Case Study: Mimi and Leo — Sudden-onset Aggression in a Cat

Written by Monika Januszewska, CCBC

A decorative cover image of two cats sitting on a couch. One cat is mostly covered with a blanket.

Summary: After years of the two cats living together with no issues, Mimi suffered an acute medical event and started to behave aggressively towards Leo. The aggression continued even after her recovery from surgery, which prompted the intervention documented here.

  • Subjects: Mimi  and Leo
  • Age: 10 years old
  • Breed: domestic shorthairs
  • Sex: Mimi  – female (spayed); Leo  – male (neutered)
  • Housing: Living together in an apartment for almost 10 years


Presenting complaint: Mimi’s aggression toward Leo

Acquired from, age at time: Both cats had been born as house cats, and adopted from private persons via the internet – Mimi  as the first one, and Leo about 6 months later. The owners don’t remember their exact age at the time of adoption, but it was probably about 2 to three 3 months for Mimi and about 4 months for Leo.

Medical history

Leo: In 2019 he began vomiting frequently. A blood test showed some liver problems; however, this was temporary. In early April 2021, soon after the onset of the problematic situation described below, he had a blood test, which didn’t show any problems. It was recommended that he lose some weight, especially as an X-ray had found lumbar spondylosis deformans.

Mimi: In the middle of March 2021, she lost her appetite and started to vomit. She was initially diagnosed with some minor digestive problems; however, after a few days she turned out to have pyometra. She was spayed to cure the condition.

Persons involved in care: Joanna – owner (adult), Tom – owner’s husband (adult)

Housing:  An apartment consisting of one small bedroom, a living room, kitchen, and bathroom. Both cats are indoor only.

Diet: Until recently the cats had free access to dry food, and once a day in the evening they were given about 70g of wet food each. Now Mimi has the same feeding routine, but Leo eats dry food for weight loss (restricted feeding). Both still get their portion of wet food in the evening.

Behavioural history

About two weeks after her surgery for pyometra, Mimi suddenly started to hiss at Leo, and then attacked him. The owners didn’t see the moment of the attack, they only heard hissing and noticed that something was going on. Joanna stated that on that day, just a few seconds before they heard the hissing, she was making a new cat tree and she accidentally dropped a heavy wooden element, which made a lot of noise. Leo was eating when it happened, and he was startled by the sudden noise to the extent that he flipped his food bowl over, the fur on his back and his tail puffed up (piloerection), and he ran away. The owners were so surprised that they didn’t notice whether Mimi was present in the room then or not, but her hissing and attacking Leo started shortly afterwards.

Since that incident, Mimi had been hissing at Leo whenever she saw him. She had also been watching him, and tried to attack him whenever he was passing by. The owners also noticed that Leo became more sound-sensitive and appeared anxious at sudden noises.

Joanna said that until recently the cats have been living together in a neutral-to-positive relationship, sometimes they were seen sleeping close to each other, cuddling, allogrooming etc., but usually Mimi preferred to relax by herself.

After the first attack occurred, both cats were examined by the vet, who didn’t find any abnormalities except Leo’s spondylosis and obesity. Mimi’s condition after the surgery was also described as satisfactory, and she wasn’t showing any signs of physical discomfort.

For about a week after Mimi attacked Leo, the cats had still been allowed to move freely around the apartment, however Mimi was watching Leo all the time, hissing and growling if he happened to be too close, and swatting at him. He had been moving around with caution, trying to avoid passing Mimi, and after a few days he also started to watch her and hiss at her when she approached him. Mimi was attacking him a few times a day, and Leo started to strike back, but there was no blood drawn. Joanna and Tom used a squirt bottle to spray the cats with water when they saw them fighting, which made Leo flee under the bed, and Mimi jump onto the wardrobe. The owners tried to anticipate such situations, and if they saw the cats staring at each other intensely and hissing, they would distract them by yelling or making other noise and separating them for a few minutes. The cats had also been separated when the owners were at work (from about 7:30 a.m. to 4:30 p.m.).

The fights got more intense, so that Joanna and Tom decided to separate the cats completely and email me for a consultation. On the next day Joanna reported that after 24 hours of separation the cats seemed a little calmer and were able to stay in one room without fighting, so the consultation probably wouldn’t be necessary. However, three days later I got another email and Joanna reported that Mimi had started to watch and follow Leo again, and if they didn’t intervene she would attack him just like before the separation. Thus, Joanna and Tom finally decided that my help was needed. I replied immediately, asking them to separate the cats and keep it like this until we could discuss the situation and set the intervention plan.

