Summary: When a cat is taken out of their home and then taken back, for example to the veterinarian, they can experience “non-recognition aggression.” Even cats who previously had a strong affiliative relationship can fail to recognize each other under certain circumstances, and show aggressive behaviors as a result. This article summarizes a small citizen science project looking at what cat guardians have tried to prevent and address non-recognition aggression in the cats in their household. 

Non-recognition aggression happens when one cat has spent some time outside the home and is viewed as a stranger or threat by a cat that remained at home. If not prevented, the cat at home might attack the returning cat, the returning cat might react defensively because they don’t understand what provoked the attack, and the situation could escalate into a fight.

The phenomenon is believed to happen only in cats. The most often cited examples involve vet visits,¹ while less often cited examples include when a cat spends time at the groomer, another house, or outdoors.² In this article, I’ll discuss possible causes of and possible solutions to non-recognition aggression. My writing is based on expert opinion, professional experience with clients, personal experience with my own cats and foster kittens, and my own informal survey of cat owners.

Non-recognition aggression was on my radar well before I became a cat behavior consultant. Our cat Rainy would become hostile to our other cats when they returned home from a vet visit. Most of the time, I was able to prevent the tension between her and our other two cats from turning into a full-blown fight. Each time I was able to reunite them, I’d breathe a sigh of relief but also renew my efforts to find a definitive solution.

In the summer of 2020, I encountered my first professional case of non-recognition aggression. Two cats were living on separate floors because of fights that had occurred after a visit to a groomer. When I was able to help reunite the two cats, I thought I finally had a handle on how to deal with non-recognition aggression.

My illusions fell apart a year later when non-recognition aggression once again become personal. In November of 2021, our oldest cat, Cinder, had a biopsy to determine if she had irritable bowel disorder, and was confined to our bedroom for a week after surgery to limit her activity. When Cinder was finally allowed to comingle with our other cats, she and Rainy got into a series of fights that were so bad that we didn’t have any other recourse but to fully reintroduce Rainy to her.

Causes

Theories abound as to the cause of non-recognition aggression³ and include the following:

• Loss of group scent
• The familiar scent of the returning cat is masked by:
  • Strange humans
  • Other animals
  • Vet smells (disinfectant, alcohol, iodine, anesthetic gas)
  • Grooming smells (shampoo, finishing spray/cologne, alcohol-based ear cleaner or witch hazel, sterilizer used on equipment between clients)
• Home cat may associate the returning cat with a negative experience because of the aforementioned unfamiliar smells
• Returning cat might smell of stress or fear
• Returning cat may have involuntary discharged its anal sacs due to stress and the discharge may contain a pheromone that signals danger to the home cat
• Returning cat looks or acts differently due to:
  • Anesthesia or sedation
  • Unusual body language
  • Pain

Solutions?

Theories also abound as to the prevention of non-recognition aggression, and include the following:

• Make your cat’s excursions as low-stress as possible
  • Give cats anti-anxiety medication before taking them out of the home
  • Carrier-train cats before taking them out of the home
• Ensure that your cat has fully recovered from sedation or anesthesia before being allowed to comingle with cats at home
• Bathe your returning cat to remove foreign odors – but this introduces other foreign odors, which some have found problematic
• Rub something with the home cat’s regular scent on the returning cat
• Take cats together on excursions
• When the returning cat joins the other cats, supervise closely
• If there is any non-recognition aggression, separate the cats and reintroduce

Until faced with the challenge of fully reintroducing our own three cats, I had thought the fail-proof preventive measure was to remove the foreign smell by exchanging scents and then keep the returning cat separate from the home cats until the former had fully groomed themself. If at that point tension still existed between our cats, I would follow up with a mini-reintroduction that consisted of scent exchanges, treats between barriers, and the supervising of initial interactions.

Insights from citizen science

Why then did none of these measures work in November 2021? Stumped for answers, and without any formal research to guide me, I decided to conduct my own informal research. I posted a survey on social media, in which I inquired about the following:

• Whether the respondent had tried each of the possible aforementioned interventions with their cats
• Which of the interventions they had tried had been completely successful on at least one occasion
• Which of the interventions had been partially successful on at least one occasion
• Which of the interventions had been unsuccessful on at least one occasion

I also asked about one particular incident of potential non-recognition aggression from the respondent’s recent past, and included questions about

• How long the cat was at the veterinarian’s office,
• How stressful the visit was for the cat
• What the respondent did to prepare the cat before their visit
• How long the cat was isolated for after the visit
• What the respondent did (if anything) to restore the cat’s baseline odor
• What behaviors the respondent observed in the cat that stayed home, when presented with the returning cat.
• Whether the respondent feels their cats’ relationship has been restored

Impact of interventions

Figure 1. Which interventions have you tried that was successful on at least one occasion?

