Differential susceptibility to furunculosis of turbot and rainbow trout and release of the furunculosis agent from furunculosis-affected fish

Abstract

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 LD50 of Aeromonas salmonicida was 3 x 105 cfu (colony-forming units) fish-1 for 25 g rainbow trout Oncorhynchus mykiss and 2 x 104 cfu fish-1 for 30 g turbot Scophthalmus maximus. The minimal lethal dose by hath for rainbow trout was 108 cfu ml-1 in fish exposed over a challenge period of 12 h. With turbot, the same mortality percentage was obtained with 105 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 of 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.