Prevalence and location of Listeria monocytogenes in farmed rainbow trout Hanna Miettinen * , Gun Wirtanen VTT Biotechnology, P.O. Box 1500, FIN-02044 VTT, Finland Received 25 October 2004; received in revised form 2 January 2005; accepted 10 January 2005 Abstract A total of 510 rainbow trout originating from fish farms in lakes and sea areas around Finland were studied for the presence of Listeria monocytogenes . Samples were studied as pools from five fish. Gill, viscera, and skin from the pooled samples were analysed separately. The individual samples were analysed later if the pooled sample was found to be Listeria positive. The prevalence of Listeria spp. and L. monocytogenes in pooled unprocessed fresh rainbow trout was on average 35.0% and 14.6%, respectively. On the other hand, the prevalence of Listeria spp. and L. monocytogenes in individual thawed fish was found to be 14.3% and 8.8%, respectively. These numbers tend to overestimate and underestimate the real situation because not all fish in pooled samples were necessarily contaminated and in some of the Listeria positive pooled samples all individual samples turned out to be Listeria free. The prevalence of L. monocytogenes varied greatly between different fish farms from zero to 100% in pooled samples and from zero to 75% according to individually studied fish samples. Some indications of the influence of weather conditions and seasonal variations that strongly affected the Listeria contamination of fish were also noticed. The location of Listeria spp. and L. monocytogenes in different parts of the fish differed with statistical significance in rainbow trout. Up to 95.6% of the L. monocytogenes and 84.5% of Listeria spp. positive samples were gill samples. Only 4.4% (2/45) of the L. monocytogenes positive samples were obtained from skin or viscera. Closer study at one fish farm revealed that there was only one L. monocytogenes ribotype present in the contaminated fish, although water and surfaces were heavily contaminated with six other L. monocytogenes ribotypes. D 2005 Elsevier B.V. All rights reserved. Keywords: Listeria monocytogenes ; Rainbow trout; Prevalence; Contamination source 1. Introduction Listeria monocytogenes is a pathogenic bacteria commonly found in soil and water environments. It seems that Listeria spp. can often be isolated from polluted waters and waters with a high content of organic material, e.g. rivers and coastal areas (Ben 0168-1605/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ijfoodmicro.2005.01.013 * Corresponding author. Tel.: +358 20722 5150; fax: +358 20722 7071. E-mail address: hanna.miettinen@vtt.fi (H. Miettinen). International Journal of Food Microbiology 104 (2005) 135 – 143 www.elsevier.com/locate/ijfoodmicro