Horse Health: Mammary Gland Tumors in Mares
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.
By Dr. Maria Shank, Livestock Disease Diagnostic Center - University of Kentucky
Published on Equine Disease Quarterly newsletter (July 2008 - Volume 17, Number 3)
Maxwell H. Gluck Equine Research Center
Department of Veterinary Science, University of Kentucky
My mare is 14 yrs old 1 year ago she presented with symptoms of mastitis (bacterial infection) and clear draining fluid but she wasn't in foal or had a foal at foot the mammary gland was not painful or ulcerated in any way. She was treated with anti-biotics but these did not clear her symptoms. A lump developed in one side of her mammary gland lab tests were done on the fluid again which was draining from the teat and came back only as bacterial infection. She developed a temperature and became depressed she was put on potassium iodide in preparation for an operation to remove the suspected granuloma the potassium iodide seemed to relieve her of infection better than anti-biotics, she was taken to theatre and the mass was removed which was full of nasty liquid which was tested and this came back with possibility of neoplasia (the mass was full of sulphites), the central part of the mass was tested and this came back as an aggressive neoplasia. We are now 3 months on and the operation site is not healing fully as it was left open to drain there is now a mass which can be felt unsure if this is scar tissue or tumour which has returned. She has had to have antibiotics several times and is now kept on potassium Iodide as this seems to be the only effective treatment which keeps her happy. My mare has now developed a thirst but other than this she is maintaining her weight, eating well, shiney coat and bright in her eyes if not quiet for her but still has enthusiasm.. She is a beautiful mare and advice is that she will lose condition when the neoplasia metastisies however the tumour which was removed was the size of a melon and externally had no effect on her she maintained her weight, shiney coat, ate well only her personaity changed and now I feel I have got used to her new personaility and have to remind myself of the horse she was although she is still a horse who knows what she wants and is still stroppy so hasn't completely lost her personality. Unfortunately as this is a rare cancer in horses prognosis is difficult I wish I had a crystal ball as I only want to do the best for my mare. Studies in to this dreadful desease would help.
