Occurrence of Salmonella Typhimurium and its monophasic variant (4, [5],12:i:-) in healthy and clinically ill pigs in northern Italy.

The serovar Typhimurium (4, [5],12:i:1,2), is the most frequently isolated serovar in case of salmonellosis in pigs in Europe and its monophasic variant (4, [5],12:i:-) has been increasingly responsible for Salmonella outbreaks in humans. A total of 25,215 samples were collected, during the years 2002-2017, from 1359 pig farms located in Northern Italy. Samples were collected from different material sources including fecal samples, rectal swabs, gut content and different organs. Salmonella was isolated in 15.80% of samples and, among the isolates, 733 were typed as Salmonella Typhimurium (ST) or its monophasic variant (MST). Over time, there was an increase of isolation of MST which outnumbered ST. Most of the strains were isolated in animals during the weaning stage and the growing - fattening period whereas the clinical cases were mainly present in young pigs after weaning. This study confirms the presence of ST and MST in pig farms although, considering the total of isolated serotypes, with lower percentages than previously reported. In the last few years, ST has increasingly been replaced by MST suggesting that MST has a competitive advantage over ST, probably due to its different antigenicity and pathogenicity which renders the infection stealthier to recognize and control.