Sick cats don't always look sick. In fact, most of the time they appear to be just fine. But if your cat suddenly stops using the litter box, the first step is to investigate possible medical issues. Here are a few key illnesses to watch for:
Urinary tract infection: If your cat enters the litter box on a disturbingly frequent basis, cries out while urinating or begins urinating in other parts of your home, a painful urinary tract infection could be to blame. Sometimes, even after an infection has been treated, a cat will refuse to use the litter box because she associates it with pain.
Kidney stones: These can be a painful and serious problem for cats. The symptoms mimic that of a urinary tract infection, but may also cause your cat to flinch in pain at the touch of her swollen abdomen.
Feline interstitial cystitis: Although this disease is neurological in origin, its symptoms include frequent and strained urination, often with little output. Other signs include a cat who obsessively licks herself where she urinates or has blood in her urine. The pain and nearly constant urge to urinate will cause your cat to relieve herself in places other than the litter box.
If any of these troubling medical symptoms are present, contact your veterinarian. Not only will these illnesses make your cat uncomfortable, in the case of feline interstitial cystitis, they could be life-threatening as well [source: ASPCA].