Thank you so much for posting this. I was able to use this information to help save one of my hens, Gwendolyn.
On October 1, 2017, I noticed that Gwendolyn was hunched over. She wasn't acting like herself. I picked her up and noticed that the area just below her vent was bloody. She had a large open sore about two inches long and one inch wide. It smelled like rotting flesh and was full of maggots. Norma (my wife) and I took her inside and cleaned her up at the basement deep sink. We soaked her in warm, soapy water. Then we removed some loose, bloody feathers and cut away a few more around the wound. We poured hydrogen peroxide on the wound and proceeded to remove the maggots. There were literally a hundred or more. It took two hours. Then I laid out pine shavings in the basement shower and put her there with some food and water. A few hours later, we removed a few more maggots and then cleaned the wound with rubbing alcohol. We bought a generic version of Neosporin (which I will refer to as antiseptic cream) and used that to cover the wound. Gwendolyn doesn't appear to be in pain but she is very weak. What is the problem? Fly strike. Why Gwendolyn? She probably had feces that stuck to her feathers which flies then laid eggs in. Some chickens are just more prone to having feces stick to their butt. I don't check my chickens closely on a regular basis though I do make sure they make it into the coop every night. She was strong enough to jump onto the roost. Beyond that, I hadn't noticed anything special about her until today.
As of October 7, 2017, Gwendolyn is making significant progress. Every day, we have been inspecting her for maggots, cleaning the wound, and applying more antiseptic cream. She has been separated from the flock and will stay that way at least until October 10, perhaps much longer. To clean the wound, at first we were using hydrogen peroxide with a syringe. After three days of that, we switched to saline. Then she gets blow dried before applying the antiseptic cream. I've also started giving her a
homeopathic antibiotic. I would have preferred giving her something stronger but as of this year, a law was passed that requires oral antibiotics for animals to now be prescribed by a veterinarian. We also bought
Alushield Bandage which I expect we will use after she is re-introduced back into the flock. Gwendolyn is the runt of the flock and sometimes she gets picked on. I wonder if one of the other chickens created a cut that made it easier for the maggots to take hold. We have
Hot Pick which we can use to help prevent this.
The attached photo shows her on October 6, 2017.