Mammary neuroendocrine tumor in a mare.

Mammary neuroendocrine tumor was diagnosed in a 10-year-old mare presenting with signs of mastitis and a possible abscess. This tumor type is extremely rare in horses and should be considered when evaluating mammary masses in mares.

Case Presentation

• A 10-year-old American Paint Horse mare was brought for evaluation of mastitis and a suspected abscess in the right mammary gland.
• The clinical signs included inflammation consistent with infection or tumor involvement in the mammary tissue.

Diagnostic Approach

• A biopsy of the affected mammary tissue was performed to investigate the underlying cause of the mass and inflammation.
• Histological examination revealed a densely cellular, infiltrative tumor with cells arranged in packets and separated by thin connective tissue (stroma).
• The microscopic features suggested a neoplasm but required further testing to classify the tumor type accurately.

Immunohistochemical Analysis

• The tumor cells were tested using a panel of antibodies typically used to identify neuroendocrine tumors.
• Neoplastic cells did not label for neuron-specific enolase or synaptophysin, common markers for neuroendocrine differentiation.
• The cells strongly labeled positive for chromogranin A, a key neuroendocrine marker confirming the neuroendocrine origin of the tumor.
• Moderate, scattered labeling was observed for S100 protein, which can be expressed by some neuroendocrine cells.
• No immunolabeling was found for cytokeratin, an epithelial marker; however, neuroendocrine carcinomas can sometimes lack this marker, so a carcinoma cannot be fully excluded.

Diagnosis and Significance

• The combination of histology and immunohistochemistry led to the diagnosis of a mammary neuroendocrine tumor.
• Neuroendocrine mammary tumors are extremely rare in horses, and this report is likely the first documented case in a mare.
• The case highlights the importance of considering neuroendocrine tumors in differential diagnoses of mammary masses in mares, especially when typical markers for other tumor types are negative.
• This diagnosis may influence treatment decisions and prognosis for affected horses.

Conclusions

• Mammary neuroendocrine tumors, while well-documented in humans and other species, are virtually unreported in equine medicine.
• Veterinarians should be aware of this rare tumor type when presented with mammary lesions that do not respond to usual treatments for mastitis or abscesses.
• Further research and case reports will be important to better understand the behavior, treatment, and outcomes of equine mammary neuroendocrine tumors.