What this means
A cat's fur hides a lot, so a lump or swelling is often found by chance while stroking or brushing. It may be soft or hard, loose or fixed, single or several. None of these features tells you what it is, and even experienced people cannot tell a harmless lump from a dangerous one by feel.
After an injury a swelling can appear at once or a few days later. After a fight or a bite the wound in the fur is barely visible, but a painful, hot abscess can form under the skin. A cat in pain does not always say so loudly: it may hide, stop eating, move less or not want to be touched.
That is why the history of the lump matters: when it appeared, whether it is growing, whether it is changing shape or colour, whether the skin over it has broken or is wet. Note it down and show it to a vet, but do not decide for yourself that it is nothing serious.
Possible causes
These are things that can cause this sign. It is not a diagnosis, and they are not equally likely for your cat.
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An abscess after a bite or scratch
After a fight or bite, a painful, warm swelling can form under the skin. It can burst and leak fluid.
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A bruise or a pocket of blood after a knock
After a fall or blow, blood can collect under the skin or in the ear flap and cause a swelling. An injury can also have damaged something deeper that you cannot see.
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A benign or malignant skin growth
A growth in or under the skin may be harmless, but it may also be cancer. Only a vet, with tests, can tell the two apart.
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A cyst or a blocked gland
A lump filled with fluid or secretions can form in the skin. A vet has to confirm that it is that and not something else.
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A growth in the mammary glands
Cats, especially unneutered or older ones, can develop firm lumps along the underside of the belly. A vet should always examine such a lump.
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A hernia or a collection of fluid
A soft swelling on the belly or in the groin can be tissue pushing through a weak spot in the muscle wall, or fluid that has collected. This too needs a vet's assessment.
What else to watch for
These observations help your vet build the picture faster. Note down what you saw and when.
- Where the lump is, how big it is and when you first noticed it
- Whether it is growing, changing shape or colour, or not shrinking
- Whether it is hot or painful, or your cat does not want it touched
- Whether it is leaking fluid or pus, or the skin has broken
- Whether your cat licks the spot, limps or hides
- Whether your cat is eating as usual and acting like herself
When to book a routine visit
Show a vet any new lump, and any lump that is growing, changing or not shrinking, because you cannot tell from outside what it is. If a swelling is hot, painful or oozing, or your cat stops eating or hides after an injury, go the same day.
Related conditions
Conditions whose entries mention this sign. The list is somewhere to read on, not a way to reach a diagnosis.
Body systems: General health