Trend of an emerging Listeria monocytogenes sequence type in Northern Italy

Objective Invasive listeriosis is a rare, life-threatening foodborne disease. In Lombardy region, Italy, twelve PFGE clusters were identified in the years 2006-2010, among which the major detected was named PFGE Cluster 11-1/2a-ST38/CC101 (Mammina et al., 2013). This cluster deserves special attention, due to its geographic and temporal size. The aim of this study was to monitor the frequency trend of emerging Listeria monocytogenes Cluster 11-ST38/CC101 in the years (2011-2013) following to the first reporting. Methods In Lombardy region, Italy, the national mandatory notification system has been integrated since 2005 with a laboratory-based surveillance network involving voluntary referral of clinical isolates, from Lombardy hospitals to a Regional Reference Laboratory at the University of Milan. All human L. monocytogenes isolates collected by the Regional Reference Laboratory, were serotyped and subtyped by Pulsed-Field Gel Electropheresis (http://www.cdc.gov/pulsenet/pathogens/listeria.html) and Multi-locus Sequence Typing (http://www.pasteur.fr/recherche/genopole/PF8/mlst/primers_Lmono.html) for cluster definition. Results In the period under study (2006-2013), 246 human isolates of L. monocytogenes were collected, among which Cluster 11-1/2a-ST38/CC101 included 42 isolates (17.1%). Their identification began in 2006 (n=2) and 2008 (n=2), peaked in 2009 (n=9), 2010 (n=16) and 2011 (n=10), and continued in 2012 (n=1) and 2013 (n=2). Through the entire period, cases were distributed in eight out of eleven provinces of Lombardy with the highest frequency in Bergamo and Milan. Two cases were pregnancy-associated and 40 were non-pregnancy associated. In this latter group, the median age of patients was 67 years (16 Male/24 Female), with 81% of cases being older than 65 years. The main clinical presentation was septicaemia, followed by meningitis and other infections. Treatment with immunosuppressive drugs and neoplasia were the main risk factor (54.1%). The fatality rate of 23 cases with a known survival outcome was 13%. Conclusion Our findings revealed a peak of cases prolonged in 2011 and a subsequent return to the initials values in the following years (2012-2013). The trend of the cluster 11-1/2a-ST38 leads us to suppose a possible outbreak, not detected by local health authorities, occurred in the years 2009-2011. Moreover, similar ST38 cases have been reported in France (1997) and in Germany (2011).