After four days of poor little Poppy acting like a wild-cat I was getting quite desperate. Parsifal, who bore the brunt of her hissing, spitting and growling, didn't know what to do and turned into a real Mummy's Cat since I was the only friendly person in the apartment. Last night he dived under the covers when I went to bed and, as far as I am aware, he stayed there for most of the night. Poppy stayed under the bed and growled. We were all miserable.
So, as a last resort, yesterday afternoon I went over to Zoo and bought one of those pheromone dispensers and plugged it in. Parsifal calmed down immediately but it took Poppy until this morning and so far we have had only one hiss from her and she spent a short time this evening playing with her favourite toy so, as well as feeling better she is obviously feeling happier.
They are still on a soft diet but I am cutting down on that now and Poppy can now go through the cat flaps without discomfort so I can shut the doors and turn on the air conditioning more efficiently again.
The pathology came back on the lump. It was "fat necrosis with Steatitis" which is uncommon in cats and very painful, either idiopathic or from an injury caused by Parsifal, although there was no external sign of claw or bite marks. The other cause is overfeeding on tuna fish but not so in her case. She has mostly been fed on kibble which is a balanced diet. So we are going to have to accept that it is idiopathic and hope that it doesn't happen again.