I was amazed to see this pic!! This is absolutely, 100% identical to what we experienced in a 3 yo Comet about a month ago!! Exact same place on her behind, exact same description of the maggots, etc. Therefore, I don't believe this could be a spider bite and we would both have such an identical occurrence! We also picked out all the maggots, cleaned it well with mild soap/water and triple antibiotic the first day, then kept it sprayed with bluekote after that. I also injected 1.5cc Tylan50 for 3 days. She acted well out with the flock, but I kept checking her abcessed wound and found that it just became very hard in that area but skin never seemed to regrow. With time, I also saw that she had less and less color in her comb and also her feathers lost color like crazy. I found that her abdomen had become swollen and hard as a rock. It felt like someone had literally put a rock in it that was too big and the skin was going to burst any minute. I really became sadly convinced that there was no saving her and called a friend who is a veterinarian from South America. Before coming to the USA, he worked for a time overseeing poultry production houses in his country and therefore has more than the usual amount of chicken knowledge for a small animal vet. Since I just cannot bring myself to kill my own birds, he kindly offered to euthanize her and then opened her up and did a thorough gross examination. He said she was full of material that looked like cheese--all the result of malfunctioning oviduct. He saw how other organs had been affected, including some fatty liver, but thought that everything was a result of the oviduct situation.
Are these two things related? Who knows? But my vet friend does want to look for powdered Tylan to put in the water as I'm suddenly having many more problems with my flock. He says the Duramycin which I suggested (easy to get at TSC) has been overused which is why he preferred the Tylan. He also says this needs to be a long-term treatment which is why he doesn't feel that injections will work.
My whole flock turns 3 years old in one week and I had never realized before how much this seems to be the age for so many problems to set in. Not a happy discovery!!