The livestock reservoir for antimicrobial resistance: a personal view on changing patterns of risks, effects of interventions and the way forward

Abstract

The purpose of this review was to provide an updated overview on the use of antimicrobial agents in livestock, the associated problems for humans and current knowledge on the effects of reducing resistance in the livestock reservoir on both human health and animal production. There is still limiting data on both use of antimicrobial agents, occurrence and spread of resistance as well as impact on human health. However, in recent years, emerging issues related to methicillin-resistant Staphylococcus aureus, Clostridium difficile, Escherichia coli and horizontally transferred genes indicates that the livestock reservoir has a more significant impact on human health than was estimated 10 years ago, where the focus was mainly on resistance in Campylobacter and Salmonella. Studies have indicated that there might only be a marginal if any benefit from the regular use of antibiotics and have shown that it is possible to substantially reduce the use of antimicrobial agents in livestock production without compromising animal welfare or health or production. In some cases, this should be done in combination with other measures such as biosecurity and use of vaccines. To enable better studies on both the global burden and the effect of interventions, there is a need for global harmonized integrated and continuous surveillance of antimicrobial usage and antimicrobial resistance, preferably associated with data on production and animal diseases to determine the positive and negative impact of reducing antimicrobial use in livestock.

Keywords: antimicrobial resistance; antimicrobial usage; food animals; interventions; productivity.

Figure 1.

Overview of some of the most important antimicrobial-resistant pathogens and the overlap between the different reservoirs (VRE, vancomycin-resistant enterococci). The human reservoir (community and hospital) and food animal reservoir is indicated. Boxes and colouring indicate the degree of overlap of specific resistant bacterial species between these reservoirs. As indicated, some pathogens such as Pseudomonas aeruginosa, are strictly confined within the human reservoir, whereas others have a mainly or partly animal reservoir, such as Salmonella and Campylobacter. The two pictures show the changes in the author's personal perception over the past 10 years (2004–2014).

Figure 2.

Antimicrobial consumption and millions of heads of pigs produced in Denmark from 1994 to 2013. Black line indicates number of heads. Bars indicate total antimicrobial consumption adjusted to mg kg⁻¹ of pork produced (grey indicates use as antimicrobial growth promoter (AGP), white used for treatment). Also depicted are important events over time, including: no sales profit in 1995 reducing the use of antimicrobials for treatment, ban of the AGP avoparcin in 1995, ban of the AGP virginiamycin and voluntary stop for all AGP use in 1998, complete stop for all AGP use at the end of 1999 and implementation of the yellow card scheme in 2010.

Figure 3.

Data on production characteristics for piglet mortality (per cent; grey triangles) and mean number of pigs produced per sow per year (black diamonds) (a), average daily gain (ADG, grey triangles) and mortality rate (black squares) in weaning pigs (b), and average daily gain (white squares) and the percentage of dead or condemned finishing pigs (black diamonds; (c)) raised in the Danish swine production system from 1992 to 2013. The ban on antimicrobial growth promoters (vertical line) was instituted on 1 April 1998, and 1 January 2000, for finishing and weaning pigs, respectively. Weaning and finishing pigs weighed <35 and >35 kg, respectively. The yellow card scheme was introduced from 1 July 2010. Based on references [18,81].