The patient was a two-year-old male budgerigar (Melopsittacus undulatus) weighing just 32 grams, referred to a specialized veterinary hospital because of a substantial swelling in the medial region of the left leg. Because small birds cannot easily undergo the extensive imaging and biopsy protocols used in mammals, the veterinary team first attempted to rule out the most common alternative diagnosis: an infectious inflammatory lesion. The bird was treated with enrofloxacin at 150 mg/L in drinking water every 24 hours for 10 days, alongside meloxicam at 0.5 mg/kg orally every 24 hours for 3 days. The mass continued to grow steadily despite the therapy, and the bird became profoundly lethargic and deteriorated physically. Given the absence of any response to antibiotics or anti-inflammatory drugs and the aggressive growth pattern of the swelling, the team concluded the lesion was neoplastic. With the owner’s consent, the bird was euthanized and a complete necropsy was performed.

The gross findings alone were striking. At the medial aspect of the left leg sat a roughly spherical tumour approximately 3 cm in diameter—remarkable in an animal whose entire body weight was barely more than the mass itself—with a soft consistency and a cream-coloured cut surface. Tissue samples from the mass and all major organs were fixed in 10% neutral buffered formalin and processed for histopathological examination. Under the microscope, the tumour was composed of mildly pleomorphic spindle cells with the blunt-ended, cigar-shaped nuclei that are the histological signature of smooth muscle differentiation. The cells were arranged in intersecting fascicles and showed only mild atypia, but the mitotic count told a far more ominous story: 65 mitotic figures per 10 high-power fields, a proliferation rate consistent with a high-grade malignancy. In some sections, tumour cells were seen invading the adjacent femur, and the kidney contained clear metastatic deposits.

Because spindle-cell sarcomas—fibrosarcoma, histiocytic sarcoma, and even schwannoma—can look nearly identical under routine haematoxylin and eosin staining, the researchers turned to immunohistochemistry to establish the tumour’s cell of origin. The neoplastic cells stained positively for desmin and smooth muscle actin (SMA), the two most reliable markers of smooth muscle differentiation, but were negative for vimentin, an intermediate filament protein widely used as a general mesenchymal marker. That combination clinched the diagnosis of leiomyosarcoma. The vimentin negativity is itself noteworthy: vimentin is commonly expressed in mesenchymal cells and its intensity often correlates with malignancy, yet studies of avian leiomyosarcomas show that expression can vary considerably, with some cases weak or negative, likely reflecting the tumour’s degree of differentiation or species-level differences in protein expression. When interpreted alongside SMA and desmin, the panel definitively separated this tumour from other spindle-shaped neoplasms.

Leiomyosarcoma is a rare malignancy in pet birds, arising from smooth muscle wherever it exists—gastrointestinal walls, uterus, blood vessels, spleen—and most commonly reported in the spleen, gastrointestinal system and reproductive tract. Sporadic cases have been described in cockatoos, African grey parrots and, particularly, budgerigars. Neoplasia overall is disproportionately reported in captive and companion birds compared with free-ranging ones, a pattern attributed to closer observation, longer lifespans, and possible genetic predisposition from inbreeding. Budgerigars stand out even among psittacines: retrospective data suggest an incidence of neoplasia in budgerigars ranging from 16.8% to 24.2%, and in one veterinary laboratory with a diverse avian caseload, budgerigars accounted for 69.7% of psittacine neoplasms and 41% of all avian neoplasms. Advances in breeding, husbandry and veterinary care have extended the lifespans of domestic birds, which paradoxically increases the prevalence of age-related disorders including cancer.

