Many tumour conditions in horses are common and affect the skin – but there are some significant internal tumours to be aware of. Often these have cutaneous manifestations either as a similar tumorous appearance (eg melanoma, lymphoma) or paraneoplastic cutaneous evidences (eg pruritus, paraneoplastic pemphigus). The management of cutaneous neoplastic diseases has improved over the past 40 years, but despite significant progress in other species in a range of neoplastic conditions, in the horse, progress lags – especially in the management of internal neoplastic disease.
The early detection of internal cancer is critical to its potential management – managing severe, disseminated tumours, whether cutaneous or internal, is easier when tumours are small and localised. However, in the equine species, cutaneous neoplasia has been consistently belittled and, therefore, owners have developed a casual approach to tumour medicine that regrettably has also transferred to the veterinary profession. This has been a major constraint on progress in cutaneous cancers.
There are many reports of single or short series of equine cases for almost all the known tumours in mammals, but these are seldom properly documented and are even less often incorporated into larger, multicentre studies that could provide genuine evidence-based information on prognosis and treatment options.
Oncology has not become a significant speciality in equine medicine. Specialist opinion can usually be obtained, and some cases can be admitted to specialist centres for treatment; this should maximise the chance of successful treatment. However, it is important to remember that no specialist will have a 100 percent success rate – any person or organisation that claims remarkable results should be viewed with suspicion.
The major emphasis in equine oncology is on:
- Equine sarcoid: A cutaneous tumour; the main complications involve the fact that it encompasses a spectrum of fibroblastic tumours, including neurofibroma, spindle cell sarcoma, fibrosarcoma and myxofibrosarcoma.
- Equine melanoma: There are variations, some of which are highly malignant, but the majority are singularly benign and have only space-occupying and cosmetic effects.
- Squamous cell carcinoma: The most aggressive form is in the stomach, but there are highly dangerous variants of the cutaneous form that occur in the mouth, nasal passages and sinuses, bladder, and preputial and vulvar regions.
- Mast cell tumour: Mostly cutaneous or conjunctival presentations, as well as in the skin of the face/head and distal limb regions.
- Lymphoma: Cutaneous forms are recognised (and these may be related to granulosa cell tumours, directly or indirectly); intestinal, mediastinal and generalised forms are encountered occasionally.
- Granulosa (theca) cell tumour: The most common reproductive tumour, it has well recognised clinical and endocrinological manifestations (Figures 1 and 2).
- Lipoma: Rare in horses but mesenteric lipomas are common in older horses, with ponies probably being over-represented. They are invariably benign, but do have serious consequences with a high proportion of strangulating intestinal obstructions due to lipoma in older horses.
These tumour types make up over 98 percent of equine tumours in the horse, with surveys of tumours largely focusing on cutaneous tumours (Jackson, 1936; Baker and Leyland, 1975). There are also several less common skin tumours such as keratoma, keratoacanthoma, sebaceous adenoma and giant cell tumour. Little evidence is available on most of these. Rarer tumours include mesothelioma, haemangioma, pheochromocytoma, intestinal adenocarcinoma, basal cell carcinoma, haemangiosarcoma, seminoma, dysgerminoma.
The main problem with many of these is that so few cases have been reported that evidence-based approaches are difficult and, in any case, most of those involving serious internal tumours are diagnosed at necropsy or are destroyed when the diagnosis is made. The “pituitary adenoma” that is usually viewed as a neoplasm of the pars intermedia is not now considered to be a tumour in most cases – it is probably best viewed as a benign but functional hyperplasia. What is certain, however, is that brain and central nervous system tumours are extremely rare in horses. Similarly, the ethmoid haematoma is widely viewed as a non-cancerous neoplasm, but a few cases have anecdotally been associated with subsequent adenocarcinoma.
Young foals may also have cutaneous haemangioma; these are both difficult to treat and highly dangerous if left. The most difficult cutaneous tumours to treat are those that are cutaneous manifestations of internal or widely disseminated tumours such as lymphosarcoma.
Despite the high prevalence of the major tumour types, little therapeutic progress has been made. The problems of individual tumour types have influenced progress. The equine melanoma occurs predominately in grey horses and, despite the often reported near 100 percent prevalence in grey horses over the age of 10 to 15 years, little is known about the condition. Grey horses are the only ones materially affected by melanoma and so there is likely little incentive to research this condition.
Squamous cell carcinoma affects various skin sites, so the numbers of cases are low – again, this slows progress. Furthermore, it commonly affects difficult anatomical sites such as the penile skin, the eyelids and the mouth, so again the therapeutic options are limited by anatomic considerations. There has also been little progress in our understanding of the equine sarcoid; the belief of many pathologists that the disease behaves as a virus infection rather than a “cancer” condition has hindered therapeutic progress.
This has led to an unhelpful and introspective attitude towards the disease with veterinarians inclined to advise benign neglect: “Monitor its progress and let me know when it gets bad.” This attitude is counter to all recognised policies on neoplastic disease in any species. The owner can ask the veterinarian: “Is this small tumour on my horse going to get smaller, easier and less dangerous with time, or is it going to get bigger, more difficult and more dangerous?” The next question could be: “Would you prefer to do something about it now while it is small, easy and relatively safe, or would you prefer to operate when it is larger, difficult and pathologically dangerous?” Given the melanoma is almost invariably benign when it is small and invariably becomes malignant, the choice is obvious.
Where melanomas occur in non-grey horses, they usually have a more malignant implication. Squamous cell carcinoma is an aggressive, often invasive tumour but only rarely does it metastasise to other organs. Cutaneous lymphosarcoma is a serious problem but the cutaneous histiocytic form has a much better prognosis than the multicentric forms. What is possible to treat depends primarily on the tumour type and extent, available technology, and skill and experience of the veterinarian.
The wide variety of treatment options implies that no single method is universally applicable or effective. The surgeon needs to consider the likely prognosis and should avoid any treatment attempts unless there is reasonable expectation of an improved outlook for the horse. Failure to remove the whole tumour usually results in recurrence and, in some cases, such as the equine sarcoid, this can be in a dramatically more aggressive form. Therefore, careful clinical assessment must be performed before embarking on treatment and the owner must be apprised of likely outcomes. Referral to a specialist centre is a valuable option in all cases.
