The potential side effects of equine medical treatments *H&H subscribers*

Modern medicines licensed for equine use have undergone rigorous testing before they become available for your vet to prescribe. All drugs, from antibiotics and anti-inflammatories to wormers and pain-relief medications, will have been thoroughly evaluated before being allowed on to the market.

One of the frustrations for vets is that there are limited numbers of effective drugs licensed for use in horses, as the costs and procedures required to license any new treatment can be prohibitive. Inevitably, even with the best treatment options, there are drawbacks — especially where a treatment is misused.

A course of antibiotics that is not completed, for instance, or something prescribed for one horse that is given to another, may do more harm than good. If a standard treatment at a dose appropriate for a large, adult horse is used in a young foal or a small pony, the result can be catastrophic.

Certainly, some drugs are safer than others, with a minimal chance of things going wrong even if an overdose is administered. Other medicines have much narrower margins of safety and need to be given exactly as directed to avoid complications.

So many factors can influence how effective a drug treatment is and whether it may cause unwelcome side-effects.

These include the age of the horse, his general health, other treatments he is receiving and the dosage.

Many treatments are not recommended for use in the pregnant mare, because of the possible harmful effects. In many cases the risk is simply unknown, so the recommendation is to err on the side of caution — especially in the early stages of pregnancy.

As an equine vet, it can be sometimes surprising to observe the reaction of owners to potential side-effects. Some are seriously worried about certain medications, but not at all about others. The level of concern may be totally disproportionate to the degree of risk.

Such fears are usually unfounded, but it can be hard to reassure when there is always a small chance that something will go wrong. The benefits of a licensed medication will always outweigh the risks; this is part of the authorisation process.

Dangerous doses

Phenylbutazone, sometimes referred to as bute, is one of the medications most commonly prescribed for horses.

There is a lot of research on the benefits and administration of such painkilling medications in horses. Even so, it seems that they are sometimes given randomly by owners, without any diagnosis, or in much larger doses than recommended.

Like all effective drugs, non-steroidal anti-inflammatory medications (NSAIDs) — particularly the widely used phenylbutazone — can occasionally cause unwanted side-effects. The two most widely-recognised complications are gastrointestinal ulceration, which can affect the mouth, stomach or intestines, and also kidney damage, particularly in a dehydrated horse.

Some horses receiving phenylbutazone or similar drugs can develop low blood protein levels in association with ulceration. Rarely, some will develop the extreme adverse reaction of right dorsal colitis — a severe form of inflammatory bowel disease. Adverse events are more likely in horses that are off their feed, dehydrated, already have some form of gastrointestinal disease or are otherwise ill, so careful monitoring including blood testing, where necessary, is important.

It can be challenging to know whether an unexpected response to a treatment is a true side-effect or adverse reaction, or just an unfortunate coincidence. After all, a horse is prescribed medication because he is already unwell, so there may be a deterioration despite — and not because of — the prescribed treatment.

Laminitis is a common condition, with many horses and ponies developing the condition frequently. Some animals that develop laminitis will have been prescribed corticosteroids (often termed steroids), yet this does not necessarily indicate cause and effect. In fact, an increasing body of evidence suggests there is no definite link between steroid usage and laminitis.

Many of the anecdotal cases where horses given steroids then developed the condition were reported before the actual causes of laminitis were as clearly understood as they are now.

We now know that metabolic dysfunction as a result of obesity or pituitary dysfunction is the key reason for the development of laminitis. Although there are, theoretically, reasons why steroids might exacerbate equine metabolic dysfunction, they cannot be blamed entirely.

Scientific evidence includes an electronic search of more than 70,000 clinical records, covering a 26-year period, which showed that apparent links between laminitis and steroids disappeared when such large numbers were studied. There is so much else to consider — the type and amount of steroids used, for instance, as well as the patient’s overall health.

There is some work demonstrating that steroid therapy represents a lower risk than routine anti-infective therapy. It is therefore unreasonable for many owners to be so wary of steroids.

Granted, steroids are a double-edged sword that can slow healing, yet they can also be a life-saving treatment — especially a one-off injection of a short-acting steroid, or tablets that can be stopped if complications ensue.

Reporting concerns

When one reflects on the risks of a treatment, the multitude of factors involved makes scientific studies hard to interpret.

The way any substance is used is crucial. If, or when, there is any adverse reaction or recognised side-effect, this should be properly documented to provide definite data. There is a robust official system, so report your concerns to protect horses in the future.

For every effective medical treatment prescribed, there may be a downside. Yet while it makes sense to consider the possible side-effects, there is no need to avoid using a useful therapy option because of anecdotal reports of problems.

It is far better to rely on scientific evidence, as well as doing an appropriate risk assessment with your vet. Proper proof is more important than remembering one case that might have gone wrong, when the circumstances of each horse under treatment may be entirely different.

Ref Horse & Hound; 10 October 2019