Certainly sounds quite a lot like the mycoplasma gallisepticum my BO has just recovered from.
She'd been going light for weeks until she actually got ill (caught it from a newly introduced hen, probably because she was so thin and weak already) and we think she'd developed a bit of an eating disorder! She was so skinny her immune system couldn't fight it off.
Tylan Soluble (at about double the recommended strength) worked for her. She had her head dunked right under three times a day and was left to drink the rest throughout the day. She wasn't quite as bad as your girl, but the original hen that she caught it from was, and she's almost completely recovered too.
She'd been going light for weeks until she actually got ill (caught it from a newly introduced hen, probably because she was so thin and weak already) and we think she'd developed a bit of an eating disorder! She was so skinny her immune system couldn't fight it off.
Tylan Soluble (at about double the recommended strength) worked for her. She had her head dunked right under three times a day and was left to drink the rest throughout the day. She wasn't quite as bad as your girl, but the original hen that she caught it from was, and she's almost completely recovered too.