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Coccidiostats and Antimicrobial Resistance

Published 1 September 2026

1. Executive summary

1.    The Advisory Committee on Animal Feedingstuffs (ACAF) met to discuss the use of ionophore coccidiostats as prophylactic and metaphylactic feed additives in the UK and their potential relationship to antimicrobial resistance (AMR). The Committee reviewed the current legislative framework, discussed developments and concerns relating to AMR associated with food production, and examined relevant evidence from scientific literature. Members also considered existing AMR surveillance arrangements and the potential implications of coccidiostat use for animal and human health, and the environment.

2.     Coccidiostats are used to manage Eimeria infections across UK livestock systems and support production efficiency, animal health and welfare. Surveillance programmes such as the UK Veterinary Antimicrobial Resistance and Sales Surveillance (UK-VARSS) and the Pathogen Surveillance in Agriculture, Food and the Environment (PATH-SAFE) currently indicate low prevalence of vancomycin resistant enterococci in livestock using low sensitivity testing methods. However, the Committee noted emerging evidence concerning resistance determinants associated with some ionophore coccidiostats, namely narasin, salinomycin and maduramicin, and highlighted the lack of systematic measurement and monitoring data using appropriate sensitive testing methods to fully assess the extent of resistance. The potential for environmental exposure pathways was also recognised.

3.    The Committee concluded that surveillance of resistance selection by coccidiostats could be strengthened by coordination with the Veterinary Medicines Directorate (VMD) and the Department for Environment, Food & Rural Affairs (DEFRA) to support the alignment of environmental monitoring, residue surveillance and risk communication. The Committee emphasised the value of a One Health–aligned approach that integrates monitoring of resistance markers in enterococci, environmental sampling and systematic data sharing across relevant regulatory bodies. ACAF also noted the importance of maintaining oversight of emerging evidence and developments in surveillance methodologies.

2. The Issue

4.    During an assessment of a dossier for the authorisation of a narasin-containing feed additive, the Committee identified peer-reviewed scientific evidence indicating that antimicrobial resistance (AMR) determinants for narasin, such as narAB, can occur on transferable plasmids in Enterococcus spp. alongside antimicrobial resistance genes. Exposure to narasin might therefore enable co-selection and dissemination of AMR traits in Enterococcus spp. Furthermore, evidence from Norway showed that discontinuing the use of polyether ionophores, alongside enhanced cleaning and disinfection practices, was associated with a decline in the prevalence of vancomycin-resistant Enterococcus faecium. Taken together, these observations prompted discussion within the Committee as to whether the routine use of ionophore coccidiostats warrants consideration in the broader context of human health-relevant AMR prevalence and expression.

5.    This paper summarises the ACAF’s discussion on the use of coccidiostats in the UK, with a particular focus on their relationship to antimicrobial resistance and the balance between animal health, environmental impact, public health protection and considerations for national food security.

3. Background

6.    Coccidiostats are pharmacologically active substances often incorporated into feed for UK livestock used to inhibit the development and reproduction of Eimeria spp., the protozoan parasites that cause coccidiosis. Although hygiene and husbandry measures are considered useful to control coccidiosis, alone they are unlikely to provide reliable control of Eimeria under commercial production conditions.

7.    Historically, coccidiostats are considered to fall into two principal groups, ionophore and synthetic compounds. Ionophoric coccidiostats act by disrupting ion transport across parasite cell membranes, leading to osmotic imbalance and parasite death. Examples of ionophore coccidiostats include monensin, salinomycin, narasin and maduramicin. Synthetic – or non-ionophoric – coccidiostats act via diverse and specific biochemical targets in the coccidia life cycle, often intracellular. Examples include diclazuril, decoquinate and amprolium.

8.    Coccidiosis is a common intestinal parasitic infection that causes varying degrees of gut pathology (depending on the infecting species, dose, and host immunity) with loss of production performance and impaired welfare. The resulting impacts on health and productivity are economically and environmentally significant for livestock farmers.

