One of the main reasons cats are taken to the vet is urinary tract issues, namely, the cat is using the litterbox frequently with little urine produced, or the cat is peeing outside of the box somethimes with blood. These are the warning signs that the might have a bladder infection, bladder stones, bladder cancer, etc., or it may be suffering from feline idiopathic cystitis (FIC).
A note on acronyms: In the early 1980s, this was usually termed FUS -- feline urological syndrome. Thanks to an article in a 1984 journal, it became FLUTD -- feline lower urinary tract disease. It has also been refered to as LUTS -- lower urinary tract signs and IC -- interstitial cystitis, which was borrowed from human medicine. I will refer to it as the new preferred term -- FIC.
FIC strikes males and females equally, although males are by far more likely to become obstructed than females. Risk factors seem to include stress, excess body weight, lack of activity, and multicat homes.
With humans, there are two types -- ulcerative and non-ulcerative. With cats, almost all are non-ulcerative, and so, biopsies of the bladder are not recommended for cats suspected of having FIC.
The actual cause of FIC is unknown. There seems to be no clear link between any virus and FIC. There seems to be some connection with bacteria, namely cats with FIC are at increased risk of bacterial UTI. The neurons assocaited with the bladder are larger and more sensitve in FIC cats than in normal cats. Abnormalities in the hpothalamus, pituitary, and adrenal axis have been observed in cats with FIC. Many cats with FIC also suffer from various other diseases leading researchers to wonder if FIC is a single disease or an expression of a larger disease process which effects other organs also. One important "cause" is early kittenhood adversity, such as being orphaned or starved.
Diagnosing FIC is by exclusion of other diseases. (The accompanying diagram shows most of the other possibilities.) There is no clear diagnostic test, although researchers have found some biomarkers which seem to be linked to FIC. Abdominal xrays are suggested since 20% with some type of urinary tract issue have crystals.
Treatment involves both drugs and environmental changes. Narcotic pain-killers are recommended during a FIC period, whereas NSAIDs are not because of decreased blood flow to the kidneys.
The main treatment is modification of the cat's evironment to decrease stress. An interesting question to help identify stress triggers is at http://indoorpet.osu.edu . Increased water intake is seen as important, but using acidifying foods is not. If the cat is fat, a safe diet needs to be undertaken. Feliway may or may not be useful with FIC cats, but it was discussed and there is research on using it with these cats. No drug, such as antidepressives, have been found useful.
Summarized from "Feline Idiopathic Cystitis", Jodi L. Westropp and C. A. Tony Buffington, pp.518-525 in Consultations in Feline Internal Medicine, v.7, 2015.