A closer look: Melanoma in Horses
Melanocyte cells are the cells which create skin pigmentation. When these cells start growing uncontrollably, they develop into masses (tumors) called melanomas.
Melanomas are either benign or malignant. Benign melanomas are not life threatening, but they can grow large enough to be uncomfortable or cause functional impairment. Malignant melanomas are life-threatening cancers that can metastasize and invade other organs. Veterinary attention is required to determine the status of melanomas and plan treatment.
Typical locations of melanomas include:
- Tail head
- Edges of the lips
- Eyes including eyelids, iris, and retina
- Parotid salivary glands in the neck
- Lymph nodes in the neck
Small, benign melanomas do not harm a horse. Some melanomas grow slowly, while others grow rapidly. Melanomas sometimes grow as big as oranges. Some melanomas appear alone, and others come in clusters or in lines along the skin.
Compared with other species, horses generally have very low or non-existent rates of most types of cancer. Of all forms of cancer that can occur in horses, melanomas are the second-most common.
All horses are at risk of developing melanomas. At particular risk are gray horses. Some research indicates that up to 80% of gray horses are affected by melanoma. Breeds with a high incidence include:
Usually, melanomas start to develop at around 3 or 4 years of age, but may not be noticed until middle age. In rare cases, horses are born with congenital melanomas.
Even benign tumors can disrupt the function of specific systems if they grow large or in groups. Horses with large or multiple melanomas on their perineum or anus, for example, have difficulty defecating. Melanomas on the eyelids make it difficult to open the eyes. Melanomas in the iris or retina disrupt eyesight. Melanomas on the lips make it difficult to eat.
Malignant melanomas are more seriously damaging. Malignant tumors cause damage to the skin as well as spreading (metastasizing) to other organs. In cases where the tumors spread, symptoms depend on which organs are affected.
The causes of melanomas in horses are not yet fully understood.
Tumor growth occurs when there is abnormal, uncontrolled cell growth. In humans, melanomas are caused by UV radiation. In horses, melanomas typically develop in places on the body where the sun does not reach, making other mechanisms of tumor development more likely.
As with all cancers, genetic and environmental factors are suggested.
The main symptom of early melanoma is small, black, firm, shiny bumps on the skin.
Testing and diagnosis
Diagnosis is based on the presence of characteristic black bumps or lumps. Biopsy or fine-needle aspiration is used to determine whether the melanoma is malignant or benign.
Steps to Recovery
Treatment varies depending on the location of the tumor, what disruption is occurring to various systems, and whether the tumor is benign or malignant.
Removing small, benign tumors may help prevent them from becoming large or becoming malignant. Large benign tumors require removal if they are disrupting the function of the structure on which they occur.
Clustered and malignant tumors do not respond well to surgery, as they tend to grow back, often in larger groups than before surgery. Treatment in these cases include chemotherapy drugs and repeated cryosurgery using liquid nitrogen.
A vaccine is available that promotes the immune response to tumors, potentially reducing their size.
Horses with small, benign melanomas have a good prognosis. Horses with large melanomas or clusters of melanomas have a fair prognosis, but can continue to have a good quality of life so long as the necessary functions of the body are not disrupted. Horses with malignant melanomas, especially where they have metastasized to other organs, with congenital melanomas, or with melanomas that disrupt the body’s necessary functions, have a poor prognosis.
There are no proven preventive measures for melanomas. A vaccine is available that helps the immune system fight tumors for horses that have melanomas. This vaccine is not recommended for preventing cancer.
Is Melanoma in Horses common?
This is a very common condition in gray horses. It is less common in other horses.
- Benign neglect