The interplay of host agent and environmental factors is often

The interplay of host, agent and environmental factors is often discussed in the context of infectious disease in animal populations, yet this interaction is less frequently considered in relation to the occurrence of antimicrobial resistance in such groups. Instead, when discussing antimicrobial resistance, attention is usually focussed on the relationship between antimicrobial use and resistance. Whilst antimicrobial use is undoubtedly the primary selective pressure for the emergence and persistence of antimicrobial resistance, and might be considered as a ‘host’ factor, the role played by other host, agent and environmental factors in the occurrence of resistance is often overlooked. The influence of such factors is demonstrated by Dr. Karla Cameron-Veas and colleagues, of the Centre de Recerca en Sanitat Animal (CReSA), Barcelona, Spain, in their paper on cholesterol absorption inhibitor of cephalosporin resistant in pigs treated with antimicrobial agents, published recently in ().
In this study, the selective effect of use of antimicrobial agents was demonstrated by an increase in the numbers of cephalosporin resistant (CR-) in faeces following the use of a single injection of ceftiofur at a therapeutic dose in piglets already colonised with CR-. However, treatment with ceftiofur was not a significant predictor of the occurrence of resistance in the study group as a whole. Instead, farm of origin (an environmental factor) had the greatest influence on the occurrence of cephalosporin resistance, with the selective pressure of antimicrobial use only exerting an effect in those herds where CR- was already present. Furthermore, the study showed that the proportion of animals shedding CR- and the numbers of CR- shed decreased with animal age (a host factor).
Resistance to third generation cephalosporins in is mediated by a number of β-lactamases, the extended spectrum β-lactamase (ESBL) and acquired AmpC-type β-lactamases being of particular current concern. These resistance phenotypes have their origins in other Gram-negative bacteria and their emergence and spread is due to horizontal transfer of the associated resistance genes on mobile genetic elements (). Therefore, in contrast to other resistance mechanisms, these are not likely to arise as a result of mutation of existing bacterial genes during antimicrobial treatment. This may explain why CR- did not emerge during treatment on all farms in this study.
Farm related factors that may influence the occurrence of antimicrobial resistance in of porcine origin include production type, housing type, hygiene status and stress (). In the study by , the authors suggest that the origin and housing of sows may be part of the farm effect observed. These findings offer some hope that farms and animals can be kept free of certain resistant organisms through strict biosecurity and management practices, such as the careful sourcing of breeding stock. However, the efficacy of such interventions needs to be evaluated fully before they can be recommended.
The decrease in the proportion of animals shedding CR- and in the numbers of CR- shed may be explained, in part, by the time elapsed after the removal of the selective pressure of antimicrobial use. However, similar results have been reported in other studies from pigs, independent of antimicrobial use, which suggest that increasing host age is associated with decreased resistance (). It has been proposed that these changes over time reflect changes in the cholesterol absorption inhibitor microbiota of the porcine intestine, but it has not been determined whether these bacteria enjoy a primary selective advantage in the juvenile intestine as a consequence of their resistance status or if this results from co-selection with other advantageous traits. In either case, colonised sows might be considered as ‘reservoirs’ of resistant organisms (with shedding at lower levels due to increased age) for the colonisation of young pigs, which in turn act as ‘amplifiers’ to allow for the persistence of resistance within the herd and environment.