October 2019



Signalment (JPC #2681629): Adult boa constrictor (Boa constrictor constrictor)


HISTORY: A 1 cm diameter skin mass that recurred 1.5 years after surgical excision


HISTOPATHOLOGIC DESCRIPTION: Scaled skin: Expanding the dermis and elevating the multifocally ulcerated epidermis is a 2 x 1.5 cm, unencapsulated, infiltrative, moderately cellular, indistinctly lobulated, pigmented neoplasm composed of round to spindle cells arranged in and packets as well as short streams and whorls on a scant to moderate fibrovascular stroma. Neoplastic cells have indistinct cell borders; a moderate amount of eosinophilic granular cytoplasm that contains variable amounts of gold-brown to green, granular, highly birefringent pigment; and an oval to elongate nucleus with finely stippled chromatin and one variably prominent nucleolus. Anisocytosis and anisokaryosis are marked. Mitoses average 10 per 2.37mm2 (10 400x HPF). Rarely, small nests of neoplastic cells extend into the epidermis (junctional activity). Vessels within the neoplasm are surrounded by variable numbers of lymphocytes, plasma cells, histiocytes, and granulocytes. There is a focally extensive area of ulceration with subjacent granulation tissue formation and adjacent epidermal hyperplasia with hyperkeratosis.


MORPHOLOGIC DIAGNOSIS: Skin: Iridophoroma, Boa constrictor (Boa constrictor constrictor), reptile.



·      Rare neoplasm of reptiles most commonly reported in snakes; composed of chromatophores (contractile pigmented cells of neurectodermal origin)

·      Snake skin contains three classes of pigment producing cells; tumors are classified according to the type of chromatophores they contain:

·      Melanophores- dark brown to black melanin-producing cells (melanophoroma or melanoma)

·      Xanthophores- reddish carotenoid and pteridine producing cells (xanthophoroma)

·      Iridophores - crystalline purine-producing cells containing refractile (birefringent) granules of guanine, adenine, hypoxanthine, or uric acid (iridophoromas)

·      Neoplasms from all three classes have been reported in snakes, including mixed chromatophoromas

·      Iridophoromas are generally benign; rare malignant forms are reported



·      Presents as ulcers or blisters on the skin

·      Underlying mass is often firmly attached



·      Typically single, white to light grey, dermal mass or plaque; typically with overlying ulceration

·      Occasionally overlying integument is hyperkeratotic



·      Pleomorphic population of cells with abundant golden brown to yellow-green pigment that is birefringent with polarized light

·      The primary indicator for malignancy of chromatophoromas is nuclear atypia; mitotic count, lymphatic invasion, level of infiltration, and degree of pigmentation or ulceration are not reliable indicators of metastasis

·      Nuclear atypia for chromatophoromas is classified as mild (<30% of neoplastic cells), moderate (30%–60% of neoplastic cells), and marked (>60% of neoplastic cells)



·      Microscopic examination is typically sufficient for diagnosis

·      Immunohistochemistry:

·      PLN2, S-100 for all chromatophoromas

·      Electron microscopy



·      Gross:

·      Granulomas due to bacteria, fungi, trauma, or septicemia

·      Parasitic masses due to plerocercoids, larval dracunculids, acanthocephalans, or spirurids

·      Steatitis

·      Cryptosporidium spp.

·      Other neoplasms: Fibrosarcoma, squamous cell carcinoma, other chromatophoromas

·      Microscopic (other chromatophoromas):

·      Melanophoroma/melanoma:

·      Amorphous granular melanin granules (melanosome);

·      These are the most common of the chromatophoromas in snakes;

·      Can be invasive with intravascular and distant metastases;

·      Immunohistochemistry: One study indicates that the sensitivity of MelanA and HMB45 is poor for reptile melanocytic tumors (Munoz-Gutierrez Vet Pathol. 2016), another reference states that both Melan-A and S-100 immunostaining are reported in snake melanophoromas (Ossiboff Pathology of Wildlife and Zoo Animals 2018)

·      Histochemistry: melanin granules stain with Fontana-Masson

·      Mosaic chromatophoroma: Combined contractile pigment granules

·      Xanthophoroma: Concentric lamellar pteridines (pterinosome); pterinosomes may stain with Fontana-Masson, making differentiation from poorly pigmented melanophoroma difficult



·      Chromatophoromas have been reported in a variety of snakes and amphibians

·      Melanophoroma and iridophoromas have been reported in amphibians

·      Reported in several fish species

·      Reported in dwarf bearded dragon (Pogona henrylawsoni)



1.    Munoz-Gutierrez JF, Garner MM, Kiupel M. Cutaneous chromatophoromas in captive snakes. Vet Pathol. 2016; 53(6):1213-1219.

2.    Ossiboff RJ. Serpentes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 907-908.

3.    Pessier AP. Amphibia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 926.

4.    Siniard WC, et al. Immunohistochemical analysis of pigment cell tumors in two cyprinid species. J Vet Diagn Invest. 2019;31(5):788-791.

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