The common sites for primary melanomas include the under surface of the tail, perineum and the external genitalia. Other sites
include the parotid lymph nodes, mammary gland, paralumbar and neck musculature, feet and legs, eye, mouth and vertebral column,
but some of these can be metastatic. About 30 percent of melanomas are malignant, metastasizing/multicentric lesions when
seen. Metastasizing lesions may be found throughout the body, including lymph nodes, body cavities, liver, kidney, heart and
gastrointestinal tract. Melanosis of the parotid lymph nodes is a common finding in older gray/white horses and may be nidus
for primary melanomas in this area. Clinical signs depend on the location of the tumor, rate of growth, and the amount and
type of tissue compromised. Primary tumors can range from less than 1 cm to 20 cm and can be flat, raised, verrucous, pedunculated
and ulcerated. There are several theories for pathogenesis of melanoma in horses, but there is no viable etiological explanation.
Despite the relatively low occurrence of neoplasia in equids, manifestations are a serious health threat.
"Right now we need to raise the bar so that equine practitioners and the general public understand that melanoma is a malignancy
and not a benign thing, like a mole," he implores. "I routinely see horses that have the advanced stage of the disease. They
are going to die from this disease, and we have very few treatments for it. We just need to recalibrate our thinking totally
about what we are going to do for it."
The overall numbers of cases are low, but this may be an artifact of sampling. Most veterinarians in practice typically don't
biopsy melanomas, so the number is likely to be substantially higher.
"The problem with diagnosing and understanding the actual incidence of melanoma in horses is that many veterinarians consider
they are benign, common lesions in pale-coated horses, and they don't need to either biopsy them or to treat them," Robertson
Increased surveillance, diagnosis and treatment could reduce the numbers of horses presented with non-treatable lesions; there
is no convincing evidence that biopsy of melanomas causes them to spread, experts say.
Squamous cell carcinoma
Squamous cell carcinoma is very common in horses, usually found on the face, external genitalia and mucocutaneous junctions.
Squamous cell carcinoma occurs commonly in skin lacking pigmentation, and this skin might be more susceptible to effects of
Squamous cell carcinomas are ulcerated, raised or verrucous, nodular growths ranging from 3 mm to more than 20 cm. The tumor
is locally invasive and spreads readily to adjacent lymph nodes. Distant metastases occur but are uncommon. SCC in horses
has less keratinization than that in dogs or cats, keratin pearls being infrequent. Secondary inflammation is common in ulcerated
Cutaneous papilloma of horses is a neoplastic disease induced by infection with the equine papilloma virus, a DNA-containing
papovavirus. The tumors are most common on the muzzle and lips of horses ranging from age 1 to age 3. Single papillomas on
the head or neck are occasionally seen in aborted fetuses and newborn foals. There is no breed or sex predisposition. Lesions
may persist for one month to nine months, then spontaneously regress. Papillomas are irregularly shaped, multiple, discrete
nodules, ranging from 2 mm to more than 1 cm, and they are gray, white or multicolored, and mottled.
Lymphosarcoma is rare in horses. There are four forms — multicentric, alimentary, thymic and cutaneous. More common clinical
signs include weight loss, fever, peripheral lymphadenopathy and anemia. Some horses show upper and lower respiratory disease.
It occurs primarily in middle-aged mares (> 6 years of age), which present with a relatively short history, weeks to months,
of growth of firm nodules on the head, neck and chest. Other clinical signs in these horses are variable but include anorexia,
inability or unwillingness to work, depressed mentation, fever, dyspnea and weight loss. Some horses with primary cutaneous
lymphosarcoma show no signs other than multiple, non-painful swellings in the skin or subcutis and peripheral lymphadenopathy.