A Randomized Trial to Assess the Effect of Fluoroquinolone Metaphylaxis on the Fecal Prevalence and Quinolone Susceptibilities of Salmonella and Campylobacter in Feedlot Cattle.

The study objective was to determine effects of fluoroquinolone metaphylaxis on fecal prevalence of Salmonella and Campylobacter and fecal prevalence of quinolone-resistant Salmonella and Campylobacter in feedlot cattle. On Day 0, cattle (n = 288) at risk for bovine respiratory disease (BRD) were randomly assigned to either a nontreated control pen (12 pens) or a fluoroquinolone-treated (enrofloxacin; Baytril® 100) pen (12 pens). Rectal fecal samples were collected from cattle on days 0, 7, 14, 21, and 28. Feces were cultured for Salmonella enterica and Campylobacter spp. using enrichment and selective isolation methods, and confirmed by serology and PCR. Susceptibilities to nalidixic acid and ciprofloxacin were determined using microbroth dilution methods. Data analyses were performed using linear mixed models. Overall, Salmonella sp. and Campylobacter spp. were recovered from 10.2% (139/1,364) and 12.4% (170/1,364) of the fecal samples, respectively. Campylobacter species included hyointestinalis, jejuni, and coli. Neither Salmonella sp. nor Campylobacter spp. prevalence was significantly impacted by fluoroquinolone treatment (p = 0.80, p = 0.61, respectively). However, Salmonella prevalence differed between study weeks (p < 0.01) with prevalence decreasing over time. Before treatment, 98.9% (91/92) of Salmonella isolates were susceptible to nalidixic acid and ciprofloxacin. All Salmonella recovered posttreatment (n = 43) were susceptible to both antimicrobials. The majority of Campylobacter spp. recovered before treatment were resistant to nalidixic acid (23/35; 65.7%) and ciprofloxacin (21/35; 60.0%). There was no significant treatment by week interaction (p = 0.85) or treatment effects (p = 0.61) on the posttreatment prevalence of Campylobacter resistance. There was, however, a significant week effect (p = 0.05), with Campylobacter resistance prevalence decreasing over time. In this 28-day study, we found no evidence that a fluoroquinolone used for metaphylaxis significantly impacts fecal prevalence of Salmonella sp. or Campylobacter spp. or the fecal prevalence of nalidixic acid or ciprofloxacin resistance.