From the Executive Editor

The truth behind antimicrobial resistance

What is the most reliable way to bring a new disease into a pig barn? ON A PIG. Are there other ways to bring in new diseases? Yes, new diseases can come in via contaminated trucks, people’s clothing, or through the air. But by far the easiest way is to bring in an infected pig.

What is the most reliable way to develop antimicrobial resistance in a person? Feed the person antimicrobials. If you take an antibiotic for an illness, your normal intestinal microflora will become resistant to that antibiotic. Many reports describe the potential of antimicrobial use in food-production animals to lead to antimicrobial resistance in humans, BUT almost none that show a direct link. There is such a long, circuitous route between antimicrobial use in pigs and the potential of antimicrobial resistance in people that it is hard to measure the link.¹ We really do not know which of the putative causes described actually occur.

So is there an association between antimicrobial use in food-production animals and antimicrobial resistance in people? This has rarely been shown. However, there are a few reports in the literature of this connection. A report by the Public Health Agency of Canada² showed the relationship between the use of ceftiofur in hatchery eggs and day-old chicks and cephalosporin-resistant Salmonella Heidelberg in both human and retail-chicken isolates. Chicken hatcheries in Quebec were using ceftiofur in an extra-label manner. After the cephalosporin resistance in Salmonella Heidelberg was identified, the owners of the chicken hatcheries voluntarily discontinued their use of ceftiofur in February 2005. Subsequently, there was a significant, measurable reduction in the resistance seen against ceftiofur in both human and retail-chicken samples.

Even if antimicrobial use in pigs is not linked to antimicrobial resistance in people, we should take the use of antimicrobials in pigs seriously because of the concern about developing antimicrobial resistance in our pig populations. We rely on having antimicrobials to treat bacterial diseases in pigs. Studies done in the 1990’s indicated that antimicrobials used on many pig farms in the USA were not being used according to label directions.³ By the time pigs leave the nursery unit, they have often been given several antimicrobial agents. Studies conducted in Ontario indicate that antimicrobial resistance is common in Salmonella cultured from the feces of finisher pigs.⁴

Dr Scott McEwen, an expert in antimicrobial resistance, discusses the fact that antimicrobials are important in raising livestock.¹ If we are to ensure the welfare of pigs, we must have access to antibiotics to treat and control infectious bacterial diseases. Also, antimicrobials are used for the efficient production of food-production animals. However, we all must be cognizant of the term “prudent use.” This means we must use the antimicrobials responsibly. What questions might you ask yourself? Is the antimicrobial necessary? Is this the best choice of antimicrobial? What antimicrobials have these pigs already been fed? Do we really have to feed the pigs a new antimicrobial or can we restrict the number of antimicrobials on this farm, or in these pigs? I recommend you review the paper delivered by Dr Anne Deckert⁵ in the research session at the AASV annual meeting. In that research, Dr Deckert classified antimicrobials according to their importance for use in humans. The results indicated that the majority of antimicrobial resistance was for antimicrobials of limited use in humans. However, there was evidence of some resistance against antimicrobials that were very important in human medicine. Keeping the welfare of the pigs paramount, we need to avoid the use of those antimicrobials when possible.

We need to be vigilant about prudent use of antimicrobials in the pigs in our care. We need to be cognizant of which antimicrobials and how many different antimicrobials are given to the pigs. The welfare of pigs relies on maintaining both access to antimicrobials and susceptibility of infectious bacterial diseases to antimicrobials.

References

1. McEwen SA. Antibiotic use in animal agriculture: What have we learned and where are we going? Anim Biotech. 2006;17:239–250.

2. Public Health Agency of Canada. Canadian Integrated Program for Antimicrobial Resistance Surveillance (CIPARS). Salmonella Heidelberg – ceftiofur-related resistance in human and retail chicken isolates. Available at: http://www.phac-aspc.gc.ca/cipars-picra/heidelberg/heidelberg-eng.php. Accessed 17 Mar 2009.

3. Dewey CE, Cox BD, Straw BE, Bush EJ, Hurd S. Use of antimicrobials in swine feeds in the United States. Swine Health Prod. 1999;7:19–25.

4. Farzan A, Friendship RM, Poppe C, Martin L, Dewey CE, Funk J. Molecular epidemiology and antimicrobial resistance of Salmonella Typhimurium DT104 on Ontario swine farms. Can J Vet Res. 2008;72:188–194.

5. Deckert A. Gow S, Léger D, Rosengren L, Dutil L, Avery B, Irwin R, Reid-Smith R, CIPARS Collaboration. Antimicrobial use and resistance: Results from the CIPARS on-farm swine program. Proc AASV. Dallas, Texas. 2009;131–135.

--Cate Dewey, DVM, MSc, PhD