19 Aug 2025
Tim Mair BVSc, PhD, DEIM, DipECEIM, DESTS, FRCVS considers how resistance has been allowed to grow on this issue and how the veterinary profession can help to fight it
Tim Mair
Job Title
Image: vchalup / Adobe Stock
Antimicrobial resistance (AMR) is a significant global public health threat that is already contributing to the deaths of millions of people worldwide. It has been described by the World Health Organization (WHO) as one of the top 10 global health threats, causing increased severity of disease, prolonged hospitalisation and increased mortality risk.
AMR occurs when bacteria, viruses, fungi and parasites no longer respond to antimicrobial medicines used to fight them. As a result of drug resistance, antibiotics and other antimicrobial medicines become ineffective, and infections become difficult or impossible to treat, increasing the risk of disease spread, severe illness, disability and death.
The close association between animals, including horses, and people provides opportunities for the transfer of resistance determinants between these species.
Although the development of AMR is part of normal bacterial evolution, antibiotic use creates selective pressure on bacterial populations and leads to the emergence of new strains with resistance mechanisms against the action of the antibiotic used.
Overuse, inappropriate prescribing in both people and animals, and inappropriate disposal of antimicrobials risks accelerating the development of drug-resistant pathogens in the environment.
Already, the organisms that cause many common human diseases such as tuberculosis, HIV/AIDS, malaria, sexually transmitted diseases, urinary tract infections, chest infections, bloodstream infections and food poisoning, can resist a wide range of antimicrobials.
Some cases of tuberculosis and gonorrhoea are already resistant to the antibiotics of last resort.
The veterinary profession has a responsibility to ensure prudent use of antimicrobials to help preserve their efficacy and contain AMR. Antimicrobial stewardship (AMS) can be considered as strategies or actions promoting responsible antimicrobial use.
In recent years, multiple national and international guidelines have been developed in companion animal medicine promoting the rational use of antimicrobials. In 2019, the Government said: “It will take the combined efforts of diverse UK public and private sector bodies as well as all members of the public to fulfil our high-level ambitions and help contain and control antimicrobial resistance. Each one of us needs to be ready, willing and engaged to play our part”1.
Antimicrobial drugs disrupt microbial environments, potentially causing ecosystem degradation and release of greenhouse gases. Microbes were responsible for creating the earth’s atmosphere, which supports eukaryotic life-forms and is essential for our normal physiological functions.
However, microbes are only generally associated with infectious diseases, and as a result our focus has been on using antimicrobial drugs to destroy them. The risks associated with their use has been limited to the development of AMR and loss of efficacy, whereas their impact on microbial environments and greenhouse gases has been mainly overlooked.
It has been suggested that a single course of antibiotics will cause the release of 9.84 tonnes of CO2; this is the equivalent of a standard car driving around the earth 1.47 times. It is not only antibiotics that are harmful to the environment – antiseptics, disinfectants, pesticides, herbicides and many food additives all contribute to AMR.
Despite growing awareness of the problem of AMR and increasing recognition of the importance of AMS strategies, antimicrobials and antibiotics are still used indiscriminately, including situations where their use is questionable or where they fail to confer a critical, or even demonstrable, benefit.
Little doubt exists that we use antibiotics too often and on a “just in case” basis.
A major problem facing the clinician is to prove beyond doubt that an infection requiring treatment with antibiotics is present.
Performing bacterial culture and antibiotic susceptibility testing takes time, and by the time that the results are available, the decision to treat or not to treat will have likely already been taken.
No commercially available tests exist that can be used to prove an active infection before the decision to implement antibiotic use is needed. Tests such as haematology and inflammatory markers (such as serum amyloid A) are often helpful to assess inflammation, but are not specific for infection, and so the results can be misleading.
The veterinary profession, including individual practices, plays an essential role in delivering the aims of the UK (and global) government(s) to limit the development of AMR and to establish effective AMS policies.
Achieving these goals requires several distinct, but related endeavours. The American Veterinary Medical Association identified five core principles of AMS, including commitment to stewardship, selection and use of antimicrobial drugs judiciously, and evaluation of antimicrobial drug use in practices. Measurement of antimicrobial antibiotic use (AMU) is an essential first step in improving prescribing practices.
Antibiotic drug use data are used to describe baseline practices, identify opportunities for improvement, guide goal setting, and measure progress. Several veterinary AMS programmes with different interventions have now been shown to have a positive influence on antimicrobial use in small animal veterinary practice.
