Equine Sarcoids

Equine sarcoids are the most commonly diagnosed tumor of equids, representing 20% of all equine neoplasms and 36% of all skin tumors in horses. Studies suggest there is no significant sex or age predisposition.

Equine sarcoids are rarely life threatening but can compromise function and be an economic concern. Sarcoids may also cause welfare dilemmas, particularly in developing countries where equids, principally donkeys, are widely used as work animals (eg, brick-carrying donkeys in India).

Sarcoids can occur as single or multiple lesions in different forms, ranging from small, wart-like lesions to large, ulcerated, fibrous growths. Six distinct clinical entities are recognized:

1. Occult—flat, gray, hairless, and persistent; often circular or roughly circular

2. Verrucose—gray, scabby, or warty in appearance and may contain small, solid nodules; possible surface ulceration; well-defined or cover large, ill-defined areas

3. Nodular—multiple, discrete, solid nodules of variable size; may ulcerate and bleed

4. Fibroblastic—fleshy masses, either with a thin pedicle or a wide, flat base that commonly bleed easily; may have a wet, hemorrhagic surface

5. Mixed—variable mixtures of two or more types

6. Malevolent—an extremely rare, aggressive tumor that spreads extensively through the skin; cords of tumor tissue intersperse with nodules and ulcerating fibroblastic lesions

Lesions can develop anywhere on the body but are most common in the paragenital region, the ventral thorax and abdomen, and the head. Frequently, they are seen at sites of previous injury and scarring. Equine sarcoids can resemble other skin tumors such as benign fibropapillomas and also other cutaneous conditions such as exuberant granulation tissue (proud flesh). An individual lesion on a horse can be difficult to diagnose, but multiple tumors (often of more than one type) with characteristic features on an individual horse make the clinical diagnosis reasonably straightforward. A definitive diagnosis can be made by biopsy; however, acquiring the sample carries the risk of triggering a considerable and uncontrollable expansion of the lesion.

Bovine papillomavirus (BPV), primarily types 1 and 2, is now considered the main etiologic agent of equine sarcoids. There may also be a genetic predisposition associated with equine leukocyte antigens; particular breeds and bloodlines appear to be more susceptible to the disease.

The mode of transmission has not been confirmed. BPV-1 has recently been detected in several common fly species (eg, housefly and stable fly), and because there is an apparent predilection for sarcoid development at wound sites, it has been proposed that flies may act as vectors as they move between wound sites on different horses. Alternatively, BPV infection may be transmitted via stable management practices, such as the sharing of contaminated tack or passed into existing wounds from contaminated pasture.

There is a wide range of therapies for sarcoids, and many tumors recur. Pedunculated sarcoids with a discernible neck are ideally suited to ligation with rubber bands or elasticized suture material, usually in combination with a topical preparation once the tumor is detached. Other commonly used treatments include cryotherapy, surgical or laser excision, and local immune modulation (Bacillus Calmette-Guérin [BCG] therapy). Surgical excision with margins of at least 0.5–1 cm is recommended. Preplaced sutures or releasing incisions are often needed for primary closure.

Local radiotherapy (interstitial brachytherapy), using permanently implanted radon-222 or gold-198 seeds or removable needles of radium-226, cobalt-60 or, more often, iridium-192 (¹⁹²Ir) implants, is a highly effective treatment for tumors less amenable to traditional therapy (eg, those on the limbs or around the eye). However, ¹⁹²Ir implants and other radioisotopes are expensive and not widely available but may be the best option for recurrent aggressive lesions. Sarcoids have a 15%–82% recurrence rate if treated by surgical excision alone. Excised sarcoids often regrow more aggressively within 6 months, which may be due to activation of latent BPV in apparently normal tissue surrounding the lesion. Larger tumors may require a combination of therapies (eg, surgical debulking or CO₂ laser vaporization followed by topical or intracavitary chemotherapy or local electrochemotherapy).

Several promising treatments now available or in the final stages of clinical trials include the use of intratumoral bioabsorbable cisplatin beads/emulsion (9% recurrence rate), the application of topical imiquimod every other day for 32 weeks or until resolution (60%), autologous implantation, and topical acyclovir creams for treatment of flat, occult sarcoids (68% response rate) or applied to the wound bed of larger tumors removed by surgical excision. Acyclovir is relatively inexpensive and has a wide safety margin, but its method of action is unknown. Other products used to treat sarcoids are topical ointments that contain heavy metals, thiouracils, and 5-fluorouracil; an escharotic salve containing bloodroot powder extract and zinc chloride; and an IV immunostimulant of nonviable Propionibacterium acnes and BCG. The development of preventive or therapeutic vaccines may form a significant part of disease control strategies in the future, but trials so far have shown limited success.

A novel therapeutic approach using small interfering RNA molecules to target viral gene expression is being investigated. This technique has been shown to selectively destroy BPV-1–infected equine skin cells in vitro.