Immunofluorescence screening of Renibacterium salmoninarum in the tissues and eggs of farmed chinook salmon spawners

Abstract A simple and rapid on site-test method was used to screen farmed chinook salmon ( Oncorhynchus tschawytscha ) broodstock for bacterial kidney disease (BKD) at the time of spawning. Kidney tissue and ovarian fluid smears were examined, using the indirect fluorescent antibody technique (IFAT). To check the validity of our field screening procedure, intra-ovum fluid, taken from random samples of 300 surface-sterile, fertilized eggs, was examined in the laboratory for the presence or absence of Renibacterium salmoninarum , using the IFAT. Prior to sampling, eggs were incubated at 15°C for 40 days in enrichment KDM-2 broth. R. salmoninarum cells were found in one out of 10 samples of pooled contents of eggs from fish showing no R. salmoninarum in the kidney tissue (these fish were assumed to have R. salmoninarum -free ovarian fluid); three out of 10 pooled samples from fish in which R. salmoninarum was detected in the kidney but not the ovarian fluid; all of the 10 pooled samples from fish with R. salmoninarum in both the kidney tissue and the ovarian fluid. The number of R. salmoninarum in the pooled egg contents was low (2–23 per 100 fields). Using the IFAT technique it was not possible to determine whether the BKD-bacteria in the eggs were alive or dead. Methods of BKD treatment and screening are discussed as is the validity of using these techniques to prevent vertical transmission of BKD in farmed salmon.