9.    The main welfare concern arises from intestinal tissue damage, which leads to pain, inflammation, and compromised digestive function. Affected animals commonly develop diarrhoea, which can result in dehydration, electrolyte imbalance and systemic weakness. This can substantially reduce wellbeing, particularly in young animals that are more vulnerable to rapid deterioration.

10.    Damage to the intestinal epithelium also contributes to malabsorption and nutrient loss, resulting in poor growth and weight loss, despite maintained or increased feed uptake. Affected animals are more susceptible to secondary infections and there is a substantial mortality risk, particularly in poultry.

11.    Whereas coccidiosis is a ubiquitous disease of all livestock species, in the UK in-feed coccidiostats are economically most important for poultry production and this position paper primarily refers to the use of coccidiostats in that context.

12.    The Committee acknowledged the positive impact of coccidiostat use within UK systems, including the prevention of clinical coccidiosis, improvements in weight gain and feed efficiency, and the indirect reduction of therapeutic antibiotic use by maintaining gut integrity and limiting secondary infections. This translates into sustained production efficiency and broader environmental co-benefits by avoiding the resource and emissions penalties associated with poorer feed conversion.

13.    Members noted that unavailability of ionophore coccidiostats could have significant negative impacts on bird welfare and AMR, highlighting a study that showed an increase in the use of antibacterial drugs for the treatment of necrotic enteritis in poultry following a ban on avoparcin (Grave et al. 2004). Unavailability of ionophore coccidiostats without an adequate alternative would also impact domestic production and supply chains, leading to a reduction in UK production efficiency, likely increasing reliance on imports from jurisdictions that continue to use coccidiostats.

14.    The routine replacement of ionophores with synthetic coccidiostats alone would be unlikely to provide a sustainable control strategy, as Eimeria spp. are well recognised to develop reduced sensitivity to synthetic compounds relatively rapidly under continued use. For this reason, ionophores have typically been retained within control programmes to support rotation and mitigation of loss of efficacy associated with exclusive reliance on synthetic products. Ultimately, removing access to ionophore coccidiostats would increase the economic and environmental costs of bird production, but continued reliance on coccidiostats carries a risk of AMR.

15.    Unavailability of coccidiostats could also increase requirement for therapeutic antimicrobials to manage secondary infections, raising selective pressure on medically important antimicrobial classes and potentially contributing further to the development or persistence of antimicrobial resistance. The Committee therefore noted the need to consider the balance between the potential consequences of insufficient control of coccidiosis and those associated with antimicrobial resistance.

16.    Coccidiostat use is regulated under retained EU law, notably Regulation (EC) No 1831/2003 on feed additives (EC,2003), alongside requirements on maximum residue limits, cross-contamination (EC, 2009), and feed hygiene (EC, 2005). Oversight is provided by the VMD and the Animal and Plant Health Agency (APHA), supported by surveillance programmes such as UK-VARSS and PATH-SAFE which monitor antimicrobial resistance trends and potential environmental exposure pathways (UK-VARSS, 2024; PATH-SAFE, 2025). The Food Standards Agency (FSA) regulates coccidiostats as feed additives and applicants are legally required to report any incidents or adverse effects to the regulator. The VMD manages coccidiostats and coccidiocides that are formulated and authorised as veterinary medicinal products. Most in-feed coccidiostats are treated as feed additives and therefore overseen by the FSA.

17.    It was previously considered that because polyether ionophore coccidiostats are not used in human medicine, their potential to select for antimicrobial resistance in pathogens of human relevance was limited. More recent evidence relating to narasin use in coccidiosis control – including findings on the occurrence of narAB on mobile genetic elements and observations concerning vancomycin-resistant enterococci – has prompted further consideration of this assumption.