Figure 2. Which interventions have you tried that were unsuccessful on at least one occasion?

Based on my informal survey, here are the calculated success rates for each intervention:

• Separate the cats: 76%
• Take both cats together: 70%
• Remove odors: 50%
• Rub group scent: 38%
• Crate training: 33%
• Anti-anxiety medication: 25%

Based on these results, I’m left wondering if one should choose an intervention from the two that were the most successful, or whether they should be used in conjunction.

Impact of length of stay at the clinic

I also tried to use my informal survey to test an observation that I’d made over the years about my own cats, which is that that the longer a cat stays at a vet clinic, the longer it will take for any foreign smells to dissipate. Whenever either Cinder or Bootsie has gone to the vet for a routine check-up, I have needed only to remove foreign smells to reunite Rainy with them. In contrast, whenever either Cinder or Bootsie has gone to the vet clinic for more than a few hours, I have needed to do a mini-reintroduction. There was even an occasion where the two had cohabitated peacefully for several days after being reintroduced, but then Rainy sniffed Bootsie as the two crossed paths in our hallway and Rainy immediately attacked her. While my survey did show that some cat owners have faced a similar experience, it also showed that others have easily reunited their cats despite a lengthy vet visit.

Limitations of the survey

I realize now that my survey was flawed in a number of ways. First, the sample size (30) is too small for me to draw reliable conclusions. Second, there’s no way for me to know whether the successes were due to the use of isolated interventions or combined interventions. Third, the respondents were relying on memory to answer my questions rather than reporting the results of recent specific incidents. Still, I am presenting the results here as a curiosity and to provide potential talking points.

Insights from cat experts

Still stumped for answers, I once again turned to cat behavior experts. After emailing a few researchers and taking copious notes on professional writings about the topic, three new theories came to mind with regard to why some non-recognition cases can take longer than others to resolve.

The first was inspired by discussions with other cat behavior consultants. They were the ones who pointed out to me that if Cinder was still recovering from being sick this might be contributing to Rainy’s reaction toward her. While there are cats that will become like a nursemaid to an ailing cat, there are also cats that will attack them due to viewing them as prey. In Cinder’s case, it took a few months for her health to stabilize. At that point, her hair grew in after losing it due to a steroid. We also figured out which foods she could eat and which she should avoid, thanks to a saliva-based food intolerance test.  Around the time her health began to stabilize, we began to make headway with reintroductions.

My second new theory regarding why some non-recognition cases can take longer than others to resolve was inspired by Dr. Nicolas Dodman, who wrote that the fundamental problem seems to involve the nature of the aggressor. He further explained that “Some cats are extremely stable and highly unlikely to fly off the handle. Others are more mercurial. The latter type is more likely to be the aggressor in non-recognition aggression.”⁴

Would I label Rainy as mercurial? In many ways, Rainy is extremely stable. We used to call her unflappable, because as a kitten she’d ignore the hissing and swatting of our other two cats. Now as an adult, she’s a certified therapy cat who loves to meet new people, readily explores new places, and handles herself with confidence in most new situations. In other ways, Rainy is less stable. She’s the one who experiences redirected aggression and will attack our other cats if she sees a cat in our yard. Rainy is also the one who can react defensively to the hiss of one of our other cats.

Perhaps it would be most accurate to call her boisterous. She is the one who demands that we feed her and give her attention, and she is the one who plays rough with our other cats. With regards to this latter label, I do see resemblances between her personality and the aggressor in my first professional case of non-recognition aggression. Actually, there are resemblances to the two “victims” too. Both of the victims by nature are insecure. If I were to revise my survey, I would ask questions about the personalities of the cats involved.

