Mechanically Induced Myopathy in Poultry
Avascular necrosis caused by pressure in heavy birds that are down because of lameness or leg deformity is seen occasionally and occurs most frequently in the breast muscle. On gross examination, the tissue is firm and pale. Histologic examination reveals swelling, hyalinization, and necrosis of fibers, with edema, heterophils, and macrophages at the periphery.
Rupture of the gastrocnemius tendon occurs in meat-type chickens, particularly roasters and breeders, and is rare in turkeys. The rupture is due to application of excess weight to previously damaged tendons (most frequently by reoviral or staphylococcal tendinitis), or it can be spontaneous. The intra- and peritendinous fibroplasias that occur after infectious tendinitis make the tendon larger but weaker because of replacement of normal tendon collagen tissue with weaker, dense, irregular connective tissue. Synechiae between the tendon and its sheath may also develop, limiting the tendon’s range of motion. Application of normal or excess weight to these previously damaged tendons results in partial or complete tearing or rupture.
In noninfectious spontaneous tendon rupture, the cause of the rupture is uncertain but may be a combination of weaker tendons and heavier body weight. Rupture of the tendon of one leg puts stress on the other tendon, and bilateral rupture is frequent. Affected birds are lame or “down on their hocks” (creepers). Hemorrhage from the injury above the hock on the back of the leg is visible as red, blue, or green discoloration of the tissues, depending on the age of the lesion. No discoloration may be visible in chronic lesions. Tissue discoloration results in condemnation of the affected part at processing (red-leg, green-leg). The ruptured tendon can be palpated as a hard mass on the back of the leg above the hock. Broiler breeders >12 weeks old are most commonly affected, and flock incidence may be as high as 20%.
The peroneus muscle originates on the proximal end of the tibiotarsus and patellar tissue, with attachments to other muscles in that area. A small band of tissue from the medial side of the muscle runs to the lateral tibial condyle. The main muscle tendon crosses the lateral side of the hock and joins other tendons that extend the hock and may affect foot and toe movement. The muscle is thin and wide, covering the anterior and lateral surface of the leg, and is covered by a heavy aponeurosis in which the tendon is embedded.
Rupture of the aponeurosis and muscle occurs as a 1–2 cm horizontal wound on the anterior and/or lateral surface of the muscle. It occurs above the middle of the tibiotarsus at the top of the ossifying tendon where the tendon attaches to the muscle. Rupture usually occurs at 10–14 weeks, the age at which turkey leg tendons become ossified, reducing the elasticity of the tissue in that location.
Rupture of the peroneus longus muscle is an uncommon muscle lesion. It is most frequent in females and may affect as much as 5% of the flock. The separation of the muscle likely occurs slowly, caused by activity such as repeated springing, in rapidly growing turkeys. Affected birds are not lame, but the resulting hemorrhage causes a red, blue, or green discoloration under the skin around and ventral to the rupture site (drumstick). This results in downgrading of the carcass at processing. The affected portion can be trimmed.