Clinicopathological findings of sustentacular cell tumors in 13 singing tournament passerine birds

Testicular tumors are rarely reported in passerine birds and are typically described as isolated cases. This study characterizes the clinical, pathological, immunohistochemical, and epidemiological features of 13 testicular neoplasms diagnosed in ultramarine grosbeaks ( Cyanoloxia brissonii; n = 12) and a green-winged saltator ( Saltator similis; n = 1), 2 South American passerine species commonly used in singing competitions. Clinical signs included coelomic distension, respiratory distress, pododermatitis, feather loss, cloacal prolapse, and decreased vocalization. Radiography demonstrated displacement of coelomic organs and, in some cases, markedly reduced gastrointestinal transit associated with tumor compression. Grossly, tumors were large, pale tan to yellow, frequently multilobulated, and caused compression of adjacent viscera. Histologically, all neoplasms were classified as sustentacular cell tumors (SCTs) based on established histomorphological criteria adapted for avian testes. Most tumors exhibited a tubular growth pattern (77%), whereas diffuse (15%) and mixed-type (7%) patterns were less common. Rare Call-Exner-like bodies and extensive contralateral testicular atrophy were observed. Immunohistochemically, KIT (CD117) and neuron-specific enolase were the most consistently expressed markers, whereas melan-A, calretinin, and GATA-4 showed sporadic labeling and inhibin-α, vimentin, and pancytokeratin were consistently negative. Liver metastases occurred in 2 ultramarine grosbeaks, demonstrating the malignant potential of SCTs in passerines. This study represents the largest series of testicular tumors reported in passerine birds and provides the first evidence that participation in singing competitions is significantly associated with the development of SCTs in ultramarine grosbeaks. Our results underscore the importance of including testicular neoplasia as a differential diagnosis in cases of coelomic distension in passerines.