Salmonella Shedding in Slaughter Pigs and the Use of Esterified Formic Acid in the Drinking Water as a Potential Abattoir-Based Mitigation Measure

SIMPLE SUMMARY: Salmonella excretion at slaughter is considered a source of carcass contamination and human infections. To assess this potential risk, a survey on Salmonella shedding at slaughter in 1068 pigs from 24 farms was carried out. Almost one-third of these farms (27.3%) shed Salmonella. The monophasic variant of Salmonella Typhimurium, an emerging serotype of zoonotic importance, was the most frequent (46.9%). Antimicrobial resistance in Salmonella isolates was common, but resistance against antimicrobials of critical importance for humans was low, with the exception of tigecycline, a new tetracycline-derivative antimicrobial used to treat severe infections caused by extensively drug-resistant bacteria that is not used in food-producing animals. An abattoir-based strategy for the control of Salmonella shedding, consisting of the addition of formic acid esterified in the form of glycerides in drinking water while waiting for slaughter, was able to significantly reduce the proportion of pigs shedding Salmonella. It appears this strategy can contribute to mitigating the burden of abattoir environmental contamination. ABSTRACT: Pigs shedding Salmonella at slaughter are considered a source of carcass contamination and human infection. To assess this potential risk, the proportion of Salmonella shedders that arrive for slaughter was evaluated in a population of 1068 pigs from 24 farms. Shedding was present in 27.3% of the pigs, and the monophasic variant of Salmonella Typhimurium, an emerging zoonotic serotype, was the most prevalent (46.9%). Antimicrobial resistance (AMR) in Salmonella isolates was common, but few isolates showed AMR to antimicrobials of critical importance for humans such as third-generation cephalosporins (5%), colistin (0%), or carbapenems (0%). However, AMR to tigecycline was moderately high (15%). The efficacy of an esterified formic acid in the lairage drinking water (3 kg formic acid/1000 L) was also assessed as a potential abattoir-based strategy to reduce Salmonella shedding. It was able to reduce the proportion of shedders (60.7% in the control group (CG) vs. 44.3% in the treatment group (TG); p < 0.01). After considering clustering and confounding factors, the odds of shedding Salmonella in the CG were 2.75 (95% CI = 1.80–4.21) times higher than those of the TG, suggesting a potential efficacy of reduction in shedding as high as 63.6%. This strategy may contribute to mitigating the burden of abattoir environmental contamination.