Inflammation of the membrane that lines the bladder is called cystitis.   

Inflammation of the urethra is called urethritis.   

Cats with FUS often have both cystitis and urethritis.

Inadequate water consumption.    

Physical inactivity.   

Intentional urine retention (which may occur if a cat holds his urine in order to void outside, or habitually waits until his litter box is cleaned before he'll use it).   

Stress.   

Blood in the urine.   

Urine with a strong ammonia odor.   

Concentrated urine.   

Frequent trips to the litter box with only small amounts of urine voided.   

"Camping out" in the litter box in a squatting position.   

Crying while urinating.   

Loss of usual litter-box habits such as urinating small amounts of urine in unusual places.   

Note:  If you are suspicious that your cat is acting constipated because he's straining in the litter box, please suspect FUS and get him checked. 

Prescription diets such as Hill's, Feline Maintenance, or C/D.   

Urine acidifiers (methionine, an amino acid; and ammonium chloride, a salt) are available in palpable paste form that the cat can lick off his paw.  Never give urine acidifiers to a cat already on a prescription diet.  It can make the cat's blood too acidic and cause a life-threatening  metabolic acidosis.    
   
 

• Urinary catheterization (passing a narrow tube through the urethra and into the bladder) to relieve the obstruction and drain the urine. 
• IV fluids and antibiotics may be given. 
• Recurrence can be prevented in the majority of cases by feeding an acidic, magnesium restricted, diet.