Actinobacillus pleuropneumoniae Eradication with Enrofloxacin May Lead to Dissemination and Long-Term Persistence of Quinolone Resistant Escherichia coli in Pig Herds

Norway has a favourable situation with regard to health status and antimicrobial usage in the pig production sector. However, one of the major disease-causing agents in the commercial pig population is Actinobacillus pleuropneumoniae (APP). In some herds, APP eradication has been performed by using enrofloxacin in combination with a partial herd depopulation. The aim of this study was to investigate the long-term effects of a single treatment event with enrofloxacin on the occurrence of quinolone resistant Escherichia coli (QREC). The study was designed as a retrospective case/control study, where the herds were selected based on treatment history. Faecal samples were taken from sows, gilts, fattening pigs and weaners for all herds where available. A semi-quantitative culturing method was used to identify the relative quantity of QREC in the faecal samples. A significant difference in overall occurrence and relative quantity of QREC was identified between the case and control herds, as well as between each animal age group within the case/control groups. The results indicate that a single treatment event with enrofloxacin significantly increased the occurrence of QREC in the herd, even years after treatment and with no subsequent exposure to quinolones.