Sarcoids are the most frequently diagnosed skin tumors in horses, donkeys, and mules. These masses are classified as locally aggressive, fibroblastic, and persistent tumors that arise in the skin and underlying tissues. While they do not spread to distant organs, sarcoids can be highly invasive to surrounding local tissues and may change in appearance and behavior over time, becoming more severe. The tumors present a significant concern for animal welfare and can have economic implications, particularly if their location interferes with the use of tack or causes functional impairment.
Defining Sarcoids and Their Etiology
Sarcoids are tumors of the fibroblasts (connective tissue cells in the skin) which undergo abnormal growth. This transformation is strongly associated with an infection by the Bovine Papillomavirus (BPV), primarily types 1 and 2, which are the established etiological agents of these tumors in equids. The presence of BPV DNA and the expression of its transforming proteins have been consistently detected in sarcoid tissues. The E5 protein disrupts normal cell regulation, inhibiting pathways that typically lead to cell death and allowing affected cells to proliferate uncontrollably.
The transmission of BPV from its natural host, cattle, to horses occurs through direct contact or, more commonly, through mechanical vectors like flies. Flies have been found to carry BPV-1 DNA, suggesting they may transfer the virus between animals, especially to sites of existing skin trauma or wounds. However, exposure to the virus alone is not sufficient to cause sarcoid development in every animal, indicating a significant role for genetic susceptibility.
Certain breeds and individual horses appear to have a genetic predisposition, often linked to specific equine leukocyte antigens (ELA). This complex interaction between viral exposure, genetic makeup, and local skin trauma determines whether a sarcoid will form. The viral DNA persists within the tumor cells, which explains why lesions can recur aggressively even after apparent removal.
Clinical Forms and Visual Appearance
Sarcoids are classified into six distinct clinical forms based on their physical appearance and behavior, which can range from subtle to highly invasive. The occult sarcoid is typically the mildest form, presenting as a flat, roughly circular area of hair loss with a gray or scaly surface texture. This appearance can often be mistaken for common skin conditions such as ringworm or a scar, making early detection challenging.
The verrucose type has a rough, warty, or cauliflower-like texture, often appearing dry and scabby. These lesions may cover large, ill-defined areas and are frequently found on the face, groin, or axilla (armpit) region. Nodular sarcoids are firm, spherical lumps felt beneath the skin. They can be solitary or appear in clusters, sometimes remaining covered by normal-looking skin or becoming ulcerated and bleeding.
Fibroblastic sarcoids are fleshy, aggressive masses that grow rapidly and often have a wet, ulcerated, and hemorrhagic surface that bleeds easily. They can attach to the skin either by a narrow stalk (pedunculated) or a wide, flat base (sessile), and are frequently found at sites of previous injury. The mixed category displays characteristics of two or more types simultaneously.
Finally, the malevolent sarcoid is a rare but aggressive form that spreads extensively through the skin and along lymphatic pathways, forming cords of tumor tissue interspersed with ulcerated fibroblastic lesions.
Diagnosis and Treatment Modalities
The initial diagnosis of a sarcoid is often based on the characteristic clinical appearance and location. A definitive diagnosis requires a tissue sample to be examined microscopically, but taking a biopsy can be risky. Invasive sampling can sometimes stimulate the lesion, causing a milder sarcoid to transform into a more aggressive fibroblastic form or promoting rapid growth.
Treatment for sarcoids is difficult, and high recurrence rates are a persistent challenge. The choice of therapy is determined by the sarcoid’s type, size, and location, often requiring a multimodal approach. Surgical excision can be performed, but if margins are incomplete, the tumor often regrows more aggressively, so it is frequently combined with other treatments to destroy residual cells.
Topical chemotherapy involves applying specialized creams or ointments directly to the lesion. Intralesional chemotherapy involves injecting drugs directly into the tumor mass. Radiation therapy is highly effective but expensive and not widely available. Immunotherapy, often utilizing the Bacillus Calmette-Guérin (BCG) vaccine, is injected into the tumor to stimulate the horse’s immune system to reject the sarcoid tissue.