One of the most common forms of cat aggression occurs between cats that live together but are separated for a short time. This type of cat attack can be emotionally upsetting to the victim cat as well as the cats' owners. In many instances, the unexpected attack results from redirected aggression when the aggressor lashes out at a surrogate target since the actual target can't be reached.
A common scenario occurs when one cat from the household goes to the groomer or veterinarian. When the cat returns home, former kitty friends either snub or outright attack the treated feline. It almost appears that the cats don't recognize each other. This is essentially true because the absent cat has an altered scent that its housemates don't recognize.
Confident kitties usually work out their shortsightedness in time. But you can speed up the process, and prevent worsening of feline relationships.
Consider Scent Communication Rules
Cats communicate with vocal, visual, and scent cues, and each cat's unique scent serves as an identification tag to other felines. When they sleep together, groom each other, or simply rub against one another in passing, cats share scent. Think of this communal scent as a system that identifies them as belonging to the same family.
When a cat visits the veterinarian, it smells weird upon its return. It's been handled by strangers, perhaps bathed or treated with medicines that smell funny, and may even be ill and smell unhealthy. The other cats don't readily recognize these new scents and may react as if an interloper is invading their turf.
When the household cats greet the returning cat with hisses, it naturally gets its back up and acts defensive, too. This situation can escalate beyond the posturing, particularly if the ill cat continues to make trips to and from the vet clinic.
Schedule Cats at the Vet Together
Try to schedule routine veterinary visits for your cats at the same time. That way, they all smell similarly after being handled by clinic staff and are less likely to attack each other when they return home
Keep a Sick Cat Separated
When a cat is ill, its body chemistry can make it smell different and other cats often change their behavior toward it even before the vet visit. Segregate an ill cat from the others, especially if they're treating it poorly. Cat-bashing just raises the stress levels of all involved, and the longer it goes on, the more time it will take to reverse.
Plan to segregate the cat returning from the vet in a room alone for at least half a day. That gives it time to self-groom and remove all the icky clinic smell from its fur, which is so offensive to its cat buddies. It also allows the treated cat time to decompress from the stress of the vet visit so it's less on the defensive.
Redistribute the Communal Cat Scent
Use a hand towel to pet the other household cats. Pay particular attention to the best-friend cat, rubbing the towel against its cheeks. This gathers the best-friend cat's signature scent, which can then be rubbed over the vet-visit cat to help re-establish the communal family scent.
Pheromones are a great way to speed up scent communication. Products, such as Feliway, contain a man-made version of the substance your cat deposits when it rubs its cheek on your leg or furniture. These products help signal that the areas is familiar and safe and can help disguise other scents or pheromones that cats may be experiencing or emitting. These pheromones can be used to make a new place or situation "feel" more familiar and safe for cats. Using these products on carriers, bedding and favorite places or as a diffuser can really improve the acceptance of the returning cat.
Problems and Proofing Behavior
Monitor the first several hours of the cats' interaction, and segregate the treated cat immediately should there be a cat-bashing/hissy incident. There's little point in reprimanding any of the cats for fighting; they're likely to become more confused than they already are.
All the cats may be more willing to become reacquainted after a cooling-off period. Don't rush to re-integrate the sick cat to the rest of the pack. It may need extra time to get over the trauma of the vet visit, and there's no urgent need to thrust it back with the rest right away.