| Summary: | Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum, is classed as a notifiable disease under the Diseases of Fish Act 1937. It is a list III disease as specified in Annex A of Council Directive 91/67/EEC (1991). The UK government has opted to maintain movement controls on fish farms and hatcheries found to be infected with R. salmoninarum, as it is believed that the bacterium is not widespread but poses a significant risk to farmed and wild fish populations. This study took a newly developed PCR technique, designed to detect the presence of R. salmoninarum in fish samples, and validated it for use in both wild fish and farmed fish surveys. Validation studies found that the technique gave false positive reactions when tested with high concentrations of Yersinia ruckeri, a common fish pathogen. The nature of the false cross-reaction was determined and the primers redesigned to ensure specificity. PCR reagents and cycling conditions were optimised to give the modified technique a sensitivity of between 10¹ and 10² R. salmoninarum cfu/mg head kidney material. A cross-sectional survey of R. salmoninarum infection in farmed rainbow trout, and wild fish populations, was carried out in ten farms and six rivers, respectively, using the validated PCR technique. Grayling (Thymallus thymallus) and brown trout (Salmo trutta) were the main wild species tested. Two fish, one grayling and one salmon (Salmo salar), were R. salmoninarum culture positive compared to 46 confirmed PCR positive fish. The highest prevalence of R. salmoninarum infection was found in grayling in rivers that had rainbow trout farms with a history of R. salmoninarum infection. One hundred and fifty fish were sampled from each rainbow trout farm, but none of the fish was found to be R. salmoninarum positive. Evidence was found, for the first time, for the presence of R. salmoninarum in an eel.
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