Your cat expresses his emotions through his tail. Just by looking at that appendage, you know if he's happy, annoyed, or frightened. If your cat has a bump on his tail that isn't usually there, observe him for symptoms that can give a clue to the nature of his injury and call your veterinarian.
Beware of a broken tail
If your cat has a bump on its tail, especially near the tip, it can mean that he broke it at one time. While you'd likely be aware of such an incident in a pet you had since kittenhood, that might not be the case in a cat who came into your life when full-grown. The lump that exists after a fracture could have resulted because the cat continued to move his tail during the healing process.
There's no real treatment for minor tail fractures — they heal on their own. More serious tail fractures can sometimes require amputation according to Animal Medical Center.
Consider a cyst or tumor
A painful lump at the base of a cat's tail could be a tumor, either benign or malignant. Your vet can't tell whether or not a tumor is potentially deadly just by looking at it, so she'll either remove the entire tumor if it's small or take a sample from a larger tumor and send specimens for testing. Samples from a large tumor are usually retrieved via fine needle aspiration, which doesn't require anesthesia.
If the tumor is benign, no more treatment is necessary, unless it's a sebaceous cyst. That's an encapsulated tumor filled with sebum — an oily substance produced by the skin's sebaceous glands — which your vet can surgically remove. If a tail tumor does turn out to be cancer, your vet might recommend tail amputation.
A large, painful swelling on your cat's tail could indicate an abscess. Outdoor tomcats are most likely to suffer from tail abscesses since most abscesses result from fighting. The tail area is a common site for bites, as the dominant tomcat might inflict a bite as the other cat flees.
Bacteria from a bite causes the site to fill with pus. Within a few days, you'll notice hair loss around the swelling. If your cat hasn't already gone to the vet to have the abscess lanced, it could break at home, releasing large amounts of blood and pus.
Your vet will prescribe antibiotics to combat infection and instruct you as to dressing the wound. Large abscesses could require the vet to drain them.
Recognize territorial overmarking
Although tom cats primarily develop stud tail — hence the name — neutered males and spayed females might also come down with this malady. The condition is caused by cats marking territory by repeatedly rubbing objects with a gland located at the base of the tail.
Technically known as supracaudal gland hyperplasia, overactivity of the gland — such as by unneutered males marking out territory — stud tail causes blocked hair follicles from the overproduction of sebum. The blocked follicles can cause matted hair, pustules, and bacterial infections according to International Cat Care.
Your vet will clip off the matted hair and recommend degreasing shampoo to get rid of the excess oil, caused by sebaceous gland overactivity. If an infection exists, she might prescribe antibiotics. If your cat isn't neutered, your vet will recommend that surgery.