Malignant Mixed Irido‐Melanophoroma in a <i>Betta splendens</i> with Concurrent <i>Mycobacterium chelonae</i> Isolation—A Case Report

A 10-month-old intact male blue Siamese fighting fish (Betta splendens) was presented to the Purdue University Small Animal Hospital with multiple bilateral masses on the caudal flank. Comprehensive evaluation using impression cytology, histopathology and transmission electron microscopy (TEM) revealed a tumour composed of a mixed-cell population of iridophores and melanophores, consistent with a mixed-cell irido-melanophoroma. The tumour exhibited clinical and cytohistological characteristics of malignancy, including rapid enlargement, cellular and nuclear atypia, tissue invasion and tumour emboli formation. Interestingly, mild mononuclear inflammation was observed, and mycobacterial culture isolated an acid-fast-positive bacterium identified as Mycobacterium chelonae through Sanger sequencing of rpoB and 16S rRNA, combined with Basic Local Alignment Search Tool analysis. This is the first case report of a malignant piscine chromatophoroma with concurrent inflammation and isolation of Mycobacterium, highlighting the need for further studies to investigate a potential association.