Here is a great article written by Dr Karen Overall, a board-certified Veterinary Behaviorist
B-14 PROTOCOL FOR THE INTRODUCTION OF A NEW PET TO OTHER HOUSEHOLD PETS
When you first bring home a new pet, expect a period of transition and adjustment for the other pets in the household. You may find that some pets hide from the new addition, whereas others might try to push it around. Sometimes the original pets will start behaviors designed to get your attention including barking, pawing, stealing items, or pushing the new addition out of the way and jumping on you. All of these can be normal and are not worrisome if they change within a week or two. If the animals in the household do not revert to normal behavior within a short time or if they become aggressive, a problem exists that will not go away on its own. The sooner you seek help from a qualified specialist, the better.
Before introducing any new pet, make sure it is healthy, has up-to-day vaccinations and that test results for fecal parasites are negative. It is particularly important that all new cats are checked for their viral titer (feline immunodeficiency virus [FIV], feline leukemia virus [FeLV]) status. Cats with positive results should not be brought into a negative household.
You can make the transition easier for new pets by using gradual introductions. The new pet should be kept separate from the other pets whenever they are not closely supervised. This advice may be extreme, but it is designed to ensure that no injuries occur and that the social system of the original pets is not suddenly fragmented. The original pet(s) should have access to the same areas of the house as previously. IF the dog was crated, the crate can still be used. IF access was restricted to the first floor, this pattern should continue. The new pet should be placed in a neutral area (den, finished basement, brightly lit bathroom) with toys, a blanket, water, a litter box if the new pet is a cat, and anything else that it might need. It is important that the new pet not be placed in an area that is considered highly desirable by the other pets. Areas of high value usually include places where the people spend a lot of time with the pets (bedrooms) or where the pets choose to stay when they are alone (around food dishes or on window sills that are good perch sites). If your dog is always crated, you can accustom the dog quickly to a new dog by crating the new dog across the room where it can be seen by the original one. As the dogs become more accustomed to each other, their crates can be moved gradually closer together until they are side by side.
Be sure that the area in which you are confining the new pet is “pet-proof.” This means that toilet seats should be down, electric cords should be tied up and put away, sockets should be protected with child guards and any valuable or fragile items should be moved. New pets will explore, and that exploration should not endanger them. If the new pet is a very young puppy or kitten, you may wish to crate it for its own protection (see handout on “Protocol for Basic Manners: Training and Housebreaking for New Dogs and Puppies”). Crates do not afford total protection from willful and determined claws and teeth of an uncrated animal, but they do greatly minimize the risk of damage.
Whenever any animal is isolated for any reason it is critical that the animal receive a lot of social attention whenever possible. This is especially true for new pets. When you come home greet the original pets (make all the dogs sit first) and let them out, if this is your normal routine. Do not rush- when people are stressed and rush they may either facilitate undesirable interactions between the pets or not be as attentive to cues about impeding problems as they otherwise would. Introduce the new pet gradually. First, spend some time alone with the new pet. They bring the new pet outside on a leash or harness and let the other pets explore him or her. If you anticipate problems, the other animals also can be on leashes or harnesses. If you have too many animals to adequately monitor under these circumstances, the new pet can be placed in a crate or cage in the center of a room and the other pets can explore the caged pet.
The best time to perform gradual introductions is when the animals are calm. Start by petting the original pets and telling them that it is “okay” only if it is truly okay; do not reward hissing, growling, or biting. When you tell a p-et it is “okay” when it is upset, you are not calming the anima- you are rewarding the inappropriate behavior. If the animals in the household are calm and either ignore each other or act friendly despite the new addition, you can feed them within sight of the new pet. This distance should be close enough that they can easily see and watch each other, but not so close that they become upset. Once you find this distance you can move their food dishes closer together by an inch a day until they are side by side. If you ever have an aggressive encounter, back off from that distance and return to the last distance where neither pet reacted. Leave the dishes there for a few days and then gradually start to move them again. Feeding and petting the animals in each other’s presence teaches them that good things happen when they are together and calm. For this to be successful, neither side can react violently. If a pet does react this way, banish that animal to a neutral zone immediately and try again when it is calm. If it again reacts violently, banish the pet for the rest of the day or evening and try later in the day or during the next morning.
Some aggressive and undesirable interactions are not violent but are still not conductive to the development of a good relationship between the pets. You can learn to watch for subtle behaviors that can signal potential problems, should the recipient of those behaviors not be able to change the course of the interaction. In dogs these behaviors include piloerection (hair lifting on scruff, neck, or back), staring, snarling, stalking, side-by-side posturing with growling or lip lifting, and pinning the other animal by grabbing its neck. Cats are masters or subtle threats, and their biggest nonvocal threats include a direct stare and an elevation of the rump and the base of the talk with or without piloerection. Hissing, snarling, and pouncing are also threats but are less intimidating to many animals than the display just described. If you believe that the new pet either is losing the contest or is terrified, or is becoming so aggressive that it might injure the original pet, separate the animals. Do not put your hands or other body parts between the animals. This is the single most common way in which people are injured by pets. Use cardboard, brooms, loud noises (whistles, foghorns), or water pistols to separate the animals. If you can identify the aggressor, banish that animal to neutral turf. If you cannot identify one aggressor, banish every animal to different pieces of neutral turf.
