It’s a story we often hear on the cat forums. Two cats that used to get along suddenly turn into enemies. The trigger? One of the cats had spent some time outside the home. Upon its return, the other cat sees him or her as a threatening stranger. The term for this kind of behavior is “non-recognition aggression”. If you have more than one cat, you could be facing this situation too someday.

Why does non-recognition aggression even happen?

I have two cats, Shadow (10 years old) and Sylvester (5 years old). Shadow has a gum disease and I took her to the vet last Friday. I brought her back in the house and put her down on the ground and Sylvester attacked her, ripped off her bandage, her collar and had tuffs of her fur under his claws.

From the cat behavior forum

Why are we seeing this type of behavior in cats? Why would a healthy cat turn on the sick cat and attack? After all, they used to be good friends before.

The answer is that they probably don’t even recognize the cat at that very moment.

This sounds strange to us as we rely on sight for recognizing other people. For cats, however, the more relevant sense is scent. The cat returning from the vet doesn’t smell like the cat they knew before. This cat is covered with strange scents to the point that he or she becomes unrecognizable. The resident cat therefore reaches a conclusion: Someone swapped his former friend with a stinky alien.

Cat behavior experts call this “Non-recognition syndrome”. Some believe that it’s not only the strange smells that were picked up by the cat while outside the home. The new smell may also be the result of the cat’s own fear. Having spent the day in unfamiliar surroundings, the returning cat could very well be very stressed and fearful. He or she may still be in pain or under the influence of anesthesia drugs, and this is something other cats could pick up on, whether through smell or body language. Those cues – invisible to us – could also trigger an aggressive response.

For some cats, the smells from a veterinary clinic may just be too frightening in their own right. Even if they recognize the returning cat, they may still lash out in response to the smells that remind them of their own scary experience at the clinic.

Is non-recognition aggression triggered only by vet visits?

Non-recognition aggression can be seen in any situation where the cats were separated. A vet visit is the most usual scenario because that’s a common occasion where one cat leaves home and returns hours or days later. The same type of behavior can also happen following a visit to the groomer but most cats don’t require such visits, making this a rare trigger.

When an indoor-only cat manages to get outside for a few hours or even days, he or she may experience the wrath of non-recognition aggression upon returning home. The longer the time spent outside, the more likely he is to pick up new and unfamiliar scents that can trigger the response in other household cats.

How to deal with non-recognition aggression in cats?

Whenever one of your cats is separated from the others, you should keep this type of aggressive behavior in mind. There are things you can do to try and prevent an onset of non-recognition aggression, but if all else fails, you need to be prepared to deal with the situation.

Deal with stress early on.

Stress makes cats more prone to aggression in general. Just like in humans, elevated levels of stress can put a cat “on edge”. When a stressed-out cat is confronted with the unfamiliar scents on the returning cat, he or she is more likely to lash out.

Read more about the strategies you can use to reduce stress.

Use care when bringing a cat back home from the vet

Don’t just waltz into your home and put the cat down on the floor. Instead, keep the returning cat in a separate room for a few hours to make sure he or she is feeling better and is more relaxed. If Kitty underwent anesthesia, make sure the effects have passed and he or she is ready to face the world again. Keeping the cat in a separate room gives him or her time to groom and regain some of the familiar scents of your home.

Before allowing the cats to meet, grab a blanket or a cloth that your cats regularly come in contact with and rub it on the returning cat. Your aim is to re-apply familiar scents to cover up the “vet stink”.

Have the cats meet each other in a gradual way. Open the door to the separate room to an extent that allows mutual sniffing but allows you to cut the encounter short by shutting the door if needed.

Remain calm while the cats meet each other and avoid staring directly at either cat. At the same time, do keep an eye on them and be prepared to step in.

What to do if one of the cats becomes aggressive

A certain amount of hostility is normal in this situation. If one of the cats hisses and then moves on, that’s ok. However, if either cat lashes out, you’ll have to break up the cat fight by placing some kind of barrier between the cats. Be careful! If one of the cats is very upset he or she can quickly re-direct that aggression towards you. Always keep your hands away from a scared or aggressive cat.

Keep calm and avoid shouting or reprimanding any of the cats, regardless of their reactions. They’re not misbehaving. They’re just being cats, acting on sensory input that you don’t have access to.

If the cats are very hostile towards each other, you may need to separate them for a few days until both sides calm down and then introduce them again, as if they’ve never met before. Use our guide: How to successfully introduce cats and follow through. Don’t rush the process to give the cats the best chance at becoming friends once again.

Have you ever experienced non-recognition aggression with any of your cats? Share your story in a comment below and let us know what happened. As always, if you need more help, share your experience in our cat behavior forum where members can support you in your efforts to deal with non-recognition aggression.