Comparison of kidney tissue and ovarian fluid from broodstock Atlantic salmon for detection of Renibacterium salmoninarum, and use of SKDM broth culture with Western blotting to increase detection in ovarian fluid

As part of a screening program to reduce the vertical transmission of bacterial kidney disease (BKD) in 1992, kidney tissue and ovarian fluid from subclinically affected broodstock Atlantic salmon Salmo salar L. were assayed for the presence of Renibacterium salmoninarum. Of the methods used, culture on selective kidney disease medium (SKDM) agar identified the highest number of samples positive for R. salmoninarum followed by the indirect fluorescent antibody technique (IFAT). The comparatively low sensitivities of an enzyme-linked immunosorbent assay (ELISA) and Western blot assay reflected the low levels of R. salmoninarum in most infected samples. Also, for many individual fish sampled, R. salmoninarum was detected either in the kidney tissue or in ovarian fluid but not in both. In an attempt to increase the level of detection, a method involving incubation of ovarian fluid cellular debris in SKDM broth followed by Western blotting was used to screen ovarian fluids in 1993. This relatively simple procedure identified more positive samples than either of the established methods of SKDM agar culture or IFAT. Although it increased the total numbers of positive samples identified by 32%, incomplete correlation among methods was observed. Therefore, to ensure that the maximum number of positive samples is detected, it is advisable to use a combination of these methods.