1.1.3 Antimicrobials used for growth promotion

In the 1940s, antimicrobial waste products fed to chickens were found to have growth promotion effects. Since then, antimicrobials have been routinely added to animal feed to increase productivity. For a long time, sub-therapeutic levels of antimicrobials in feed were believed to be a nutritional supplement.

The concern with using antimicrobial growth promoters (AGPs) is two-fold:

1. Dosing is at sub-therapeutic levels
2. Consumption of medicated feed is relatively uncontrolled in that individual animals consume varying amounts of feed.

This creates an ideal environment for the selection of resistance in gastrointestinal bacteria. Resistant bacteria may transfer to people via handling animals or on food products or enter the environment via faeces. In some countries, animal faeces is also used as fertiliser for food crops or may be used to supplement nutrients in aquaculture systems (Figure 2).

Figure 2 Example of integrated poultry and fish farming where chicken manure drops into the fishpond

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A photograph of a number of hens caged together on a platform suspended over a fishpond such that their manure can fall from the cage and enter the water below. Fish are feeding at the water surface immediately below the hen cage. This image illustrates the potential for antimicrobial resistant microbes to transfer between species and thus highlights concerns regarding the use of antimicrobials as growth promoters in livestock.

Figure 2 Example of integrated poultry and fish farming where chicken manure drops into the fishpond

Once it became apparent AGPs contributed to AMR in bacteria carried by animals, nations began imposing regulatory restrictions and prohibitions on their use, with the European Union (EU) beginning the trend in 1997. Today, many countries, including many LMICs, have banned the use of AGPs. According to the latest OIE report on antimicrobial use in animals, 77% of responding countries indicated they did not use any AGPs in animals, either with or without regulatory provisions to impose the demands (OIE, 2020).

There have been unintended consequences from banning AGPs in food animals. After the AGP bans came into effect in the EU, the trend for using antimicrobials for preventative purposes increased. The prohibition of AGPs showed up weaknesses in animal husbandry and biosecurity practices on farms. The AGP bans in the EU demonstrated that decisions of this magnitude need to be carefully considered and affected farming sectors supported through a transition phase to improve husbandry and biosecurity practices. This is especially important in LMICs where farming conditions can be challenging. In LMICs, where regulatory structures are often inadequate, the effectiveness of AGP bans may be limited (Schar et al., 2018).

Not all AGPs are controversial. For example, the ionophore class of antimicrobials (e.g. monensin, salinomycin, narasin and lasalocid), frequently used in food animals for growth promotion and parasite control (e.g. coccidia), are widely seen as low risk for AMR development in bacteria carried by people (commensal or pathogenic). There is no evidence that resistance is transferrable to human pathogens or that co-selection for resistance to other antimicrobials classes occurs. Also, owing to their toxicity to people, ionophores have never been used in human health (Landers et al., 2012).

People who advocate for AGPs in animals argue that their use is critical for food security and the economic viability of individuals and nations reliant on agriculture. However, others say that caution should be taken with the continued use of antimicrobials in food animals, given the risk posed to human health. One thing is universally agreed upon: alternatives to AGPs such as vaccination, improved nutrition, hygiene and biosecurity are urgently needed.

We’ll discuss more about these opportunities to reduce the burden of AMR in food animals throughout this module.