When good goes evil

Question
Due to expense, I've recently spayed two of my cats late in life.  One of my cats was 4 and the other 2.  Directly after surgery., my sweetest cat and turned on my 14 yr. old siamese (who's been spayed since a kitten.)  She will lunge after her at any cost, even if she's in my arms.  In the rare occasion I don't have them separated she will attack this cat viciously.  I'm having to keep these two separated in different rooms.  It's breaking my heart locking my old friend up in a room.  Help!

Answer
Hi, Nici.  I'm guessing the spay probably doesn't have to do with it.  Increased aggression isn't usually seen, it usually works the opposite way.  

I wonder if something happened between them when you weren't there.  Sometimes if two cats are in the same room and something spooks them - say a loud noise or seeing another animal outside the window, they will come to associate each other with that negative experience and will turn on each other.  Usually, separating them completely for a couple weeks and then reintroducing them slowly will mend things.  

If not, I think I'd consult the vet about possibly using a mild sedative for your younger cat.  You could use it for a short time to help her calm down.  Or an antidepressant for a couple of months might be the way to go.  One antidepressant is preferred for severe aggression in cats.  It's called Clomicalm.  Treatment usually lasts 4-6 months and then can be discontinued without the behavior returning.