Initial observations

As the COVID-19 pandemic was underway, the consultation was to be conducted online, and I was only able to see the cats on the videos that the owners were supposed to send me. Before the session, I sent them a questionnaire with several questions assessing Mimi’s and Leo‘s overall welfare and environmental management, and then scheduled the video conference.

The questionnaire included questions about:

  • Mimi’s and Leo’s adoption history
  • Their health status since the moment of adoption until now
  • Feeding management
  • The owners’ and the cats’ daily routines
  • Litter box management and the cats’ elimination behaviours
  • The cats’ relationship so far (until the current problems occurred)
  • What are the cats” favourite spots and hiding places
  • Favourite play time activities of each cat and their frequency
  • Types of vocalisation observed
  • How the owners manage fights between Mimi and Leo

I also asked Joanna to make a home tour video, so that I could see the cats’ resources and their location. This would include litter boxes, food and water bowls, scratching posts, hiding places, elevated spots, etc.

The owners had also made some videos since the conflict started, so I was able to see the cats’ behaviour as it had been before the separation. I asked them not to bring the cats together in order to make more videos, as it would worsen the situation and stress both cats even more.

After receiving the questionnaire back and watching the videos, I was able to make an initial assessment of the cats’ welfare. Both Joanna and Tom worked away from home, leaving at 7:30 a.m. and coming back home around 4:30 p.m. The cats apparently were sleeping most of the time when they were absent, as the litter boxes usually seemed hardly used. Joanna used to have cats in her family home in the countryside, but she was a child at that time, so she didn’t have much experience as far as feline needs were concerned, and Tom had never had a cat or any other pet before. Both of them needed education on cat behaviour and needs, so in my intervention I was going to address not only the current problems, but also the overall environmental management influencing the cats’ welfare.

As far as toileting conditions were concerned, there were two very small open litter boxes – one in the bathroom and the other one in the bedroom, where Mimi had been isolated. Before the cats were separated, both boxes were located in the bathroom, side by side. They were filled with corn litter, which is not the substrate I usually recommend, but as Joanna stated that neither of the cats had ever had any problems with litter aversion, I decided not to address this.

The food bowls were also placed in separate locations, Leo’s in the kitchen and Mimi’s in the bedroom, as far away from her litter box as possible. Before the separation the owners didn’t notice any conflicts around the mealtimes. The cats had free access to medium-quality dry food, and every evening they also got about 70 grams of wet food (meat pieces in jelly) each. Shortly before Joanna and Tom contacted me, Leo’s feeding routine had changed. He had been put on restricted-calorie dry food and was allowed to eat three meals a day. However, he usually didn’t eat everything, so in fact there were always some kibbles in his food bowl.

The relationship between Mimi and Leo was seen by the owners as rather neutral, and occasionally positive, when the cats were allogrooming or cuddling while asleep. However, Mimi usually sought more isolated spots, e.g., the top of the wardrobe, and Leo preferred the sofa or the windowsill lined with the blanket. Mimi sometimes (but rather rarely) liked to jump onto the top of the cupboards in the kitchen. They had one small cat tree (about 1.2 m tall) with scratching posts and two platforms.

As far as play time was concerned, the owners reported that the cats were not very playful, and they had never been interested in playing with fishing rod type toys, balls, or mice toys. Leo used to like chasing laser light when he was younger, and now he sometimes gets interested in his feeder ball, whereas Mimi got interested in toys only after the surgery and started to like playing with wand toys.

Until recently, the cats had only been vocalising (meowing) toward the owners when they wanted to get some extra treats, and occasionally when any of the apartment doors were closed, as they were used to having unrestricted access to the entire apartment. Besides that, they had also been purring when petted. The owners reported that when Mimi began to hiss and growl at Leo and soon he also began to hiss at her, that was the first time they had ever heard their cats vocalising like this.

In the videos I could see Mimi usually staying in some elevated locations (most often the cat tree), and intensely watching Leo, who was either sitting or lying on the sofa, on the chair or on the floor. In one of the videos, as soon as she saw Leo walking toward the bathroom where his litter box was, she jumped down and launched in his direction. Then hissing could be heard (it was impossible to tell whether it was Mimi or Leo, or maybe both), Leo fled to the corner and stayed there, crouched, whereas Mimi returned to the cat tree.

