Open wound on the breast

[Jkelley]

My 4 year old Orpington hen has developed a pretty large 'mass' on her breast. Not really open since it looks like it's started to heal, but is scabbed over. She's been doing o.k. so far, but now her breathing has become labored. Recently I lost 2 others (with zero sign beforehand other than they started to act lethargic the day before), and a 3rd started behaving almost as though he had a stroke- also lethargic, off-balance, and just lays there. Any ideas? I've never lost a bird prior not predator related, and now I've lost 3, possibly 5 in less than a month. None of the others showed any trauma or wounds and all were vaccinated for Mareks. Help!