Cancer knows no boundaries for the animals that it will affect and certainly horses are no exception. Gray horses are genetically at risk for developing melanomas, primarily of the skin. It is reported that over 70% of Gray horses over the age of 15 years will develop melanoma. In some patients, these tumors can become numerous and quite large. They can also be located in areas on the body that can lead to problems, such as difficulty urinating and defecating. Additionally, these tumors can become more invasive and can spread to other parts of the body. Effective treatment options are limited. Recently, immunotherapy has become a promising therapy option for horses with melanoma. The melanoma vaccine that is used in both dogs and cats may also be effective in some horses. The vaccine is generally well tolerated and side effects are uncommon. At CancerVets, the oncologists can answer your questions regarding this type of treatment for horses.