Well this has not only got me but the owner totally stumped. Yesterday I had to take one of the two cats I’m looking after to the vets…
For the poor cat it went well and horrible. We got good news in what ( had been wrong) kidney stones completely disappeared yes here is the first BUT the vet had her own dog walking around free in the practice and there was another dog probably in a backroom in a kennel barking totally nuts. So poor. kitty was totally freaked out and when the vet took him out of the carrier to make an x-ray he bolted and her dog chased the cat ( aaagh was so angry about this but in a country where i don’t speak the language enough to really voice my irritation) Vet otherwise very calm and hugged ct when she caught him so here comes the 2nd BUT
Have had cats all my life and cared/sat voor many on THS so experienced everything even to being mauled hed to toe by a cat ( scars to prove it). But since we returned home yesterday teatime. it’s now 11 the next day the other cat in the house has beensuper aggressive towards the lets say vet cat and seems to be regarding him as a totally new and unknown to her cat.
Hissing. swiping at him and being generally unfriendly/aggressive/constant growling Have had plenty of experience even with my own cats of reactions after vets but it’s usually limited to uuugh. you stink etc of vet and for a few hours they walk round each other. But I think due to the dog incident. the vet-cat was so pumped with adrenaline that he not only smelt of that but als of the disinfectant so a double whammy for the poor thing. And he is such a sweetie so he is completely confused by her behavior as am I.
I’ve got Feliway classic. and give them treats together so they have visual contact but sometimes it’s like she fazes out of realising who he is so they walk past each other then she turns. sees him and thinks ggrrrrr. Oh yes forgot that i’m not happy with him. Hope this calms down soon. Been in constant touch with the owner and it sounds like she is acting the same as when the boy cat was first brought to the house.
Any tips would be welcome
Hi @Maggie
As someone that has looked after over a 100 cats and volunteered at a shelter I think you’ve nailed it. The cat has come back with a different scent which is provoking the other cat to attack. Hopefully as the cat washes itself and the scent fades the cats will go back to normal.
This happened to my cats after one got lost outside for a day (they were indoors cats)! The other one went mad at her. We thought it was a scent issue that she smelled weird from being outside.
yes richten also my idea but never experienced it so extreme as this. The other cat is taking the aggression to the extreme over reaction level even towards me a little it’s as if she sees, him and myself (partially) as totally new and untrustworthy beings even though i’ve already been here 9 days in this sit. But from experience I know the status quo just needs to be re-established and hopefully they’ll just go back to enjoying their staring out of windows and chasing toys.
I have seen it take a number of days unfortunately @Maggie for cats to go back to normal. Just keep doing the feeding together thing.
Yes by the amount of her reaction expecting that to take a few days. Luckily their mum comes home ( late) tomorrow night so hoping that will at least help to calm her a bit more. But feel so sorry for the poor guy ( a lovely chunky marmelade/ ginge) he really does not get why she’s being weird
To calm down the “weird one” and to protect the “poor guy” from the weirdness, it might be a good idea to keep them separate until the owner comes back. 
@Maggie, this happens every time I take one cat but not the other to the vet. The cat is confused because their housemate looks the same but smells really weird. I do have a way to fix it which I use myself, YMMV on whether you feel comfortable using the strategy on someone else’s cats, but: I make them smell the same. I spray a little perfume on my hand and give them each a little rub on the top of the head. It literally works every time for me.
Yes been doing that not with perfume but with stroking one then other and even wiping them with blanket he uses. I guess we’ll just have to be patient and. Just give them a few days to re-adjust.
I had a long readjustment with 2 of my foster cats once. One of them was spayed and when I brought her home after surgery, her brother was really hostile for days. Normally it only takes a few hours for the cats to get used to each other again.
I recall a similar scenario with my cat’s Cheyenne and Dakota. In the early 90’a I adopted sibling kitties bro and sis who had been an owner surrender at the San francisco SPCA. Their previous owner had to rehome them due tona terminal illness. Anyway, long story short. I kept them inside for a month although they adjusted quickly before letting them out to explore, they had brrn outdoor cat’s even though the SFSPCA discouraged it. Once they went out Dakota became king of the backyard and Cheyenne queen of me and the house. Soon after when he’d come in she wouldn’t let him eat, hissed and would swipe at him. It was pretty obvious his new strange scent didn’t go well with her.
@Maggie This was going to be my suggestion, too. Just stroke the both cats with some of their bedding. Distract any signs of aggression with toys and games. They’ll soon get back to normal.