Development of macrolide-resistant Campylobacter in broilers administered subtherapeutic or therapeutic concentrations of tylosin.

The use of antimicrobials in food animal production, particularly those commonly used to treat infections in humans, has become a source of debate in recent years. However, limited data are available regarding the development of resistance following the subtherapeutic or therapeutic administration of antimicrobials in animal production. The objective of this study was to evaluate the effect of the administration of therapeutic and subtherapeutic concentrations of tylosin on the erythromycin susceptibility of Campylobacter jejuni and Campylobacter coli isolated from the ceca of treated broilers. In three replicated studies, day-of-hatch chicks were exposed to macrolide-susceptible C. jejuni or C. coli. At 2 weeks of age, tylosin was administered at subtherapeutic (22 ppm, continuously in the diet) or therapeutic concentrations (529 ppm, in the drinking water for 5 days). Broilers were sacrificed weekly. Total and erythromycin-resistant Campylobacter spp. were enumerated from individual ceca plus cecal contents. Overall erythromycin resistance was observed at a higher frequency (P < 0.01) among C. coli isolates (70.8%) than among C. jejuni isolates (36.8%) following tylosin administration. Across Campylobacter species, erythromycin resistance was observed at a higher frequency (P < 0.001) when tylosin was administered at subtherapeutic (62.7%) than at therapeutic (11.4%) concentrations. Subtherapeutic administration resulted in the recovery of 83.3 and 56.1% erythromycin-resistant isolates compared with only 33.3 and 7.9% of the isolates expressing erythromycin resistance following the administration of therapeutic concentrations for C. coli and C. jejuni, respectively. Further studies are needed to determine the factors involved in the apparent difference in the acquisition of macrolide resistance in C. coli compared with C. jejuni.