Kvistholm Jensen and colleagues analyzed 559 clinical Listeria monocytogenes isolates from 2002-2012 using 2-enzyme PFGE and conducted serotyping. They found clustering of some common patterns that could either be a large linked source. or maybe a few types from unrelated foods. Not sure.

In Denmark, the annual incidence of listeriosis increased from 0.5 cases per 100,000 population in 2002–2003 to a peak of 1.8 cases in 2009 and 0.9 cases in 2012, and is now among the highest incidences reported globally (8,9). Similar increasing trends have been reported from other European countries during the same period (4). The high but variable incidence calls for further examination of the possible explanations. We retrospectively analyzed trends related to patient data and PFGE- and MLST-types of L. monocytogenes strains occurring in Denmark during 2002–2012. In addition, we assessed the possible association between clinical aspects of the disease and strain genotype.

Our findings show that retrospective typing of isolates gives new insight into the epidemiology of listeriosis. By PFGE typing, we found a high diversity of L. monocytogenes in clinical cases but also a small number of frequent types representing a substantial fraction of all cases. Possibly, these types represent epidemiologically linked cases (outbreaks) or, alternatively, ubiquitous types present in many unrelated food sources and infections