In another video I could once again see Mimi on the cat tree, watching Leo, who was sneaking into the room very cautiously, looking around. In yet another short video Leo was sitting on the chair, about 4 metres from Mimi, who was lying on the cat tree platform, and they were staring at each other, which Joanna reported was how they had been spending most of their time now. As I have already mentioned, I asked Joanna to separate the cats permanently (for the time being) and to not let them have any contact. However, the owners were so impatient to check if the situation had improved that from time to time they let Mimi out of the bedroom.


The most important antecedent and direct cause of Mimi’s aggression toward Leo was probably the sudden noise made by the wooden element falling on the floor. Leo startled and the fur on his back and tail underwent piloerection, which is an agonistic behaviour usually in response to a suddenly appearing threat, accompanied by an increase in adrenaline levels.1 It is possible that Mimi, witnessing his behaviour and seeing his body language, had perceived this as directed toward her and reacted accordingly, trying to increase the distance by hissing, and at the same time warning Leo not to move any closer. This would have been hard to achieve if he (as we can only guess) bumped into her while he was fleeing to one of his hiding places. As a result she might have responded with a similar body posture, which made him even more frightened – not only of the sudden noise, but also of her. Thus we might call it a somehow misdirected fight or flight response, and as they continued to see each other on the following days, with only a short separation time, they remained aroused for longer. Mimi, to gain control over her environment and make it predictable again, started to watch Leo and tried to deter him from moving around by hissing and launching at him. Leo, on the other hand, was still under the influence of that acute noise, which now had made him oversensitive to sudden noises. Additionally, he was trying to avoid Mimi, whose behaviour he perceived as threatening, and eventually started to fight back, which resulted in the situation’s escalating.

However, I couldn’t overlook the overall environment, as it certainly had a great impact on Mimi’s and Leo’s welfare and their relationship, so it needed to be addressed properly irrespective of the present problem. Since the resources that had been accessible to the cats were scarce and of rather poor quality, it was possible that there had been a silent conflict underway for a longer time, despite the moments of allogrooming and cuddling. The owners, being absent most of the day and having little knowledge of feline needs, might not have been aware of this fact.

They also probably didn’t realise that Mimi’s withdrawal from interactions with her surroundings might have been caused by her worsening condition. The same applies to Leo’s disinterest with play – both his excess weight and spondylosis might have made it more and more difficult to engage in play. Moreover, the owners’ lack of understanding of how important the opportunity to play is for cats made them neglect this feline need. I see a similar mindset quite often; many owners perceive play as an activity belonging to the kittenhood – probably analogous to our human childhood and the fact that we usually “grow out of” the need to play. However, for cats, play offers a chance to engage in hunting behaviour, which is an innate need present in their ethogram. If the cats are kept indoor-only, they have no possibility to meet their hunting instincts other than by playing. Being able to complete the hunting sequence (wait – crouch – approach – pounce/chase – kill) on regular basis in the form of play gives the cat very needed mental stimulation.

Mimi and Leo most probably had had so few toys at their disposal so far, that they simply got bored with them, as habituation is said to take place very quickly.2

It is also possible that the cats were trying to avoid each other most of the time, as it must have been difficult for them to share the territory. The litter boxes, which are a very important resource for cats, were way too small and additionally placed too close to each other in the bathroom. Thus, not only were they uncomfortable and not allowing for a normal elimination behaviour, but also they might be perceived by the cats as one elimination location, which made them a resource too scarce for two adult cats. Very limited access to vertical spaces might have made Leo and Mimi feel frustrated and insecure, especially Leo, who hadn’t been able to get higher than a windowsill by himself, and until now he wasn’t even able to access the cat tree without the owners’ help (Joanna was making a new cat tree adjusted to Leo’s capabilities when the incident had happened). If we add to this picture a significant lack of play, we can assume there was a considerable level of frustration in both Mimi and Leo.

Moreover, I suspected that Mimi, feeling better after the surgery, began to perceive her environment differently – she now not only wanted to play and to interact with her owners, but it is possible that she also got more conscious of the deficits of the resources. That might have been an additional reason she had now been making multiple efforts to control her territory by watching Onyx all the time and attacking him when he was moving around (e.g., while he was trying to get to the litter box – a highly valuable resource).

