The study of cancer in the horse and other equids is substantially less advanced than in other companion animal species, making equine oncology a relatively poorly understood clinical specialty. Despite this, neoplasia in the equid is common with many surveys finding neoplasia involving the skin to be most prevalent. One such survey by Knowles et al. (2016) looked at 964 samples sent to the University of Bristol for histological examination over an 18-year period and found 24 percent of these samples to be equine sarcoid, followed by 19 percent squamous cell carcinoma and 6 percent melanoma, while lymphoma represented only 14 percent of diagnoses. Similarly, a North American survey of neoplasia in 125 donkeys (Davis et al., 2016) found equine sarcoid to represent 72 percent of all tumours observed. Non-cutaneous neoplasia appears to be much less common in horses than in other species.
Sarcoids are fibroblastic tumours of the skin and are divided into six types: occult (Figure 1), verrucose (Figure 2), nodular (Figure 3), fibroblastic, mixed (Figure 4) and malevolent. The classification between these types is generally made based on appearance and their clinical behaviour. Malevolent sarcoids are thankfully quite uncommon but are aggressive and will become rapidly invasive. Predilection sites for any form of sarcoid tend to be where there are relatively superficial blood vessels (Figure 5).
Diagnosis is generally made presumptively based on clinical appearance, with the index of suspicion being significantly greater if multiple lesions with the appearance of sarcoid are present on the horse. However, in certain circumstances biopsy and careful histopathological examination is indicated. Such situations would include lesions which appear in less common anatomical sites or whose appearance varies from that which would be expected. In these cases, owners should be warned prior to this being undertaken that, should histopathology confirm sarcoid to be present, it is likely that treatment of the site will be required. The reason for this is two-fold; one, even an excisional biopsy of a small lesion is unlikely to have achieved sufficient tissue margins to be curative; and two, sarcoids which are inactive can become substantially exacerbated by any kind of trauma, including the surgical trauma of biopsy, meaning aggressive regrowth at the site is a possibility. Other differentials would include but are not limited to melanoma, granulation tissue, mast cell tumour, papilloma, fibrosarcoma, cutaneous lymphoma and squamous cell carcinoma.
There is no one gold standard treatment for all sarcoids, and treatment options are usually decided based on the type of sarcoid present and its anatomical location. From the ambulatory vet’s perspective, we are even more limited with what we can do but it is of course important to keep in mind the range of modalities which may also be available via referral as new protocols are often being considered. In general terms, topical chemotherapy in the form of AW5 cream is probably most commonly used in ambulatory practice and generally has good success following treatment. The concentration of the cream and the frequency of application will depend on the type and site of the lesion. Equine Medical Solutions, led by Prof Knottenbelt, is extremely helpful in providing guidance and protocols for sarcoid treatment and indeed AW5 can be sourced only from there. It is of course an off-licence treatment and the owner should be clearly counselled with regard to the potential complications. A degree of pain and swelling is clearly to be expected but in a small number of cases more extreme reactions can be seen. For lesions which cannot be treated with AW5, such as some on the face, other topical treatments such as imiquimod and 5-fluorouracil may be appropriate. The use of topical bleomycin which is used to treat certain human skin cancers is also now being used in some lesions, for example those close to the eye.
Immunotherapy is possible for nodular type periocular sarcoids. Such treatment most commonly involves intralesional injection of Bacillus Calmette–Guerin (BCG) at defined intervals with the intention of stimulating local cell-mediated immune responses and inducing cytotoxic T cell and natural killer cell activity against tumour cells. Success rates of over 80 percent have been reported but it should be noted that there is a risk of anaphylaxis, and animals should be pre-treated with dexamethasone and flunixin meglumine intravenously and monitored carefully with adrenaline available for around 30 minutes post-treatment.
Squamous cell carcinoma
Probably the most common sites for squamous cell carcinoma (SCC) as seen in first opinion practice would be penile SCC (Figure 6A) and ocular SCC, usually that involving the third eyelid (Figure 6B). Although SCC can be seen in animals as young as one year old, most will be seen in adults.
In penile SCC, small early lesions may be suitable for debulking under local anaesthesia and standing sedation in conjunction with cryotherapy and topical 5-fluorouracil treatment at five- to seven-day intervals. More advanced lesions will likely require referral for distal amputation or in some cases, more radical surgery. Enlarged inguinal lymph nodes may be palpable but may not always represent metastases as sometimes local infection associated with penile SCC will cause lymphadenopathy. Inguinal lymph nodes can be difficult to palpate due to fat deposition in this area also. In any case, distant metastases are rarely described.
In horses suspected of having SCC of the third eyelid, the third eyelid can be easily removed under standing sedation and local anaesthesia, with the removed tissue then being submitted for histopathology. Occasionally, cases will present at a much more advanced stage of disease such as the aged gelding in Figure 7, where SCC which appeared to have arisen from the third eyelid had then progressed to extend over part of the cornea. In this case enucleation was undertaken and the horse survived for a number of years without any related complications. Some corneal SCC may be treated by surgical debulking or keratectomy, with or without follow-up topical chemotherapy. Strontium plesiotherapy has also been used in the UK. Referral to an ophthalmologist would give more information on prognosis for individual cases where the cornea or other structures of the eye are affected.
Lymphoma is the most common type of neoplasia of the haematopoietic system in the horse. It can be classified into: multicentric, alimentary (Figures 8 and 9), mediastinal, cutaneous and solitary tumours of extranodal sites. It most commonly affects horses between 4 and 10 years old with clinical signs seen varying dependent on the body system involved. That said, weight loss, lethargy, depression, ventral oedema, lymphadenopathy and recurrent pyrexia are commonly reported.
Clinical approach would begin with thorough physical examination including rectal examination as well as the submission of blood for haematological and biochemical analysis. Furthermore, ultrasound of the thorax and abdomen is useful to attempt to identify organ involvement, additionally with a view to biopsy any masses although this will of course depend on the ease of access of such masses. Lymphoma rarely exfoliates into the thoracic or abdominal cavity but centesis of these cavities is often performed with the resulting samples undergoing cytological examination and yielding a diagnosis in a small number of cases.
Sadly, since horses are generally diagnosed with lymphoma at a late stage of the disease, there is no information regarding treatment responses. Solitary masses can be excised but prognosis will depend on other staging of the tumour. Chemotherapy protocols have been reported for horses but not in sufficient numbers to make any wide statements about their success.