Moderate severity

Feline Mammary Hyperplasia (Fibroadenomatous Hyperplasia)

A rapid, benign, hormone-driven swelling of one or more mammary glands, usually in young unspayed queens or cats given progestin hormones. It is not cancer, but the enlarged glands can become huge, uncomfortable and infected.

⚠️ Common symptoms

Sudden, rapid swelling of one or more mammary glandsFirm, sometimes dramatically enlarged breast tissueWarmth, redness or discomfort over the swollen glandsSkin over the glands stretched, ulcerated or weeping in severe casesOccurs typically in young entire queens, sometimes during or after pregnancy or a false pregnancyCan also appear in cats (of either sex) given progestin-type hormone medicationSigns of pain, fever or illness if the tissue becomes infected

Feline mammary hyperplasia — known more formally as fibroadenomatous hyperplasia — is a benign but often dramatic swelling of the mammary glands caused by the hormone progesterone. It is one of those conditions whose appearance is far more frightening than its nature: an owner may notice one or several breasts ballooning in size over just a few days, yet the underlying process is an overgrowth of normal tissue, not a tumour.

The condition is classically seen in young, entire queens, because their bodies produce progesterone naturally after a heat, during pregnancy, or during a false pregnancy. The mammary tissue is exquisitely sensitive to this hormone and responds by proliferating rapidly. The same thing can happen in cats of either sex, including males and neutered cats, when they are given progestin-containing medications — which is an important reason such drugs are used cautiously.

Although it is not cancer, the condition should never be ignored. The glands can grow so large and firm that the skin stretched over them splits, ulcerates and becomes infected, and a severely affected cat can become genuinely unwell. This is also why veterinary assessment matters: a rapidly enlarging mammary mass in a cat always needs proper evaluation to distinguish this benign, treatable condition from mammary cancer, which is far more common in older cats and behaves very differently.

The good news is that treatment is usually straightforward and effective. Removing the hormonal drive — by spaying an entire queen, or by stopping a progestin medication — allows the swollen tissue to regress, often over several weeks, sometimes with the help of anti-progesterone medication and supportive care for the skin. Spaying is also the best way to prevent it happening in the first place.

🔍 Causes

Feline mammary hyperplasia is an excessive but benign growth of mammary tissue driven by the hormone progesterone. It is seen most often in young, entire (unspayed) female cats, in whom natural progesterone rises after a heat cycle, during pregnancy or during a false pregnancy. It can also develop in cats of either sex, including neutered cats and males, that have been given progestin-type hormone drugs (for example some medications used to suppress heat or treat skin conditions). The mammary tissue responds to the hormonal signal by proliferating rapidly, which is why the swelling can appear so suddenly and grow so large. Crucially, despite its alarming appearance, it is not a cancer.

🛡 Prevention

The most effective preventive step is neutering female cats, which removes the natural hormonal cycle that drives the condition and avoids the surges of progesterone that trigger it. Where hormone medications containing progestins are used, they should only be given under veterinary direction and at the lowest effective dose, as they are a well-recognised trigger. Being alert to sudden mammary swelling in a young unspayed queen, or in any cat receiving hormone treatment, allows early veterinary assessment before the glands become severely enlarged or ulcerated.

💊 Treatment

Treatment depends on the cause and severity. Because the condition is driven by the hormone progesterone, the mainstay of treatment in an entire queen is spaying, which removes the ovarian source of the hormone and usually leads the swelling to regress over several weeks. If the condition was triggered by a progestin medication, that drug is stopped under veterinary guidance. Vets may also use specific anti-progesterone medications to speed regression. Supportive care is important when the glands are very enlarged: pain relief, careful cleaning, and antibiotics if the stretched skin has broken down and become infected. Surgery to remove tissue is usually avoided unless there is severe, non-healing ulceration, as the glands typically shrink once the hormonal drive is removed.

When to see a vet: Contact your vet promptly if you notice sudden swelling of one or more mammary glands, particularly in a young unspayed female cat or any cat receiving hormone medication. Although the condition is benign, the glands can enlarge dramatically and the overlying skin can ulcerate and become infected, so it needs veterinary assessment to confirm the diagnosis, rule out mammary cancer, and start treatment. Seek urgent care if the swollen tissue is ulcerated, very painful, or your cat is feverish, off its food or unwell.

⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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