Comparison of Prevalence, Antimicrobial Resistance, and Occurrence of Multidrug-Resistant Salmonella in Antimicrobial-Free and Conventional Pig Production

Abstract

Conventional swine production evolved to routinely use antimicrobials, and common occurrence of antimicrobial-resistant Salmonella has been reported. There is a paucity of information on the antimicrobial resistance of Salmonella in swine production in the absence of antimicrobial selective pressure. Therefore, we compared the prevalence and antimicrobial resistance of Salmonella isolated from antimicrobial-free and conventional production systems. A total of 889 pigs and 743 carcasses were sampled in the study. Salmonella prevalence was significantly higher among the antimicrobial-free systems (15.2%) than the conventional systems (4.2%) (odds ratio [OR] = 4.23; P < 0.05). Antimicrobial resistance was detected against 10 of the 12 antimicrobials tested. The highest frequency of resistance was found against tetracycline (80%), followed by streptomycin (43.4%) and sulfamethoxazole (36%). Frequency of resistance to most classes of antimicrobials (except tetracycline) was significantly higher among conventional farms than antimicrobial-free farms, with ORs ranging from 2.84 for chloramphenicol to 23.22 for kanamycin at the on-farm level. A total of 28 antimicrobial resistance patterns were detected. A resistance pattern with streptomycin, sulfamethoxazole, and tetracycline (n = 130) was the most common multidrug resistance pattern. There was no significant difference in the proportion of isolates with this pattern between the conventional (19.5%) and the antimicrobial-free systems (18%) (OR = 1.8; P > 0.05). A pentaresistance pattern with ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline was strongly associated with antimicrobial-free groups (OR = 5.4; P = 0.01). While showing the higher likelihood of finding antimicrobial resistance among conventional herds, this study also implies that specific multidrug-resistant strains may occur on antimicrobial-free farms. A longitudinal study with a representative sample size is needed to reach more conclusive results of the associations detected in this study.

Longitudinal Study of Distributions of Similar Antimicrobial-Resistant Salmonella Serovars in Pigs and Their Environment in Two Distinct Swine Production Systems

“…Salmonella isolation and confirmation. Isolation and confirmation were performed as previously described (7,9). Briefly, the samples were preenriched by adding 90 ml of buffered peptone water (BPW) (Difco, Becton, Dickinson) to cups containing either fecal or environment samples, whereas 30 ml of BPW was added to each bag containing either MLN or carcass swabs.…”

“…Emergence and persistence of antimicrobial-resistant (AR) foodborne pathogens due to routine use of antimicrobials for therapeutic, preventive, and growth purposes in conventional swine production are a major public health concern (4). Multidrugresistant (MDR) Salmonella strains, exhibiting resistance to thirdgeneration cephalosporins, have been reported in commercial pigs (4)(5)(6)(7). With growing consumer concerns over AR in bacterial pathogens from commercial swine that are given antimicrobials for prophylaxis and treatment, the demand for antimicrobial-free (ABF) and organic products has increased over the past decade (8).…”

“…With growing consumer concerns over AR in bacterial pathogens from commercial swine that are given antimicrobials for prophylaxis and treatment, the demand for antimicrobial-free (ABF) and organic products has increased over the past decade (8). However, previous studies have highlighted the occurrence of MDR Salmonella in both ABF and organic food animal production despite the apparent absence of antimicrobial selection pressure (7,9,10).…”

“…North Carolina is the second-largest pork-producing state in the United States next to Iowa, with a 14.4% contribution to the national inventory (14). Studies in swine production systems have been conducted in North Carolina to report Salmonella prevalence (6,7,9). Interestingly, a higher Salmonella prevalence (16 to 29%) has been reported in the swine farm environment than in fecal samples within the same farm (12).…”

Macrolide Resistance in Microorganisms at Antimicrobial-Free Swine Farms

“…A better understanding of this complex relationship is needed to provide a basis for developing moreeffective measures to control the prevalence of antimicrobial resistance. One means for investigating the factors influencing the prevalence of resistance is through comparisons between conventional farms and organic, antimicrobial-free farms (12,13,18,25) or the wilderness (14,19).…”