Detection of humoral antibodies to Renibacterium salmoninarum in rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar challenged by immersion and in naturally infected populations.

Humoral antibodies to heat-stable antigens of Renibacterium salmoninarum (Rs) were detected by enzyme-linked immunosorbent assay (ELISA) in rainbow trout Oncorhynchus mykiss and in Atlantic salmon Salmo salar challenged by immersion. A slow antibody response was found: 3% (1/30) was positive 4 wk after immersion and 72% (26/36) was positive after 8 wk. All 30 fish sampled after 4 wk were found to be infected, as determined by bacterial culture and/or the presence of soluble antigens in the kidney. At 6, 8 and 12 wk after immersion the proportion of positives indicated by ELISA was 58%. The Rs infection was detected by cultivation in 36% of sampled fish collected on the same occasion. Elevated antibody titres to Rs were detected in samples from both Atlantic salmon (59% in 1 farm) and from rainbow trout (20% in 1 of 5 sampled farms) in naturally exposed populations all of which classified positive for bacterial kidney disease (BKD). Elevated antibody titres were detected among sampled fish from populations of rainbow trout and salmon with clinical BKD. Samples collected from farm populations of rainbow trout, salmon and brown trout Salmo trutta, exposed to Rs but without clinical BKD, were negative in the ELISA, although Rs bacteria or soluble antigens were detected at the same sampling. The antibody ELISA method cannot be recommended for general fish health monitoring purposes, but may be a valuable tool for monitoring the disease progression during controlled experiments.