Detection of Flavobacterium psychrophilum in eggs and sexual fluids of Pacific Salmonids by a polymerase chain reaction assay:: Implications for vertical transmission of bacterial coldwater disease

Ovarian fluid and eggs were collected from returning salmonid species during normal spawning operations and examined for the presence of Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD). A nested polymerase chain reaction assay (PCR) that targets the 16S rRNA gene of F. psychrophilum was used for detection of the pathogen. Samples from coho salmon Oncorhynchus kisutch, chum salmon O. keta, Chinook salmon O. tshawytscha, and steelhead O. mykiss were taken from four fish culture facilities in the Pacific Northwest. Ovarian fluids were pelleted by centrifugation, and yolk material from within the eggs was aseptically removed and prepared for testing by PCR. Flavobacterium psychrophilton was detected in ovarian fluids and unfertilized eggs from species sampled at all facilities. This confirmed the use of PCR as a valid assay for the detection of F. psychrophilum in returning salmonid broodstock of all species tested. More importantly, PCR confirmed the presence of the pathogen within egg contents, not just in the surrounding fluids or on the egg surface.