What makes this case exceptional is the metastatic pattern. In birds, leiomyosarcomas generally metastasize rarely, and when spread does occur it most often involves the liver, lungs, kidneys or spleen. Bone metastasis is exceedingly unusual; in human medicine, the most common distant sites for leiomyosarcoma are lung, liver and bone, with the kidney reported far less frequently as either a primary or secondary site. The authors believe this is the first documented case of leiomyosarcoma in a budgerigar with metastases to both bone and kidney. Direct local invasion of the femur by the leg mass is biologically plausible—aggressive soft-tissue sarcomas commonly erode adjacent bone—but the renal metastasis implies haematogenous dissemination, raising intriguing questions about avian circulatory anatomy.

Indeed, the authors emphasize that avian metastatic behaviour cannot be assumed to mirror mammalian patterns. Beyond the systemic circulation, birds possess specific anatomical pathways that can route tumour cells in unexpected directions, most notably the renal-portal shunt, which directs blood from the hindlimbs to the liver before it enters general circulation; this portal system has been implicated in metastatic deposition patterns in other avian cases, including a sarus crane in which a myxoid leiomyosarcoma spread via renal and hepatic portal circulation. Moreover, birds lack large, organized lymph nodes, diminishing their lymphatic filtering capacity and potentially reshaping how disseminated tumour cells distribute through the body. Some studies further suggest that poorly differentiated leiomyosarcomas, particularly those with myxoid or dedifferentiated features, are more prone to metastasize—a tendency linked to genetic alterations and cellular transdifferentiation that enhance invasiveness. Metastasis in avian leiomyosarcoma typically manifests late, after substantial local tumour progression, which underscores why early diagnosis and prompt intervention are critical for limiting disease spread.

The clinical lessons extend beyond oncology. The diagnostic workup in this case illustrates a common trap in avian medicine: soft-tissue masses in small birds are often first assumed to be abscesses or granulomas, and empirical antibiotic therapy is frequently the initial response. The failure of antimicrobial and anti-inflammatory treatment, combined with relentless growth, was the pivotal clue that a neoplasm was present. For a species in which surgical, chemotherapeutic and radiotherapeutic options remain limited and refined, and in which a 3 cm tumour on a 32 g patient is essentially unresectable, the prognosis at presentation is typically grave. The authors argue that earlier recognition—prompted by vigilance for non-responsive swellings and willingness to pursue biopsy—may reduce metastatic risk and lower mortality, although they caution firmly that conclusions drawn from a single case cannot be generalized.

The report also situates itself within a growing catalogue of avian leiomyosarcoma cases assembled through a comparative literature review. Prior documented cases in budgerigars include a beak-associated leiomyosarcoma positive for α-SMA, a caudal abdominal tumour positive for desmin and SMA, a cutaneous wing tumour with bone metastasis and osteoid metaplasia positive for desmin, SMA and vimentin, and a hind-limb tumour with disseminated metastasis to spleen, lung, kidney and liver. Related species add further datapoints: a cockatiel with a myxoid oviductal leiomyosarcoma that was vimentin-negative, a zebra finch with an intestinal tumour, pigeons with pulmonary, ileal and disseminated cutaneous forms, an African goose with metastases to kidney, brain and liver, and a broiler chicken with gizzard involvement at just 57 days of age. The immunohistochemical profiles across these cases vary—vimentin is positive in some and negative in others, while desmin and α-SMA positivity recur almost universally—reinforcing the view that a panel approach, rather than reliance on any single marker, is essential for definitive diagnosis in poorly differentiated avian spindle-cell tumours.

The team behind the report—based at Shahid Bahonar University of Kerman in collaboration with Ashian Veterinary Hospital and a clinical pathology laboratory—acknowledges the inherent constraints of a single-case study and calls for research on larger groups of budgerigars to clarify the epidemiology, risk factors and clinical features of leiomyosarcoma in these birds. For now, the case stands as a vivid reminder that even the smallest companion animals can develop malignancies with the same aggressive biology, the same diagnostic complexity, and the same capacity for distant spread as those seen in human medicine—and that the tools of modern pathology, from mitotic counting to immunoperoxidase staining, are as informative in a 32-gram parrot as they are in any mammal.