Epibulbar melanoma in a foal

Equine intraocular melanocytic neoplasia

Objective

To describe the clinical appearance, histopathology, and treatment of equine intraocular melanocytic neoplasia in adult horses.

Animals and procedure

A retrospective review of medical records was conducted. Data recorded included signalment, ocular examination findings, physical examination findings, therapeutic interventions, and case outcomes. Histopathologic characteristics of enucleated globes were evaluated. A Student's t-test was used to evaluate differences in the interval from diagnosis to last known outcome between horses receiving therapeutic interventions and horses undergoing monitoring alone.

Results

Of the 55 horses included, Arabian was the most common breed (15/55, 27%). Gray was the most common coat color (85%). Physical examination was completed for 75% of horses at time of diagnosis, and of those, 67% had cutaneous melanoma. The interval from diagnosis to last known outcome was not different (P = 0.312) between horses that underwent monitoring alone (median: 2.0 y) and those that received treatment (mean: 2.25 y).

Conclusion

Equine intraocular melanocytic neoplasms are highly associated with cutaneous melanoma and gray coat color, and they are more prevalent than previously published reports suggest.

Clinical relevance

A complete ophthalmic examination is indicated for all horses with cutaneous melanoma. Additional research into the timing and rationale for treatment of intraocular melanocytic neoplasia is necessary.

Melanocytic Ophthalmic Neoplasms of the Domestic Veterinary Species: A Review

Melanocytic neoplasms in veterinary species occur in various ophthalmic locations including the eyelid, conjunctiva, cornea, sclera, anterior and posterior uvea, and orbit. Histology usually provides the definitive diagnosis for melanocytic ocular neoplasias. The degree of tissue invasiveness and anaplastic cellular characteristics are more reliable indicators of biological behavior than is mitotic index in most ophthalmic melanocytic tumors. Melanocytic neoplasias of the eyelid are predominantly benign in canines and equines, though in felines, there is the potential for metastasis, especially if the conjunctiva is involved. Limbal melanocytic tumors are predominantly benign in all the studied species, though there is a bimodal occurrence with this tumor type in canines, where those that appear in dogs younger than 4 years tended toward active growth, whereas those that appear in dogs older than 8 years tended to progress more slowly, and may not require therapy. The most common location for melanocytic ocular neoplasias in both canines and felines is the anterior uvea. Feline diffuse iris melanoma in particular has a higher incidence of metastasis than does canine nodular anterior uveal melanocytoma. In contrast, posterior uveal melanocytic tumors are rare in both canine and feline species and are considered benign. Orbital melanoma is rare in both canine and feline species; however, it generally carries a grave prognosis owing to its malignant nature. Knowledge of the general biological behavior and its variability among locations within the eye and between species is essential in therapeutic planning.