Cat people: Cat Aggression?

[Paige]

My cats have been unexpectedly tolerant of my foster cats and we introduced them slowly and carefully. There was peace. My cats ignored the newbies and newbies were appropriately avoidant and deferential.

We went out of town for 5 days and one of my cats has suddenly become aggressive with the visitors. They are no longer safe together. She stalks them. She threatens them, and she will try to chase and fight. It’s uncharacteristic. We’ve been back for about 3 weeks and she’s no better.  Both mine and the fosters stayed at home with a petsitter in separate rooms while we were gone so she shouldn’t have forgotten their scent or anything. 

What is going on? We have feliway everywhere and calming collars. They used to work but don’t help anymore. We only allow one foster cat up at a time and it doesn’t matter who it is- she’s not having it.  She used to touch noses with any of them and let them sit by her. 

I expected some regression but this is worse than she’s ever been. She’s about 8 yrs old and healthy. She has no aggression towards people or our other cat. Sometimes she randomly attacks the dog, but that’s not new.  
 

 

 

 

[Jean in Newcastle]

The first advice with unusual behavior would be a health check. Cats hide pain but it can come out in aggressive behavior. I am not sure what to suggest if the vet finds nothing wrong 

Edited August 17, 2021 by Jean in Newcastle
Typo

[Clarita]

Short answer: If health isn't an issue, it may be that they need a re-introduction. During that time rub blankets and towels that smell like the aggressor cat all over the offending cats. 

The maybe scientific answer to this is cats identify their gang (family, group they hang out with and tolerate) by smell. Specifically they rub their scent all over their gang. When cats are together a lot they smell like one another. If the cat stops smelling its scent on the other cat it becomes not part of its group. 

Our cat did this to our other cat when our other cat needed a teeth cleaning. She came back after 1 day smelling like the vet and our other cat was aggressive toward her. They've been together 6 years, We spent a few days rubbing blankets and towels that smell like the cat who stayed home and our family all over the cat who went to the vet. Eventually the aggressor cat went back to treating her adopted sister normally.