Sunny adopted his new family when his previous owners moved house and left him behind. He was very charming and they quickly fell in love with him and took him in, but he became more and more aggressive towards their dog Milly, which started after he had some nasty injuries to the base of his tail consistent with having been attacked by a dog or fox. A traumatic event (being attacked) caused Sunny to be very afraid and defend himself in the only way he could.
Initially we found a way to keep Milly safe (she was much more at risk than Sunny in this case!) by keeping her downstairs and Sunny upstairs, where he had access to the garden through a window so they never came in contact. As Sunny settled into his new life (he had been a stray for several months) and his general confidence increased he was ready to start treatment. Sunny was very slowly introduced to Milly, with her on a lead and him in a cat carrier (which we had taught him to love by feeding him chicken in it so that this didn't add to his stress when we started treatment). We started off at a great distance so that Sunny (and Milly!) could be rewarded for being calm. We gradually brought the two closer and closer, over many weeks. Sunny is now able to tolerate being right next to Milly without showing any signs of fear. We started in the garden because we needed the distance for Sunny to feel safe, so the next stage will be to continue working on this in the house. Sunny is making really good progress and is an altogether more relaxed (and demanding!) cat.