A sarcoid is a tumor found on the skin of horses, donkeys, and mules. It is one of the most common skin neoplasias seen in horses. It is generally benign and non-life-threatening but can be locally invasive. Sarcoids can have several different appearances and sometimes look like a thickened and bleeding area (ulceration) that may crust over as it heals. Other skin lesions, such as the equine papilloma, can be confused for sarcoids. However, the papilloma will go away on its own over time, while the sarcoid will rarely regress.
Symptoms and Types
There are six different classifications of sarcoids based on their appearance.
1. Nodular sarcoids
- Firm, raised circular nodules
- 5 – 20 mm in diameter
- Usually in the sheath/groin area and eyelids
2. Fibroplastic sarcoids
- Proliferative, fleshy, and ulcerative
- Usually along the groin, lower legs, and eyelid
3. Verrucous sarcoids
- Wart-like appearance
- Can occur anywhere along face, body, and groin area
4. Occult or flat sarcoids
- Flat, circular thickened areas
- May also appear as small nodules 2 – 5 mm in diameter
- Usually found on the neck, mouth, eyes, and inside of the thighs and upper forelegs
5. Malevolent sarcoids
- Appear as multiple nodules
- Are locally invasive and occasionally infiltrate the local lymphatic system, appearing like cords under the skin
6. Mixed sarcoids
- Lesions appear as a mix of the ones mentioned above.
- This type is more typical of a sarcoid that has been on the horse for a long time, or has experienced some sort of trauma.
There has been no specific cause identified for sarcoids, but the bovine papilloma virus (BPV) is thought to be a potential contributor. Both BPV types 1 and 2 have been associated with the formation of sarcoid disease in horses. Research has yet to determine the method in which the disease is transmitted, but several theories exist.
One theory is that skin that has been wounded previously is more prone to development of sarcoid. Another is that flies act as a source of transmission (vector) of the virus, as they land on wound sites on various animals. And still others believe that sharing contaminated tack or equipment between infected horses and other animals will transmit the virus.
There is no indication of a sex or breed predilection for sarcoids. However, greater than 70% of these tumors develop in horses younger than 4 years. Sarcoids may arise spontaneously or sometimes at a site of previous trauma.
A veterinarian may be able to diagnose the lesions by appearance and location of the tumors only; however, a positive diagnosis can only be made using a biopsy of the skin. In some cases, a veterinarian may not wish to take a skin biopsy, as it may make the sarcoid irritated and worsen.
Other potential skin lesions should be ruled out, and your veterinarian will check for other skin conditions such as fungal infections (dermatophytosis) and warts (papillomas).