Wooden breast myopathy (pectoralis major muscle myopathy, "wooden breast") is a worldwide emerging myopathy in commercial, rapid growth, high-breast-yield broiler chickens. This myopathy primarily affects the superficial pectoral muscle (pectoralis major). Affected chickens do not exhibit clinical signs, but palpation of the breast musculature reveals firm superficial pectoral musculature. Pectoral muscles are pale, have white striations, and, in more severe cases, coalescing areas of fibrosis. These lesions are more prominent in the anterior part of the pectoral muscles. The fascia overlying the muscles can be expended by edema fluid and occasionally hemorrhages in severely affected broilers. Polyphasic myodegeneration and regeneration with a lymphocytic phlebitis are the main microscopic lesions. Fibrosis develops secondarily.
The pathogenesis is presently unknown. Selection for fast growth rate and increased total breast meat yield has affected muscle morphology with an increase in myofiber diameter, reduction in connective tissue and blood supply, and altered muscle metabolism. These changes may result in increased myofiber susceptibility to degeneration, necrosis, and limited regenerative capacity, ultimately predisposing to myopathy.
This myopathy results in significant economic losses due to decreased meat quality and condemnation of the affected muscles.
Type II glycogen storage disease (acid maltase deficiency, Pompe disease) has been reported in Japanese quail. Symptoms start between 4 and 6 weeks of age, and affected quail show progressive myopathy with reduced ability or inability to lift their wings, fly, or get back on their feet after being placed on their back (flip test). Glycogen deposition occurs in skeletal muscle, cardiac muscle, and smooth muscle, as well as in the brain and spinal cord. Both red and white muscle is affected, but lesions are more pronounced in the pectoralis superficialis (white muscle).