Histopathological description of iridophoromas resembling skin nodule syndrome in Siamese fighting fish Betta splendens

Intensive breeding of the Siamese fighting fish Betta splendens, a species that inhabits marshlands in Thailand and other countries in south-east Asia, was traditionally focused on game fighting because the males are very territorial, but has evolved to become mainly devoted to ornamental purposes. Recently, a preliminarily named 'skin nodule syndrome' (SNS) has been described affecting this fish species in Thailand, with multiple bacterial infections suggested, including 4 species of Mycobacterium spp., although the etiology remains elusive. Here we describe the histopathological and immunohistochemical characteristics of 2 iridophoromas in 2 male Siamese fighting fish that strongly resemble the lesions described for SNS. Immunohistochemical analysis yielded negative results for Melan-A, PNL-2, and S-100, likely due to species-specific reasons. The results for molecular detection of mycobacterial DNA were also negative in both cases. The published histological lesions of SNS are very similar to the 4 chromatophoromas that have been reported affecting this species since 2015. Taken together with the present cases, we hypothesize that the preliminarily named SNS in fighting fish may be characterized as invasive chromatophoromas, although the true etiology remains to be elucidated and could include infectious and non-infectious causes. Further studies are necessary to examine whether commercial breeding of Siamese fighting fish has resulted in a possible genetic origin.

Cutaneous iridophoroma in a Green iguana ‬‬‬‬‬‬‬‬‬‬‬‬‬(Iguana iguana)‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬

An 11-year-old intact male Green iguana (Iguana iguana) was referred for treatment of a probable iridophoroma based on previous cytopathology. A periocular mass was present near the right medial canthus. Computed tomography did not show any sign of metastasis. Clinicopathologic abnormalities included lymphopenia and hyperproteinemia. Cytologic and histologic evaluations of the mass were consistent with iridophoroma. Complete surgical excision of the mass was not possible without removal of the orbit due to local tissue involvement. Recovery and suture removal were unremarkable. Adjunctive radiation therapy was recommended, but not performed. A year later, the surgical site had healed well. To our knowledge, this is the first reported chromatophoroma cytopathology in a Green iguana. Chromatophoromas should be included in the differential diagnoses of pigmented skin tumors in reptiles. Early surgical excision is useful to limit local tissue destruction and metastatic potential.

HISTOLOGIC, IMMUNOHISTOCHEMICAL, AND ELECTRON MICROSCOPIC CHARACTERIZATION OF A MALIGNANT IRIDOPHOROMA IN A DWARF BEARDED DRAGON (POGONA HENRYLAWSONI)

A dwarf bearded dragon (Pogona henrylawsoni) was presented with a white subcutaneous mandibular mass and multiple nodules in the oral mucosa, heart, liver, kidney, intestine, and visceral fat. Histologically, the tumor consisted of densely packed spindle-shaped cells with brow intracytoplasmic pigment that exhibited white-blue birefringence with polarized light. Immunohistochemical staining was negative for S-100 and weakly positive with melan A. Electron microscopic examination revealed cytoplasmic irregular and oblong empty spaces, laminated and often arranged into short stacks, compatible with reflecting platelet profiles typically seen in iridophores. However, in unstained ultrathin sections, electron-dense crystalline material was present, which filled the empty spaces described for stained sections before. Based on histology, immunohistochemistry, and biologic behavior, a malignant iridophoroma was diagnosed. To the authors' knowledge, iridophoromas in lizards have rarely been characterized by using electronic microscopy. Moreover, this is the first description of an iridophoroma in a dwarf bearded dragon.