Why is my cat attacking my other cat after the vet?

Squirt and Trouble are brothers, littermates who were brought into my life 9 years ago by their feral mama, who somehow decided that my backyard looked like a safe place to set up camp. They were about 10 weeks old when I caught my first glimpse of them.

At first, I saw only their mother, who looked skinny and hungry. I quietly brought out a plate of food for her, which she immediately buried her face into once I had retreated to what she thought was a safe distance. After eating some of the food, I heard her chirp… and out from underneath a hibiscus bush came adorable, hungry little kittens!

Eventually, I adopted and socialized the boys (mama kitty disappeared before I had a chance to catch her). Trouble was a big, confident kitten, with a jet black coat that literally shines blue in the sunlight. Squirt was the runt of the litter, a quiet and gentle gray tabby with beautiful stripes. They were sweet, funny, and inseparable.

That is, until “The Incident”.

My Introduction To Non-Recognition Aggression Syndrome

About 6 months ago, I was walking across the floor in bare feet and happened to step on, of all things, a tooth. As it turned out, it was an intact canine tooth belonging to Trouble that had somehow snapped off at his gumline. As a former Veterinary Technician, my “Oh, $*@&” alarm went off, since a retained tooth root will eventually become infected if it isn’t removed. I immediately made an appointment at the vet for emergency dental surgery.

After Trouble’s surgery (during which he not only had his tooth root successfully taken out, but also 2 other damaged teeth pulled), I went to pick him up at the vet. Unfortunately, he had been the last surgery on the schedule, so he was still pretty groggy from the anesthetic by the time I got there. Since I had several years of experience recovering veterinary patients who were coming out of anesthetic, rather than leave him there overnight, I was able to take him home to finish waking up.

Here’s where I made my first mistake. After attempting to keep him in a dimly-lit, quiet room for a few hours to finish recovering (during which he freaked out that the door was closed and kept clawing at the door and rubbing his nose under the door crack until the fur literally rubbed off), I decided he was awake enough to go out into the rest of the house with supervision.

Unfortunately, when I opened the door and Trouble bolted out of the room, Squirt was on the other side of the door. I can only imagine what Squirt must have thought when he saw his brother coming straight at him with his fur puffed up, smelling like anesthetic, and looking wild-eyed like Doc Brown from “Back to the Future”. Squirt immediately freaked out, hissed, and ran upstairs to hide.

For the next few days, Squirt avoided Trouble like the plague. The problem was, Trouble wanted his brother for comfort, so he insisted on following Squirt everywhere. This led to a few hissing and growling matches between them, which I thought would eventually subside.

About 2 days later, I was in the kitchen when I heard the most ungodly sound I’ve ever heard coming from the living room. In a matter of seconds, both cats had run upstairs, screaming and slamming their bodies into the wall all the way up. I ran after them for what seemed like an eternity, and by the time I got upstairs, Trouble had cornered Squirt in the bathroom.

There was urine all over the tile floor, with clumps of black and gray fur everywhere. But what I’ll never forget was the look on their faces. Squirt was utterly terrified, laying on his back with his feet in the air, ready to defend himself to the death, while Trouble looked heartbreakingly confused, like he didn’t quite understand what had just happened.

What Is Non-Recognition Aggression Syndrome?

Simply put, Non-Recognition Aggression Syndrome in cats occurs when one cat is inexplicably aggressive to another cat in the household after both have been separated – usually after a trip by one cat to the vet, or after time spent in a boarding kennel. This aggression can lead to vicious attacks against each other, and the aggression can also be redirected to humans in the household (which fortunately did not happen to us).

This behavior, which seems to be unique to cats and does not seem to occur in dogs, is not completely understood, even by veterinary behaviorists. Each case is different, but in most cases, the behavior can escalate to ongoing feuding between the cats, which can lead to the cats’ relationship being permanently broken if something isn’t done to stop it.

Possible Causes of Non-Recognition Aggression Syndrome

There are 2 main theories as to why Non-Recognition Aggression Syndrome occurs, especially in cases when one cat has been to the vet and returns home.

1. The returning cat looks or acts differently due to anesthesia or sedation. The cat may appear wobbly, or walk or behave differently, which can be alarming to the other cat.
2. The returning cat, who has been touched by strange humans, or picked up the odors of other animals, disinfectant, iodine, alcohol, or anesthetic gas while at the vet’s, smells differently. Since cats recognize each other by smell first (not by sight), the returning cat can appear to be a total stranger when he comes home. The resident cat then thinks, this may look like my buddy, but they don’t act or smell like my buddy… they must be an intruder!
3. Sometimes while at the vet, cats can also become so frightened that they involuntarily express their anal glands, which are small sacs located next to the anus. The contents of the anal glands contain strong-smelling pheromones that can linger even after the cat has been cleaned. When the returning cat comes home literally smelling of fear, it can frighten the other cat enough to prompt an attack.

