Liz Buchanan BVSc MRCVS
Hello. Thats very hard to call, from two photographs of inflamed skin over the internet with no history. It looks hot and sore, but it's difficult to say whether we're looking at a reaction to fleas, a midge-attack, an allergy, some kind of trauma, self-trauma or something else. Does it extend to the lateral shoulder, too? - that's quite an area. Trauma should always be seen quickly by a vet and if your cat is ill in themselves, the same applies. If your cat might be scratching and making this worse, please also bear in mind that skin can deteriorate significantly over a very short time; your dedicated emergency vet or even in some cases a suitably qualified pharmacyst may help to triage the case if your own vets is shut. Best of luck in getting to the bottom of things.