Mammary Hyperplasia

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Also know as: Fibroadenomatous Hyperplasia — Fibroepithelial Hyperplasia


Mammary hyperplasia is a disorder characterised by rapid and dramatic enlargement of multiple mammary glands in cats.

It typically occurs in young, cycling females but is also occasionally seen in males and spayed females. It has been related to the high levels of progesterone in pregnancy, but can also occur with exogenous progesterone administration. It is a benign condition, but must be differentiated from mammary neoplasia.

Clinical Signs

There is usually rapid onset of signs, usually with enlargement of all the mammary glands and involvement of both epithelial and mesenchymal tissue. The swelling is generally not painful, although secondary infection of the glands may occur. There may then be pain, erythema and ulceration of the glands.

Milk production may occur even though most affected cats are not lactating.


The main aim is to differentiate this condition from mammary neoplasia.

The rapid and dramatic symmetrical enlargement of the glands is usually diagnostic, however it might be useful to confirm this by performing a biopsy and histopathology of the mammary tissue.

Histologically, there is marked proliferation of the intralobular duct system. The branching ducts are lined with epithelial cells often three to four layers thick. There is no evidence of stromal invasion of epithelial cells. This is the appearance of mammary tissue in early pregnancy when it is first coming under the influence of progesterone.

Cytology of mammary secretions can also be performed and this fluid should be void of any neoplastic cells.


The source of the progesterone should be withdrawn if possible and the condition will resolve in a few weeks.

Unspayed females should be spayed. A flank incision may be preferable to avoid the hyperplastic mammary tissue in the midline.

Cats not receiving exogenous progesterone usually have remission within a few weeks.

Other symptomatic treatments include:

Analgesics: if the cat is in pain

Mastectomy: if ulceration of the glands has occurred or if remission is taking time

Prolactin inhibitor: it is not licensed in cats and has many side effects including vomiting and anorexia.

Progesterone Receptor Antagonist: can also be used although it is not licensed and may cause abortion in pregnant queens.

Mammary Hyperplasia Learning Resources
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Norsworthy, G. (2011) The Feline Patient John Wiley and Sons

Moulton, J. (1978) Tumors in domestic animals University of California Press

Arndt, J. Feline Mammary Hyperplasia UC Davis publication

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