Clinical and Pathologic Manifestations of Ocular Melanosis in the Cairn Terrier Dog

A.E. Mentzer1A, M.Kiupel1B, B.A. Steficek1B, J.A. Render2, S.M. Petersen-Jones1A. ADept. Small Animal Clinical Science, BDiagnostic Center for Population and Animal Health, 1Michigan State University, East Lansing, MI; 2Pfizer Global Research & Development, Ann Arbor, MI.

Purpose: To describe the clinical phenotype, histopathologic features and mode of inheritance of ocular melanosis in the Cairn Terrier breed of dog.

Methods: Records of ophthalmological examinations of 169 affected Cairn Terriers were reviewed. A pedigree of affected families was constructed. 18 enucleated globes were examined by light microscopy, immunohistochemistry (IHC), or transmission electron microscopy (TEM).

Results: Bilateral ophthalmoscopic changes were first noted between 4 and 12 years of age. Initially the iris root was thickened and pigmented patches appeared in the sclera/episclera overlying the ciliary body. Pigmented particles were released into the aqueous and deposited ventrally to obscure the drainage angle. Eventually secondary glaucoma developed. Histological examination revealed that large plump pigmented cells had infiltrated the entire uvea, obliterated the iridocorneal filtration angle, and transversed the sclera into the episclera. The infiltrating cells were negative for melan-A, vimentin, synaptophysin, SMA, MSA, chromogranin, PGP9.5, and MNF 116. Some cells were MITF positive. TEM showed pigment laden cells had melanosomes of which a few were stage 2 with most stage 3 and 4. The cells had a single distinct, cleaved, centrally located euchromatic nucleus. No intermediate filaments were noted and only small numbers of mitochondria were seen. Pedigree analysis shows that the condition appears to be inherited in an autosomal fashion, but the exact mode of inheritance could not be proven.

Conclusions: Ocular melanosis in the Cairn Terrier is characterized by a slowly progressive infiltration or proliferation of pigment-laden cells primarily affecting the anterior segment. The positive MITF staining and ultrastructural features suggest that the cells are melanocytes, although the exact cell lineage remains to be determined.