From: Clinical Medicine for Small Animals. Dr. Karen Overall DVM, ACVB (Mosby 1997)
(Note: these are handouts that the author has given permission in the book for Veterinarians to copy and distribute to clients
Generally, inter-cat aggression occurs either between cats that have been recently introduced, or between those cats known to each other since kitten hood. It occurs when one of the cats becomes socially mature (sometime between two-five years of age), or when one cat perceives that the other cat in the household is becoming socially mature. Owners often comment that the cats lived together perfectly well for the first several years before suddenly becoming aggressive.
Treatment for this disorder focuses on establishing a social order that is tolerable for all cats involved, without danger of injury. The cats may never be best friends. But they may be able to get along together with a minimum of stress. Sometimes, the only solution is to find one of the cats another home. One thing is for sure, you should avoid getting any other cats. That would only added more fuel to the fire.
The basic protocol for solving this problem is as follows (source: Overall, K; Clinical Behavioral Medicine for Small Animals, 1997):
1. Make sure all cats are neutered.
2. Trim all the nails as short as possible.
3. Whenever the cats are not directly supervised, they must be separated. The cat that is the aggressor should be banished to a less valuable or less desirable area. This does not mean a dark closet, a basement, outside, or a garage. Doing this would only teach the cat to avoid you. Instead, if the bulk of the aggression occurs in a bedroom or in front of a favorite window, let the cat that is being victimized have the valued area and put the aggressor into a neutral area like a spare room. Remember to provide water, food and litter boxes for all the cats.
4. Try to find out if there is a distance at which the cats can see each other, but at which they will not react aggressively while they eat. If such a distance exists, then there is a reasonable chance of being able to convince the cats to tolerate each other. You start by putting a dish of food for each cat that they love at this safe distance. This may mean that you have to change the feeding schedule or the type of food given. Several times throughout the day, bring the cats out and feed them at the safe distance. Let them eat this way for several days. Then gradually start moving the dishes closer a couple centimeters at a time until the cats can eat calmly side-by-side. If at any time you reach a distance in which the cats start showing aggression, anxiety, or fear, move the dishes back to the safe distance and start over. If you cannot succeed in getting the cats to eat side-by-side, let them eat at the distance at which they are happy. Remember, the goal is to decrease the anxiety-especially for the victim. Be very observant for any of the subtle signs of aggression-like staring. Watch how fast the cats eat. If the victim bolts the food and leaves, or doesn't want to eat, there are probably threats involved.
5. If marking and appropriate elimination is involved, use the appropriate behavioral modification in addition to what is outlined in this protocol.
6. Only allow the cats to freely mingle under the following circumstances:
-when they have the bell on their collars that allows you to distinguish between the individuals
-if you are able and willing to visually monitor the situation at all times
-if you carry a water pistol, compressed air canister, a whistle or a fog horn at all times. At the first sign of aggression, you must interrupt the cats by directing the device towards the aggressor. Use common sense in choosing the device to use.
-if the threats escalate to frank aggression, do not reach between the cats. You will get hurt and make the situation worse. If you must get involved, throw a blanket over the animals, or use a broom or a piece of cardboard to separate them.
7. Use harnesses and leashes for all involved cats. If there are two or more people in the household, you can take turns monitoring the cats. If you are alone, attached the leash of the aggressive cat to the furniture and hold the leash of the other cat. The cats should be restrained at a distance at which they cannot touch each other even if they lunge. Find a food treat that they love. Any time the cats ignore each other praise them vigorously and given the treat. If the aggressor voluntarily looks away from the victim, reward that. If the victim stares at the aggressive cat reward that. Do not give a treat to any cat that shows signs of aggression, fear, or anxiety. These include shaking, cringing and hiding.
8. Use a harness to correct the cat verbally or with a startle at the first sign of any aggression. If the aggression continues, banish the aggressive cat to the undesirable area.
9. Use the harnesses to arrange the cats so that they can't reach each other. Then alternate between the involved cats and groom and massage them. Start with the victim. The goal is to get them to not react to each other. Any cat that react aggressively is banished. You can couple a favorable response to food treats. If the cats ignore each other gradually begin to move them closer together. They should not become distressed or aggressive by the moves. If they do, separate them and try again at a greater distance.
10. If the cats are able to lie side-by-side without becoming distressed or aggressive and if they can eat together, you can leave them alone for gradually increasing amounts of time. If you notice at any time that either cats is injured or avoiding the other cat repeat the previous steps. Some cats will never tolerate being close together but can live peaceful and separate lives in the same house.
11. Cats generally require and use more space than the average house or apartment gives them. Adding three-dimensional space can help in the form of kitty condos, cardboard boxes, beds and crates in all rooms once the reintroduction of the cats has begun.
12. Some cats may benefit from antianxiety medication. If medication needed, it is used to complement the behavior modification process and not to replace it.