Causes
There are many causes of urethral obstruction in cats, but the two most common are uroliths and urethral plugs. Urethral plugs consist of a soft, compressible material that contains variable quantities of minerals, cells and cellular debris, and mucus-like protein. Many factors interact to produce uroliths and urethral plugs; viruses, bacteria, diet, decreased water consumption, physical inactivity, urine retention, stress, and urine pH may all contribute.
The most common mineral associated with urethral obstructions in cats is struvite. Previously, the ash content—and more recently, the magnesium content—of the diet was thought to be a primary cause of struvite formation in the urine. However, researchers have found that urine pH is a more important contributing factor; urine that is acidic provides a less favorable environment for the formation of struvite uroliths and crystals. It also was discovered that cats who are fed multiple small meals throughout the day, or who are allowed to eat their food free-choice, routinely tend to produce a more acidic urine—again less favorable for struvite formation—than cats fed only one or two large meals per day.
Mineral deposits also have a greater tendency to form in urine that is highly concentrated or that is retained in the bladder for long periods of time. To encourage adequate water consumption, and thus the formation of urine that is more dilute, fresh water should be available to cats at all times. Fastidious cats sometimes avoid using a dirty litter pan, so owners should provide clean litter boxes to encourage regular and frequent urination.