Thus the misdirected fight or flight response might have been an immediate antecedent of the agonistic behaviours expressed by Mimi, and eventually, by Leo as well, but it might also have been a more indirect trigger, making Mimi look at Leo as a rival to the resources.

Additionally, the fact that the cats hadn’t been able to reduce their arousal levels because they were exposed to each other’s presence over and over again made the situation more complicated, as this might have consolidated the memory of those aversive interactions and made it more difficult to overcome.3

That’s why in my intervention plan, besides addressing the primary complaint, I wanted to advise the owners about introducing some changes in the environmental management of the cats.

Intervention recommendations

My intervention plan was to temporarily separate the cats to let them calm down and allow them to benefit from the environmental and cognitive enrichment I was recommending. Then I was going to reintroduce them through controlled interactions, using a screen door, and ultimately move on to gradual desensitizing and respondent counter-conditioning of both cats to each other’s presence. The final step was to let them interact without the screen door between them, and to gradually extend the time they spend together. I didn’t want to rush anything, as moving too quickly might have forced us to go back to the previous stage, and as a result the whole process would last longer and cause unnecessary stress in the cats and disappointment in the owners.

Consult 1

We had our consultation over Zoom and started by going over all the information I had received so far. I pointed out the possible causes of the conflict and explained how environment management influences feline behaviour. What many owners don’t realise is that cats are territorial animals and predators, even if they have lived all their life in a small apartment – and that was true for Joanna and Tom, who perceived their cats as nice furry creatures who “lived only to be cuddled.” My first recommendations applied primarily to lowering the levels of arousal in both cats and to introducing some environmental modifications to enhance their sense of security and to reduce their need to compete over resources. If both Mimi and Leo noticed that in their environment there was “enough of everything” for two cats, their level of frustration should decrease, so our additional steps would be made easier. I instructed Joanna and Tom about the importance of giving the cats an opportunity to express hunting behaviour and, using pictures and videos, I showed them how proper play with a cat looks. I also wanted to address Leo’s excess weight and suggest some feeding management modifications, as his condition certainly affected his overall fitness and wellbeing. Moreover, as obesity in domestic cats is often associated with boredom and/or chronic stress, I believed that there were some aspects of environmental management that need to be modified.

Recommendations after Consult 1

– I recommended keeping Mimi and Leo separated all the time, and not checking every now and then what their attitude toward each other is, for about a week – of course this didn’t mean that each of the cats wasn’t allowed to interact with the owners. I asked Joanna to let me know after one week how the cats had been behaving (for example, scratching the door, appearing anxious when hearing or sensing the other cat behind the door), and based on that I was going to decide if we could go to the next step, controlled contact.

– In order to lower Mimi’s and Leo’s reactivity, I recommended 225 mg of Zylkene (an over-the-counter calming supplement) once a day.4,5

– As play is said to have beneficial effects on reducing stress levels,6 I strongly recommended daily play sessions with each of the cats, using long fishing rod type toys and wand toys, and trying to imitate the hunting sequence. The person holding a fishing rod or a wand should move it in a way resembling the cat’s prey’s movements – this triggers their hunting behaviour, and encourages them to engage in this activity. Play hunting should provide mostly mental stimulation, which is beneficial for the cat’s wellbeing. I instructed them to adjust the intensity of play to each cat’s physical capabilities, though I never recommend tiring the cat physically, as it increases their mental arousal.

– I recommended modifying feeding strategies by introducing small meals of good quality wet food when the owners are at home, and leaving dry food in active feeding toys, food puzzles, etc. when they are absent. I provided Joanna with some DIY food puzzle tutorials, because she had decided to make them herself. I also advised the owners that some wet food might be placed on feeding mats, not only to make eating slower and diversified, but also because licking is a displacement behaviour with calming effects.

– As litter box is a very important and highly valued resource in an indoor cat’s territory, I strongly recommended purchasing two big litter boxes (the length of at least 1.5 cat’s length and width at least 1 cat’s length). The “golden rule” says that the number of litter boxes should equal the number of cats plus one, but due to limited space in the apartment this would be hard to meet.

