female cat agression

Question
We brought our 4 year old female cat home from the vet today after being spade.  My 1 year old unspade female cat attacked her.  She has never been agressive in any manner before.  Why now?
What can I do, right now I have them separated.

Answer
Hi Debbie,

Well, for one your 4 y/o came home smelling funny with tons of fear pheromones and the like from other dogs and cats there. Vets offices are never pleasant for animals. Also, she was sedated and not quite herself when she came home. Shall we say all of this probably confused the 1 year old. Keep them separate until your 4 y/o is rested and sutures have no chance of being opened. She just had surgery and although kitties act like it is any other day, they really are experiencing pain as any being would that had internal organs removed.
This may be a week and by then she will have licked herself and gotten her normal smells back.
Some people have had luck with Feliway a cat product that emits pleasant pheromones. You might want to give the younger one Calming chews by Pet Naturals with L-theanine. Do not exceed recommended dosage. One or two treats maybe a half hour before they are re-introduced.
Things will be fine again once your four year old is back to herself.

Good luck,
Shanti