Equine sarcoid

What it is

The sarcoid is a locally aggressive tumour of the dermal fibroblasts, with changes in the overlying epidermis. It is associated with infection by bovine papillomavirus types 1 and 2 (BPV-1 and BPV-2), whose DNA is found in almost all sarcoids; the virus does not complete its cycle in the horse but persists in the tumour cells. How it is transmitted is unclear: flies and contact with wounds are suspected, and sarcoids often appear at sites of damaged skin.

It does not metastasise, but it can grow large, ulcerate, become infected and multiply. Incomplete treatment, including handling or partial biopsy, can activate the lesion and make it grow faster. Several clinical types are recognised, and they can coexist in the same horse:

• Occult: an alopecic area of slightly thickened, scaly skin.
• Verrucous: a dry, rough, wart-like lesion.
• Nodular: firm subcutaneous nodules, with normal or thinned skin over them.
• Fibroblastic: a fleshy, ulcerated mass that bleeds easily.
• Mixed: a combination of the above.
• Malevolent: rare; spreads along the lymphatic vessels of the skin, forming cords of nodules.

Who it affects

Horses, donkeys and mules of any breed, most often young adults. There is a genetic predisposition: some families and breeds are more affected. Horses with one sarcoid tend to develop others.

Anatomy involved

• Skin: the tumour arises in the dermis. Typical sites are areas of thin, sparsely haired skin: the head (periocular region, lip commissures, ears), the inguinal region and medial thighs, the prepuce, the axilla, the chest and the ventral neck and abdomen. Wounds and scars are frequent sites.
• Eyes and eyelids: periocular sarcoids are the hardest to treat, because surgery with wide margins can deform the eyelids and leave the eye unprotected. They affect the eyelids and medial canthus, not the globe itself, but their growth compromises eyelid closure and protection of the cornea.

Clinical signs

• One or more skin lesions of the types described, growing slowly or, after trauma or incomplete treatment, rapidly.
• Ulceration, bleeding and secondary infection in fibroblastic forms, which attract flies.
• Depending on location, problems with the saddle, girth or bridle, or with eyelid closure.

Diagnosis

• Mainly clinical, from the appearance, location and history.
• Biopsy confirms the diagnosis, but partial biopsy can activate the tumour; it is preferably done with complete excision of the lesion or when the result will change treatment.
• PCR for BPV DNA in lesion samples supports the diagnosis.
• The differential diagnosis includes exuberant granulation tissue, squamous cell carcinoma, cutaneous habronemiasis, papillomas and dermatophytosis.

Treatment and prevention

There is no clearly proven treatment of choice, and the choice depends on the type, size, number and location of the lesions:

• Surgical excision with wide margins, often with electrosurgery or laser.
• Cryosurgery.
• Intralesional chemotherapy with cisplatin or carboplatin, or electrochemotherapy.
• Immunotherapy with intralesional BCG vaccine, especially for nodular and fibroblastic periocular sarcoids.
• Topical cytotoxic or immunomodulatory treatments, such as imiquimod.
• Radiotherapy (brachytherapy), with good results in delicate areas such as the periocular region, but with limited access.
• For small, stable lesions, monitoring without treatment is sometimes chosen, because some remain inactive for years.
• Prevention: fly control and protection of wounds.
• The horse is a food-producing species in many countries. In the EU its passport states whether it is excluded from the food chain, and this determines which medicines may be used and with which withdrawal periods; many of the products listed are not authorised for equids.

Key studies

• Offer et al. (2024), systematic review of sarcoid treatments. Only 10 studies met the criteria (histological confirmation of lesions and at least 6 months of follow-up). Reported regression ranged from 28% to 100% per sarcoid and from 9% to 100% per horse, with risk of bias from "some concerns" to "critical". The authors conclude that there is insufficient evidence to recommend one treatment over another and call for randomised placebo-controlled trials.
• Haspeslagh et al. (2016), retrospective series of 230 equids with 614 sarcoids treated under a standardised protocol. Overall success (complete regression without recurrence for at least 6 months) was 74.9%, and highest with electrosurgical excision (86.8%). Intralesional platinum chemotherapy, cryosurgery and topical acyclovir had more failures, as did horses with several sarcoids. Treatment was not randomised but chosen by lesion type.
• Christen-Clottu et al. (2010), randomised blinded trial in 53 horses (444 sarcoids) treated with subcutaneous mistletoe extract (Viscum album) (32) or placebo (21) for 105 days. Complete or partial regression occurred in 41% of treated horses versus 14% of controls; complete disappearance of lesions (38% versus 13%) was not significant. Diagnosis was clinical, without histological confirmation of all lesions.
• Beermann et al. (2024), randomised, double-blind, placebo-controlled trial in 45 horses: oral mistletoe extract, subcutaneous extract or placebo for 7 months. There were no significant differences in the primary endpoint; complete regression was 27%, 21% and 13%, and the comparison of the oral route with placebo gave an OR of 2.16 (95% CI 0.45–10.42). It did not confirm the benefit of the 2010 study; it is too small to detect moderate differences.
• Haspeslagh et al. (2017), double-blind trial with 49 occult and verrucous sarcoids treated twice daily for 6 months with 5% acyclovir cream (24) or placebo (25). There were no differences in success rate, diameter, surface area or appearance of the lesions. The number of lesions was small and only mild sarcoid types were included.