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, 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 development of AMR is likely to be different. Therefore, a study was conducted to compare in-feed vs. in-water administrations of chlortetracycline (CTC) and or tiamulin on fecal prevalence and AMR profiles of gut commensals (Escherichia coli, Enterococcus spp.) and foodborne pathogens (Campylobacter and Salmonella) in nursery piglets. A total of 1,296 weaned piglets, allocated into 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 were collected randomly from 5 piglets from each pen during pre-treatment (days -7, 0), treatment (days 7, 14) and post-treatment (days 21, 28) phases. Bacterial isolations and species identifications were done by culture and PCR, respectively. From d 0 to 14 and 0 to 28, there was an antibiotic × route of administration interaction (P < 0.05) observed for average daily gain (ADG). The interactions were a result of pigs fed CTC having increased ADG compared to CTC in-water, whereas pigs provided tiamulin in-water had increased ADG compared with tiamulin in feed. An antibiotic × route of administration interaction (P < 0.05) was also observed for gain:feed (G: F) from d 0 to 14 and 0 to 28. Pigs provided tiamulin in the feed had the poorest G:F, whereas G:F was not different among the other treatments. From d 0 to 14 and 0 to 28, providing CTC in the feed or water or tiamulin in the water improved (P < 0.05 or P < 0.10) ADG compared to pigs fed the control diet.

In summary, our study shows that providing CTC or tiamulin in the water improves growth in nursery pigs. The microbroth dilution method with SensititreTM plates were used to determine antimicrobial susceptibilities. Overall prevalence of Campylobacter and Salmonella were 18.2% (262/1,440) and 3.0% (43/1,440) respectively. Speciation of Campylobacter isolates by PCR indicated C. hyointestinalis (17.9%; 258/1,440) and C. coli (0.3%; 4/1,440). Campylobacter isolates were resistant to tetracycline (98.5%), ciprofloxacin (89.3%), and nalidixic acid (60.3%). Salmonella isolates were susceptible to azithromycin and totally resistant (100%) to ampicillin, streptomycin, sulfisoxazole, tiamulin, and tetracycline. Providing CTC in feed with or without tiamulin or tiamulin provided in the water demonstrated to improve nursery pig growth performance. Administering antibiotics via water may prove to be a more practical strategy due to flexibility of its usage. In-feed and in-water route of antimicrobial administration did not influence the fecal prevalence and antimicrobial susceptibilities of Campylobacter and Salmonella in this population of nursery piglets. Fecal resistome was not influenced by either antimicrobials or their route of administration in this population of piglets.