Avascular necrosis of muscles, caused by pressure in heavy birds that are unable to stand because of lameness or leg deformity, occurs occasionally, most frequently in the breast muscle. On gross examination, the tissue is firm and pale. Histological examination reveals swelling, hyalinization, and muscle fiber necrosis, with edema, heterophils, and macrophages at the periphery.
Rupture of the Gastrocnemius Tendon in Poultry
Rupture of the gastrocnemius tendon occurs in meat-type chickens, particularly roasters and breeders, and is rare in turkeys. Rupture is either due to excess weight applied to previously damaged tendons (most frequently by reoviral or staphylococcal tendinitis) or is spontaneous.
The intra- and peritendinous fibroplasia that occurs after infectious tendinitis makes the tendon larger but weaker, because normal tendon collagen tissue is replaced with weaker, dense, irregular connective tissue. Synechiae between the tendon and its sheath can 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.
The cause of noninfectious spontaneous gastrocnemius tendon rupture is uncertain; it might be due to a combination of weaker tendons, heavier body weight, and/or tendon rupture in hens resulting from male aggression when the ratio of males to females is too high. Rupture of the tendon of one leg puts stress on the other tendon; as a result, bilateral rupture is common.
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 (see gastrocnemius tendon rupture image). Chronic lesions might show no discoloration. Tissue discoloration (red leg, green leg) results in condemnation of the affected part at processing.
Courtesy of Dr. T. A. Abdul-Aziz.
Rupture of the gastrocnemius tendon can be palpated as a hard mass on the back of the leg above the hock. Broiler breeders > 12 weeks old are the most commonly affected, and flock incidence can be as high as 20%.
Rupture of the Peroneus (Fibularis) Longus Muscle in Turkeys
The peroneus muscle in turkeys 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 to join other tendons that extend the hock and can affect foot and toe movement.
The peroneus 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 manifests as a 1- to 2-cm horizontal wound on the anterior and/or lateral muscle surface. It occurs above the middle of the tibiotarsus, at the top of the ossifying tendon, where the tendon attaches to the muscle (see rupture site image).
Illustration by Dr. Gheorghe Constantinescu.
Peroneus muscle rupture usually occurs at 10–14 weeks, when turkey leg tendons ossify and the elasticity of the tissue in that location decreases.
Rupture of the peroneus longus muscle is rare in turkeys. It is most common in females and can affect as much as 5% of the flock. In rapidly growing turkeys, separation of the muscle likely occurs slowly from activities such as repeated springing (physical displays such as strutting during the spring to attract mates).
Birds with a ruptured peroneus muscle are not lame; however, the resulting hemorrhage causes red, blue, or green discoloration under the skin around and ventral to the rupture site (drumstick), leading to downgrading of the carcass at processing. The affected portion can be trimmed.
Key Points
Rupture of the gastrocnemius tendon results in lameness and in birds sitting on their hocks. It occurs in broilers and, rarely, in turkeys.
Rupture of the gastrocnemius tendon is either spontaneous or secondary to reoviral or staphylococcal tenosynovitis.
Rupture of the peroneus longus muscle is rare in turkeys and results in downgrading of the carcass at processing.
For More Information
Crespo R. Developmental, metabolic, and other noninfectious disorders. In: Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, associate eds. Diseases of Poultry. 14th ed. Wiley Blackwell; 2020:1286-1329.
