Salmonella in fish feed; occurrence and implications for fish and human health in Norway

Globally, Salmonella is one of the most important food borne zoonotic pathogens. Norway has a favourable situation regarding Salmonella, as these bacteria are virtually absent in food producing animals and domestically produced food, including products from fish. Consequently, Norway has a low incidence of domestically acquired salmonellosis in the human population. Some Salmonella serovars can however occasionally be found in animal feed and its ingredients, as well as in the feed production facilities. During the years from 2000 to 2004, the prevalence of Salmonella in samples of feed materials varied from 0.14 to 0.33%. Of environmental samples obtained at Norwegian fish feed production facilities, 3.78% of the examined samples were positive. During the same period, the prevalence of Salmonella in Norwegian ready to use compound fish feed were shown to be 0.3%. The predominant serovars found in fish meal were S. Senftenberg and S. Montevideo. The same serovars were dominating in isolates from the production environment, and could in these cases be considered “house strains”. In ready to use compound fish feed, the most common serovars were found to be S. Senftenberg, S. Agona, S. Montevideo and S. Kentucky. Under natural rearing conditions for farmed Atlantic salmon in Norway, and with low concentrations of Salmonella in the feed, the risk of transmission to humans via fish products is minimal. Epidemiological data on salmonellosis from the Norwegian Surveillance System for Communicable Diseases, show that the most common serovars in fish feed ingredients, fish feed and fish feed factories accounts for approximately 2 % of clinical Salmonella isolates from domestically acquired cases in Norway. There is no evidence for transmission of Salmonella from fish feed to humans. However, there is little information on the risk of Salmonella cross-contamination from fish feed, the ingredients and the factories to other parts of society, including wildlife. A probable cross-contamination between fish feed factories and seagulls have been described. The prevalence of antimicrobial resistant Salmonella strains isolated from fish feed, its ingredients and the production environment have so far been very low.