In this myopathy, there is degeneration, necrosis, and fibrosis of the deep pectoral (supracoracoideus) muscle in heavy meat birds (chickens, turkeys), secondary to excessive muscle activity (exertional myopathy).
The deep pectoral muscle functions to elevate the wing. Although well-developed in modern meat birds, it is little used. After episodes of prolonged wing flapping (such as occurs during handling, when a lame bird uses its wings to assist ambulation, or when a bird is placed on its back), the muscle swells within the dense fascia normally surrounding it. This swelling collapses vessels supplying the muscle, leading to ischemia, tissue hypoxia, and muscle necrosis (compartment syndrome). The lesion can be produced artificially by stimulating the deep pectoral muscle to contract and can be prevented by surgically opening the fascial sheath covering the muscle.
The myopathy may be unilateral or bilateral with the central ⅓–⅔ of the muscle affected. Early, the involved muscle is pale, swollen and edematous. Later, affected tissue is sharply demarcated from adjacent, viable muscle. Eventually, it is encapsulated, resulting in dry, green, necrotic muscle enclosed in a thick fibrous capsule. The defect can be identified by a depression of the breast over affected muscles or by transillumination of carcasses at slaughter.
Flock incidence as high as 25% has been reported, with few birds developing the disease before 24 wk of age. Deep pectoral myopathy occurs frequently in turkey breeder hens during artificial insemination. The disease is usually subclinical, and major loss is from downgrading or condemnation at processing.
Incidence can be decreased by careful handling of susceptible birds to prevent excessive wing flapping and, as a longterm method, by selective breeding because the condition is heritable. Supplementing rations with selenium, vitamin E, or methionine has not influenced incidence.
Last full review/revision March 2012 by Arnaud J. Van Wettere, DVM, MS, DACVP