What Is Feline Mammary Hyperplasia?
Feline mammary hyperplasia, also known as feline mammary fibroadenomatous change or mammary fibroadenoma complex, is a benign but potentially alarming condition in which the mammary glands undergo rapid, exaggerated growth. Unlike mammary tumours, this condition is not cancerous, but the enlargement can be dramatic enough to cause significant discomfort, skin complications, and considerable anxiety for owners.
The condition is closely tied to progesterone exposure. Understanding this link is key to both diagnosing the cause and choosing the most effective treatment.
The Role of Progesterone
Progesterone exerts a powerful stimulatory effect on mammary gland tissue in cats. Unlike in dogs, where oestrogen is the primary driver of mammary development, the feline mammary gland is acutely sensitive to progesterone. When progesterone levels are elevated — whether naturally or through exogenous administration — the mammary glands respond with rapid proliferation of both glandular and fibrous tissue.
This proliferative response is mediated through progesterone receptors within mammary tissue, and is potentiated by locally produced growth hormone. The mammary glands produce their own growth hormone under progesterone stimulation, creating a local hormonal environment that drives disproportionate tissue growth.
The condition predominantly affects young, intact female cats, typically occurring during the luteal phase of the reproductive cycle (the progesterone-dominant phase following ovulation) or during pregnancy. However, it can also affect male cats and neutered females when exposed to exogenous progestins.
Exogenous Progesterone as a Cause
One of the most common causes of mammary hyperplasia in veterinary practice — particularly in male cats and neutered females — is the administration of synthetic progestins. These drugs are used in cats for various purposes, including oestrus suppression, management of skin conditions, and behavioural issues such as urine spraying.
The synthetic progestins most commonly implicated include:
- Medroxyprogesterone acetate (MPA)
- Megestrol acetate
- Proligestone
Depot (long-acting injectable) forms are particularly associated with mammary hyperplasia because they deliver sustained high levels of progestins over weeks to months. Any cat receiving these medications should be monitored carefully for mammary changes, and the risk-benefit balance of progestin use should be carefully considered.
Clinical Presentation
The hallmark of feline mammary hyperplasia is rapid, often dramatic enlargement of one or more mammary glands. The change can develop over just a few days to weeks. Affected glands may become very large relative to the cat's body size, sometimes involving multiple gland pairs.
The enlarged tissue is typically firm and non-painful on palpation in mild cases, though significant enlargement can cause:
- Discomfort or pain due to tissue tension and weight
- Skin ulceration, necrosis, or secondary infection where the overlying skin becomes stretched and compromised
- Restricted movement in severely affected cats
- Self-trauma from grooming or discomfort
The glands do not typically produce milk unless the cat is pregnant or pseudopregnant. There is no systemic illness in uncomplicated cases, though secondary infections can cause fever and malaise.
Distinguishing Hyperplasia from Mammary Tumours
Feline mammary tumours are unfortunately common — approximately 85 to 90% of mammary tumours in cats are malignant — so distinguishing benign hyperplasia from a malignant lesion is critically important. This cannot be done reliably on clinical examination alone.
Key points of differentiation include:
- Age: hyperplasia is more common in young cats (under three years), while tumours more often affect middle-aged to older cats
- Context: recent progestin exposure or an intact cycling female strongly supports hyperplasia
- Speed of development: hyperplasia often grows very rapidly; tumour growth may be more gradual (though not always)
- Consistency: hyperplasia tends to produce firm, rubbery tissue; malignant tumours may be irregular, ulcerated, or adherent to underlying tissue
Fine needle aspirate (FNA) cytology can be informative but is not always definitive. Histopathological examination of a biopsy sample is the gold standard for diagnosis. Any suspected mammary mass in a cat warrants veterinary investigation without delay.
Treatment Options
The treatment approach depends on the underlying cause.
Withdrawal of Exogenous Progestins
When hyperplasia is linked to synthetic progestin administration, stopping the drug is the most important first step. In many cases, gradual regression of the mammary tissue occurs over weeks to months following cessation. However, depot forms cannot be instantly reversed, so the clinical team must monitor the cat closely during the regression period and address any secondary complications.
Aglepristone
Aglepristone is a progesterone receptor antagonist (anti-progestin) licensed for use in cats in several European countries, including the UK. It competitively blocks progesterone receptors, effectively halting the hormonal drive behind tissue proliferation. It is considered the medical treatment of choice for naturally occurring hyperplasia in intact cycling cats, and has also been used to accelerate regression in progestin-induced cases.
Ovariohysterectomy
In intact cats, spaying removes the source of endogenous progesterone and is typically curative. It is often combined with aglepristone treatment to initiate tissue regression before surgery. Spaying at the time of mammary hyperplasia is not always straightforward due to the hormonal status of the cat, so timing is discussed on a case-by-case basis with the attending vet.
Surgical Debulking
In severe cases where skin necrosis or severe discomfort is present, surgical reduction of mammary tissue may be required as a welfare measure, even before hormonal stabilisation is achieved. This is a more complex procedure and carries risks in hormonally active cats.
Long-Term Considerations
Cats that have experienced mammary hyperplasia are at risk of recurrence if exposed to progesterone again — either through a subsequent cycle, pregnancy, or repeat progestin administration. Spaying is strongly recommended for intact cats following resolution of the acute episode. Future progestin use should be avoided in all affected cats where possible, with alternative treatments sought for any underlying condition that was previously managed with synthetic progestins.
While the condition itself is benign, its potential to cause significant physical harm through skin breakdown and secondary infection means that veterinary management and close monitoring are essential throughout the course of the condition.