A. Cats are notorious for getting abscesses on their back. These are caused by puncture wounds that heal too fast and cause a bacteria filled swelling that is usually warm to the touch. Sometimes these can rupture and start to drain. If this is an abscess the treatment for them is to lance/drain them and give oral antibiotics and anti-inflammatory medication from your vet.
I would try to find out the cause of the injury and try to keep kitty away from the source so injury in the future can be avoided. And if kitty is allowed to go outside, it is best to keep them indoors so that don't fight with other stray/feral cats which can cause future abscesses as well.
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