILITY TO FURUNCULOSIS OF TURBOT AND RAINBOW-TROUT AND RELEASE OF THE FURUNCULOSIS AGENT FROM FURUNCULOSIS-AFFECTED FISH

The different susceptibilities to furunculosis of turbot held in sea w ater and of rainbow trout in fresh water were compared using intraperi toneal bath, and intragastric exposures. The intraperitoneal LD(50) of Aeromonas salmonicida was 3 x 10(5) cfu (colony-forming units) fish(- 1) for 25 g rainbow trout Oncorhynchus mykiss and 2 x 10(4) cfu fish(- 1) for 30 g turbot Scophthalmus maximus. The minimal lethal dose by ba th for rainbow trout was 10(8) cfu ml(-1) in fish exposed over a chall enge period of 12 h. With turbot, the same mortality percentage was ob tained with 10(5) cfu ml(-1) after an exposure for 12 h. By both chall enge methods turbot therefore proved to be more susceptible than rainb ow trout to A. salmonicida. Both species proved refractory to challeng e by the intragastric method. The release oi bacteria from exposed fis h was studied and the recovery of culturable A. salmonicida was only p ossible from dead or moribund rainbow trout. It is also interesting th at only the more resistant rainbow trout appeared to become carriers o f A. salmonicida following exposure to the pathogen. The implications for farming are discussed.