A hard lump in a cat's tail may be caused by a break in one of the many tail bones, an abscess, tumour or cyst. Any of these conditions will require an accurate diagnosis by a licensed veterinarian. Once you and your veterinarian determine the cause of the lump, you can treat the problem.
Cat tails get a lot of action and attention. Since a happy cat usually carries its tail high, the tail tends to get stroked, pulled by infants and small children, and tugged or chewed on by other animal playmates. It's also the last part of the cat to leave a room and can be accidentally closed in doors or windows.
A hard lump on a cat tail may indicate a newly broken bone, or an old fracture that has healed on its own. If your cat has broken its tail near the base, or closest to the spine, it may have difficulty lifting it properly and may drag it. Using the litter box could also be painful for the cat. Cats with a newly broken tail may not only exhibit pain at the injury site, but can become constipated and have urinary tract infections due to pain the tail causes when they try to eliminate. A newly broken tail will be painful to when you touch it. A lump left over where an old fracture has healed is typically not painful.
Abscess and Wounds
Cats that spend any time outdoors or live in a house with multiple cats or other pets often get bitten on the tail. Bites may occur as a result of rough play, territorial disputes or battles of unknown origin. A bite, or more to the point a puncture wound resulting from a bite, can close over, trapping material inside the wound which then becomes infected and swells to a hard or soft mass. These abscesses can occur on any part of your cat's body and will need veterinary care to resolve. The veterinarian will usually open and drain the abscess as part of the treatment process.
Injuries can also cause swelling of blood under the skin, or hematomas, where the injury occurred. Since the tail has little in the way of fat, especially near the tip, these bruises may feel raised and hard. Lumps caused by injury will typically be painful when touched.
A hard lump could also be a tumour. The texture of a tumour or cyst can depend on its cause. Tumours can be benign or malignant and can appear any place on your cat's body. Since the tail has little in the way of fat, a hard lump that is diagnosed as a tumour can be growing on the tail bones, skin or other soft tissue. If your veterinarian suspects that the lump is a tumour, he may perform tests to diagnose the exact cause. Some tumours resolve with basic treatment, while others may need to be surgically removed.