4. Coccidiosis and AMR risk

18.    Members noted that plasmid-mediated co-localisation of ionophore resistance determinants with genes conferring resistance to antimicrobials such as vancomycin, tetracycline and macrolides represents a biologically plausible mechanism through which co-selection and horizontal dissemination of multiple antimicrobial resistance traits could occur, even though ionophores are not used in human clinical medicine. The Committee observed that the presence of resistance determinants on mobile genetic elements may, under certain conditions, facilitate their maintenance and transfer within Enterococcus populations (Frederiksen et al., 2024; 2025). While the operation of these mechanisms in UK livestock systems is not fully characterised, available evidence suggests that coccidiostat use could be a plausible contributory factor.  It was further noted that, while the use of coccidiostats could potentially contribute to the emergence or persistence of such determinants, the actual frequency, directionality and ecological significance of these events are not well characterised in current evidence.

19.    The Committee examined recent literature linking specific ionophores to selection of resistance determinants in clinically relevant enterococci. Evidence was found to show that narasin, salinomycin and maduramicin can select for the narAB resistance mechanism (Simm et al., 2020). Members also referred to a long-term study from Norway indicating that narasin use in broilers may be a risk factor for a persistent reservoir of vancomycin resistant enterococci (VRE) (Frederiksen et al., 2024).

20.    Current UK arrangements mirror retained EU law governing authorisation, safety assessment and monitoring of coccidiostats, and these authorisations explicitly consider safety of the target animal, consumer protection, environmental impact and the potential contribution to AMR. This framework reflects the pragmatic judgment that hygiene and husbandry measures alone are unlikely to prevent coccidiosis at commercial scale, which has been the rationale for the continued use of coccidiostats. However, Members agreed that there needs to be a more dynamic balance between the public health concerns regarding AMR and healthy livestock production.

21.    The Committee recognised that coccidiosis is a ubiquitous challenge across UK livestock systems, and coccidiostats have a role in maintaining incidence and prevalence at manageable levels. If the availability of ionophore coccidiostats were reduced, increases in clinical disease and associated secondary infections would be expected, which could in turn lead to greater use of therapeutic antibiotics (coccidiocidal products and antibiotics for the secondary infections). At present, the authorised coccidiostats are not related to antimicrobial classes used in human medicine, whereas some veterinary antimicrobials are.

22.    Coccidiosis vaccines are a recognised component of poultry health management in the UK, particularly for commercial layers and broiler breeders, where vaccines are used to induce immunity against important Eimeria species. Vaccination can induce lifetime protection and reduced reliance on in-feed coccidiostats, with avoidance of drug-residue concerns in eggs and meat. However, vaccines are more expensive than traditional coccidiostats, immunity is specific to the Eimeria species against which they are directed, they require careful administration and depend on controlled cycling of vaccine oocysts in the environment (Price, 2012). Logistical challenges related to formulation and delivery of multivalent live vaccines make production and consistent field performance demanding.

23.    Members noted that the harmonised EU surveillance of enterococci in the animal domain ceased in 2014 and is now undertaken only on a voluntary basis. Consequently, AMR data are sparse and insufficient to support trend analysis at the EU level. In the UK, enterococci (E. faecalis and E. faecium) were added to the AMR surveillance programme for slaughter broilers and turkeys in GB in 2022 as an indicator species for resistance in Gram-positive bacteria. In 2024, this was also extended to Northern Ireland for E. faecium isolates in broilers. This includes testing against an expanded panel which includes human only antibiotics (such as vancomycin) as well as  antibiotic classes used in animals (such as tetracyclines, macrolides and aminoglycosides). Members agreed that the existing AMR surveillance data provide only limited insight into the likelihood that resistance determinants selected under farm-level conditions could disseminate more widely or become established within clinically relevant bacterial populations. The available UK surveillance indicates a very low prevalence of vancomycin-resistant enterococci in livestock and minimal evidence of transmission into the food chain (UK-VARSS, 2025, Food Standards Agency & FERA 2021). However, members emphasised both the use of methods with low sensitivity and the emerging evidence consistent with the co-selection and persistence of vancomycin-resistant enterococci associated with the use of certain polyether ionophore coccidiostats, particularly narasin. The Committee therefore recommends enhanced surveillance and improved communication across organisations and surveillance programmes to better understand the potential for these mechanisms to contribute to antimicrobial resistance under UK conditions.

