There are many reasons cats go the bathroom outside of their litter box. The first thing to consider is a medical problem. Take the cat to the veterinarian to rule out any physical problem like a urinary tract infection.
If the veterinarian does not find a medical problem, we can assume it is a behavioral problem. To stop the inappropriate behavior, you will first need to figure out what is causing it. Below are some of the common causes and some suggestions for correcting the problem.
Identify the reason for the aversion and work on changing it. If the cat has developed an aversion due to a scary experience, the litter box may need to be relocated. It is sometimes necessary to provide a new box that is very different from the old one. If there are multiple cats in the household, there should be multiple boxes.
The cat may still occasionally use the litter box but has developed a preference for another surface. Fine-grained and softer substrates (beds, carpets, piles of clothes, towels) are preferred by most cats. Surface preferences can also develop secondarily to litter box aversions.
Sometimes this occurs after the owner moves the box (the cat may still use the old location). It is also likely that the area where the box is kept is undesirable for some reason. Perhaps there is too much activity, or the box is located so that there is only one way out or the cat cannot see all around while going to the bathroom. The box may be too isolated in an out of the way scary place (basement), or it may be located near unpredictable noises (near furnace or other large appliance).
Change the location of the litter box to meet the cat’s preferences. It may be necessary to put the litter box in the place the cat is relieving himself and slowly move it (inches a day) to the new desired location.
Inappropriate bathroom behavior may result from one cat being harassed and threatened while in the box by another cat. The cat becomes afraid to go to the litter box if it is then vulnerable to attack or harassment. Some cats simply will not use a box that another cat has gone to the bathroom in.
Have multiple boxes in multiple locations. In all locations the cat should have several escape routes and good visibility all around the box. Work on improving the relationship between the cats using positive reinforcement (food treats whenever they are together).
If the cat is afraid to move comfortably in the house, he may start to go to the bathroom inappropriately. This is often seen in cats that are new to a house and not yet comfortable with the environment.
Use of harsh punishment by the owner could cause the cat to hide whenever the owner is present and thus go to the bathroom close to the hiding place. Fear of the family dog who chases the cat could contribute to this problem.
Identify the source of the fear and anxiety. Stop all non-contingent punishment, move the box away from scary noises, and use desensitization and counter conditioning to teach the cat to be less fearful of things in the household.
Olfactory cues (odors) from other animals that have soiled the area may contribute to inappropriate bathroom behavior.
Reduce odors with an enzymatic cleaning product. Restrict the cat’s access to soiled areas during the cleaning process.