AVIAN/FUR/EXOTIC SPECIES

What are ‘white striping’ and ‘wooden breast’ in broiler chickens?

Emily Martin, Alexandra Reid, Marina Brash

Breast muscle myopathies are emerging worldwide in the broiler industry, but more recently in North America. Two forms of breast muscle myopathy are known as ‘white striping’ and ‘wooden breast’. Both affect the pectoralis major (breast) muscle and are described in several fast-growing commercial broiler strains. Carcasses with breast muscle myopathies result in economic losses because of carcass downgrading (sent for further processing), condemnation of whole birds or portions, or impact consumer choice.

White striping is grossly described as visible white lines of variable quantity and thickness running parallel to the muscle fibers (Fig. 1) - these lines are composed of adipose tissue. In severe cases, white striping can also have increased connective tissue and variable degrees of myofiber degeneration and regeneration.

Wooden breast is described grossly as a hardening of the breast muscle, primarily in the proximal fillet (pectoralis major) but can affect the whole muscle. Other observations include pallor, surface hemorrhages, and a sterile surface exudate. Histologically, there is polyphasic myodegeneration with regeneration and cellular infiltration with increased interstitial adipose and connective tissue (Fig. 2)

White striping and wooden breast can be found either alone or in combination. Opinions vary as to whether these are 2 distinct myopathies or whether they represent a disease spectrum with white striping as the initial lesion progressing to wooden breast. There is no clear etiology for these myopathies. Etiologies that have been examined include, vitamin E or selenium deficiency, associated pathogens, genetics, and environmental factors.

In wooden breast, the findings are not consistent with nutritional myopathy given that the pectoralis muscle is hard on palpation and there are no lesions noted in other muscle groups such as the gizzard or heart.

The leukocyte counts in white-striped birds do not differ from unaffected birds suggesting that there is no systemic infection. In cases of wooden breast, even though there appears to be an exudate over the breast, no associated pathogens have been reported.

In earlier studies, researchers developed a theory that growth selection resulted in larger diameter muscle fibers with decreased capillary blood supply to the muscle, reduced connective tissue spacing between muscle fibers and bundles, and increased myofiber degeneration. Physiologically, the breast muscles are composed of fast-twitch aerobic muscles that, as part of anaerobic respiration, produce lactic acid that is removed in the circulation. With reduced connective tissue spacing, there are fewer capillaries and less lactic acid being removed from the tissues. Lactic acid build-up could result in decreased pH, muscle damage, and satellite cell mediated regeneration. However, a more recent genetic study revealed that the selection for breast yield may have some role. Environmental factors (i.e. nutrition, management) may play a significant role in the expression of the myopathies.

Muscle fiber formation is complete at hatch, therefore muscle growth is related to the satellite cells associated with the existing muscle fibers. In the hatchery, increased incubation temperatures at certain times may influence myoblast activity and breast muscle yield. Further study is required to determine if this impacts the later development of myopathies. Once the birds are in the barn, there are multiple studies that indicate that feed restriction (during the first 2 weeks of life or at 13-21 days of age) resulted in a change in satellite cell activity associated with increased muscle fiber necrosis and fat deposition in the pectoralis major muscle. Another study showed that feeding of low-energy diets reduced both growth rate and the occurrence of white striping.

Overall, the lesions of white striping and wooden breast result in product quality issues that are economically significant. The etiology appears to be multifactorial and further study is needed.   

Figure 1. White striping (arrows) in breast muscle (photo courtesy of Dr. Alexandra Reid, OMAFRA).	Figure 2. Histologic section of breast muscle showing polyphasic myodegeneration and interstitial fibrosis consistent with wooden breast (20X).