The Impact of In-Water vs. In-Feed Chlortetracycline and Tiamulin Administration in Piglets on the Fecal Prevalence and Antimicrobial Resistance of Salmonella

Antimicrobial Resistance (AMR) in bacteria is a growing public health concern in the US and around the world threatening the continual use of antimicrobials. In pigs, the oral route, either in-feed or in-water, is by far the most common route of administration of antimicrobials. Because the distribution of the antibiotic in the gut and the dosages are different, the impact of in-feed vs. in-water administration of antibiotics on the prevalence of pathogens, such as Salmonella, and the development of AMR are likely to be different. Therefore, a study was conducted to compare in-feed vs. in-water administrations of chlortetracycline (CTC) and/or tiamulin on the fecal prevalence and AMR profiles of Salmonella in nursery piglets. A total of 1296 weaned piglets, housed in 48 pens (27 piglets per pen), were assigned randomly to six treatment groups: Control (no antibiotic), in-feed CTC, in-water CTC, in-feed tiamulin, in-water tiamulin, or in-feed CTC and tiamulin. Fecal samples (n = 1440) were collected randomly from five piglets from each pen during the pre-treatment (days 7, 0), treatment (days 7, 14), and post-treatment (days 21, 28) phases. Salmonella enterica isolation and identification were completed by culture and PCR methods. The microbroth dilution method with SensititreTM (ThermoFisher Scientific, Lenexa, KS, USA) plates was used to determine the antimicrobial susceptibility and resistance of Salmonella strains. The susceptibility and resistance were interpreted based on the Clinical and Laboratory Standards Institute guidelines. The overall prevalence of Salmonella was 3.0% (43/1440). All isolates belonged to Salmonella enterica subsp. enterica serovar Typhimurium. Salmonella isolates were susceptible to azithromycin and resistant (100%) to ampicillin, streptomycin, sulfisoxazole, tiamulin, and tetracycline. Neither antibiotic, CTC or tiamulin, nor the route of administration, in-feed or in-water, had an effect (p > 0.05) on the occurrence of resistant Salmonella in the feces of piglets.