Epidemiological and molecular characterization of Salmonella enterica serovar Napoli isolated in Northern Italy (2009-2013)

Objectives .One event that has emerged since the beginning of this century has been the increase of cases of human infection due to Salmonella enterica serovar Napoli in Italy. The aims of our study were to describe the spatial and temporal distribution of S. Napoli human infections in Lombardy region between 2009 and 2013, and to assess clonality of the strains isolated from different sources by using Pulsed-Field Gel Electrophoresis (PFGE). Methods. All S. Napoli cases in Lombardy region reported from two surveillance system in the five years period, were included in our study. The data sources were national mandatory notification system (MAINF) and laboratory-based surveillance system (IT-Enter-net). Therefore, it was possible to realize a database that collects the reported cases by both systems. Moreover, a random sample of isolates (10%) collected by the Regional Reference Laboratory were sub-typed by PFGE according to PulseNet protocol and compared with 19 animal samples isolated by Istituto Zooprofilattico Sperimentale and Istituto Superiore di Sanità. Results. The comparison of the two databases detected a total of 645 observed cases, including 475 isolates detected only by IT-Enter-net system, 44 cases notified only by MAINF system and 126 cases recognized by both surveillance sources. These data revealed that IT-Enter-net network sensitivity was higher than MAINF system (91.5% vs 8.5%), since it is used to detect even the out-patient cases. It was possible to observe an increase in reported cases and a steady state in the following years. During the study period, the highest isolation rates were for infants (0-4 years, 47.9%) and for children (5-14 years, 21.7%). There was no evident differences between males and females. The average annual incidence rate was 1.3/105 inhabitants in Lombardy region, considering the observed cases by both sources. Figure 1 shows the distribution at the province level of the mean incidence rate in the five years period: the highest incidence rate were observed in the provinces of Como, Lecco and Varese. Every year, the largest proportion of S. Napoli isolates was observed between June and October. S. Napoli isolates compared by PFGE exhibited high levels of diversity (twenty-six XBa pulsotypes with a similarity value higher than 80%) between human and animal sources. Conclusion. Salmonella Napoli is relatively uncommon in Europe, while our data reveal that it ranks 3rd in the Lombardy region (17% of Italian population). Concerning possible source of infection or reservoir, the results did not point out any, but our hypothesis is that the environment can act as the main reservoir for S. Napoli, and from there it can spill over to animals and humans. Further studies are needed to increase the knowledge on the ecology and on the main risk factors for human infection.