First reported mammary neuroendocrine tumor described in a mare

A newly published case report is putting a rare equine diagnosis on the radar: mammary neuroendocrine tumor in a mare. The report, published July 3, 2026, in the Journal of Veterinary Diagnostic Investigation, describes a 10-year-old American Paint Horse mare evaluated at the Virginia-Maryland College of Veterinary Medicine for mastitis and a suspected abscess in the right mamma, only for biopsy to reveal an infiltrative neoplasm with neuroendocrine features. The authors state that, to their knowledge, this is the first reported mammary neuroendocrine tumor in a horse. (madbarn.com)

That first-presentation-as-mastitis detail matters because it fits a broader pattern in equine mammary disease. Clinical commentary in Equine Veterinary Education has emphasized that initial distinction between mastitis and mammary neoplasia in mares can be challenging, and review literature has likewise noted overlapping clinical signs between mastitis and mammary tumors. Earlier case reports and reviews also underscore how uncommon equine mammary tumors are overall, which helps explain why inflammatory disease may be the first working diagnosis in practice. (aaep.org)

In this case, histology showed a densely cellular, infiltrative tumor with cells arranged in packets supported by thin stroma. On immunohistochemistry, the neoplastic cells were positive for chromogranin A, had moderate scattered S100 labeling, and were negative for neuron-specific enolase and synaptophysin. They also did not label for cytokeratin, although the authors note that a neuroendocrine carcinoma still cannot be excluded on that basis. Their conclusion was a mammary neuroendocrine tumor, and their practical takeaway was straightforward: this entity should now be considered among differential diagnoses for mammary tumors in mares. (madbarn.com)

The report lands in a field with very little precedent. Published equine mammary tumor literature includes invasive ductal carcinoma, adenocarcinoma, tubulopapillary carcinoma, and metastatic mammary carcinoma, but not, until now, a neuroendocrine mammary tumor in a horse. Older reports also reinforce that equine mammary malignancies can be aggressive and may present late, while more recent commentary points to major knowledge gaps around pathogenesis, prognosis, and even basic epidemiology in mares. (journals.sagepub.com)

Expert reaction specific to this paper appears limited so far, which isn't surprising given how recently it was published. But the surrounding expert commentary is consistent on two points: equine mammary neoplasia is easy to miss early, and tissue diagnosis matters. The 2021 clinical commentary from the University of Cambridge described equine mammary neoplasia as an area full of “known unknowns,” especially around diagnosis and prognosis, while a Canadian case report stressed that confirmation of suspected equine mammary neoplasia in a live animal requires surgical biopsy. (aaep.org)

Why it matters: For veterinary professionals, especially equine ambulatory clinicians, internists, surgeons, and diagnostic pathologists, this case broadens the diagnostic frame for mares with chronic, unusual, or treatment-refractory mammary enlargement. A mare presenting with mastitis-like signs may still have neoplasia, and this report suggests that standard morphology alone may not be enough to classify the lesion without immunohistochemistry. The chromogranin A-positive, synaptophysin-negative pattern in this case is also a useful reminder that marker panels can behave imperfectly in rare tumors and across species, so negative results on some expected markers shouldn't automatically close the case. That may be especially relevant in equine pathology, where species-specific validation of many antibodies remains limited. (madbarn.com)

There are also communication implications for care teams and pet parents. Because mammary neoplasia in mares is rare and can mimic infection, clinicians may need to set expectations early that a seemingly routine mastitis workup could escalate to biopsy, referral, or more advanced pathology. In rural or field settings, that can affect cost, logistics, and timing, particularly when the lesion doesn't respond as expected to empiric treatment. (pmc.ncbi.nlm.nih.gov)

What to watch: The next step for the field is more case accumulation: additional reports could help define biologic behavior, metastatic potential, treatment response, and the most reliable immunohistochemical approach for suspected equine mammary neuroendocrine tumors. Until then, the most practical shift is diagnostic awareness: if a mare's “mastitis” doesn't fit, biopsy sooner rather than later may be the most important lesson from this report. (madbarn.com)