Indoor cat spent accidental night outside

Hi all,
My indoor cat, Pippa spent the might outdoors last night. I only discovered her outside when I noticed an odd moving shadow at the window while showering. It was Pippa who clearly wanted in! I opened the back door, called her name and she came running. However she ran right into a large fight with my sweet foster kitty Donna.
Up until the this point Donna and Pippa have gotten along very well. They snuggle and groom each other. Donna has been living with us for about a month and they have only had some tiny little scraps. The 2 fights that they have been in this morning are at whole new level, I have never heard the 2 of them scream and hiss like this before.

I have done some reading, and the consensus seems to be that Pippa smells different because of the time outdoors and that things should settle down when she smells normal again .
I am just wondering if there is anything else I can do for these girls? They were just so sweet and loving before, they really seemed to care for each other.
Currently they are separated, with Donna in the living room and Pippa with the remainder of the apartment.
I should mention that they are both fixed and that Pippa is 5 or 6 (she was a stray from the spca) and Donna is 2. Pippa has always been the dominant cat up until now, but it appears to be Donna who starting the fights.

Thanks very much for any help you can give!

try rubbing them both with the same towel, back and forth until they both smell like each other. 

thanks for the suggestion.  I gave that a go and also brushed them with the same brush (both of them really like to be brushed).  Things are still pretty rough between them unfortunately.  Going to keep trying this technique and see if that will help them. 

mjrm said:

Thanks very much for any help you can give!

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I would consider acting like they are being introduced for the first time, and i would add the use of feliway spray for Pippa (see www.feliway.com) by spritzing any place she sleeps or hangs out.  Below is my standard intro guide.

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There are several steps to a successful introduction, the goal being BFFs, not enemies or angry at you (especially the resident cat).  A careful introduction raises the stress level in incremental steps, allowing both cats, especially the resident cat time to acclimate to the stressor before being introduced to the next level.  You are going to move the "bar" closer and closer to the resident cat until the final step, a supervised face-to-face, becomes  a fender bender and not a car crash.

Step one: Complete separation, putting the new cat is a small room like a bathroom with food, litter and water.  Do not let the cats see each other - too much stress too soon.  Give the new cat time to adjust.  Give both cats time (a week+/-) to get used to this.  They will know each other is there.  Start feeding the resident cat nearer to the door, adjusting daily until he is at the door eating. Do voluntary scent exchange by rubbing the new cat's cheeks on a sock and then offering the sock as a gift to the resident. Don't force him to smell the sock, don't rub it on him. Observe his behavior and allow it.   Rub a clean sock on his cheeks and offer it to the new cat.  Continue to do this but never force either cat to interact with the other cat's sock.

When they are reasonably calm with everything in step one go to:

Step Two:  Allow the cats to see each other.  Two baby gates stacked on top of each other in the open door is a great way.  Cracking the door open and blocking it into position so they can't get through the door is another way.  With many cats the stress of this will make them revert, but it would have been much worse if you had started with this step.  Continue as if this was step one, but now with them seeing each other.  When they are both calm, no hissing or growling, you can go to:

Step Three: After eating meals and feeling satisfied (full stomach = less aggressive) and trimmed nails, you can start to do brief supervised introductions face to face.  Watch their body language and reactions and increase their time together until you are confident that they can manage on their own.

In General, treat the resident cat like he is King.  Don't do things to make him jealous. Don't discipline either cat for showing aggression, punishing them for what they feel is a normal behavior (and is normal for them) just raises the stress.  And follow your cats' lead on the speed of the introduction, there are no rules other than to listen to them.

http://www.thecatsite.com/a/introducing-cats-to-cats

http://www.aspca.org/pet-care/virtual-pet-behaviorist/cat-behavior/introducing-your-cat-new-cat

http://www.catbehaviorassociates.com/a-simple-little-trick-to-use-during-new-cat-introductions/