Detection and identification of virulent Yersinia ruckeri: the causative agent of enteric redmouth disease in rainbow trout (Oncorhynchus mykiss) cultured in Fars province, Iran

From the winter 2002 to spring 2006, 126 moribund rainbow trout with clinical signs of external body haemorrhages around and within the oral cavity were sampled from 10 rainbow trout farms situated in the northwest and west of Fars province, Iran and examined for the detection and identification of Yersinia ruckeri, the causative agent of enteric redmouth disease. Fish kidneys were cultured aseptically on brain heart infusion (BHI) agar plates and incubated at 25ºC for 48 h. Using conventional biochemical tests, Y. ruckeri was detected in 7 fish (5.5% of total fish sampled). This was also confirmed using specific polymerase chain reaction (PCR) assay. The 16S rDNA PCR assays produced amplicons of 409 bp when applied to Y. ruckeri isolates as well as a reference strain. Results of antibiogram tests on Y. ruckeri isolates showed a high susceptibility to enrofloxacin, norfloxacin, ofloxacin, trimethoprim and oxytetracycline. In pathogenicity tests, dilution of 4 × 108 colony forming unit/ml of Y. ruckeri by immersion route in challenge experiments showed 70 ± 8.2% mortality during 14 days post-infection. Experimentally infected fish showed typical haemorrhages in mouth, blackening of skin, exophthalmia and a wide haemorrhages on the internal organs.