Why is my cat aggressive towards his sister after a vet visit?

Cats will often do this if they have been separated for a period of time, especially if one cat looks or smells different - your female cat may have strange smells on her from her time outside. Most of the time, this reaction is mild and the cats will get along again after a day or two. If there is any serious fighting or if his aggressive behavior towards her does not seem to be improving, then you may need to separate them and gradually reintroduce them to each other - feel free to ask for additional help on this if needed :)

This happens quite a bit actually. Could be territorial aggression - she left and now he is trying to tell her the house is his OR it could be that he feels threatened due to the way she smells when she returns. I suggest you start using some Feliway products. I would spray the carrier when you take the cats to the vet and use the diffusers (multicat) in the house. ( https://www.feliway.com/us ) ( https://www.feliway.com/us/Why-Use-Feliway/how-to-stop-cats-fighting )

There does seem to be an unusual phenomenon that when one animal attends the vet for a time its companion can react badly on its return. Possibly due to unfamiliar smells. Usually it settles after a week or so. If not you could try bathing the cat or invest in a pheromone diffuser to try and settle them.

Did Pernell go to the vet recently or leave the house for a period of time? When cats go to the vet or leave the house for a period of time (such as going to a kennel), they can come back smelling differently and that would cause the existing cat to view the returning cat as a stranger. Then that would lead to aggression. Or it could be that Gattino isn't feeling well. If he was in pain that would causing him to react aggressively to Pernell. I'd recommend a check up with the vet for Pernell to make sure everything is ok. I hope this helps!

Oh dear, how stressful! Unfortunately it can be common for cats to lash out when one cat in the household suddenly smells or acts different - as they tend to do when they come home from the vet. Most of the time they go back to their normal interactions once the strange smell (in this case, from the vet clinic) wears off and the cat relaxes and acts like herself....but in your case, there's nowhere for Tribble to hide and isolate herself while she readjusts to being home. The most important thing right now is to not allow them to fight or have negative interactions. As long as they can be kept separate while they both settle down, they will probably do ok - but if they continue hissing and growing, and worse yet if it escalates into a fight, that could permanently damage their relationship. I would absolutely keep them separate until they seem to be relaxing. I understand that'll be challenging in a 1 BR living space - but if possible, you could put 1 in the bathroom with her own litterbox & food, and then have them switch places tomorrow so they can get reacquainted with each other's smells. If that's not possible, you may need to crate them both while you work on their reintroduction. Perhaps you could borrow dog kennels so they can each have their own living space while they're confined. I know this isn't an easy answer - but the most important immediate intervention is to do what you need to do, to keep them from fighting while the stress levels come down! Finally, adding a pheromone diffuser such as Feliway Multicat may help promote a calming atmosphere. I hope this helps!

Thank you for submitting your question regarding your cats. I suspect that Stella is stressed by the return of her housemate. Callie likely smells and acts a bit different after being gone for so long. I would keep them separate to allow them time to readjust to each other. They should get along fine, but it will take some time. I hope this information helps!

Thank you for submitting your question regarding your cats. This can be common after a trip to the veterinarian. The kitten will be covered in different smells. This can cause stress in the older cat resulting in her to act out in aggression. Keep them separate unless they are fully supervised. They should be fine with each other in a few days. In the meantime, you can try a pheromone diffuser to help calm them. Feliway is a good brand. I hope this information helps!