OCCURRENCE OF LISTERIA-MONOCYTOGENES IN FRESH-WATER FISH FARMS AND FISH-SMOKING PLANTS

Three Swiss fish farms, farming rainbow trout (Oncorhynchus mykiss), a nd their affiliated smoking plants were analyzed for the presence of L isteria spp. 590 samples were collected from the farming environment ( raceway water, sludge), faecal content and skin of the fish, fish duri ng processing, and the processing environment. Listeria spp. were foun d at prevalences of 2.3% in plant A, 31.6% in plant B (mainly L. monoc ytogenes), and 13.8% in plant C (mainly L. innocua). This high contami nation rate in plant B may be explained by the following facts: (i) fa rm B uses river water flowing through agricultural land; (ii) plant B rears fish in earth ponds instead of concrete ponds or raceways; (iii) fish from farm B had not been denied feed prior to slaughter; and (iv ) total lack of regular mechanical and chemical cleaning in the fish f arm B and processing plant B. In all three plants samples taken after smoking but before packaging did not contain Listeria spp., although i n plant B and C the raw fish was contaminated. Hygienic defaults durin g packaging can lead to contaminated ready-to-eat products, detected i n plant B (L. monocytogenes) and plant C (L.innocua) with one sample e ach. To minimize a possible health hazard to the consumer, it is of gr eat importance to prevent postprocessing contamination of smoked fish. Finally, means of preventing Listeria contamination during farming, s laughtering, processing and storage are suggested.