24.    The Committee noted that policy interventions in some countries were followed by reductions in frequency of AMR. Although causality cannot be inferred, the discontinuation of use of narasin as well as concomitant hygiene improvement in Norway was followed by a dramatic reduction in prevalence of vancomycin-resistant enterococci in poultry (Simm et al., 2019).

25.    UK surveillance suggests a relatively low prevalence of VRE in poultry and no current evidence of VRE in pigs, contrasting with high rates in human clinical isolates, for reasons that remain unclear (APHA, 2023; UK-VARSS, 2023). However, among other possible explanations, this might be partly attributed to the UK-VARSS testing strategy being based on the use of less sensitive testing methods that use non-selective media. Therefore, Members agreed that this does not preclude the possibility that changes in use patterns, management practices or environmental exposures could influence these dynamics over time.

26.    The Committee stated that although the mechanistic and ecological basis for concern is credible and warrants enhanced surveillance, including more sensitive testing with selective media, and that the direct human health burden attributable to coccidiostat associated AMR remains uncertain with current UK evidence. The Committee supports continued monitoring and advocates increased coordination across programmes.

27.    The Committee noted the absence of standardised environmental AMR surveillance and discussed environmental selection as a potential pathway, particularly in settings such as areas fertilised with farmyard manure containing coccidiostat residues (Marutescu et al., 2022). Potential ecotoxicological effects, including impacts on aquatic environments and food webs, were highlighted, alongside considerations of occupational exposure for farm workers, reinforcing the need for enhanced surveillance alongside farm-level stewardship (Yardimci et al, 2025).

28.    The Committee recognised the substantial overlap of the roles of the FSA, VMD, DEFRA and APHA and stressed the importance of inter-agency internal consultation for the effective management of the AMR threat from these products. While ACAF’s remit encompasses the risk assessment of feed additive issues and the food safety interface, further input is required regarding medicines regulation, environmental risk assessment, residue surveillance, and the broader One Health implications that sit within VMD and DEFRA’s statutory responsibilities.

5. ACAF’s position

29.    Some Committee members had expressed concerns regarding narasin, maduramicin and salinomycin due to strong evidence of co-selection by a narAB gene which confers resistance to these ionophores and appears likely to co-select for vancomycin resistant enterococci (VRE). However, the extent to which this mechanism occurs and how it is facilitated by the use of these coccidiostats is uncertain.

30.    To ensure proportionate risk management, the Committee recommends enhancing AMR surveillance to include sentinel monitoring of resistance profiles of enterococci in poultry and the farm environment, with explicit attention to mobile genetic elements and co-selection markers associated with ionophore resistance. ACAF members proposed exploring ways to improve the measurement of use of coccidiostats in feed to allow for more accurate analysis.

31.    ACAF advises the FSA to engage with VMD, DEFRA and APHA to scope potential alignment on data collection and risk communication and to explore options for incorporating farm-level environmental sampling into ongoing programmes. Improved co-ordination between APHA, UK-VARSS and PATH-SAFE is also of great importance.

32.    Give the potential risks, the Committee strongly supports the development and use of alternative strategies for coccidial control.

33.    The Committee will revisit its position following feedback from partner bodies and as new surveillance outputs emerge, including finalised PATH-SAFE analyses, to ensure that its advice remains proportionate, current, and aligned with the One Health framework.

6. Abbreviations

Abbreviation	Meaning
ACAF	Advisory Committee on Animal Feedingstuffs
AMR	Antimicrobial resistance
APHA	Animal and Plant Health Agency
DEFRA	Department for Environment, Food & Rural Affairs
EC	European Commission (used in references to EU regulations)
EU‑OSHA	European Agency for Safety and Health at Work (OSHwiki)
FSA	Food Standards Agency
PATH‑SAFE	Pathogen Surveillance in Agriculture, Food and the Environment programme
SAC	Scientific Advisory Committee
UK-VARSS	UK Veterinary Antibiotic Resistance and Sales Surveillance
VMD	Veterinary Medicines Directorate
VRE	Vancomycin resistant Enterococci

7. References

Aarestrup FM, Seyfarth AM, Emborg HD, Pedersen K, Hendriksen RS, Bager F, 2021. Effect of Abolishment of the Use of Antimicrobial Agents for Growth Promotion on Occurrence of Antimicrobial Resistance in Fecal Enterococci from Food Animals in Denmark. Antimicrob Agents Chemother. July;45(7):2054-9.

