Indoor Cats Went from Friends to Enemies

Hi - I have 2 male neutered cats. Luke is 6. Leo is 5. I have had both since they were kittens. They are strictly indoor cats. Both are very friendly and active cats. They have always gotten along very well.

In July I started noticing that Luke was hissing at Leo. It was the first time I ever heard him hiss so it was really unusual. There was nothing different going on in our lives or any other sort of change. I didn't really think much of it though because it was so infrequent.

So he started hissing. It started as like a once a week thing, then it graduated to about twice a week.

At the end of August we moved and since the move the hissing has gotten worse and he is now doing very unprovoked attacks against Leo (very clear it is not the typical play-fighting they used to engage in). It can happen just because Leo walks by. There is no question that Luke is now the aggressor against Leo.

When we moved he was hissing about once a day and now I can hear it as many as 20 times a day, along with these attacks. They used to eat out of one of those dual dish bowls (you know the kind that have a side for food and a side for water) and I would put food in each side and each would eat from one side. Once we moved, Luke started growling when Leo would get near the food and sometimes swat him so they now eat in separate dishes although just a few feet away from each other. That seems to make mealtime ok.

I plan on taking Luke to the vet to have him checked out but I'd like as many opinions/solutions as possible. I haven't noticed any other change in Luke to indicate a health issue - he's just as active as ever, eating/drinking normally, etc.

I don't think the move is the issue because it started before then but I'm guessing the move has aggravated it. I don't think it is a scent issue because they are both indoor cats. I did give Leo a bath though just in case and that didn't work. I have also tried Feliway, playing individually with Luke more (although probably not as much as I should) and putting Luke in a different room when he hisses. None have helped.

Also, I should add that there has been no aggression toward any humans, just Leo.

Any idea why these two friends are now enemies and what can I do?

Thanks for everything so far. I hadn't thought about the possibility that Leo could be sick. I actually had them both already scheduled to go into the vet at the same time so we'll be able to look into that.

I tried Feliway and there has been no change. I even tried a collar with phermones and that hasn't worked.

I will definitely try bathing both of them and we'll see how that goes.

As for the move, it certainly is a possibility that he sensed something was up but we weren't that fast in terms of getting our stuff together to move so it's not like we had lots of stuff out in July that would suggest something was going on.