Abstract

Melanoma (chromatophoroma) is a pigmented tumor arising from the chromatophores of the skin and epidermis. Isolated cases have been reported in snakes, but large retrospective studies have not been done. This report describes the clinicopathologic features of melanomas in snakes submitted to a specialty diagnostic service. From November 1994–April 2010, 4663 snakes were submitted to Northwest ZooPath, including 54 melanoma submissions representing 35 species. Affected snakes included 29 colubrids, 15 vipers, and 13 boids. Of these, four were repeat biopsies and five were follow-up necropsies (relative prevalence=46/4663=1.0%). San Francisco garter snakes (Thamnophis sirtalis tetrataenia) were the most commonly represented species (4). Five snakes died, seven were euthanatized, and disposition was unknown for 34. All tumors were initially diagnosed from skin biopsies, and were described as solitary or multiple white, yellow, brown or black, solid or cystic lesions from the dorsum (11), side (8), ventrum (9) or unknown location (22). Age ranged from 2–27 years and average age was 14 years, but age was unknown for 16 adults. No sex predilection was noted. None of the snakes were reported as clinically ill at the time of initial biopsy. Twenty-eight tumors were incompletely excised at first biopsy, three at second biopsy, and seven were euthanatized due to recurrence at the surgical site or regional skin. Survival after initial histologic diagnosis was known for nine snakes, ranged from 2 to 118 months with average of 24 months, and metastasis was noted in seven of these snakes. Two biopsied tumors had evidence of vascular invasion, and histologic evidence of visceral metastasis was noted in 11/16. Well-differentiated and anaplastic, poorly differentiated tumors had metastatic behavior. Regardless of cell morphology and degree of differentiation, cutaneous melanomas in snakes should be considered malignant with potential for local tissue invasion and visceral metastasis, although survival time after initial diagnosis may be relatively long compared to that of malignant melanoma in other classes.