There are great many reasons why a cat may not use the litter tray. The first port of call is always your veterinarian to rule out a medical problem. If all is well, then you must look at possible behavioural reasons why your cat is refusing the litter tray. Below I will outline some possible behavioural and medical reasons why cats will refuse to use their litter tray.
Dirty litter tray - Cats are fastidiously clean and may refuse to use a tray that is not cleaned as often as it should be. As a rule of thumb, you should have one litter tray per cat, plus one spare. Although in multiple cat households this is not always possible. Regular scooping of your tray, along with changing the litter frequently should help.
- Placement of the litter tray is also important. Just as humans like privacy, so do cats. Placing the litter tray in a busy area may result in your cat's refusal to use the litter tray. Also, some cats will enjoy the privacy of a fully covered litter tray, while this may be too confining for other cats. Generally I prefer to have a combination of both trays to suit my cat's different preferences.
- Cats do not like to go to the toilet near where they eat. So make sure the litter box is placed far from your cat’s food and water.
- Doesn't like the brand/type of litter in the tray.
- Stress is major cause of inappropriate urination in cats. Factors causing stress in cats can be moving house, the introduction of a new family member (pet or human), a neighbours cat roaming your garden etc. Helping your cat overcome these stresses may well result in the inappropriate urination stopping, however this isn't always the case. Sometimes the behaviour can become so ingrained in the cat that even once the stress has been removed, the behaviour continues. If this is the case it is a good idea to seek advice from your veterinarian as to how to re-train your cat. Some cat owners have had great success using Rescue Remedy on their cats, although this isn't advisable until you have spoken to your vet.
- Declawing sometimes leads to refusal to use a litter box. Often the cat's paws are tender and scratching around in cat litter can lead to pain and discomfort. In this case it is advisable to find a softer litter which is less harsh on your cat's feet.
- Urolithiasis - Uroliths are rock like stones found within the urinary tract causing irritation and secondary infection. Struvite (also referred to as magnesium ammonium phosphate or MAP) account for approximately 65% of stones, 20% are calcium oxalate , other stones include calcium phosphate, ammonium urate, silica, and cystine. The stones are named after their mineral formation and are caused by varying reasons. It is important that your veterinarian identifies which stone(s) your cat has and treats accordingly.
- Cystitis is defined as inflammation of the bladder. It may precede or be secondary to urolithiasis (stones). Other possible causes include; idiopathic (no known cause), bacterial infection.
- Megacolon is a condition in which the colon becomes abnormally dilated and enlarged and loses it's ability to contract. It is associated with constipation or obstipation. There are two forms of megacolon; congenital (present from birth) or acquired. The most common form of acquired megacolon is idiopathic (cause unknown) although it is believed to be improper activation of smooth muscle within the colon and rectum.  Other causes , dietary (ingesting non-digestible objects which become impacted), injury (ie; pelvic fracture), refusal to defecate because of a dirty litter tray which results in fecal matter building up and distending the colon, anal sac impaction which also leads to refusal to defecate, tumors and neurological disorders.