Caused by the bacterium Flavobacterium psychrophilum, RTFS is widespread and occurs frequently at water temperatures between 10-14°C. It affects rainbow trout ranging in size from 0.2g - 100g, leaving necrotic lesions on the skin surrounding the dorsal fin and tail. In very small fish no clinical signs are apparent and death occurs due to septicaemia. RTFS can lead to high mortalities in fry and larger fish in freshwater hatcheries and on-growing sites. An average early stage mortality of 10% in rainbow trout could be considered normal in many production sites.

Although disease outbreaks have been recorded in Europe since the late 80s, the lack of knowledge regarding the epidemiology of Flavobacterium psychrophilum has limited the development of disease management tools. Antibiotics are currently the only course of action to contain disease outbreaks. In the UK, RTFS is an important issue as rainbow trout are the second most produced finfish by harvest volume.

Now, a new RTFS vaccine is due to be tested during field trials to improve fish welfare in the rainbow trout industry and secure the necessary license for commercial use. Dawnfresh, one of the UK's largest rainbow trout producers, and the University of Stirling's Institute of Aquaculture (IoA), are leading the initiative, while Scottish trout farmer Kames Fish Farming and aquaculture research firm Tethys Aquaculture will also be working on the project with funding from the Scottish Aquaculture Innovation Centre (SAIC). The vaccine was developed as part of a five-year European project called 'TARGETFISH' that aimed to prevent important fish diseases in European aquaculture through a targeted vaccination strategy.