Feline Mammary Hyperplasia: Telling It Apart from…

A young queen whose mammary glands swell to 1.5 cm or to 18 cm within two to three weeks of oestrus usually has mammary hyperplasia, a benign progesterone-driven proliferation. After ovariohysterectomy it remits completely in 92% to 94% of cases, but full resolution takes a median of about 49 days, with a range of 28 to 180. The urgent part is not the swelling itself; it is proving that the swelling is not mastitis or a mammary carcinoma, because 80% of mammary masses in cats are malignant.

What the swelling actually is

Feline mammary hyperplasia — also called fibroepithelial or fibroadenomatous hyperplasia — is a rapid, benign proliferation of glandular epithelium and surrounding stroma, not a tumour and not an infection. Progesterone receptors are present in the tissue, and progesterone or synthetic progestins drive the growth, probably through local growth hormone and IGF-1 production. Two forms are described, lobular and fibroepithelial, and both can affect one gland or the whole chain. Histology shows no inflammation and no necrosis, which is exactly what separates it from mastitis and neoplasia on a biopsy.

When it appears

The classic patient is a sexually intact female under two years old, two to three weeks after oestrus or a pseudopregnancy, when the corpus luteum is producing progesterone. Pregnant queens can develop it in late gestation. Cats of either sex given long-acting progestins such as medroxyprogesterone acetate or megestrol acetate can develop it too — in that group the interval from injection to first signs is reported at 7 to 30 days. There is no breed predisposition, and the onset is acute: owners typically notice a change over a few days, not months.

Three findings that separate it from mastitis and cancer

Mammary hyperplasia is diffuse and symmetrical, involves most or all glands, and feels firm rather than hot. Mastitis is usually seen in a lactating queen, is painful and warm, often affects one gland, and comes with fever and purulent discharge. Mammary carcinoma is a discrete, hard nodule that has been present for weeks and does not shrink when progesterone is withdrawn. No milk is produced by hyperplastic glands, even in a pregnant cat.

Feature	Mammary hyperplasia	Acute mastitis	Mammary carcinoma
Typical patient	Intact queen under 2 years, or any cat on progestins	Lactating queen, first weeks postpartum	Middle-aged to older queen
Onset	Days, 2 to 3 weeks after oestrus	Days, with fever	Weeks to months
Distribution	Diffuse, symmetrical, most glands, 1.5 to 18 cm	Usually one gland, warm and painful	Single firm nodule, irregular
Discharge	None; no milk	Purulent or serous	Variable, often none
Response to spaying	Regresses in 92 to 94%	Persists until treated	No regression

What severe cases look like

Large glands stretch the skin until it ulcerates or becomes necrotic, and secondary infection follows. Oedema of the ventral abdomen and both hind legs is common and makes the cat look far worse than the histology suggests. Cats with necrotic areas can become febrile, anaemic, tachycardic and anorexic. Because the glands may measure 18 cm across, a simple physical examination is often enough to decide that this is not a small nodule to watch — but a fine-needle aspirate or biopsy is still what confirms the diagnosis before any decision about breeding is made.

Removing the progesterone source

Spaying is the definitive treatment when the queen is not intended for breeding, and a flank approach is preferred because the enlarged glands make a midline incision awkward. In a series of 79 cats treated by ovariohysterectomy, total remission was recorded in 73 (92.4%); a larger review quotes 94%. Regression usually begins within three to four weeks and complete remission is reached in 28 to 180 days, with a median around 49 days in cats whose hyperplasia came from endogenous progesterone. A residual pad of connective tissue often remains after the swelling collapses, and oedema can take several more weeks to resolve, so apparent "persistent growth" at three weeks is not automatically a failure.

Antiprogestin when surgery is not the first choice

Aglepristone, a progesterone receptor blocker, is the medical option for queens that are to be bred, or when growth continues after surgery. Reported success is close to 100%, with complete resolution averaging 3.9 weeks and ranging from 1 to 11 weeks; the dose and schedule are set by the veterinarian, typically repeated weekly until the glands are normal. Regression can start within days of the first injection. Side effects are usually limited to transient discomfort at the injection site, and queens treated this way have gone on to conceive and deliver normal litters — but recurrence is well documented, especially in cats previously given long-acting progestins.

Progestin-triggered cases behave differently

When the trigger was an injected progestin rather than the cat's own corpus luteum, the disease is more stubborn. In the 79-cat series, persistent mammary growth after spaying occurred in 19 cats (24%), and all but one of those had received injectable progestin. Remission took 99 days in the progestin group versus 49 days in the endogenous group, and all six deaths in the series occurred in progestin-treated cats. If a cat has had megestrol acetate or medroxyprogesterone, plan for a longer course, add antiprogestin early rather than waiting, and warn the owner that five weeks of treatment may be needed.

Recurrence and the breeding question

Hyperplasia that is treated but leaves the ovaries in place can return in a later oestrus or pregnancy, and one case report describes recurrence during gestation in an 11-month-old Maine Coon that had responded to aglepristone the first time. Queens that have had the disease are generally not recommended for breeding, and a cat that develops it while pregnant may not be able to lactate for her litter, so hand-rearing supplies should be ready. Spaying after regression removes the recurrence risk entirely.

What to do in the next 24 hours

Photograph the glands and note the date of the last oestrus — the two-to-three-week interval is diagnostic information.
Check for fever, pain on touch, purulent discharge, skin ulceration and appetite loss; any of these moves the case from "monitor" to "seen today".
Do not apply heat, do not express the glands, and do not start antibiotics without a diagnosis; mastitis treatment and hyperplasia treatment point in opposite directions.
Ask for cytology or biopsy before agreeing to any surgery beyond spaying, because 80% of feline mammary masses are not benign.

For the two conditions this one is most often confused with, see mastitis in cats and mammary tumours.

This article does not replace veterinary diagnosis. Any mammary swelling in a cat needs an examination, and drug protocols must be set by the treating veterinarian.

Frequently asked

How long does feline mammary hyperplasia take to resolve after spaying?

Regression usually starts within three to four weeks, but complete remission takes longer: in one series of 79 cats the median was about 49 days, with a range of 28 to 180 days, and total remission was recorded in 92.4% of cats. Cats previously given injectable progestins took about twice as long, 99 days versus 49. A residual pad of tissue and ventral oedema can persist for several more weeks after the swelling collapses.

How can I tell mammary hyperplasia from mastitis in my cat?

Hyperplasia is diffuse and symmetrical, affects most or all glands two to three weeks after oestrus, feels firm but not hot, produces no milk and usually comes without fever. Mastitis is typically seen in a lactating queen, is warm and painful, often affects one gland, and comes with fever and purulent discharge. Cytology or biopsy settles it when the picture is mixed.

Can a cat with mammary hyperplasia be bred later?

It is possible — queens treated with aglepristone have gone on to conceive and deliver normal litters — but recurrence is documented in later oestrus cycles and during pregnancy, and one case recurred during gestation after a first successful treatment. Most clinicians advise against breeding a queen with a history of the disease, and a cat that develops it while pregnant may be unable to nurse her litter.

Feline Mammary Hyperplasia: Telling It Apart from Mastitis and Cancer