My favourite hen is gasping for breath. She's been getting worse over about 2 months. The vet has run out of ideas and is puzzled as to what is wrong with her. All the other hens are fine, she's had 2 lots of antibiotics and been treated for gapeworm and it's made no difference. She's had no other symptoms, she's been laying and feeding and has no discharge from eyes or beak. Just struggling to breathe and she's now starting to get tired. Any ideas what else might cause this?