Case 180427-2

Five-year-old, female giant garter snake (Thamnophis gigas)).

A 0.3-cm-diameter, black, slightly-raised lesion was noted on the caudal tail approximately one year ago. The zoo keepers recently noted that the lesion appeared to be larger.

A 1.3 x 1 cm section of scaled skin was received, which had a 0.7-cm-diameter, slightly-raised, black focus.

Five sections of scaled skin are examined in which a variably well-demarcated region of the epidermis and superficial dermis is infiltrated by a densely cellular, expansile neoplastic population of round to spindloid cells, which form dense sheets in the epidermis and loose sheets in the underlying superficial dermis. Neoplastic cells have indistinct cell borders and a moderate amount of eosinophilic cytoplasm with numerous dark brown granules (melanin). Nuclei are round to oval with finely stippled chromatin and one to two variably distinct nucleoli. Anisocytosis and anisokaryosis are mild to moderate, and there are no mitotic figures noted in ten 400x fields. The neoplasm and surrounding dermis are infiltrated by moderate numbers of lymphocytes, plasma cells, heterophils, and macrophages filled with similar pigment to that in the neoplastic cells (melanin). Vessels in the deep dermis are surrounded by moderate numbers of lymphocytes and plasma cells. The collagen in the superficial dermis is segmentally pale eosinophilic and smudgy. The epithelium immediately adjacent to the neoplasm is mildly hyperplastic.

Immunohistochemistry: PNL-2: Neoplastic cells exhibit variably strong cytoplasmic and membranous immunoreactivity.

Skin: Melanophoroma with moderate, chronic, lymphoplasmacytic, heterophilic dermatitis and segmental epithelial hyperplasia.

Chromatophoromas are neoplasms of the pigment cells (chromatophores) in the dermis of reptiles and fish. The overall prevalence of these tumors in snakes is low, and the average age of affected snakes is 12.9 years. Among the various types of pigment cells, melanocytic neoplasms are the most common in snakes. The macroscopic presentation of these neoplasms is highly variable, and can range from nodular masses to flat, pigmented plaques. It is common for these neoplasms to be locally invasive into the subcutis, skeletal muscle, or bone; however, metastatic disease was only reported in 29% of cases in a recent article. The same article notes that moderate to marked nuclear atypia appears to be consistently present in cutaneous chromatophoromas with a high risk of metastasis, while mitotic count, lymphatic invasion, the level of infiltration, and the degree of pigmentation or ulceration were not reliable predictors of metastasis with these tumors. Immunohistochemistry can be performed in these neoplasms in order to confirm diagnosis in poorly-pigmented tumors. S-100 and PNL-2 are the most useful markers for melanocytic neoplasms in snakes.

References