Urinary bladder infections, known as cystitis, are the number one killer of cats. In cats the infection is known as FUS (Feline Urologic Syndrome). In dogs, it is both less usual and far less serious. It is caused by magnesium and phosphorus crystals on the urine that irritate the bladder and urethra. The first symptoms are of frequency, running back and forth to the litter box or in and out of the house. The animal strains and may void a few drops, but it is itching and burning, rather than fullness, that makes her think she needs to go. There may be traces of blood in the urine. A pet who suddenly starts wetting indoors or missing the litter box, likely has a urinary-tract infection and should be checked immediately.