If the new pet is sitting in close proximity to the other pets and everything seems to be going well, tell all the animals that they are good and give them all small food treats and petting, if they like to be petted. This works best if you have two people so one can hold the new pet while the other deals with the other animals. If you are working with two people, switch roles so that the new pet does not associate its rewards with only one person. This can still be accomplished with one person by using leashes, harnesses, and crates. Leashes can be tied to furniture or doorknobs that are at a distance that will allow the pets to sniff each other and react, but not so close as to permit them to lunge at and injure one another. Never leave a tied pet unsupervised even for a minute; it could strangle and die.
The entire time that you are doing this exercise- and it could take hours or weeks- make sure that each pet has 5 to 10 minutes alone with you each day when all you do is pay attention to that pet. This attention could be grooming, playing with a toy, or just petting and massage. Make sure that he pet is happy and relaxed at these times. If you know in advance that you are getting a new pet, you may want to establish these periods of individual attention in advance of the new arrival. If these periods follow a regular schedule, the pets will learn to anticipate them. It may decrease their anxiety about the new addition because they can rely on them.
Once you are able to get the pets to react to each other in a positive manner or not to react at all when restrained, remove the restraints. Be vigilant and keep a water pistol, foghorn, air canister, or whistle with you to interrupt any dangerous situations. If the animals are all behaving well, remember to reward them with praise and treats.
Once you have done the above, you are ready to let the animals out of your sight. Put a bell on the new animal by sewing a bell it its collar so that you always know where it is. This will allow you to spy on any potentially problematic interactions and to interrupt them before they create problems. During this period when you are beginning to provide the pets with free access, remember to provide additional water dishes, litter boxes, beds, and toys so that you minimize competition an the potential for aggressive interaction.
The keys to success are patience and observation. It is critical that the animals are not inadvertently encouraged to become hostile ore nervous in each other’s presence by well-meant but misplaced reassurance for inappropriate behaviors. Expect that the social system may shift. The dog that you always thought of as the “boss dog’ may not only be relegated to a lower position, but may also prefer that. Let the animals set their own pace. In many cases the pets never become close companions but are reasonably content leading separate lives under the same roof. This is far more preferable to frank aggression. Do not push the animals too hard or push for relationships they clearly do not want; this could backfire and you could undo most of the good behavior that you had achieved.
If your pets have lived in the same household but have begun to have some problems with interaction, the previously mentioned protocol can also help them (for more detailed information for dogs, see “Protocol for Dogs with Interdog Aggression”). The pet that is the victim of the aggressive behavior should be fed, walked, and given attention before the aggressor. This reinforces its right to some valued status. If confinement of one pet becomes necessary, confine the aggressor in a neutral or lower quality room. Do not confine the aggressor where it would rather spend time; this only convinces the animal that the contest is meritorious. When you reintroduce the pets, do so gradually as described previously. Move from introductions under controlled circumstances to ones in which the animals are being monitored from a distance. Let their behaviors tell you when you are ready to progress. Put a bell on the collar of the aggressor. At the first sign of any aggressive behavior, and definitely within 30 t0 60 seconds of the onset of the behavioral progression, startle the aggressor with a foghorn, air canister, or water pistol. This means that you should not wait to startle the cat until it has pounced on the kitten, but that you startle tit as soon as it stares at the kitten. Timing is everything. The startle must be sufficient that the behavior is aborted but not so profound that the animal becomes terrified. At that time reassure the victim, and after all animals have been calmed, engage them both in behaviors that are incompatible with aggression (i.e., feeding and petting). IF the aggression persists, banish the aggressor until it is calm, and then try again. If the aggression continues, banish the aggressor until later in the day or the next morning.
If the aggression – either between new pets or pets already in the household – continues, you can try a behavioral modification technique called “flooding”. Done incorrectly this can be very traumatic and damaging. Consider consulting a behavioral specialist to see if this is necessary. It can be a wonderful last resort. In flooding, one animal is kept confined or otherwise restrained while it is reacting inappropriately in the presence of the other animal. It is kept in that restrained or confined situation until the level of the inappropriate reaction diminishes by at least 50%. Obviously, you could not keep an animal on a leash for days without respite, but an aggressive animal can be crated for an extended period with food, water, toys, and litter box, if necessary, and a blanket while the other animal is either locked in a room with it or placed in a similar cage facing the aggressor. If one animal is loose, you should realize that it could injure the caged animal or be injured by sticking its paws through the crate. If the animals become more aggressive and upset, flooding does not work and is counterproductive and should be stopped. Usually the effect is a positive one, and the crated aggressor realizes that the other animal also has a right to share the house. This technique is a last resort and should not be attempted without qualified advice.
Finally, pharmacological intervention may succeed where other therapies have failed. There are many newer anxiolytics available which, when used correctly and prescribe by qualified individuals, may be useful adjuvants to behavioral and environmental modification. In very extreme cases of interanimal aggression in which all other therapies, including pharmacological, have failed, the best, kindest, and safest solute may be to place one of the animals in a new home.
Checklist for Introducing a New Pet to Other Household Pets
1. Separate the pets when they are unsupervised.
2. Crate one or more of the pets.
3. Pet-proof the home.
4. Gradually introduce the pets using food and rewards.
5. Introduce the pets during quiet times by using leashes and harnesses.
6. Use water pistols, air canisters, foghorns, or whistles to interrupt any aggression.
7. Be familiar with the physical signs of impending aggression and know how to safely interrupt such behavior.
8. Put a bell on the new animal when you are ready to introduce it to the household unsupervised.
10. New home?