Multidrug resistant Salmonella enterica isolated from conventional pig farms using antimicrobial agents in preventative medicine programmes

A longitudinal study was conducted to investigate the presence of multidrug antimicrobial resistance (multi-AR) in Salmonella enterica in pigs reared under conventional preventative medicine programmes in Spain and the possible association of multi-AR with ceftiofur or tulathromycin treatment during the pre-weaning period. Groups of 7-day-old piglets were treated by intramuscular injection with ceftiofur on four farms (n = 40 piglets per farm) and with tulathromycin on another four farms (n = 40 piglets per farm). A control group of untreated piglets (n = 30 per farm) was present on each farm. Faecal swabs were collected for S. enterica culture prior to treatment, at 2, 7 and 180 days post-treatment, and at slaughter. Minimal inhibitory concentrations of 14 antimicrobial agents, pulsed-field gel electrophoresis and detection of resistance genes representing five families of antimicrobial agents were performed. Plasmids carrying cephalosporin resistant (CR) genes were characterised. Sixty-six S. enterica isolates were recovered from five of eight farms. Forty-seven isolates were multi-AR and four contained blaCTX-M genes harboured in conjugative plasmids of the IncI1 family; three of these isolates were recovered before treatment with ceftiofur. The most frequent AR genes detected were tet(A) (51/66, 77%), sul1 (17/66, 26%); tet(B) (15/66, 23%) and qnrB (10/66,15%). A direct relation between the use of ceftiofur in these conditions and the occurrence of CR S. enterica was not established. However, multi-AR was common, especially for ampicillin, streptomycin, sulphonamides and tetracycline. These antibiotics are used frequently in veterinary medicine in Spain and, therefore, should be used sparingly to minimise the spread of multi-ARThis work was supported by project AGL2011-28836 from the Ministerio de Economia y Competitividad of Spain, the Departamento de Innovación, Empresa y Empleo del Gobierno de Navarra (project reference IIQ14064.RI1) and Fundación Caja Navarra, Spain (project reference VATC 2014-0411 y VATC 2015-70628). Contract of LMG was supported by the Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) and the European Social Fund. Contract of VG was supported by Fundación Caja Navarra, Spain (project reference VATC 2015-70628). We acknowledge the Centres de Recerca de Catalunya programme for financial support, and to John G. Wild for review of English