One of my ladies had a tendency to get mites and we couldn't figure out what was happening. Mites like wet warm areas. They will get under the feathers and cause a lot of irritation for the poor bird. It's why I'm so crazy about the diatomaceous earth since it was so inexpensive and worked in two days. I would just go out, take the earth with my hand, then kind of massage it into the affected areas. My Oko seemed to absolutely love the attention a the massage (she loved having her wattle massaged and being petted behind her ear). That being said, my girls are kept outside in a heated coop. My neighbor who shares the fence with the hens had a pipe that was leaking water and attracting the little critters. That was eight years ago and we haven't had mite, roach, flea, etc. problems.
I would be curious as to what type of infection. Was it an infection based the scratching? The itchiness based on the mites? Thank you for sharing the information! I want to keep it in my list of potential issues.