Cystitis in Cats.
Cystitis is inflammation of the urinary bladder. This is a common condition in cats and causes painful urination. The signs that people notice most commonly are frequent attempts to urinate, small amounts of urine produced at a time, urinating outside the litter box, and blood in the urine.
There are three common causes for this painful, bloody urination:
Idiopathic cystitis. This is the most common cause of cystitis in cats. We don't know why cats get this but it is not due to bacterial infection and antibiotics are not helpful. A lot of cats will recover from this disease with no treatment. This causes confusion because if a cat with this disease is given antibiotics and then gets better, people incorrectly think that the improvement was due to the antibiotics.
Bladder stones. Bladder stones are a less common cause. Some stones can be dissolved with diet. Some require surgery to remove.
Infection. Bacterial infection of the bladder is common in older cats but is almost always associated with an underlying condition - either diabetes or kidney disease. Because of this, it usually needs to be treated with an effective antibiotic for at least 6 weeks.
The signs for all of these are similar. They can't be distinguished by clinical signs alone.