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 water and of rainbow trout in fresh water were compared using intraperitoneal 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 bath for rainbow trout was 10(8) cfu ml(-1) in fish exposed over a challenge period of 12 h. With turbot, the same mortality percentage was obtained with 10(5) cfu ml(-1) after an exposure for 12 h. By both challenge methods turbot therefore proved to be more susceptible than rainbow trout to A. salmonicida. Both species proved refractory to challenge by the intragastric method. The release oi bacteria from exposed fish was studied and the recovery of culturable A. salmonicida was only possible from dead or moribund rainbow trout. It is also interesting that only the more resistant rainbow trout appeared to become carriers of A. salmonicida following exposure to the pathogen. The implications for farming are discussed.