The results presented here were obtained from farms located in Northern Italy in a high-density pig population area and regularly (at least once a year) checked for enteric pathogens. We focused on ST and MST, considering the prominent role of these two serovars in the pig population (4, 12).
ST and MST represented 4.92% and 9.73%, respectively of the Salmonella serovars isolated. These figures are not consistent with recent reports suggesting that ST and MST represent between 40% and 50% of the Italian isolates, with MST increasing from 9.66–46.34% in the last ten years (16, 17). A similar increasing has also been reported in other countries (12).
The distribution of the studied serovars during the considered period highlighted a predominance of ST on MST in the first period and, since 2008, a reversion of this tendency with MST becoming more and more predominant over the years. These data are in accordance with recent reports where the increasing prevalence of MST is well documented, in particular in the United Kingdom, Poland and Malta (6, 18, 19). In particular, in the United Kingdom, MST represented 60.7% of the Salmonella isolates obtained from a surveillance program in pigs in 2015 (20).
It is conceivable to hypothesize that MST has a selective advantage over ST. It was suggested that a number of factors (i.e. involvement of prophages and antigenic changes) can cause a reduced immune response to MST in herds when compared to ST (4). More recently, a comparative whole-genome sequencing and phylogenomic analysis of MST isolates from the United Kingdom and Italy during the period 2005–2012, revealed a high level of microevolution that may affect antigenicity, pathogenicity, and transmission (21). We recently explored the ability to induce an immunity using an inactivated MST (22) or an attenuated variant of ST (23, 24), yet we found a limited cross reaction between MST and ST.
When considering the production stage, both ST and MST showed their highest presence in the weaning and growing period as reported previously (25, 26). A comparison between the prevalence of ST and MST in different production stages showed no significant differences. Overall, these findings suggest a higher level of susceptibility in younger pigs, irrespective of the serovars involved. Pigs can become infected at any production stage but the decline of maternal antibodies after weaning makes younger pigs more susceptible to the infection (27).
When considering the association between clinical signs and isolates, we observed that clinical signs were associated more to ST than to MST and that most of clinical cases were present in young pigs after weaning. These data, although only approaching significance, are supportive of a lower virulence of MST compared to ST. In addition, it is worth mentioning that data obtained in our laboratory seem to show that MST is less virulent than ST in either pig or mouse models of experimental infections (Alborali G.L. and Pasquali P., personal communication).
The phage-typing highlighted the prevalence of four types representing about 70% of the typed isolates (DT193, DT120, U311, U302) and this has been a common feature of European isolates for the last twenty years (11, 28, 29). DT193 has to be regarded as an important phage type also for ST, considering its increase in Europe in the last years (30) and its role in human cases of salmonellosis.