In our case, I believe that Squirt, who is quieter and more shy, became so afraid of Trouble that he began slinking around acting fearful and submissive, which may have somehow actually provoked Trouble to attack him. Cat behavior is complex, and can definitely be difficult to understand!

What To Do If Your Cats Experience Non-Recognition Aggression

First, safely separate the 2 cats immediately. Although you will probably be alarmed, do not yell at the cats or raise your voice. The cat who seems to be the aggressor should be herded into a separate room to give him time to settle down.

How you handle things afterward becomes very important. These strategies can be used to give the cats a chance to recover and, hopefully, return to normal.

• Keep the cats separated in different parts of the house, with no visibility to each other, for as long as it takes for them to act calmly again. Be patient – this can take hours, days… or weeks. Each cat should have his own litter box, food and water, and bedding. Never let the cats “work it out” themselves by fighting. This will only lead to more severe, and sometimes permanent, aggression.

• During the initial time-out, don’t try to soothe the cats; just leave them alone to give them time to calm down on their own.

• Over the next few days, give the cats a chance to reacquaint themselves with each other’s scent without actually seeing each other. You can do this by “scent-swapping”: exchange items that smell like the other cat, such as their beds or toys. You can also take a sock, place it over your hand, and rub one cat for a few minutes, then place the sock on the floor in the other cat’s room. This gives each cat a chance to smell the other without having to actually see them.

• During the time-out periods, play with each cat daily and give them both lots of attention. Playing, especially with wand toys, gives them an outlet for their aggression and redirects their focus onto something other than each other.

• Gradually reintroduce the cats through a crack in the door, or with the use of a pet or baby gate so they can get a glimpse of each other without being able to have direct physical contact. Whenever the cats see each other without acting aggressively, give them lots of treats and praise so they come to associate good things with being in each other’s presence.

• Only when you are certain there will not be any further attacks, allow the cats to be near each other while supervised. If there are any signs of aggression, separate them and try again the next day. BE PATIENT and don’t rush them. Every cat is different.

I’d like to be able to say that this process was quick and easy with Squirt and Trouble. It wasn’t. I had to keep them separated for at least a week, then it took 3 more weeks to gradually re-introduce them. I used a tall pet gate (which was a lifesaver), and used the guest bedroom as a “safe zone”, swapping them in and out of that room until they got used to each other’s smells again. I played with each cat separately for 30 minutes a day while they were apart, and also gave them treats like crazy so they learned that every time they saw each other, they got the best food ever.

Eventually, they were reunited, and thankfully, they are now back to being best friends.

Can Non-Recognition Aggression Be Prevented?

The short answer is, sometimes, but not always. That being said, there are some things you can do if you know you will be taking one cat to the vet.

• Make sure that the cat returning from the vet’s office is fully recovered from sedation or anesthesia before coming home.

• Keep other cats separated from the returning cat for at least 2 days. This may seem like overkill, but trust me, it works. This gives the returning cat time to lose any strange smells he may have picked up at the vet (especially if he had gas anesthesia, which cats can continue to exhale from their lungs for several hours after surgery), and give him time to settle back into a routine.

• If the returning cat will tolerate it, you can try bathing him before reintroducing him to the other cat(s). However, most cats don’t like to be bathed, so it may be more practical to just let the scents wear off on their own. Or you can try using unscented baby wipes (be sure they have zero fragrance added) to wipe him down.

• After bathing, rub something from the cat’s regular scent back on him, like a toy or blanket.

Wiser And More Prepared For Next Time

As luck would have it, about 4 months after our episode of Non-Recognition Aggression Syndrome, Squirt also needed to have a tooth pulled. At first, I’ll admit, I freaked out a bit, especially since we had just gotten the household back to normal. But this time, I was ready. When Squirt came home, I kept the boys separated for 3 days, gradually reintroduced them on Day 4, and, thank goodness, everything was fine.

Cats are complex and territorial, and sometimes their behavior can be a bit perplexing. But when we make an effort to see the world through their eyes, it can help us understand what drives them, what they fear, and what we can do to help them overcome some of the challenges of living in our world.

For more valuable tips on how to reintroduce cats, check out this article from cat behavior expert Pam Johnson-Bennett.

Have your cats ever experienced Non-Recognition Aggression Syndrome? If so, how did you handle it? Please share your story with us in the comments below!