cat hissing

Question
We adopted a stray cat (who we found out was fixed when she went for a check up) a few weeks ago. We slowly introduced her to our two fixed males and all was fine.  One cat quietly ignored her mostly and the other would play with her chasing and interacting just fine. All was well.  Cruz, the one that played with her had to go to the vet (diagnosed with FLUTD) and was kept at the hospital over night. When we brought Cruz home yesterday Pearl, the newest cat, started hissing at Cruz and also at the other cat.  It is continued on this morning.  What should I do?  They had all gotten along and now there is no harmony. Please advise.

Answer
Hi
this could be trauma aggression. this is a strange behavior as the cat who shows the aggression doesn't have to be the one that something happened to but they are stressed out by something that has happened around them. i have heard on several occasions of this behavior happening when another cat is ill or has returned from the vet, so it is not unusual.
The problem is that the cat now needs to feel secure and safe again so that they can stop feeling stressed and showing it via aggression and the only way to do this is to separate them from the other cats for a few days or even a week by putting them in a separate room with their litter tray and bedding and only being let out to be fed. It sounds like a punishment but it is not it will help your cat to feel secure again in a smaller environment without the worry of the other cats. then after a while when they have settled you can start to reintroduce the other cats under your supervision for short period so time, perhaps distracting them with treats or a game.

I know this sounds  a little long winded  but it can make a big difference in how the cats react to each other in the future.

best wishes Kate