Serotype distribution and antibiotic resistance of Salmonella in food-producing animals in Shandong province of China, 2009 and 2012

• We characterised the Salmonella serovars in food-producing animals in Shandong, China.
• Salmonella on poultry carcasses were from the cross-contamination in abattoir.
• Antibiotic resistance rates in 2012 were significantly higher than those in 2009.
• In 2012, 41.5% of Salmonella were resistant to ciprofloxacin.
• First report of multidrug resistant Salmonella Indiana isolated from ducks

The aims of this study were to investigate the serotype distribution, genetic relationships and antibiotic resistance of Salmonella from food-producing animals in Shandong province of China in 2009 and 2012. A total of 362 out of 1825 samples from chickens, 53 out of 445 samples from ducks, and 50 out of 692 samples from pigs were positive for Salmonella. Isolates were subjected to serotyping, antibiotic susceptibility testing (15 antibiotics) and pulsed-field gel electrophoresis (PFGE). The most common serotypes recovered in the chicken samples were Enteritidis (n = 294, 81.2%) and Indiana (n = 45, 12.4%). For ducks, Cremieu (n = 25, 47.2%), Indiana (n = 13, 24.5%) and Typhimurium (n = 9, 17%) were frequently isolated. In the pig samples, Derby (n = 29, 58%), Typhimurium (n = 9, 18%), and Enteritidis (n = 6, 12%) were the most common serovars. PFGE results indicated that clonal dissemination of each serovar was prevalent, and that the Salmonella found on the poultry carcasses was caused by cross-contamination in the abattoirs. More than 99% of the Salmonella isolates collected were resistant to at least one antibiotic. The Salmonella resistance rates for 15 antibiotics in 2012 were significantly higher than those in 2009. In 2012, the highest resistance was to nalidixic acid (95.9%), followed by sulphafurazole (78.2%) and ampicillin (72.3%); the lowest levels of resistance were to kanamycin (40.1%) and amikacin (38.7%). Additionally, 41.5% and 42.2% of the Salmonella were resistant to ciprofloxacin and ceftiofur, respectively. Noticeably, 25% of the serovar Enteritidis and all of the serovar Indiana were resistant to at least 10 antibiotics in 2012. The increasing trend of antibiotic resistance in Shandong province indicates the need for more careful use of antibiotics.