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Treating tumours in horses

In the second of a two-part article on equine skin cancers, treatment options and welfare considerations are discussed

11 January 2019, at 10:46a.m.

Before embarking on treatment of any skin tumour, the clinician needs to be aware of the pathological behaviour and implications of the type of tumour and location. This creates challenges since many tumours are poorly characterised in horses and there is considerable debate about the true implications. For example, a penile carcinoma in a five-year-old horse has considerably different implications, behaviour and prognosis from the visually similar condition in older geldings. Further, even common tumours have a variable clinical behaviour. It is well known that many sarcoids remain static for years, but others show highly aggressive behaviour within days or weeks of being subjected to accidental or intentional trauma.

Many different tumour types look similar but behave in different ways. Similarly, some tumours such as the occult and verrucose sarcoid can easily resemble benign skin diseases such as dermatophytosis or viral papillomata (see Figure 1 above). A localised mast cell tumour on the face can be clinically indistinguishable from a localised lymphoma (see Figure 2 above), but the treatment options and the prognosis are different. The clinical behaviour of the tumour is critical in assessing whether treatment is warranted (see Figure 3 below). Tumours of the same type can behave in different fashions on different horses and can even show different pathological and clinical behaviour at different sites on the same horse.

Many tumours have morphological variations – the equine sarcoid is the best example of this. The sarcoid is a frequent challenge since the name “sarcoid” encompasses a spectrum of morphological types (see Figure 4). The sarcoid is recognised as having six major clinical forms and some of these broad types have different sub-types. The reason that tumour classification of this nature is important is that often (and particularly in the case of the equine sarcoid and the cutaneous forms of squamous cell carcinoma) treatment options are different for various types of tumours. For example, some proliferative carcinomas can be easily and effectively removed surgically, while others that have a more destructive nature will not respond to surgical interference.

FIGURE (5) A highly complex mixture of tumours on the eyelids.
FIGURE (5) A highly complex mixture of tumours on the eyelids.

This creates difficulties because of the location and different demands for sarcoid (blue arrow), melanoma (white arrows) and squamous cell carcinoma (green arrow). Note also that the sarcoid lesion in the lower lateral palpebrum might easily be confused with a proliferative carcinoma

It is important that clinicians remember also that a horse can have more than one tumour type (see Figure 5) and so each recognisable lesion must be positively identified and classified to allow effective treatment. While many treatment modalities for cutaneous lymphoma are equally applicable to several tumour types, there are significant differences.

Radiation is the gold standard treatment for many tumours, but tumours respond differently to this approach. The equine melanoma is, for example, much more refractory to radiation than the sarcoid, and the squamous cell carcinoma is highly sensitive to beta or gamma radiation.

Many tumours look alarming but have little implication, while others may seem benign but have serious implications both for treatment and for the overall prognosis for the horse (see Figure 6).

FIGURE (6) This alarming neoplasm developed quickly and certainly gave all the impressions of a serious tumour. It was in fact simply a nasal polyp that was easily removed by simple manual tension. The horse showed no recurrence over 12 years at least
FIGURE (6) This alarming neoplasm developed quickly and certainly gave all the impressions of a serious tumour. It was in fact simply a nasal polyp that was easily removed by simple manual tension. The horse showed no recurrence over 12 years at least

In many cases, the early diagnosis of a “tumour” condition is counterproductive for the horse since the natural attitude is that cancer equates with suffering and even if the horse is apparently unaffected, there is a strong tendency to carry out euthanasia.

What are the options?

In general terms, benign and accessible tumours can often be treated effectively even when multiple modalities must be used. The concept of a “magic bullet” in cancer medicine, ie a treatment that will resolve the issue in one hit, should not be foremost in the mind of an attending clinician – there is no magic about oncology, but there is also little evidence-based information to provide a basis for rational decision making. The reality is that most cutaneous tumours are amenable to some forms of treatment but as soon as complications arise in respect of the pathological behaviour, size, location and the tumour type itself, there will inevitably be limitations. The old dogma of tumour “monitoring” is often no longer an acceptable option. The clinician needs to ask themselves whether monitoring is in the best long-term interests of the patient and its owner.

Tumours that have no effect on the welfare and wellbeing of the patient and which will not develop into a serious welfare, clinical and therapeutic problem later can probably be justifiably left alone. This can also be applied by conscious decision when the owner cannot afford treatment, when facilities do not exist or when the temperament of the horse precludes treatment. The other groups of tumours that can be left alone are those that have an impossible/untreatable clinical behaviour and those occurring in impossible sites.

This benign neglect does not mean that the clinician has no further interest in the case. The only predictable thing about the clinical behaviour of tumours is that they are unpredictable, and this applies more to the equine sarcoid than almost any other tumour type affecting the skin of horses. Furthermore, there is an inherent responsibility for the clinician to make regular assessments of the tumour and to keep up to date on any new information regarding that tumour type. Even a benign tumour in an impossible site will probably dictate that treatment is contraindicated.

FIGURE (7) This 14-year-old Warmblood gelding developed aggressive malignant sarcoid and the owner was subjected to irrational and ill-informed criticism on welfare grounds. The horse performed well throughout its subsequent life and was apparently unaffected by the condition
FIGURE (7) This 14-year-old Warmblood gelding developed aggressive malignant sarcoid and the owner was subjected to irrational and ill-informed criticism on welfare grounds. The horse performed well throughout its subsequent life and was apparently unaffected by the condition

The presence of a “serious” tumour should not mean that the horse necessarily requires immediate euthanasia – there are many factors that need to be considered (Figure 7). Early diagnosis of the most serious tumour types could be viewed as counter to a horse’s best interests – the horse may be destroyed at a stage when it can still lead a good life. This sentiment is somewhat counterintuitive because early diagnosis is a critical issue in most “treatable” tumours. An owner may be justifiably aggrieved that a tumour has been ignored, but communication and discussion is a major help in clinical decision making. There may be limitations in financial aspects, management and even in-patient compliance. At the same time there is a responsibility of the profession to encourage evidence-based trials of treatments and to properly publish the results in peer reviewed journals rather than on the internet.

This is part two of the third article in a series written by veterinarians from the Vets with Horsepower team, in partnership with Norbrook Laboratories. The articles are excerpts from the continuing professional development lectures delivered during a recent charity ride.

A full list of references is available on request

Derek Knottenbelt, OBE, BVM&S, DVMS, DipECEIM, MRCVS, is an equine internal medicine specialist and a Diplomat of the European College of Equine Internal Medicine. Derek ran a sarcoid referral service for over 20 years, established Equine Medical Solutions and is a consultant at the University of Glasgow.

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