A tumor is an abnormal mass of tissue that arises when cells multiply without the normal regulatory constraints governing growth and death. Tumors are classified as benign (locally contained, non-invasive, and generally not life-threatening) or malignant (invasive, capable of spreading to distant sites via the lymphatic or vascular systems — a process called metastasis). In horses, the most clinically significant tumors are the melanoma, squamous cell carcinoma (SCC), equine sarcoid, and lymphoma.
Melanomas are disproportionately common in gray horses, with estimates suggesting that more than 80% of gray horses over 15 years of age have at least one melanoma. They occur most frequently under the tail, around the anus, on the sheath of male horses, and at the parotid salivary gland. Gray horses carrying the Greying-with-age (G) allele are predisposed; the tumors grow slowly and may be benign for years before some undergo malignant transformation.
Equine sarcoids are the most common skin tumor of horses worldwide, thought to be associated with bovine papillomavirus types 1 and 2. They are locally invasive but do not metastasize. Treatment options include topical chemotherapy, immunotherapy, laser ablation, and cryotherapy; recurrence after incomplete treatment is common.
Squamous cell carcinoma affects sun-exposed areas and mucous membranes — the eye, the sheath, the vulva, and around the lower-limb skin affected by SCC in horses with white markings. Early recognition and excision dramatically improve outcomes. Any unusual growth on a horse, particularly one that is rapidly enlarging, ulcerated, or causing functional impairment, warrants prompt veterinary evaluation.