Evaluation of the risk factors for shedding Salmonella with or without antimicrobial resistance in swine using multinomial regression method.

Impacts • Development of resistance against antimicrobial agents in Salmonella on swine farms is a multi-factorial phenomenon. • Multinomial approach helps to differentiate the risk factors associated with shedding susceptible and drug-resistant Salmonella sp. and thus the appropriate intervention can be applied to each group. • Using mash or liquid feeding as well as screening commingling animals and replacement gilts for Salmonella can be considered possible solutions to limit Salmonella on swine farms. Summary A multinomial logistic regression method was used to investigate the risk fac- tors for antimicrobial resistance (AMR) in Salmonella isolated from faecal sam- ples collected on 80 Ontario swine farms in Canada. The samples were classified into three groups including Salmonella-negative samples (S)), Salmo- nella-positive samples without AMR (S+AMR)) and Salmonella-positive sam- ples with AMR (S+AMR+). The samples collected directly from pigs had a greater chance to be positive for Salmonella with AMR compared to those sam- ples collected from the pen floor. The odds of culturing Salmonella with or without AMR was higher if pelleted feed was used compared with mash or liquid feed (P < 0.001). The faecal samples collected on farrow-to-finish farms had a significant lower chance of testing positive for Salmonella with multidrug resistance than the samples from grow-finisher farms (P = 0.004). The chance of culturing Salmonella without AMR on farms with a continuous system was higher than on farms with an all-in/all-out system (P = 0.009). However, there was no significant association between the flow system and recovery of Salmo- nella with AMR. The larger farms were more likely to be in S+AMR+ group than in S) group ( P< 0.001) whereas herd size did not appear as a risk factor for being in S+AMR) group compared with S) group. These findings indicate that although on-farm antimicrobial use is one component of resistance, there might be other farm management factors that also affect the development of emerging resistant bacterial foodborne pathogens on swine farms. Finding dif- ferent risk factors for shedding Salmonella with or without antimicrobial resis- tance would help to take the appropriate approach to each group if a control programme were to be implemented or an intervention applied.