A total of 50 commercial swine finishing barns were recruited and sampled from 5 states during June – October 2011. A total of 30 fresh fecal samples were collected from the floor of each barn, 5 samples from 6 different pens, when the animals were at the end of the finishing phase and within 30 days of marketing. At the time of collection, barn managers were given a survey to capture data relating to antimicrobial use practices for the sampled group of animals beginning in the farrowing phase.
Fecal samples were shipped overnight to The Ohio State University for bacterial culture of E. coli and Salmonella, and selective enrichment with media containing cephalosporins:
E.coli: For blaCMY-2 selective E. coli culture, a 4 gram aliquot of feces was added to 36 ml of nutrient broth containing 4 µg/ml cefoxitin and incubated at 37° C for 24 hours. The samples were then streaked onto MacConkey agar plates containing 8 µg/ml ceftriaxone and incubated at 37° C for 24 hours; lactose positive colonies were considered presumptive positive for harborage of blaCMY-2. For blaCTX-M selective E. coli culture, a 4 gram aliquot of feces was added to 36 ml of nutrient broth containing 2 µg/ml cefotaxime and incubated at 37° C for 24 hours. The samples were then streaked onto MacConkey agar plates containing 4 µg/ml cefepime and incubated at 37° C for 24 hours; lactose positive colonies were considered presumptive positive for harborage of blaCTX-M.
Salmonella: A 4 gram aliquot of feces was enriched sequentially in tetrathionate and Rappaport-Vassiliadis R10 broth and then streaked onto xylos-lysine-tergitol 4 agar plates. An isolated black colony was streaked onto a MacConkey agar plate and confirmed as Salmonella using inoculation into triple sugar iron agar and urea broth as well as polyvalent antisera testing. The confirmed Salmonella isolates were streaked onto selective MacConkey agar plates containing 8 µg/ml ceftriaxone and 4 µg/ml cefepime to identify presumptive harborage of blaCMY-2 and blaCTX-M respectively.
All presumptive isolates were screened by Polymerase Chain Reaction (PCR) to confirm harborage of blaCMY-2 and blaCTX-M genotypes. Pulsed Field Gel Electrophoresis (PFGE) was utilized for comparison of isolates for clonality. Answers from the antimicrobial use survey were compiled and analyzed for association with recovery of blaCMY-2 and blaCTX-M genes using Chi-square and Fisher’s exact tests.
A total of 1,495 fresh fecal samples were collected from 50 commercial swine finishing in 5 states between the months of June and October 2011. The barns contained an average of 1,236 pigs with an average pen density of 35 pigs/pen and had an average of 96 days on feed in the finishing barn.
The antimicrobial use survey (Table 1) indicated that ceftiofur treatment in the early phases of production was common with 58% of farrowing barns and 72% of nursery barns administering the antimicrobial. In the finishing phase, 48% of the barns administered...