Equine sarcoids are the most commonly diagnosed tumor of equids, representing 20% of all equine neoplasms and 36% of all skin tumors in the horse. Recent studies suggest that there is no significant gender or age predisposition.

Equine sarcoids are rarely life threatening but can compromise the use of an animal which could prove to be a major economic concern. They may also have significant welfare implications, particularly in developing countries where equids, principally donkeys, are widely used as work animals.

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 2 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 occur anywhere on the body but are most common in the paragenital region, the ventral thorax and abdomen, and the head. They frequently occur 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, house fly 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 be passed into existing wounds from contaminated pasture.

There is no universally effective therapy for sarcoids, and many tumors recur. Pedunculated sarcoids with a discernable 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 employed treatments include cryotherapy, surgical or laser excision, and local immune modulation (Bacillus Calmotte-Guérin therapy). Radiotherapy, most commonly involving 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 are expensive and not widely available. Sarcoids frequently recur if treated by surgical excision, 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 followed by topical chemotherapy).

Recently, several promising new treatments have become available, or are in the final stages of clinical trials. These include the use of intra-tumoral cisplatin beads/emulsion and application of topical imiquimod. There have also been reports of successful application of acyclovir creams for sole use in the treatment of flat, occult sarcoids or applied to the wound bed of larger tumors removed by surgical excision. Acyclovir's method of action is unknown, but it is relatively inexpensive and has a wide safety margin. The development of preventive and/or therapeutic vaccines may form a significant part of disease control strategies in the future, but trials so far have shown limited success.

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

Last full review/revision July 2011 by Margaret Finlay