PCR-Based Detection of Yersinia ruckeri Infection in Rainbow Trout Fish

The use of a real-time PCR primer/probe set to observe infectivity of Yersinia ruckeri in Chinook salmon, Oncorhynchus tshawytscha (Walbaum), and steelhead trout, Oncorhynchus mykiss (Walbaum)

Yersinia ruckeri is the causative agent of enteric redmouth disease (ERM), a common pathogen affecting aquaculture facilities and implicated in large losses of cultured fish. Fisheries scientists continue to gain a greater understanding of the disease and the pathogen by investigating methods of identification and pre- and post-infection treatment. In this study, a real-time PCR probe set for Y. ruckeri was developed to detect daily changes in the bacterial load during pathogen challenges. Two species of fish, Chinook salmon, Oncorhynchus tshawytscha, and steelhead trout, Oncorhynchus mykiss, were exposed to two strains of Y. ruckeri (Hag and SC) during bath challenges. A subset of fish was killed daily for 14 days, and the kidney tissue was biopsied to enumerate copies of pathogen DNA per gram of tissue. While Chinook exposed to either the Hag or SC strains exhibited similar pathogen loads, those exposed to the Hag strain displayed higher mortality (∼66%) than fish exposed to the SC strain (∼24% mortality). Steelhead exposed to the Hag strain exhibited a greater pathogen load and higher mortality (∼42%) than those exposed to the SC strain (<1% mortality). Steelhead challenged with either strain showed lower pathogen loads than Chinook. The study illustrates the efficacy of the probe set to enumerate Y. ruckeri bacterial growth in the kidneys of fish. Also, strains of Y. ruckeri display species-specific growth patterns that result in differential mortality and pathogen load.