Does this chicken roost on the floor or on a roost at night? Is she walking and getting around okay? Is there any swelling of her foot and leg joints? That looks like a breast blister that has abscessed. These are common in birds who roost on the floor, and the bedding may be rough or wet. It also occurs in chickens with lameness or mycoplasma synovitis (MS,) an infectious disease of the joints. The breast blister can be treated with Vetericyn, chlorhexidene, betadine, and plain Neosporin ointment can be applied daily. Here is some information from the University of Florida about MS:
Synonyms: MS, infectious synovitis, synovitis, silent air sac Species affected: chickens and turkeys. Clinical signs: Birds infected with the synovitis form show lameness, followed by lethargy, reluctance to move, swollen joints, stilted gait, loss of weight, and formation of breast blisters. Birds infected with the respiratory form exhibit respiratory distress. Greenish diarrhea is common in dying birds (see Table 1). Clinically, the disease in indistinguishable from MG. Transmission: MS is transmitted from infected breeder to progeny via the egg. Within a flock, MS is spread by direct contact with infected birds as well as through airborne particles over short distances. Treatment: Recovery is slow for both respiratory and synovitis forms. Several antibiotics are variably effective. The most effective are tylosin, erthromycin, spectinomycin, lincomycin, and chlorotectracycline. These antibiotics can be given by injection while some can be administered in the feed or drinking water. These treatments are most effective when the antibiotics are injected. Prevention: Eradication is the best and only sure control. Do not use breeder replacements from flocks that have had MS. The National Poultry Improvement Plan monitors for MS.
I had a rooster who was in a large pen with 2 other roosters, and he was lowest in the pecking order. He developed a breast blister because he constantly stayed on the roost in his pen, since he was bullied (I soon removed him.) Any prolonged pressure on the keel bone will do that whether anything else is wrong. Sometimes staphylococcus, strep or other bacteria can infect these. so I would probably clean it out with chlorhexidene, betadine, or other disinfectant. Plain Neosporin ointment can be applied daily. Vetericyn would also be a good one to use. Here is a link to read: https://books.google.com/books?id=g...ge&q=breast blisters Teresa morishita&f=false http://www.thepoultrysite.com/diseaseinfo/40/breast-blister/