Indoor male-female fighting

Question
My Male 3yr old cat just developed crystals and a urinary infection on Sunday.  We took him to the vet and put him on medication which he currently takes morning and evening.  Because we have 2 cats, and he was urinating everywhere due to his spasms we had to move him into the bathroom so he could be closely supervised on whether he became blocked and stopped messing up my house.  He was only alone for 2 days in that bathroom with the girl cat (litter-mates) having roam of the house.  2 nights ago, we had let him out because he was 100% better and still back to normal.  She threw a complete fit and started hissing and growling any time he was in the same room as her.  She would be fine when he was out of sight and start up again when he came into view but as of this morning, she has started hissing and swatting at me.  I have read all morning into what to do but I thought I would get another opinion.  Usually the information is regarding new cats being introduced however, these are brother & sister Russian Blues.  On a side note, she got FREAKED out last summer (when I tried to put her outside) and pulled this same stunt but she was over it in 24 hrs.  Can you help me stop the recent madness or tell me NOT what to do so that I scar them for life?

Answer
Rachel,

Being brother and sister do not enter into it. After a cat is weaned they do not recognize brothers and sisters, or mothers and fathers, as such. A cat has no concept of 'family', which is why there is so much incest. A cat recognizes littermates and other cats in the household by smell, not by sight. Though sometimes cats will recognize solid colors and be friendly or aggressive until they  actually can smell the cat to know if it's friend or foe.

Your male cat may have absorbed a medicinal smell from the medication so your female may think he is a different cat. When it wears off things should be fine. Rubbing her fur with a towel then rubbing his will help (but NOT vise-versa). I have never tried it, but some people have had success with putting a small dab of vanilla on both foreheads so the cats smell the same. You can put him in a cat carrier and put him in the center of a main room in the house, kitchen, etc. That way she can come up and smell him with no harm to him. She may then realize who he is and her behavior may stop.

Another possible cause for her behavior may be Redirected Aggression.  That happens when a cat sees a strange cat outside and becomes territorial but can't get to the strange cat to defend her territory. You may not see the animal that the cat sees. A cat with Redirected Aggression turns on the closest animal member of the family and threatens them or fights with them as if they were the other cat. They have been known to attack other family cats, even though they may have lived and gotten along together (as yours is doing), the family dog, and even their owners. If that is the cause, you need to block the window past the cat's eye level of the window she can see another cat out of (out of sight, out of mind). If it is allowed to go on your cat may develop a permanent personality change. Put the cat in a room by herself for awhile to calm down when she behaves like that.

It is possible too that your female's behavior is a mediacl issue and not a behavioral one. If your male's urinary crystals were caused by too much ash in his food or from fish products, then your female may be having urinary tract problems too even if she isn't peeing everywhere. Or something else may be wrong. If a cat has pain the reaction is similar to your female's behavior.

There is a very popular cat calming spray and plug-in you can use called Feliway or Comfort Zone with Feliway. It is available at pet stores, vet's offices (they use it too), or on-line. It copies relaxing pheromones that cats produce from rubbing their faces on things. And there is a calming product that you can add to the cat's food or water that reduces anxiety called Bach's Rescue Remedy. It is available on-line and in health food stores. Here is a link about it: (copy and paste, or type the whole links into your address bar)
http://www.bachflower.com/Pets.htm

If your female is still being aggressive the vet can put her on "kitty Prozac" for short term (usually 2 weeks). It is very popular for cats that are stressed, being aggressive, or have emotional issues. You may want to ask your vet about it.

Note: Urinary tract infections and urinary crystals are commonly re-occurring so keep an eye on your male for any signs in the future.

I hope this helps.

Tabbi