This is not accurate. While there certainly are flies that are necrophagic (eat only dead tissue), there are also a large number that are just sarcophagic (eat live flesh). This is why they can only use very specific species for medical wound debridement
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So, pretty sure I can tell you what is going on in loose terms. I do not think you are going to like what I have to say however.
I had pretty much this same thing happen with one exception, my animal did not develop any kind of external wound.
The worms are not nematodes. The flies you are seeing are not fruit flies.
Five or so years ago, I had an animal that went off feeding on rat weans and would only take hopper mice. He had been a picky feeder so I was not unduly alarmed. Around the same time, I had a sudden explosion of what I thought were fungus gnats. Turns out, the two were related.
It took me a while to figure this out
I never bothered to key them out or anything but I am fairly certain that these flies/maggots are the same ones that can sometimes be found feeding on the feces of our animals if you are not quick enough with clean up. I still do not know how my animal was initially colonized but once he was, they continued to grow and multiply within his body. I discovered this more by accident. I had noticed that every time I cleaned this animal’s tub, there were always flies and maggots in there. Then, one day right after I cleaned his tub and put him back in it, he defecated as I was sliding the tub in and it was full of maggots and even adult flies. I immediately took the animal an put him in a clean tub and added warm water to see if I could induce another movement. That worked, all too well, and a torrent of maggots/flies came out. I repeated the warm bath process a few times, each time resulting in expulsions. When the bath was no longer producing results, I gently palpated down the stomach of the animal and that ended up pushing out even more of them. I put him back in a clean tub for the day
The next day I repeated the warm bath and palpation.
On the third day, when I palpated, a huge amount of blood and gore came out. The animal expired shortly there after.
At no point during this whole process did the animal appear to be in distress other than changing his feeder preference (hindsight, I am guessing larger meals were too uncomfortable for him to pass through his digestive tract but hoppers were not).
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Unfortunately, based on that sort of squishy, full water balloon-look your animal has on the back half of his body, my guess is that it is fairly full of maggots. If you were to palpate down toward its vent I suspect you would get a large discharge of them. I am not sure there is any way to “cure” this. An antihelminthic could work, but at that size, getting dosage correct could be tricky. And even if you did kill them all of, then you would have an animal with dead maggots filling its body cavity which would inevitably lead to sepsis