One cat gets aggressive after other cat gets allergy shots

We have two cats. Both over 8 yrs old. They've been housemates since their youngest kitten days and have always been best friends. One of them developed allergies when we moved into a new house three years ago and last week we finally began giving him allergy shots. Much like a human, he intially gets shots everyother day in increasing dosages. I was freaked about giving the shots but ironically that turned out to be the least of my problems.

The first two shots were for tiny amounts. The third shot was the first one to be of any quantity, to my mind. Well, the night of that third shot, there was a big fight. Now, they would always rassle and someone might yelp, but all was forgiven almost immediately and they went on being buddies. Well this one was bad and there was another later on that night before it became clear they needed to be separated. Since I wasn't around them when the fights started I had no idea what might have precipitated it. Also, for whatever reason, I suspected the cat with the allergies was the aggressor.

The next day I let them be together again and things were okay for a while but I monitored them closely. Long story short, there was another fight but it turned out to be the other cat attacking the one getting the allergen shots. Observing over the last couple of days, I noticed the one cat would more or less get upset when he sniffed the other's butt area. That set him off. If the one walked by him and the butt wasn't "in play" nothing happened.

It seemed to me that, since nothing else here had changed, the ramp-up of allergens in his system must be sending off some sort of offensive scent to the other cat. I spoke with the allergy vet and they'd never experienced or heard of this. But what else could it be? Has anyone had any experiences that might shed some light on this?

I'm assuming it's likely a steroid in the shot? Those will affect hormones to some degree (different types of steroids affect different ones), and whenever hormones are affected scent can/will be as well.

Simply put, the medicine is making the cat receiving them smell odd to the other cat.

You could try reintroduction techniques aimed at helping disguise scents - vanilla, rubbing a towel on the cats to distribute their scent on each other, etc. And you can also try things like feliway diffusers to help calm the other cat.

I hope you can get it settled. But be assured that it's not a completely odd occurrence, there have been other past threads in the behavior section about cats who have been put on medications and the other cats responding to them differently.

The shots are organic allergens much like a human receiving allergy shots; a hair of the dog, if you will.

Odd you should mention the steroids, tho. The one getting shots (his allergies give him rodent ulcers) has had cortisone shots in the past but there has never been a reaction from the other cat when he got those. I finally decided to go with the allergen shots because I feared the diabetes potential with steroids.

My vet recommended the Feliway. It does not seem to be working. I got both the diffuser and the spray. I will continue trying it though, if only because of how much money I spent on it! Someone else recommended the vanilla. That is intriguing. I will be trying that. Also recommended was Rescue Remedy which I'd never heard of.

The hardest part right now is keeping them separate and getting some sleep at the same time. Thanks for your input.

Could there be other medical problems with the cat receiving the allergy shots? Allergies are a type of immunity disorder, and sometimes when a cat has a slightly lowered immune system from allergies, they are susceptible to other things.

If the behavior is prompted by butt sniffing, I would suspect either a urinary tract infection or anal gland issues. You said you were visiting an allergist. Are they also a vet or is that someone else? I suggest a vet visit to rule out other illnesses.

The last two weeks have seen a number of vet visits and phone calls. The "allergized" cat had a bad fever a week and a half ago and just finished his antibiotics yesterday so I'm guessing that would have taken care of anything that he might have had. The allergist is a dermatology specialist (I assume she's a vet) and she has been in contact with my regular vet. I've asked them both about this aggression issue and no one has a clue. I just wish I could verify my theory that the allergens are causing this.

If all medical issues are cleared, has anyone suggested using a dab of "pure" vanilla on the top of tail base area of both cats so everyone "smells" the same. Important to use pure vanilla and not an alcohol based vanilla.

Just a thought that might be worth a try.

Yeah, as I mentioned above, someone else suggested vanilla and it sounds interesting/plausible. I just need me some pure as all I have at the moment is alcohol-based.

Of course, this morning after a somewhat decent night's sleep requiring certain accommodations on my part, of course, I let them out together and all seemed well though I maintained constant vigilance. There were full-body passes without any meow-moans or anything. They ate together normally. All that. But then it would happen where the offending cat would suddenly take attention of the other, get all upright and on a mission, approaching him and start the meow/moan. Out of nowhere. So I don't know how much off it's this scent thing I'm trying to hang my hat on.

It's making me sad. They were such good friends.