– Adding more vertical spaces and making these spaces available to both cats was essential as well, so I suggested at least making the existing places more accessible for both cats, and asked Joanna and Tom if they might consider adding a few shelves leading to the wardrobe, bookshelf or cupboard tops, and also adding a comfortable platform shelf (or shelves) that might be accessed from the cat tree, thus giving the cats more resting places. Since there was a huge wardrobe in the bedroom, but unfortunately its top was stacked with suitcases and boxes, I asked the owners to remove some of them and rearrange the space over there, so that it enabled Mimi to use it as her resting place.

– Since the only safe hiding place on the floor level seemed to be under the bed, I recommended adding 2 cardboard boxes (one in the bedroom and one in the living room) as additional hiding places, which might not only give the cats possibility to hide, but also to observe their surroundings while being away from the centre of attention (“I can see, but I cannot be seen”). Each box was supposed to be of the size enough for a single cat to be comfortable, but not allowing two cats to fit in – this recommendation was for safety reason, so that neither of the cats could ambush the other one inside the box. I also recommended leaving an open cat carrier as a hiding place, which additionally would teach the cats that it is just one more safe spot, and not something associated with distressing veterinary visits. Originally I also suggested adding a tunnel and at least two covered cat beds, but I later modified this recommendation, as I could see that the owners were reluctant, not only due to the lack of space, but also for financial reasons. Giving Mimi and Leo access to the cardboard boxes and to the cat carrier seemed to be a more reasonable (and affordable) alternative.

– As a preparation for the next step I asked the owners to think of installing a screen door between the living room and the bedroom, enabling the cats to have controlled interactions. However, I asked Joanna and Tom not to use it yet.

Consult 2

Our next consult was scheduled in about a week’s time, to analyse the modifications that Joanna and Tom implemented and to talk about the next step, depending on Mimi’s and Leo’s behaviour. However, only two days after the first video conference I learnt that the owners had already installed the screen door and let the cats interact, which hadn’t ended well, because Mimi started growling and lashed towards the door, and Leo was hissing and he ran away, so after a few minutes the owners had closed the door. Thus, I recommended prolonging the separation, so that the cats were truly isolated for one week, and stressed the importance of not allowing them to see each other on a random basis, as they would never be able to calm down. What I wanted to achieve first of all was reducing overarousal of the nervous system in both cats, and improving its functioning through environmental enrichment, which has been shown to have a beneficial influence on the brain, decreasing aggression in social contacts and leading to increased serotonin levels and thus improving the mood and overall being.7

Luckily, the owners also bought new cat toys and both cats got very interested and were eager to play (which shows clearly that it is sometimes the matter of appropriate toys and educating the owners). Mimi got access to the top of the wardrobe in the bedroom, which she had been using regularly now. They also ordered a new, much bigger litter box, but only one (for Leo), as they stated that there was no space for a huge box in the bedroom – I couldn’t help but accept that. Both Mimi and Leo started to receive 225 mg Zylkene a day. They also were being given more wet food, and the first DIY food puzzles were being prepared by Joanna.

Consult 2 also was a video conference, during which I asked the owners to allow the cats to see each other through the screen door for a moment, because I wanted to see whether anything had changed. I could see that both Mimi and Leo approached the screen door and were staring at each other, but after about 10 seconds Leo startled, hissed and ran away to the bathroom. Mimi stayed calm throughout the interaction. The door was closed immediately afterwards, so the whole interaction lasted about 20 seconds. Neither Mimi nor Leo showed any anxious behaviours after the door had been closed, and they resumed their normal activities (relaxing on the bed and on the armchair, respectively), which I perceived as a positive indicator that the level of arousal hadn’t been very high, though it existed anyway. However, since the memory of the incident and its association with the other cat was still quite vivid for Leo, and he was the one who seemed most affected during the incident (both by sudden noise, and then by Mimi’s aggressive reaction), I asked for another week of complete separation. The sight of Mimi seemed to act as a stressor here, so we definitely had to control it, and let the time work for us for the next few days. I also wanted to give Leo more time to benefit from the new cognitive and environmental enrichment – regular play time, food puzzles, and a cardboard box that he liked from the start, plus his huge new litter box, which had made him really happy and resulted in more frequent visits there.

