An investigation into the effects of in-feed zinc oxide on antimicrobial resistant bacteria
Project 13-011 - Lead Researcher: Bob Friendship
Research Summary – Project 13/11
Researcher: Bob Friendship
Graduate student: Mackenzie Slifierz, PhD, University of Guelph
Project funded in 2013 and completed in 2014 EXECUTIVE SUMMARY
Staphylococcus aureus is a common bacteria that is the leading cause of skin infections and pneumonia in people. The increase in drug-resistant bacteria such as Methicillin resistant Staphylococcus aureus (MRSA) have increased the burden on healthcare systems due to an inability to find an effective antimicrobial therapy. Antimicrobial resistance has also raised concerns about the responsible use of antimicrobials in human and livestock medicine. Although antibiotic use in livestock production has been associated with MRSA in pigs and humans, the persistence of MRSA in pigs raised without exposure to conventional antibiotics indicates that there are other factors that contribute to a bacteria becoming resistant to treatment. Widely acknowledged for its antimicrobial properties, it has been suggested that Zinc oxide (ZnO), may be associated with antimicrobial resistance. In an effort to reduce or eliminate antibiotic usage while still controlling post-weaning diarrhea, some starter diets now contain high levels of zinc oxide. It is possible that feeding high levels of zinc oxide (ZnO) in starter rations could create selective pressure so that drug resistant bacteria predominate, such as methicillin resistant Staphylococcus aureus (MRSA).
To investigate this possibility, University of Guelph researchers conducted two studies to determine if high levels of zinc in the feed impacts the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in pigs. Their first trial followed two groups of piglets on a farm naturally colonized with MRSA. Group 1 was fed a diet with low levels of zinc (100 mg/kg) while the second group was fed high zinc levels (3000 mg/kg). None of the pigs received antibiotics while on the trial. The results showed that the pigs fed high zinc levels had significantly greater numbers of MRSA positive samples for up to two weeks post-weaning than the pigs fed low levels of zinc. All of the positive MRSA samples collected from the farm tested positive for the zinc resistance gene. In their second trial, researchers took samples before weaning and again three weeks later on 390 pigs from 26 commercial Ontario farms. The results showed that farms feeding high concentrations of zinc, as well as those who frequently disinfect their nursery pens were more likely to have pigs carrying MRSA. In this study, two thirds of the MRSA samples collected contained the zinc resistant gene.
Overall, exposure to high levels of in-feed ZnO was associated with an increase in the frequency and persistence of MRSA among pigs, particularly during the early phase of the nursery. Steps to reduce the use of antibiotics on pig farms may have had inadvertent effects. The researchers recommend that the swine industry practice the responsible use of ZnO in feed.