Distribution and dissemination of antimicrobial-resistant Salmonella in broiler farms with or without enrofloxacin use

BACKGROUND: Salmonella is a major zoonotic food-borne pathogen that persists on poultry farms, and animals undergo reinfection with endemic strains. The present study aimed to investigate the characteristics and dissemination of antimicrobial-resistant Salmonella within and between broiler farms that used enrofloxacin and those that did not. RESULTS: Cloacal and environmental (litter, feed, and water) samples from two selected flocks in each of 12 farms owned by the same company were collected three times over a 30-day period of two production cycles during 2015–2016. The rate of Salmonella isolation was 7.8% (123/1584). Nine Salmonella serotypes (116 isolates) and seven untypable isolates were identified, and Salmonella Montevideo was the most prevalent serotype. Azithromycin-resistant (17.9%) and colistin-resistant (3.3%) isolates were detected, and multidrug-resistant isolates (43.1%) were also observed. No isolate was resistant to enrofloxacin or ciprofloxacin; however, intermediate resistance to enrofloxacin was significantly higher (P < 0.05) in farms that used enrofloxacin than in those that did not. The rate of multi-drug resistance among litter isolates (25/44, 56.8%) was significantly higher (P < 0.05) than that among cloacal swab (24/67, 35.8%) and feed (4/12, 33.3%) isolates. Pulsed-field gel electrophoresis (PFGE) analysis of strains of the same serotype was conducted to determine their epidemiological relationship. The PFGE types were classified into 31 groups with a 100% correlation cutoff in dendrograms for Salmonella Montevideo isolates, which showed 100% genomic identity based on age, sample type, flock, and production cycle within and between farms. CONCLUSION: The present study highlights the occurrence of horizontal transmission and cyclic contamination with antimicrobial-resistant Salmonella in broiler farms owned by the same company. Litter may be a good indicator of indoor environmental contamination with antimicrobial-resistant Salmonella on farms. Additionally, enrofloxacin use may be one of the factors promoting resistance towards it in Salmonella.