From what I had learnt from Joanna, he had tended to urinate not more than twice every 24 hours in the previous litter box, which proved how uncomfortable it must have been for him to use it. He also had his diet modified, so for the previous week he had been eating (and so had Mimi) more wet food, which certainly also resulted in more urine production, so the new comfortable litter box was definitely useful. Of course, Mimi took advantage of all the above enrichment as well while staying in the bedroom, with the exception of the litter box, because there was no room for the new one in the bedroom. Both cats had been on 225 mg Zylkene for about a week now, and I recommended extending this for at least next month, as Zylkene can be taken for about two months before reassessing the cat again.8

In this video meeting I started to talk to Joanna and Tom about preparing the cats to go through the process of desensitization and counter-conditioning, as relying only on passing time and mere habituation (which would be based on Leo‘s repeated exposure to Mimi, and vice versa, until they both would become used to each other’s presence and would stop responding with arousal) to improve the situation would take a very long time and might not ever be successful.

I explained to Joanna and Tom that when we would desensitize the cats to each other’s presence, we would let them see each other from a distance that wouldn’t be stressful to either of them for a limited amount of time. Then we would gradually either decrease the distance or increase the time of exposure (i.e., the amount of time they are seeing each other), so we would slightly increase the strength of the stimulus. In this way both cats would gradually get used to each other’s presence. Simultaneously with this gradual exposure we would start to build positive associations with the presence of the other cat by respondent counter-conditioning. We would thus pair the aversive stimulus (Mimi’s presence) with a new, pleasant stimulus (e.g., food), and in this way we would change Leo’s response to Mimi’s presence from fear and arousal to more positive association (“When I see her, something nice happens”). I already learnt from Joanna that Leo had always been very food-oriented, so food was a good choice to help him start to build pleasant associations with Mimi. Eventually, these positive associations were expected to replace the fear and arousal, and cause Leo and Mimi to feel better in each other’s company.

In practice the above would mean that from the moment Mimi and Leo would see each other through the screen door, Joanna would start delivering very small treats to him, one after another, for as long as the exposure would last (that could be for a few minutes, depending on both cats’ reactions). During first sessions the delivery of treats should be frequent, so that Leo would not have time to think of Mimi being there, though of course he would be able to see her. Since this was respondent counterconditioning, there was no mention about reinforcing behaviour, as I didn’t want Joanna to ask Leo for another behaviour instead of looking at Mimi. My aim wasn’t operant counter conditioning, i.e., replacing one behaviour with alternate incompatible behaviour and reinforcing the latter – all I wanted is to build a new, positive association with no specific behavior from the cat.

Recommendations after Consult 2

– I suggested they continue with the separation for the next week, as well as to remember to engage in regular play time sessions and daily Zylkene for each of them.

– I asked Joanna (she seemed more engaged, probably because she had more time than Tom, and Mimi and Leo were more her cats than his) to think about Leo’s favourite treat, which might be used in counter-conditioning, and she decided that freeze-dried meat treats were the most attractive for him.

– When that weeklong separation ended, I suggested trying to open the door (with the screen door still separating the cats) again, and this time luring Leo with food to move as far away from the screen door as possible (it was our way of establishing safe distance, allowing for Leo’s exposure to the stimulus, i.e., Mimi, without a stress response from him), and start  the procedure I had concisely described for Joanna:

Leo sees Mimi. Very small treats start flowing, one after another, for just 2 to 3 minutes. Observe both cats for possible signs of stress. If any occur, stop the exposure and close the door sooner. Try to take a video (or ask Tom) for future reference. After 2 to 3 minutes the session ends = treats stop flowing. Repeat this sequence about three times a day and make the session slightly longer every other day. Send me videos from each day.

As English is neither my nor her native language, I translated an article for her on the “open bar/closed bar” technique from Pethelpful, so that she could review this procedure for herself.9

– I also suggested placing some obstacles (e.g., cardboard boxes or tunnels) on both sides of the screen door, so the cats didn’t feel so exposed to each other, and were able to partly hide from the view if they had such a need.

We scheduled our next online consult for a week later, but of course we were going to keep in touch via email, as I was supposed to get videos as well as reports describing both cats‘ behaviour.

From the videos I got from Joanna every day I could see that Leo responded very well. He was lying on the cat tree while looking Mimi, and eating his treats at the same time. At first his body language indicated some anxiety (tense muscles, wide open eyes), but it decreased with every session. As far as Mimi was concerned, she was mostly interested in getting behind the screen door, and was hardly paying attention to Leo. Just like I asked Joanna, she gradually made the sessions a little longer, and I also advised her to start delivering treats a little less frequently.

