The epidemiology of antimicrobial resistance monitoring in pigs in relation to the growth promoter avilamycin

Field: Veterinary (Health Sciences)

The objectives of this study were to monitor resistance to the antimicrobial growth promoter avilamycin on pig farms, with changes in resistance over time and in relation to avilamycin use of particular interest. The aims were to consider how best to measure resistance and to determine which organisms in the faecal flora, in particular which Enterococcus species, were expressing resistance. Resistance to therapeutic antimicrobials used in human medicine was also to be assessed. In considering how best to measure resistance, standard statistical methods were used and novel epidemiological techniques were also developed. The findings suggested that standard statistical formulae should be applied to calculating sample numbers for antimicrobial resistance studies and that the organism, antimicrobial and animal population of interest should be clearly defined. Furthermore, the epidemiological models suggested that the sensitivity and specificity of the tests used must be defined and that the current practice of testing a small number of colonies from a small number of animals means that resistant organisms will be missed if the prevalence of resistance is very low, and that changes in resistance below 5 per cent prevalence cannot be monitored with accuracy. When the test used is not 100 per cent specific then the current practice of confirming the presence of resistance based on one bacterium testing positive is potentially misleading. The relationship between resistance and antimicrobial use on farms was shown to be difficult to assess due to the many factors potentially influencing the prevalence of resistance. The use of Slanetz and Bartley medium was shown to be effective in isolating Enterococcus species with a prevalence of isolation ranging from 0.5 to 1 but to be poorly specific for this genus. E. faecium, E. faecalis, E. hirae and E. durans were shown to be capable of expressing resistance to avilamycin and the relative proportion of these species was found to be different on different farms. Minimum inhibitory concentrations (MIC) of avilamycin ranged from 1mug/ml to >128mug/ml in the enterococcal isolates tested. The large number of Escherichia spp. and smaller number of Yersinia spp. isolates tested were resistant to avilamycin with MIC >128mug/ml whilst the MIC in a small number of Campylobacter spp. isolates ranged from 8mug/ml to 128mug/ml. Avilamycin resistant enterococci were isolated from all four farms studied and resistance had persisted or been reintroduced on one farm where avilamycin had been withdrawn from use two years previously. This was only detected when faeces were screened on avilamycin-containing medium and not by conventional individual isolate MIC determination. The prevalence of resistance to a panel of human therapeutic antimicrobials was assessed in enterococci and Escherichia spp. but few conclusions could be drawn due to the small sample numbers studied. The molecular basis of avilamycin resistance was determined in 4 enterococcal isolates. Two E. faecium and one E. faecalis had substitutions in the gene encoding ribosomal protein LI6 but one E. hirae and one E. faecium had sequences identical to the sensitive reference strain. In summary, avilamycin resistance was detected on all 4 farms studied using conventional microbiological techniques but was difficult to quantify, and it was not possible to measure changes in prevalence over time with accuracy using these methods.