Help! Indoor cat suddenly aggressive toward other indoor cat

Any advice anyone can give would be appreciated. My 2 indoor cats, a Siamese and a Tonkanese have lived together in harmony for close to 4 years, since they were kittens. They are both neutered males. Within the past week the smaller Tonk has gotten violently aggressive toward the larger Siamese. The Siamese has kidney disease and goes to the vet weekly for fluids. This problem seemed to start after I brought the Siamese home from the vet last week. The Tonk acted as if he smelled something strange on the Siamese. The next day he started hissing. Each day it got progressively worse and they ultimately started having violent scratching, sceaming fights. As of yesterday I can't even keep them in the same room any more. To make matters worse, I have a trip to Hawaii planned in 2 weeks and don't know what to do. I usually have a pet sitter come but don't know if I should leave them at home, separated in different parts of the house, or board one (double the expense) If the problem is going to persist, it's not going to be a livable situation, not to mention added stress to the Siamese with kidney disease. But I hate to give one of them away. Has anyone out there experienced something like this and if so, is there any hope of things going back to normal?
Thanks much, Gail

Hi Gailmu, I don't have a specific situation like this, but I definitely have dealt with aggressive cats, so I might be able to give it a shot. Also, I am bumping for other more experience *personnel* on the board to jump in as well

Anyway, I would say that if the Siamese is getting meds, he may be giving off a different scent than the Tonk is used to, therefore Tonk is not only adjusting to the Siamese's illness (which I think might have been a provoker, but I'm no expert, so I don't know for sure!) but to a different scent that he's not used to.

Defnitely keep them separated but try to re-introduce them...rub the Siamese's scent on a blanket or a shirt, and do the same for the Tonk, then switch them up. Try to integrate the same principles as introducing cats that have never even met with them -- cats are very scent-oriented, and you saw that the Tonk was already acting like he *smelled* something different...probably the meds do something with his pheromones. Anyway, try to get them to interact and play together (if the Siamese is up for it) but I think there is hope.
Believe me, if my two can get along in the same room, there is certainly no reason that two cats who do like each other can 't get to know one another again

Someone hear told me to put pure vanilla extract on each of my two cats who don't get along. (between their shoulder blades, where they can't lick it off0
Even in my situation I saw an improvement.

Then they both smell the same.
SO before you go to the vet put some on Tonk, then on the car ride home put some on the other...... it will be interesting to see if it works. Being this does all sound like Tonk is thinking the other one is a totally new cat, being he doesn't smell familiar.

Good luck,
Donna

There is actually three places on the cats to put the extract- and just a small amount- under their chins, between their shoulder and the base of the tail-

When one cat is ill his scent changes- therefore the other cats don't recognize the sick one and treat it as an intruder, which is what is happening here. You want them both to smell the same and take away that threat- thats where the vanilla extract comes in handy- You need to do it several times a day