Cat aggressive toward my other cat

Question
I have three cats. Two (age 11) were adopted from a shelter as adults about 9 years ago - not siblings. Two years ago I found a cat wandering in the street pregnant so I took her to a shelter where she gave birth. I eventually adopted her. Her name is Lucy and she is about 3. She got along with my other cats just fine for a 1 1/2 years but in June she turned into a psycho. Every time she even sees my cat Sabrina she violently attacks her. She just this second came from across the room and lunged at Sabrina who was on the window sill. They all used to get along so I don't get it. My other cat and Lucy don't care for each other but they exist peacefully. Before Lucy got aggressive she used to hide behind the curtains & jump out at Sabrina but Sabrina would just run away & Lucy would leave her alone.  It has evolved into violent attacks.  We've taken both of them to the vet & all they say is to contact a behaviorist which I guess we will have to do.  We've tried Feliway and Rescue Remedy but nothing works. We squirt Lucy when we can but she's not even phased by it anymore. We can't live forever with a litter box in our bedroom & we can't keep them separated for the rest of their lives. Our poor cat Sabrina shouldn't have to live in fear even when Lucy is locked in another room.  This is making life miserable for all of us.  Any suggestions? We're desperate!!!

Answer
Hi Heather,

First of all let me offer my sympathy, it is hard when you do a good deed and then it seems to go wrong on you.

However cats don't usually change behavior this suddenly without a medical reason. I know you said you brought her to the vet, but I think you should get a  second opinion and a full blood panel - including a thyroid panel (T4 and free T4). Increased aggression is a common symptom of hyperthyroidism. Cats with hyperthyroidism often start to lose weight but eat more, act restless and sometimes aggressive, and drink and urinate more. However many cats do not show any symptoms or only some of the symptoms. I have known cats whose only symptom of hyperthyroidism is  aggression. Most cats develop hyperthyroidism after 8 years of age or older.

I have had 2 cats develop it in the past year - one was 14 and the other 9. The 14 year old showed the "classic" symptoms and he did become more aggressive to the other cats. The 9 year old showed no symptoms at all, her problem was detected in her routine annual blood test - after 8 years old I do annual thyroid tests on my cats.

So rule that out first by getting a full blood panel for Lucy and make sure it does T4 and free T4 - because early hyperthyroidism may show up as normal T4 but elevated free T4.

If Lucy checks out, do the same for Sabrina if you have not already had a full blood panel. If she is sick, Lucy may be attacking her because she smells something different - could be anything from kidney failure to cancer. In nature, cats may attack and drive away a wounded or sick cat because a weak animal may attract predators to the area. So if all the vet did was a quick look and refer you to the behaviorist, get another opinion and insist on a blood panel.

If both cats are totally healthy and nothing has changed in their environment (routine, diet, etc) I would suspect that there might be a new cat visiting your yard. You didn't say but I am going to assume your cats are indoors only. If not I do recommend that. If there is another cat visiting your yard, Lucy may be displaying displaced aggression. She sees a strange cat and wants to attack him, but she can't get to him so she attacks the nearest cat. It sounds strange, but it's a common problem - cats get into fight or flight mode and adrenaline floods their system, and they stop recognizing friend from foe and just go after their companion. The best way to fix this particular problem is to see if there is indeed a new visitor, and if that's a stray find a way to get him a new home... and if he's someone else's pet either talk to them about keeping him away, or put out a repellent device or powder. Here are some ideas for repelling other cats from your yard:

Shake Away Cat Repellent (Nontoxic)
http://www.critter-repellent.com/cat/cat-deterrent.php

Motion trigger air sprayer
http://www.multivet.net/en/products/ssscat/

Don't bother squirting Lucy - in fact it may make the problem worse, because anything unpleasant she will simply associate with Sabrina. Instead, separate them for a while and re-introduce them gradually as if they were new to eachother. Supervise them and if Lucy start showing signs of being aggressive, distract her with a toy or food - do not punish. Make sure every time Lucy is near Sabrina it is a positive experience. Don't allow them to interact unsupervised for a while. It is worth taking the time over this stage to regain harmony in your home!

Good luck, please do let me know how it goes, I think you can post responses or follow ups here.