Discussion

Sarcoids:
Sarcoids are common, locally invasive, fibroblastic skin tumours. Six types have been described, including occult, verrucous, nodular, fibroblastic, mixed and malevolent.
The most common sites of development include the head and neck, extremities, and ventrum. Inactive sarcoids may become aggressive if disrupted by injury, biopsy, or inappropriate treatment.
Bovine papillomavirus (BPV) types 1 and 2 are causally
associated with the development of sarcoids. Horses with certain equine leucocyte antigen (ELA) haplotypes are at higher risk. Sites of skin trauma or open wounds are more likely to become affected by a sarcoid. A proposed mechanism is that flies may act as vectors of BPV in disease transmission.
A presumptive diagnosis of equine sarcoid is based on clinical appearance, especially the presence of more than one lesion with characteristics of sarcoid. A definitive diagnosis requires histopathology, but biopsy-induced trauma or irritation may exacerbate the lesion and induce proliferation. Histopathologically, equine sarcoid may be confused with fibroma or fibrosarcoma.
There is no uniformly effective therapy for equine sarcoid. Surgical management (including conventional excision and laser excision), cryotherapy, hyperthermia, radiotherapy, chemotherapy, immunotherapy, topical immune modulation and antiviral agents are used with variable degrees of success.

Melanoma:
Melanoma is a common, pigmented, infiltrative neoplasm most commonly seen in grey horses. Recently, the concept of equine melanoma as a form of malignant neoplasia or neoplasia with malignant potential has become more accepted, with some sources claiming that at least 66% of equine melanocytic tumours eventually become malignant.
Four manifestations of equine melanotic disease have been described: melanocytic nevus, discrete dermal melanoma, dermal melanomatosis and anaplastic malignant melanoma. Discrete dermal melanoma and dermal melanomatosis, collectively referred to as dermal melanoma, represent the largest majority of melanomas. Discrete dermal melanomas exist as single masses in typical or atypical locations. Surgical excision is curative in most cases. Dermal melanomatosis is seen in grey horses involving multiple cutaneous masses, with at least one of the masses presenting in a 'typical' location. These typical sites include the ventral surface of the tail, anal, perianal and genital regions, perineum, and lip commissures. These masses may not be as amenable to surgical resection and may be associated with visceral metastasis. Discrete dermal melanomas and dermal melanomatosis are generally histologically indistinguishable, presenting as indistinct, heavily-pigmented tumour cells in the deep dermis. It is likely that these diagnoses exist as a continuum.
Lesions may exist for many years and cause no clinical problems. However, this changes when lesions enlarge and coalesce. Surgical debulking of advanced lesions is difficult and, many times, unrewarding.
Cimetidine has been used as a therapy (oral administration of
2.5 mg/kg bwt every 8 h for a period of 4 - 12 months). However, the efficacy of this treatment has been questioned. Cisplatin can also be used. A sesame oil-based cisplatin formulation with added epinephrine is administered in a series of 4 intratumoural administrations at 2 week intervals. Therapy is less effective in larger, more advanced tumours. Cisplatin-containing biodegradable beads alone or in combination with other therapies can also be used. A preliminary study to evaluate the therapeutic potential of an autochthonous vaccine has also been performed. The vaccine was administered every other week for 6 weeks and then every 6 weeks for an undisclosed period of time. Tumour regression occurred in 11 out of 12 horses.

Squamous cell carcinoma:
Squamous cell carcinoma (SCC) is an invasive tumour of squamous cells of the skin and mucosa of the paranasal sinuses, respiratory tract, genitourinary tract, and gastrointestinal tract. The most common location of SCC is the periorbital region. Other sites of SCC formation include the genitalia, face and ear pinnae, perianal region, and extremities. The mean age of horses with periorbital SCC is 13 years, while older horses tend to develop genital SCC (mean of 20 years). The nature of SCC ranges from slow growing, benign tumours to rapidly-growing, highly malignant and invasive tumours.
A novel equine papillomavirus, Equine Caballus Papillomavirus-2 (EcPV-2) has been recently identified as a likely aetiological agent for the development of equine mucocutaneous SCC. EcPV2 DNA was detected in 100% of ocular and genital SCC lesions and genital papillomas, and about 50% each of ocular, penile and vulvovaginal swabs from healthy horses.
Definitive diagnosis requires histopathological examination in the form of an impression smear or biopsy. Local lymph nodes should be assessed clinically (including iliac lymph nodes) and if lymph node involvement is suspected, fine needle aspirates and/or biopsies should be performed when possible to assess metastasis.
Several modalities have been recommended for treatment of equine SCC, which is most successful when treatment is initiated early in the course of disease. Surgical management, cryotherapy, hyperthermia, radiotherapy, chemotherapy and photodynamic therapy are used.

Mast cell tumours:
Mast cell tumours are uncommon tumours. They are also sometimes described as mastocytosis or mastocytomas, because in many cases it is unclear whether the proliferation of mast cells in the lesion is truly neoplastic.
A wide range of ages of affected horses has been reported from one to 18 years (mean age 9.5 years). There does not appear to be any breed predilection. Male horses may be predisposed.
Solitary cutaneous tumours are usually slow growing, nonmetastatic and superficial. The most common locations are the head (lip, nostril, jaw, periorbital area), trunk, and limbs. In the limbs, they are often calcified and may be visible radiographically. The skin overlying the mast cell tumour is usually intact, although alopecia, hyperpigmentation or ulceration may be present. Occasionally the lesions may sporadically discharge caseous material.
Diagnosis is achieved by either fine needle aspiration or surgical excisional biopsy. Histologically, these tumours show a well-differentiated cellular pattern, with coalescing nodules and sheets of mast cells. Eosinophilic infiltration is a common feature.
Old lesions may demonstrate fibrosis and dystrophic mineralisation, with only small areas of mast cell accumulation; these lesions may be misdiagnosed as calcinosis circumscripta.
Surgical excision of cutaneous mast cell tumours is usually
curative, and it has been reported that even incomplete surgical excision or biopsy may be followed by spontaneous remission.
Other reportedly effective treatments include intra- and sub- lesional injection of corticosteroids (e.g. 5-10 mg of triamcinolone acetonide or 1 0-20mg of methylprednisolone acetate), cryosurgery or radiotherapy. There are also anecdotal reports that cimetidine (5 mg/kg bwt q. 24 h, per os) and intra-lesional injections of distilled water or cisplatin may be effective.