Mammary Gland Tumors in Mares

Mammary neoplasia (tumors) in the mare is exceedingly rare. To date, published cases consist of reports of six single cases and one report each for two, three, and four mares. Of published cases, all tumors were malignant except one report of a benign (noncancerous) adenoma. Previous abattoir studies report an incidence of 0.11% to 1.99% for equine mammary tumors.

Published reports of mammary neoplasia in mares have several characteristics in common. Mares presented with unilaterally or bilaterally enlarged mammary glands that were firm, usually painful, and typically ulcerated and/or draining serosanguinous or purulent material. Clinical signs progressed over weeks to months. Pregnancy and lactational status varied; age ranged from 12 to 21 years. Initial diagnosis of mastitis and treatment with antibiotics and anti-inflammatory agents initially resulted in some improvement of discharge and swelling, but clinical signs always recurred.

At the University of Kentucky Livestock Disease Diagnostic Center from 1994 to March 2008, 11 cases of mammary neoplasia were diagnosed. Most cases were submitted for necropsy; a small number were biopsy samples. All eleven cases were malignant, and complications resulting from primary neoplasia or metastasis were the cause of death or euthanasia in most cases. Often there was a history of weight loss, depression, and laboratory results indicating other organs were affected by neoplasia, such as the liver and kidneys. More than half of the cases had gross and/or microscopic evidence of metastasis such as lymph node enlargement or neoplastic nodules in other organs. Based on these results, it appears that equine mammary neoplasms are much more likely to be malignant than benign, and hence carry a poor prognosis for long-term survival.

Definitive diagnosis of mammary neoplasia is made with microscopic examination of excised mammary tissue. Cytologic examination of fine needle aspirates and/or mammary discharge is usually unrewarding in terms of confirming or ruling out neoplasia. Most neoplastic lesions have an inflammatory component, especially with chronic and/or ulcerated masses. For that reason, observation of large numbers of inflammatory cells and bacteria does not help to differentiate between neoplasia with an inflammatory response and true inflammation of the mammary gland. A core or excisional biopsy taken from affected tissue some distance away from sites of ulceration and drainage is recommended. Samples taken from ulcerated tissue may show only inflammation and necrosis and not contain enough neoplastic tissue for definitive diagnosis.

The treatment of choice for mammary neoplasia in the mare is total removal of mammary tissue and any accessible lymph nodes. Disease may initially appear unilaterally but subsequently become bilateral or show evidence of bilateral disease grossly or microscopically. Unfortunately, by the time a definitive diagnosis is made and surgery is performed, metastasis to regional and distant lymph nodes and tissues has most likely occurred. Most mares do well immediately after surgery then have recurrence of neoplasia at the excision site or evidence of metastasis such as lymph node enlargement, weight loss, and malaise. Mares are often euthanized due to poor prognosis or deterioration due to metastases. In a mare with enlarged mammary glands and poor response to antibiotic therapy, neoplasia should be considered a differential diagnosis.

Contact: Dr. Maria Shank, 859/253-0571; Livestock Disease Diagnostic Center, University of Kentucky, Lexington, Ky.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents. More articles from Equine Disease Quarterly...

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