Antibiotic resistance is an emerging clinical problem that is recognised internationally as one of the largest threats to human and animal health. All major health and veterinary organisations are working to try and limit the development of resistance so that effective antibiotics can be retained for use in clinical practice.

What are antibiotics?

Antibiotics are drugs that kill bacteria by disrupting their normal biology. They either directly kill bacteria (bactericidal) or stop them from growing (bacteriostatic).

Either way, an adult horse or pony will mount its own immune system attack on the bacteria helping to remove the infection from the body.

Antibiotics are relatively safe as the mechanism by which they destroy bacteria has no direct effect on your horse, although side effects can arise either due to killing of ‘good bacteria’ or unexpected consequences of the medicine.

The most severe side effects include allergic reactions, skin reactions or diarrhoea.

Diarrhoea is the greatest cause for concern and happens when the antibiotic destroys the ‘good bacteria’ that enable adult horses to digest plant material within their large intestine. While your horse is receiving a course of antibiotics, passing a single, soft (cow pat like) dropping is not usually a cause for concern. But if this progresses to a persistent watery diarrhoea, it can rapidly become life threatening and you should consult with your veterinary surgeon.

Why and when might antibiotics need to be used?

Not all infections caused by bacteria require antibiotic treatment. Your horse’s own immune system is effective in controlling the growth of many infections, without the need for antibiotics. If your veterinary surgeon has dispensed antibiotics, it means that they have determined that your horse has a serious enough infection that requires treatment.

What does antibiotic resistance mean and how has this occurred?

Antibiotics are not always harmless and can sometimes cause more problems than they solve. These effects may extend beyond your own horse due to antibiotic resistance.

Antibiotic resistance means a previously susceptible bacteria no longer dies when exposed to an antibiotic that killed it effectively in the past. It occurs when bacteria change their biology so that they are no longer susceptible to a particular antibiotic or group of antibiotics. Once exposed to antibiotics, the susceptible bacteria die, and the resistant ones live and multiply.

Unfortunately what also happens is that the commensal bacteria (those that live in harmony and synergy throughout the body), also can develop antibiotic resistance as antibiotics don’t just target pathogenic (disease-causing) strains. Commensal bacteria also have the ability to form a multitude of resistant genes, and once resistant genes have been formed, the commensal bacteria can transfer the genetic information to other commensal forms as well as pathogens, ultimately adding to the antibiotic resistance problem.

What does it mean for our horses, both now and in the future?

Resistant bacteria are not more harmful or damaging to your horse; they just cannot be treated effectively with a type of antibiotic that was previously effective and this may limit their future effectiveness in all horses.

Bacteria can pass resistance genes on to future generations, as well as acquire resistance to multiple or even all available drugs - even those to which the bacteria have not been directly exposed. The result of this is the development of “super bugs”.

Resistant bacteria can be transmitted between animals or from animals to people, further exacerbating this issue.

How should veterinary surgeons respond to this concern?

Firstly, not all bacteria are harmful. Most bacteria live in harmony with your horse. They exist on every surface of their body, both inside and out. Many of these commensal or ‘good’ bacteria help us by limiting the growth of potentially harmful bacteria. If we inadvertently destroy these good bacteria, we could make a horse more susceptible to life threatening infections.

Whenever it is practical and feasible, your veterinary surgeon should take samples or swabs for bacterial cultures and sensitivity testing at a lab which can help choose very specific antibiotics that target the problem, which will likely achieve the best clinical outcome.

Is there anything that owners can or should do?

If during a course of antibiotic treatment, you observe that your horse appears better but you still have remaining medicine it is vitally important that the full course of antibiotics is completed and given at the stated frequency. Stopping early may leave some bacteria present at the site of infection that will regrow, leading to recurrence of disease that may be less susceptible (resistant) to this or other antibiotics.

If you think your horse is not improving after a few days, you should contact your veterinary surgeon, who will review whether this is the correct drug for your horse. They may need to use different type of medication or obtain samples to determine the most effective antibiotic protocol to use going forward.

If your veterinary surgeon refuses to dispense antibiotics for a particular disease this is usually because:

a) It is not caused by a bacterial infection. Viruses (like the common cold in people) and equine asthma are examples of conditions that mimic the signs of bacterial disease, but where bacteria play no role.

b) It is a self-limiting infection that does not require treatment. Bacterial diseases, including strangles, are often effectively controlled by the animals own immune system without the need for antibiotics.

c) The horse’s immune system has already controlled the infection. In some diseases (eg strangles/foot abscess) the body mounts an effective immune response that controls an infection through the formation of an abscess. Once an abscess has burst, the animal has already effectively controlled that infection and antibiotics will have no real benefit in most cases.

What is the likely outcome – is it an issue that can be prevented/solved with the introduction of new drugs?

When first discovered, antibiotics were considered miracle drugs, contributing to increased human life expectancy. But antimicrobial resistance (AMR) began to occur within a relatively short time of widespread antibiotic use. This wasn’t originally a problem since 29 new drug classes were developed between 1929 and 1969; however, since then, lack of discovery of new drugs poses a sobering contrast to the rapid and rising resistance of bacteria.

At the moment the vast majority of new antibiotic drugs that are developed are used as a priority in human medicine to treat critical infections like MRSA and Clostridium difficile.

There are very few (if any) new equine specific antibiotics currently being developed.

Limiting antimicrobial resistance is a collective responsibility for both horse owners and equine veterinary surgeons. It is important to respect the decision of your veterinary surgeon if they do not believe your horse needs antibiotics for a particular condition.

Good hygiene standards are important when treating horses with infections to prevent transmission to other horses and when managing horses with wounds to prevent contamination arising from other horses. Good hand washing with soap and water can go a long way to reducing disease spread.

Aoife Byrne Dr.Med.Vet, MRCVS Cert AVP, qualified from Szent Istvan Universit Faculty of Veterinary Science in Budapest in 2007 and did an equine internship on the Curragh for 12 months before working for an equine ambulatory practice.

She followed this with a stud season at the Beaufort Embryo Transplant Centre. Aoife then worked at Rowe Equine & The Equine Eye Clinic for a few years before moving to Norfolk where she now works for Chapelfield Vets equine clinic.

From a horsey family in Carlow, Aoife enjoys hunting side saddle in the winter and showing in the summer.