Heavy toms are primarily affected, although transport myopathy also develops in hens, especially in flocks in the upper midwest of the USA. About 5% of all flocks are affected, and morbidity within the flock is 2–20% but can occasionally be as high as 70%. Transport myopathy occurs sporadically but is most common during fall and early winter. A high incidence has occurred in sequential flocks from the same farm. Incidence is likely to be higher in flocks raised in confinement than in range flocks.
The cause is unknown, but transport myopathy is associated with increased body size and weight, increased transport time to processing plant, cool ambient temperatures, and valgus leg deformities. The pathogenesis is unknown but presumed to be similar to exertional myopathy.
Often, only one leg is affected. No evidence of external trauma is seen. Skin over edematous subcutaneous tissue is pale, feather follicles are less visible, and the skin slips easily over underlying muscle when moved. Occasionally, there is crepitation. Affected areas are dark when the edematous areas contain blood. Typically, when lesions are cut, the edematous subcutis is a few to several millimeters thick and is amber, occasionally green, or rarely red. Purulent exudate is absent, which distinguishes transport myopathy from cellulitis. If hemorrhage is present, the adductor muscle usually is torn. Removal of affected legs at processing results in carcass downgrading. Microscopically, acute multifocal muscle necrosis is found, primarily in the adductor muscles. Sometimes subacute or chronic lesions are seen, suggesting earlier episodes of myopathy. Serum CK increases sharply between farm and processing.
Programs designed to improve leg strength and conformation and to reduce trauma during transportation help reduce the incidence of this myopathy. Supplemental vitamin E also may be useful. If possible, flocks with a high incidence of valgus leg deformities should be marketed early at a processing plant nearby.
Last full review/revision March 2012 by Arnaud J. Van Wettere, DVM, MS, DACVP