Detection of Yersinia ruckeri in rainbow trout blood by use of the polymerase chain reaction

We evaluated a polymerase chain reaction (PCR) method for detecting Yersini a ruckeri, the bacterial pathogen causing enteric redmouth disease (ERM), i n blood of rainbow trout Oncorhynchus mykiss. Identification of the PCR pro duct was confirmed by Southern blot hybridization with a P-32-labeled oligo nucleotide probe matching a sequence within the small subunit ribosomal RNA gene of Y. ruckeri. Following a 1 h immersion of rainbow trout in water wi th 4.5 x 10(6) colony-forming units of Y. ruckeri l(-1), the PCR was positi ve for all blood samples from 1 h (first sample) to 5 d and was negative fr om 9 to 30 d (last sample). Fish in this experiment did not show signs of d isease, probably because they had been vaccinated against Y. ruckeri. To te st this method with naturally infected fish, 42 rainbow trout from hatcheri es were examined. Four of these fish had clinical signs of ERM and were inf ected with Y. ruckeri based on bacteriological culture. The PCR method dete cted Y. ruckeri in blood, intestine, liver, and trunk kidney from the 4 fis h with ERM and from 5 additional rainbow trout that were bacteriologically negative for Y. ruckeri. Three of 5 rainbow trout from streams receiving ef fluent from hatcheries were positive for Y. ruckeri when tested with PCR, a lthough there was no growth of Y. ruckeri on culture plates inoculated with the same samples. Samples were successfully stored for 1 wk in lysis buffe r at 25 degreesC. This study demonstrated that a nonlethal blood sample can be used with PCR to detect Y. ruckeri.