APHA, 2023. Great Britain avian report – disease surveillance and emerging threats 2023. Animal and Plant Health Agency.

EC (European Commission), 2003. Regulation No 1831/2003 of the European Parliament and of the Council on additives for use in animal nutrition

EC (European Commission), 2005. Regulation (EC) No 183/2005 of the European Parliament and of the Council laying down requirements for feed hygiene

EC (European Commission), 2009. Commission Regulation (EC) No 124/2009 establishing maximum levels for the presence of coccidiostats or histomonostats in food resulting from the unavoidable carry‑over of these substances in non‑target feed

English surveillance programme for antimicrobial utilisation and resistance, ESPAUR, Report 2024 to 2025

EUOSHA, 2013. Workplace health risks caused by drug-resistant microorganisms, European Agency for Safety and Health at Work (OSHwiki).

Food Standards Agency & FERA. (2021). What is the Burden of Antimicrobial Resistance Genes in Selected Ready-to-Eat Foods?. FSA Research and Evidence.

Frederiksen, R.F., et al., 2024. Polyether ionophore resistance in a One Health perspective. Frontiers in Microbiology.. 15:1347490.

Frederiksen, R.F., et al., 2025. Genomic characterization of vancomycin-resistant enterococci in Norwegian poultry PLoS One. 4;20(6):e0324789.

Grave, K., et al., 2004. What has happened in Norway after the ban of avoparcin? Consumption of antimicrobials by poultry. Preventive Veterinary Medicine. 62(1):59–72.

Martins, R.R., et al., 2022. Coccidiostats and poultry: A comprehensive review and current legislation.. Foods. 11, 2738.

Martins, R.R., et al., 2025. Impact of cooking procedures on coccidiostats in poultry muscle.. Antibiotics. 14:586.

Marutescu, L.G., et al., 2022. Insights into the impact of manure on the environmental antibiotic residues and resistance pool.. Frontiers in Microbiology. 13:965132.

PATH-SAFE, 2025. Pathogen Surveillance in Agriculture, Food and Environment (PATH-SAFE) Programme.

Price, K.R., 2012. Use of live vaccines for coccidiosis control in replacement layer pullets. Journal of Applied Poultry Research, 21, 679-692,

Simm, L., et al., 2019. Significant reduction of vancomycin resistant E. faecium in the Norwegian broiler population coincided with measures taken by the broiler industry to reduce antimicrobial resistant bacteria. . PLoS ONE 14 (12): e0226101.

Simm, L., et al., 2020. NarAB is an ABC-type transporter that confers resistance to the polyether ionophores narasin, salinomycin and maduramicin, but not monensinFrontiers in Microbiology. 11:104.

SONAAR 2024. Scottish One Health Antimicrobial Use and Antimicrobial Resistance in 2024

Swedres-Svarm, 2024. Sales of antibiotics and occurrence of antibiotic resistance in Sweden.. Solna/Uppsala ISSN2001-7901 ISSN 2001-7901 Swedish Veterinary Agency article no. SVAKOM230.2024

UK-VARSS 2024. UK Veterinary Antibiotic Resistance and Sales Surveillance Report Supplementary Material 5

UK-VARSS 2024. UK Veterinary Antibiotic Resistance and Sales Surveillance Report

van den Bogaard, A.E., et al., 2000. The effect of banning avoparcin on VRE carriage in The Netherlands. Journal of Antimicrobial Chemotherapy. 46(1):146–148.

Yardimci, M., et al., 2025. Acute and chronic toxicity of the coccidiostat amprolium to Daphnia magna and its implications for aquatic contamination from livestock waste. Kafkas Univ Vet Fak Derg. 31(5):689–696.