Purpose: This study examined impacts of therapeutic regimens for administration of enrofloxacin (a fluoroquinolone antibiotic) in calves on AMR of Campylobacter.
Methods: Three calves (5-mo old steers) were treated subcutaneously, first with low (5mg/kg for 3 days) and after 10 days with high (12.5 mg/kg once) doses of enrofloxacin. Fecal samples were monitored for Campylobacter prior to the first treatment (period T1), between first and second treatment (T2) and subsequent to the second treatment (T3). Campylobacter spp. were analyzed by enrichment, direct plating and enumerations on selective media (mCCDA), and characterized for resistance to the (fluoro)quinolones ciprofloxacin and nalidixic acid. To detect gyrA mutations associated with resistance, a gyrA fragment was amplified and sequenced.
Results: Resistance prevalence differed among sampling periods (p < 0.0001), with T1 isolates being C. jejuni susceptible to nalidixic acid and ciprofloxacin, while most T2 and T3 isolates (100 and 75%, respectively) were nalidixic acid-resistant (NalR). However, 96.84% of these NalR isolates were ciprofloxacin-susceptible and 94.74% appeared to be C. fetus, reported previously to be frequently NalR but susceptible to ciprofloxacin. Their gyrA sequence was highly conserved with C. fetus gyrA and lacked mutations associated with (fluoro)quinolone resistance in C. jejuni. These findings suggest a shift in Campylobacter spp. populations in response to enrofloxacin treatment, with NalR campylobacters, primarily C. fetus, predominating subsequent to the administration of the antibiotic.
Significance: The data suggest complex outcomes to therapeutic antibiotic administration in food animals. These data will contribute to development of science-based recommendations for food animal veterinarians to maximize antibiotic efficiency while minimizing AMR.