and its link to antimicrobial resistance.
Ontario Veterinary College, University of Guelph.
Zinc is a necessary component of a pig’s diet. Feed containing 50-125 ppm of zinc is suitable for normal growth and development. However, zinc is also used therapeutically in swine production as a treatment for post-weaning diarrhea. The therapy involves the addition of high levels of zinc (2,000-3,000 ppm) to the feed for 2-3 weeks after weaning. Pigs are fairly resilient to high levels of zinc (up to ~3,000 ppm), but the gut bacteria are prone to the antimicrobial properties of zinc therapy; although the full mechanism of this therapy is still being researched.Due to increasing pressure to reduce the use of traditional antibiotics in swine feed, zinc therapy has become a popular antibiotic alternative that is now widely used in Ontario swine production. Our recent survey of 22 nursery herds from 9 counties in southern Ontario showed that 68% of the farms were using a nursery feed with high levels of zinc (>2000 ppm). It was also found that some herds were using zinc at levels exceeding 3,000 ppm which can cause zinc toxicity in pigs.However, despite being a popular antibiotic alternative, a high level of in-feed zinc may inadvertently be causing resistance to antibiotics. Some bacteria (Staphylococcus spp.) carry a zinc-resistance gene which is genetically linked to other antibiotic-resistance genes within a ‘genetic cassette’ (1). When these bacteria are exposed to high levels of zinc they will get this genetic cassette from other bacteria in order to acquire resistance to zinc.
However, the genetic cassette contains other resistance genes and so the bacteria will acquire resistance to other antibiotics despite not being exposed to those antibiotics. That is, it is possible for bacteria to become resistant to antibiotics even under ‘antibiotic-free’ conditions.
Our research has shown that this event can occur on commercial swine farms. We conducted a randomized-controlled trial with 110 pigs under antibiotic-free conditions; half were given a minimal zinc diet (100 ppm) and half were given a high zinc diet (3,000 ppm). It was found that pigs on the high zinc diet were significantly more likely to carry the ‘superbug’ named methicillin-resistant Staphylococcus aureus (MRSA) than pigs fed the minimal zinc diet (2). Additionally, our study of 22 nursery herds in southern Ontario showed that farms using a high zinc diet were more likely to carry MRSA, and these bacteria commonly carried the genetic cassette which contains resistance genes for both zinc and antibiotics.’
This research highlights the complexity of antimicrobial resistance. There are multiple factors which contribute to antimicrobial resistance and this is one scenario which demonstrates that an antibiotic alternative can still cause antibiotic-resistance in bacteria. Conserving the therapeutic activity of antibiotics is important for both human and animal health, but antibiotic alternatives should always be validated with research, otherwise these alternatives may just be contributing to the problem.
1. Cavaco et al., 2010. Antimicrob Agents Chemother. 54(9):3605-8.
2. Slifierz et al., 2014. Zoonoses and Public Health. doi:10.1111/zph.12150.