Sarcoids are locally aggressive ﬁbroblastic cutaneous skin tumours which can occur anywhere on the horse’s body. However, their predilection sites are the axilla, inguinal region, ear, periocular region and the penile sheath. They are the commonest cutaneous neoplasm in horses affecting up to 6 percent of horses in the UK (Ireland et al., 2013), with all ages of horses affected. If a horse has one sarcoid it is predisposed to getting more. Rarely, they can spontaneously resolve and it is this trait which has likely led to multiple anecdotal reports of success with otherwise completely unproven treatments. This can be dangerous as a failed treatment or trauma can trigger more aggressive behaviour of the sarcoid. If the sarcoids are extensive they can debilitate and metabolically exhaust the horse and, in some locations (eg the eyelids), they can cause important functional problems; in these locations, treatments can be limited. Sarcoids can also affect the commercial value of a horse.
The diagnosis of sarcoids is largely based on clinical appearance. A biopsy can be taken to conﬁrm the diagnosis but this risks aggravation resulting in transformation to a more aggressive type. There are six different types of sarcoid (Table 1; Knottenbelt, 2005).
As with all neoplasms, early treatment is more successful and broadens the possible treatment options.
This can be tempting with a very small and insigniﬁcant sarcoid, usually an occult or verrucose type. If this approach is adopted, very careful monitoring is required as sarcoids can rapidly progress, especially with trauma. It should be borne in mind that treatment when smaller results in reduced morbidity and costs, and improves outcomes.
This is usually performed using elastrator rings. It works well provided there is no extension of the sarcoid below the ring. The suitable sarcoids are type A1 and B1 nodular sarcoids and type 1a ﬁbroblastic sarcoids. This can be combined with topical or intralesional chemotherapy.
This is a mixture of ﬂuorouracil, thiouracil, heavy metal salts and a steroid. Typically, the treatment involves four to ﬁve treatments 48 to 72 hours apart. There is a marked response during treatment (Figure 1) and as such AW5 is highly controlled, only being dispensed for individual horses for speciﬁc sarcoids to be applied only by a veterinarian. Used correctly, non-recurrence rates of around 74 percent have been reported (Knottenbelt, 2019). It must not be used near vital structures or where the horse could gain access to the treated sarcoid with its muzzle.
Blood root ointment
This is a twice-daily topical ointment containing Sanguinaria canadensis, a North American plant extract. It is reported to have cytotoxic and immune-modulatory effects. It is relatively inexpensive and one study found 66 percent of sarcoids under 2cm diameter regressed completely (Wilford et al., 2014); it was less effective on previously treated sarcoids.
Fluorouracil (5%) ointment
This has been shown to be effective in treating some super-ﬁcial verrucose or occult sarcoids and it can be used as an adjunctive treatment following surgical treatments (Knottenbelt and Kelly, 2000).
Conventional excision is risky with high recurrence rates reported. This is likely because the extent of inﬁltration of the sarcoid into the surrounding tissue is impossible to deﬁne. Nevertheless, if the excised tissue is submitted for histopathology the presence of margins can be assessed. The author will usually combine sharp excision with electrochemotherapy (ECT; see later) with further ECT treatments administered if margins are incomplete (Figure 2). A one cut, one blade technique should be used alongside new instruments and re-gloving to close.
The author uses a diode laser to excise sarcoids where anatomic location allows. The laser vaporises sarcoid cells and generates an area of coagulative necrosis around the margin. This prevents seeding of the operative site during surgery and sterilises the wound bed after excision has been completed. In addition, the thermal injury extends the margin. It can be performed standing with local anaesthesia or under general anaesthesia (Figure 3). A recent study found a non-recurrence rate of 83 percent (Compston et al., 2013) following laser excision. Sarcoids on the head and neck were 1.6 times more likely to recur and verrucose sarcoids were four times more likely to recur. The excision sites are usually left to heal by second intention as they frequently dehisce if primary closure is attempted. Protracted healing can be problematic especially in high motion areas.
This therapy involves injecting a cytotoxic drug (the author uses cisplatin) into the sarcoid before applying an electric current through the tissue (Figure 4). This increases the permeability of the cell membrane to the drug resulting, in the case of cisplatin, in a four-fold increase in the cytotoxic effects. The resulting necrosis remains localised and the drug only affects cycle cells (by mitotic death), not cells in a quiescence state, such as muscle and nerve cells. The treatments must be performed under general anaesthesia and between one and seven are required, usually three. Nevertheless, the reported 99.5 percent four-year non-recurrence rates (Tamzali et al., 2012) are attractive (Figure 5).
High dose radiation (HDR) brachytherapy
Radiotherapy undoubtedly has very high success rates but the availability and high cost of treatment reduces its use. Recently very high success rates were reported in eight horses with high dose radiation brachytherapy (Hollis and Berlato, 2018); an advantage of this form of radiation therapy is that it can be delivered under sedation. Two doses are delivered one week apart.
Numerous treatment options are available for this complex skin disorder but careful consideration of the type, location and extent of the sarcoid and funds available will enable selection of the most appropriate treatment for the patient. It is important that owners are warned that any treatment can fail and even make the sarcoid worse. Treatments continue to progress and outcomes are improving; certainly, early treatment improves the chances of resolution.