I have no ideas other than what you're doing but this happened to me years ago. My Tortie went in to be spayed, and when she returned the next day, all the other cats hissed at her and avoided her. But soon it was back to normal.

Hi lily...
The youngest one has always been a pill when her mom goes to the vet. Even when the mom is only gone briefly for a dental hygiene appointment, her daughter would growl and hiss for ten days! I was hoping that after ten days, all would be back to normal, but this has been going on for weeks. I'm really starting to fear that they'll never be friends again. : (

I think cats respond to hormonal as well as odor changes. Our cats loved our newly found kitten until she reached sexual maturity. Then, they stalked her and hated her. She had kittens, we had her spayed and now they love her again. They also love the kittens which will be 'fixed' next month before they reach sexual maturity.

Your cats are just reacting to a change in your other cat's smell -- she was someplace they weren't and they don't like it!!!

Do you think if you tried to reintroducing them to each other (the way you would if you brought a new kitty in to the house) that might help?

If you have to bring your cat back to the vet, you might consider bringing them both in just so the younger one picks up the same scent.

My guess is it's a normal cat social issue. I had the same thing happen with my Mama cat and her only ever kitten. Who until Mama cat had to have surgery were inseparable. Younger cat would go to Mama cat (even though she was nine years old) to be groomed and curl up against.

She approached Mama cat after the surgery and frankly I don't supposed Mama cat felt well or wanted to be messed with and didn't get a good reception. Younger cat started the hissing and then we had the Hatfields and McCoys for a month or two. Younger cat's light bulb came on and she discovered she could get territorial and then started bullying Mama cat and chasing her off the beds they once shared. Just establishing the dominant cat position, or trying to.

IOW just a disruption of their year's old normal routine and cats don't like disruptions of routine.

Thanks for your responses, folks.

Ordinarily, they both go to the vet together- it wasn't feasible for the hospital stay, however. If I can ever get them into the same carrier again, they're both going at the same time.

The younger one seems frightened of mama kitty. I'm glad she isn't trying to dominate her, that would really upset the apple cart. Right now, she often bails off the couch when her mom approaches. Mama has always been the dominant cat.

Sigh. Hopefully, younger cat will come to her senses at some point.

Just keep doing what you've been doing. Try to get the cats together but distracted from each other as often as possible by feeding them as close to each other as the younger one will tolerate, playing with them together with wand toys or a laser pointer, and/or sprinkling catnip leaves around the floor so that they can roll around on the leaves near each other. Leave cardboard boxes and paper bags around the floor close to each other that the cats can get into and "hide". Do the same with cushy bedding materials. Get imaginative in finding ways to get them hanging out in the same space without focusing on each other. In time, that will translate into the youngster associating her mom's presence with fun/comfy/good things again.

It's also extremely important that the human members of your household NOT overreact to the youngster's behavior, because your cats will definitely pick up on and respond to human stress. Ignore the youngster's temper tantrums. Keep the household atmosphere relaxed and happy in spite of her hissy fits. If you all put a calm and comfy vibe out there into your household environment, that will help relax the youngster, as well.

Take a breath. Chant a mantra. Give it time. All will be well.

Laurie

Thanks so much, Laurie...really fantastic suggestions! The catnip thing never even occurred to me. I wondered about the younger one picking up on our stress...I'll try to keep things low key around the two of them. : )