My third new theory regarding why some non-recognition cases can take longer than others to resolve was inspired by Dr. Sarah Ellis and Dr. Stephanie Borns-Weil, both of whom view non-recognition aggression as perhaps better explained as redirected aggression. Until encountering this idea, I’d viewed the two phenomena as different. Once the prospect had been raised that they could be versions of the same phenomenon, I searched for professional writings on redirected aggression.

I found a scientific study,⁵ the two conclusions of which provide food for thought.

• Cats that redirected their aggression were more likely to have a fear of loud noises, less likely to be outdoor cats, and more likely to be from small households (no more than two people) than were control cats.
• In most situations, case cats had adopted a defensive body posture immediately before the incident of redirected aggression, which suggested that the underlying motivation was fear.

In addition to the aforementioned study, I also found a presentation given at the World Small Animal Veterinary Association World Congress Proceedings, 2001, by Niwako Ogata.⁶ The speaker noted that the most common stimuli leading to redirected aggression are the presence of another cat, a dog, or visitors, an unusual odor, high-pitched noises, or being outdoors unexpectedly. Two of her main points stood out to me:

• The stronger the trigger for a level of arousal, the longer it takes to recover.
• Repeated exposure to the trigger reinforces the panic response and then prognosis becomes poor.

Finally, I found an article by the American Behavior Associates. In it, an example was given of two litter mates that had been best friends but started to fight after a visit to the vet. The article labeled the incident as “redirected aggression.” Redirected aggression can actually occur in dogs but is believed to be short-lived, in contrast to cats where it can result in long-term relationship problems. A few of the article’s points stood out to me:

• Redirected aggression in cats with a formerly positive relationship usually has a good outcome.
• As in the Ogata article above, repeated exposure to the trigger reinforces the panic response and the prognosis becomes poor.
• The more the cats “practice” these hissing/growling behaviors, the more ingrained they become.
• It’s not uncommon for resolution to require several months of management and behavior modification until the cats can get along again.
• Prevention is important.

Based on the above sources, I’d now have several new questions to pose if I were to redo my survey. For example, how sound-sensitive is the aggressor? Rainy is so sensitive to loud noises that she’ll flee the second she suspects anyone of being a handyman. How many members are there in the household? Our three cats live with just my husband and me. What was the relationship between the cats before the non-recognition incidents? Although Rainy has slept with and even groomed both of our other cats, she has a far better relationship with Bootsie than with Cinder. Of the kittens we fostered that experienced non-recognition, all had strong relationships and all quickly overcame their fear of each other. Finally, how long were the cats together after an incident before they were separated?

What’s next?

One year after we had to fully reintroduce our cats due to non-recognition aggression, my husband and I still live with gates dividing our home. We have learned about anti-anxiety medications, the importance of providing cats with escape routes, the value of pattern games, the benefits of regular and structured enrichment including play, and the necessity of always monitoring cat body language to know when to take a step back and when to take a step forward. Thankfully, our cats are now at the place where they can be together if supervised, and so we do believe they may one day be reunited.

Prevention would be the best answer for any and all cat owners faced with the challenge of this phenomenon, but for that to happen more research is needed. I’d love to hear your case studies, for more discussion to occur on this topic, and for a cat researcher to undertake a correlated study on this topic.

References

1. Wilson, J. (n.d.) Aggression in cats after a trip to the veterinarian. Cat World website, last accessed 2/13/23
2. Non-recognition aggression in cats. Tufts Catnip (2008), last accessed 2/13/23
3. How to deal with non-recognition aggression in cats. The Cat Site website (2017), last accessed 2/13/23
4. Dodman, N. (2022) Quoted from personal correspondence
5. Amat, M., Manteca, X., Le Brech, S., Ruiz de la Torre, J.L., Mariotti, V.M. Fatjó, J. (2008) Evaluation of inciting causes, alternative targets, and risk factors associated with redirected aggression in cats. Journal of the American Veterinary Medical Association 233:4, 586-9
6. Ogata, N. (2001) Feline Redirected Aggression. World Small Animal Veterinary Association World Congress Proceedings.

Allison Hunter-Frederick is a certified cat behavior consultant and trainer, cat therapy handler, and pet education blogger. She is also the mother of three fur kids and several revolving foster cats, a host mom to an international student, and the wife of a supportive husband. Through her business, Allison Helps Cats LLC, she helps cat owners improve their relationships with their cats. Allison has presented for animal welfare organizations, been published in local and national publications, and volunteers with her local humane society and her local cat shelter.