A possible outbreak of listeriosis caused by an unusual strain of Listeria monocytogenes.

During 1987, a cluster of 23 cases of listeriosis due to an unusual serotype of Listeria monocytogenes was identified. All the 'epidemic' strains (designated serovar 4b(X)) were indistinguishable by serotyping, phage-typing and monoclonal antibody-typing. Clusters of cases due to this type have not been identified previously in Britain at such a high frequency. In an epidemiological survey, patients were interviewed so as to identify risk factors but none common in all cases was identified.