The Veterinary Antimicrobial Resistance and Sales Surveillance (VARSS) report, which is published annually by the VMD in the UK, uses information provided by veterinary pharmaceutical companies to report sales data for animals. However, these data mainly relate to farm animals and do not include the use of antibiotic preparations licensed for use in a different species (including humans) or extemporaneous preparations, which are commonly used in horses.
The VARSS report also provides no data on the indications for AMU. Studies of AMU in horses are essential to provide clearer understanding of the types and antibiotics most commonly used and the reasons for their prescription.
The antibiotics used in animals can be categorised in several ways, the most relevant being based on the importance of individual drugs to human health care.
The two commonest classification systems include the WHO list of medically important antibiotics (previously known as the WHO Critically Important Antimicrobial List for Human Medicine)2 and the European Medicines Agency (EMA) categorisation of antibiotics for use in animals3.
The WHO categorises antibiotics as antibiotics authorised for use in humans only, highest priority critically important antimicrobials (HPCIAs; includes ceftiofur, enrofloxacin and marbofloxacin, and polymyxin B), critically important antimicrobials (includes aminoglycosides, rifampin and clarithromycin), highly important antimicrobials (includes tetracyclines, penicillins and sulphonamides) and important antimicrobials.
The EMA classifies antibiotics into four categories (A to D). Category A antibiotics are labelled “avoid”, but may be given to companion animals under exceptional circumstances (includes rifampin); category B (“restrict” – use should be based on antimicrobial susceptibility testing, wherever possible; (includes ceftiofur, enrofloxacin and marbofloxacin, and polymyxin B); category C (“caution” – should be considered only when no antibiotics in Category D could be clinically effective; includes aminoglycosides and macrolides), and category D (“prudence”; includes penicillins, sulphonamides and tetracyclines).
The starting point to develop AMS strategies is to measure or audit the use of antibiotics in veterinary practice, including the use of antibiotics considered to be essential for human health. Once this is done, then reasons why antibiotics have been used in different situations can be critically assessed and changes aimed to reduce and refine AMU can be considered.
Schemes that have been developed to measure AMU in equine practice include the following.
The studies mentioned have highlighted difficulties in acquiring detailed practice data, as well as variations in antibiotic use between practices depending on factors such as the populations of horses examined in individual practices (for example, proportions of racehorses and breeding horses/foals, and so forth).
While such data are important to understand the overall use of antibiotics nationally and, therefore, the development of national antibiotic stewardship programmes, the more granular data at individual practice level is vital to enable practices to develop their own bespoke stewardship programmes and quality improvement initiatives, as well as benchmarking their antibiotic use against other similar practices.
The introduction of surveillance initiatives such as the Equine Veterinary Surveillance Network (tinyurl.com/47jxydx8) and VetCompass (tinyurl.com/2v6wz9kf) that collect data directly from practice management systems will have value in providing individual practices with their antibiotic use data, which can be used at that level to inform antibiotic stewardship programmes.
In addition, initiatives such as VetTeamAMR (tinyurl.com/5n8ypsus) developed by RCVS Knowledge can assist individual practices in auditing their antibiotic use and undertaking quality improvement projects. The VetTeamAMR project also provides detailed educational material relating to antibiotic use and resistance.
The generation of data on antibiotic use in equine practice will allow not only the development of focused antibiotic stewardship programmes, but also the creation of specific studies and programmes aiming to refine and reduce antibiotic use.
Several such studies have been successfully undertaken in small animal practice. By using different interventions and analysing their effects on antibiotic use compared with control groups (without interventions), such studies have demonstrated the ability to positively influence and decrease antibiotic use. One apparent benefit of running such studies is the apparent positive effect that simply being enrolled into a study has on reducing antibiotic use within practices.
At an individual practice level, initiatives that can be instituted that assess AMU in detail and promote AMS include:
Ultimately, it is down to all veterinary practices and individual veterinary surgeons to critically assess their own use of antibiotics to help in the fight against AMR.
Tim Mair is a specialist in equine internal medicine and equine surgery (soft tissue) based at the Bell Equine Veterinary Clinic in Kent, where he divides his time between clinical work and fulfilling his role as equine veterinary director of CVS. He has particular interests in equine internal medicine (including oncology), soft tissue surgery and advanced imaging, is a BEVA council member and has been editor of Equine Veterinary Education since 1996.
