Maintenance of Carbapenemase-Producing Enterobacteriaceae in a Farrow-to-Finish Swine Production System

Carbapenemase-producing Enterobacteriaceae (CPE) threaten both agriculture and public health. While carbapenems are restricted in food-producing animals, other -lactams, such as ceftiofur, are frequently applied in livestock. While the relationship is not fully elucidated, ceftiofur use may provide selective pressure that promotes carbapenem resistance. Recently reported in U.S. livestock, plasmid-mediated CPE are also present in livestock in Europe and Asia. We previously reported the rare carbapenemase gene, bla(IMP-64), in the environment of a large farrow-to-finish swine operation. To better understand CPE in this swine production system, in 2016 we followed a cohort of 350+ pigs over 5 months from late sow gestation to the final finishing phase. We screened both environmental and fecal samples for CPE using our selective enrichment protocol, with resulting phenotypic CPE isolates further characterized. Of 55 environmental and 109 sow fecal samples collected from a farrowing barn on our initial visit, 35 (64%) environmental and 15 (14%) sow fecal samples yielded isolates of multiple Enterobacteriaceae species carrying the metallo--lactamase gene bla(IMP-64) on an IncQ plasmid. The frequency of IMP-64-positive environmental (n=32), sow fecal (n=30), and piglet fecal swab (n=120) samples was highest for all groups when the market pig cohort was between 1 and 10 days, with observed prevalence of 97%, 28%, and 18%, respectively. After weaning, bla(IMP-64) was detected in a single environmental sample from a nursery pen, with no CPE recovered in the finishing phase. Used in U.S. swine production to treat and control disease, ceftiofur is administered to piglets on this farm at birth, with males receiving a second dose at castration (approximate to day 7). Once introduced into animal agriculture, the common use of ceftiofur may provide the selection pressure required for CPE dissemination throughout large, intensively managed food animal populations housed in animal-dense environments.