Resistance to critically important antibiotics is widespread among Salmonella and Campylobacter in Canadian chickens, a recent study found. Published in PLOS One, the study was conducted by researchers from the Public Health Agency of Canada (PHAC), the University of Montreal, and the Canadian Food Inspection Agency (CFIA).

Antimicrobial resistance (AMR) is a major threat to global public health, and its emergence in foodborne bacterial pathogens is complicated by the overuse of antibiotics in food animal production.

Since 2002, PHAC’s Canadian Integrated Program for Antimicrobial Resistance Surveillance (CIPARS) has been monitoring AMR in food animals, and has demonstrated the widespread dissemination of drug-resistant Salmonella and Campylobacter in Canadian chickens. In 2014, the Chicken Farmers of Canada adopted a strategy to eliminate the preventive use of Category I antimicrobial classes (i.e., cephalosporins and fluoroquinolones), and in 2018, eliminated the use of Category II high-importance antibiotics (i.e., aminoglycosides, macrolides, lincosamides, penicillins, streptogramins, and trimethoprim/sulfamethoxazole).

To understand the present status of AMR among foodborne pathogens in Canadian broiler chickens along the production continuum, a cross-sectional study was conducted between 2018 and 2021 to evaluate the national prevalence of AMR in Salmonella, Campylobacter, and Escherichia coli. Cecal samples were collected from healthy, spent broiler breeders at slaughter (across 290 flocks), and from slaughtered broiler chickens (across 24 slaughter plants). Fecal samples were collected from farm broiler chickens (across 132 flocks per year). Samples were tested for the presence of foodborne pathogens and for antimicrobial susceptibility.

E. coli was recovered from 95.6–97.4 percent of the samples across the three production stages. Salmonella-positive samples ranged from 10.7 percent in broiler breeders and 46.4 percent in farm broilers. The rate of Campylobacter-positive samples ranged from 22.3 percent in slaughtered broiler chickens to 73.2 percent in broiler breeders. In all, C. jejuni was the predominant Campylobacter species found across the three poultry production stages, but the proportion of C. coli was substantially higher in broiler breeders than in broilers at the farm and slaughter plant levels. Meanwhile, Salmonella serovars differed between the three production stages, but S. Kentucky was the most predominant.

Overall, the findings indicated the persistence of bacteria in the poultry production chain that are resistant to very important Category I antimicrobials—fluoroquinolones and third-generation cephalosporins—with AMR levels ranging from very low (as low as 2.4 percent in E. coli) to high (up to 24 percent in Campylobacter). Multidrug resistance (MDR) in E. coli isolates ranged from 7.5 percent in broiler breeders to 21.8 percent and 22.1 percent in slaughtered and farm broilers, respectively. However, MDR in Salmonella (2.4–3.7 percent) and Campylobacter (0.2–1.1 percent) remained low, and resistance to colistin and meropenem was rarely or not detected.

Resistance to ciprofloxacin (a fluoroquinolone) was found in approximately 20–24 percent of Campylobacter isolates across the three poultry production stages, the highest being in farm broilers. Additionally, tetracycline resistance was prevalent in Campylobacter isolates across all three production stages, but was significantly higher in slaughtered broiler chickens (44.9 percent) than in broiler breeders (30.7 percent) or farm broilers (30.8 percent).

For Salmonella, broiler breeders had the highest resistance to most antimicrobials tested, with higher levels of resistance detected for ampicillin and tetracycline. Fully antibiotic-susceptible Salmonella was lowest in broiler breeders (16 percent) compared to farm and slaughtered broiler chickens (both 42 percent). High to very high levels of tetracycline resistance were found (51.9 percent–77.4 percent), and low to moderate levels of resistance to ceftriaxone were detected across all production phases (5.7–11.3 percent). For last-resort drug colistin, one resistant Salmonella isolate was recovered from a broiler breeder, and three were isolated from slaughtered broilers.  

E. coli isolates showed low-level resistance to ceftriaxone, and occasional E. coli isolates were non-susceptible to ciprofloxacin.

A positive observation was the low prevalence of ceftriaxone-resistant E. coli across all three production stages (less than 10 percent), suggesting that the 2014 ban on the preventative use of cephalosporins in broiler chickens may be effective. Salmonella isolates had similarly low levels of resistance to Category I antimicrobials.

Interestingly, the study found E. coli, Salmonella, and Campylobacter in broiler breeders that were resistant to antimicrobials at levels lower (in the case of E. coli) or higher (in the case of Salmonella) than those found in farm and slaughtered broiler chickens. The researchers hypothesize that the higher levels of resistance in E. coli in broiler breeders could be due to their antimicrobial exposures and shorter lifespans. For Salmonella, vertical transmission of AMR genes has been observed in the literature, and the persistence of certain serovars in breeders that are tetracycline-resistant may have contributed to the levels of AMR in Salmonella from farm and slaughtered broilers.

The study’s authors include Hiddecel Medrano, M.P.H.; Sarah Hill, Ph.D.; Martine Boulianne, D.V.M., Ph.D.; Teresa Cereno, D.V.M.; Anne E. Deckert, Ph.D.; Audrey Charlebois, Ph.D.; Sheryl P. Gow, D.V.M., Ph.D.; Kathryn McDonald, M.P.H.; Richard J. Reid-Smith, D.V.Sc., D.V.M.; and Agnes Agunos, D.V.M., Ph.D., D.A.C.P.V.