Prevalence of multidrug-resistant Salmonella on commercial dairies utilizing a single heifer raising facility.

The objectives of the current research were twofold: (i) to determine the prevalence of multidrug-resistant (MDR) Salmonella in the various classes of dairy cattle and (ii) to determine if comingling of calves from multiple farms at a heifer feedlot serves as a transmission vector for Salmonella back to the dairy farm. Four large commercial dairies in the southwestern United States were sampled in October 2005 and again in March 2006. Fecal samples were collected from hutch calves, 12- and 24-month-old heifers, lactating cows, dry cows, and cattle in the sick-fresh pen and cultured with brilliant green agar supplemented with novobiocin (BGA(nov)) to estimate the overall Salmonella prevalence, or with tetracycline (BGA(tet)) to estimate MDR Salmonella. Antimicrobial susceptibility testing was conducted with the National Antibiotic Resistance Monitoring System (NARMS) testing panel, and a portion of the isolates were serotyped. Salmonella prevalence among groups ranged from 0 to 96% positive, with the highest incidence observed in the hutch calves and cattle in the sick-fresh pen. Twenty-eight different serotypes were identified with serotype Reading accounting for the majority of isolates cultured on BGA(tet). Nearly all (100 of 103) isolates cultured on BGA(nov) and screened for antibiotic resistance were pan susceptible, whereas over one-half (64%) of the isolates cultured on BGA(tet) were MDR. Forty isolates displayed the ACSSuT resistance pattern, and 36 isolates displayed the MDR-AmpC pattern of the 72 isolates examined following culture on BGA(tet). The incidence of Salmonella cultured on BGA(tet) was low (9%) in all heifers and only one MDR isolate was cultured (from a 12-month-old heifer), suggesting the risk of transmission of Salmonella from the heifer feedlot back to the dairy is low. Results of this research suggest the incidence of MDR Salmonella, found primarily in hutch calves and cattle in the sick-fresh pen, is low in comparison to the overall Salmonella prevalence.