Lessons from an outbreak of listeriosis related to vacuum-packed gravad and cold-smoked fish

The first lesson learned from this outbreak was that vacuum-packed rainbow trout is not only an excellent medium for the growth of Listeria monocytoge nes, but may also cause human listeriosis. Another lesson is that one singl e fish processing plant may spread multiple clonal types of L. monocytogene s by selling contaminated products to consumers. Thus, when investigating f ish-borne outbreaks of listeriosis one should identify and type several iso lates of L. monocytogenes from each food and environmental sample, since mu ltiple clonal types might be present. The outbreak described in this paper involved at least eight human cases, three clonal types of L. monocytogenes , and lasted for 11 months. During the outbreak investigation, L. monocytog enes was also isolated from another brand of rainbow trout found in the ref rigerator of one of the patients. These latter isolates belonged to a clona l type not associated with the outbreak. However, this clonal type is of co nsiderable interest since it has been associated with foodborne outbreaks o f listeriosis in several countries, and is also the second most common clon al type among human clinical isolates of L. monocytogenes in Sweden. Beside s the described outbreak, it is likely that vacuum-packed, cold-smoked and gravad rainbow trout have been involved in additional cases of foodborne Li steriosis in Sweden. (C) 2000 Elsevier Science B.V. All rights reserved.