Prevalence and Antimicrobial Resistance of Salmonella enterica Shed from Range and Feedlot Cattle from Post-weaning to Slaughter

The objectives of this study were to (1) estimate the prevalence of Salmonella in beef cows and steers at postweaning, finishing and slaughter, and (2) determine antimicrobial resistance of isolates, and (3) assess the association between resistance and presence of class 1 integrons. Fecal samples were collected from 48 cows and 48 steers at multiple sampling periods, and mid-line sponge samples collected from steer hides before slaughter. Bacteriological culture, antimicrobial resistance tests, and polymerase chain reaction testing were performed. Salmonella prevalence varied from 8% (3/38) to 92% (35/38) in cows and from 28% (13/47) to 100% (24/24) in steers, with higher estimates at postweaning than at finishing and slaughter. Of the 200 isolates recovered, the majority (56%) were resistant to 2 or more antimicrobials. Class 1 integrons were detected in 98 isolates, of which 88 (90%) carried the conserved sequence for aadA resistance gene encoding for streptomycin/spectinomycin resistance. Study findings revealed that most steers shed Salmonella at postweaning, indicating widespread exposure either before or after weaning. The recovery of multidrug resistant isolates and presence of class 1 integrons carrying the aadA resistance gene further underscores the dilemma and public health significance associated with veterinary use of antibiotics such as streptomycin in beef cattle.