Consult 3

Now the time had come to engage Tom. I originally planned to go on with respondent counter-conditioning Mimi in the same way as with Leo, but unfortunately she had turned out to be completely uninterested in treats of any kind, so we had to come up with other pleasant stimulus, and that happened to be play.

Thus, while Joanna was on one side of the screen door, counter-conditioning Leo with treats, Tom was supposed to play with Mimi in the bedroom. This would help Mimi build a positive association with Leo’s presence, and at the same time Leo would start being desensitized to the sight of Mimi’s moving more energetically.

They set the camera so that I was able to watch Tom playing with Mimi and Leo’s reaction. I asked Tom to start playing as far from the door as possible (which wasn’t easy, as the bedroom was tiny), to make this stimulus a little weaker. Luckily, Mimi engaged in play, and she didn’t seem to notice Leo on the other side of the screen door. I was able to see that Leo got slightly tense for a few seconds, but he continued to eat the treats that were being delivered by Joanna, and after a while he seemed quite relaxed once again. Tom continued to play with Mimi for maybe 2 or 3 minutes, then the door was closed and Joanna stopped delivering the treats to Leo.

I asked Joanna and Tom to repeat this sequence a few times a day, at different times (preferably once in the morning before they leave for work, and two or three times in the afternoon, in the evening and at night, before they go to sleep). I was aware, however, that the morning session might be difficult, as the owners were usually rather short on time. Just like before, I asked the owners to send me the videos from their daily sessions. I also asked Joanna to engage Leo in play activity every now and then during the DS/CC session, as he seemed more and more happy with his new toys, and he also was watching Mimi’s fishing rod toy flying with great interest. Thus, that could be another way of building positive associations, and Mimi might also get used to seeing him moving, and not only lying down and eating treats.

I also decided to address an important territorial issue, to prevent potential problems when the screen door would be removed. I have noticed, from my numerous other consults, that if the cats are separated for longer, and then brought together again, they tend to establish new division of the territory, presenting defensive behaviours (blocking the entrance, chasing the other cat away etc.) if the other cat tries to enter, or even is in the vicinity of their room. That’s why I always recommend regular room-swapping at some point in time – its role is to let the cats leave their scent in the other cat’s room, and also to feel secure and self-assured in this part of the territory that hasn’t been used by them for so long. Therefore I advised Joanna and Tom to continue with the exercise of Mimi playing in the bedroom, Leo getting treats in the living room to make more positive associations, but at the same time, when we made sure that Leo wasn’t showing any anxiety watching Mimi play I recommended swapping the rooms. Thus, Leo would stay in the bedroom and be getting treats from Joanna, and Mimi would play with Tom in the living room. Then the door would be closed, so that each cat might be able to relax in that new part of the territory.

This time we didn’t schedule next video conference but agreed to be in contact via emails and telephone calls. Certainly, in case any problems popped up, another online meeting would be arranged. Video follow-ups were to be continued every day. I asked Joanna and Tom to let me know how room-swapping progressed and how the cats responded to it. Then we would set up a session to bring the cats together.

Recommendations after Consult 3

– Continue with Zylkene, regular play sessions (besides the desensitisation and counter-conditioning sessions), and have in mind the need to further enrich the environment (as the shelves I had recommended hadn’t been ready yet).

– Continue with DS/CC sessions and include play sessions with Leo as well, and if everything proceeds as intended for the next three days, begin room-swapping

– After room-swapping, Joanna was to make treat delivery more frequent once again, as I expected Leo to get stressed by the change of place, so the pleasant stimulus had to be more intensive again. I also asked Tom to play with Mimi in the living room farther from the screen door, thus increasing the distance and at the same time decreasing the aversive stimulus strength (i.e., her presence).

After the first room-swapping both cats turned out to be a little anxious, even Mimi, who had been rather stable so far. As for Leo, from the video I received suspected that watching Mimi from a different perspective and her moving more freely around the living room while playing had made him more tense and watchful again. Luckily, his love for treats had soon won. I was a little surprised when I saw Mimi’s behaviour in the first video, as she was playing eagerly, but from time to time she was staring in Leo’s direction, as if she had been distracted by something. Joanna had noticed that as well, and when we talked about it on the phone, we had come up with the idea that Mimi had probably been distracted by the sound of treats falling on the bedroom floor (different than in the living room). As soon as Joanna started to deliver the treats in a different manner (putting them quietly on the floor), the problem had disappeared. Just like I had advised, Joanna introduced play sessions interchangeably with treat sessions for Leo, and neither of the cats seem to mind it, as both were so absorbed with their own hunting.

After about week the cats didn’t seem to care if they stayed in one room or the other. When the DS/CC session was over, they didn’t mind taking a nap or even using the other cat’s litter box, which turned out not to be an issue for that litter box’s “owner.”

It was now the time to think of opening the screen door, so we scheduled Consult 4. Originally I planned only to guide the owners via email as to how it should take place, but seeing how anxious they were becoming, I decided that our video contact might be useful, as I would be able to calm them down (even by merely engaging them in the conversation).

Consult 4

Before our video conference I asked the owners to prepare a towel or a cardboard sheet, and to have it around just in case the cats started to fight and had to be separated.

The plan was as follows: With Leo in the living room and Mimi in the bedroom, Joanna and Tom would engage in the DS/CC procedure, and after a few minutes Tom would end the play session, spend a few more minutes with Mimi, but without interacting with her, and then just get up and quietly leave the room, leaving the screen door slightly open.

I wanted Joanna to deliver treats to Leo if he was interested (it turned out later that he wasn’t), but otherwise neither she nor Tom were supposed to interact with cats – no petting, cuddling, calling their names, etc., as these would be unnecessary stimuli.

The whole situation was planned for just a few minutes, as my aim was to keep the cats below their threshold, so I wanted their interaction to end before they would get anxious.

After Tom had left the screen door open, Mimi stayed in the bedroom for about five more minutes – she hadn’t noticed that she was able to leave. In the meantime, Leo settled in for a nap on the windowsill, so when Mimi finally entered the room, he wasn’t aware of it, until she had made some noise walking around. That made him a little anxious, but it was far from a startle response. He jumped on the floor and approached Mimi, and for about 2 minutes they we walking around the apartment, sniffing at each other from time to time. Both Joanna and Tom got even more anxious than the cats, so I had to distract them a little by engaging them in small talk, and at the same time observing the cats. After exploring the living room for a few minutes, Mimi ate some food from Leo’s food bowl, went back to the bedroom, jumped on the top of the wardrobe (which might indicate that she had gotten a little anxious but was able to control it without resorting to aggression). I asked Joanna to close the door and let the cats relax.

I recommended repeating this procedure two to three times a day, and if neither of the cats was getting anxious, the duration of their interactions might be slightly increased.

Joanna was sending me videos every few days; she was still getting anxious at times, which was quite unnecessary because nothing bad had been happening between the cats. There had even been a few times when both Mimi and Leo engaged in allogrooming, so everything seemed to be going into the right direction.

The moments when the screen door was open were longer and longer, even at night, althought the cats were still separated when the owners were at work.

After two more weeks Joanna told me that it seemed that everything had returned to normal, and so they had tried to leave the cats together, unseparated all day long, and it worked fine (not good that she hadn’t contacted me first, but I knew she was already feeling quite confident about the cats at that time).

Thus I might say “Mission accomplished,” although I am always ready to offer advice if needed.

My suggestions for ongoing maintenance for Joanna and Tom were the following:

– Remember about environmental enrichment and make sure the cats have the resources that make them feel safe in their territory.

– Continue with play sessions with each cat separately (as this will reduce the risk of competition, and above all, hunting is a solitary activity for cats, so play should also be), and don’t forget it should be a part of their daily routine.

– As both Mimi and Leo are senior cats, be conscious that their health might start to deteriorate – take them to the vet at least once a year.

– If by any chance a similar incident ever happened again, separate the cats immediately and keep it like that until they contacted me or other cat behaviour consultant.

I also encouraged Joanna and Tom to enroll to my webinars on feline behaviour. However, I believe that they have gained quite a great deal of understanding of feline nature thanks to our emails and conversations.


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TO CITE: Januszewska, M. (2022). Case study: Mimi and Leo — sudden-onset aggression in a cat. The IAABC Foundation Journal 25, doi: 10.55736/iaabcfj25.9