By courtesy of Aviagen
Introduction
Green Muscle Disease is a hidden problem in modern-day broiler chickens. Green Muscle Disease (or Oregon Disease) is a common name given to a degenerative muscle disease known as Deep Pectoral Myopathy (DPM). The condition is characterised by necrosis and atrophy of the tenders (i.e. supracoracoideus or minor pectoral muscles). The lesions often affect both tenders and vary in colour, progressing from a pinkish haemorrhagic appearance to a grey-greenish discolouration as illustrated in Figure 1. 
DPM was first described in mature breeder turkeys and broiler breeders, but is being seen more in meat-type chickens, especially those selected for breast muscle development. The affected muscles are discarded during de-boning, resulting in saleable yield losses. However, the major issue with DPM is that if the birds are marketed as whole carcases or parts, the problem is rarely detected during processing, resulting in consumer complaints and making the cause of the problem difficult to identify.
The condition is not associated with any infectious agent and therefore has no public health significance other than affecting the aesthetic appearance of the meat.
DPM is rarely detectable during processing if the birds are marketed as whole carcases or parts
Why Does DPM Target Broiler Breast Muscles?
- The pectoral muscles in avian species are associated with flight and the deep and superficial pectorals work in synergy, one to raise the wing and the other to lower it.
- The anatomy of these muscles is, however, intrinsically different in that the inner fillet has a tough outer sheath which is made up of dense fibrous tissue and is inelastic.
- The outer or major muscle is simply surrounded by loose connective tissue that moves easily over the muscle surface as the muscle profile changes.
Contraction of the major pectoral muscles (the breast fillet) and the minor pectoral muscles (the tender) are responsible for the up- and down-strokes of the wings. During contraction, these muscles expand with increased blood supply (i.e. muscle pumping). The expansion of the minor pectoral muscle, by as much as 25% in volume, is problematic because this muscle is confined in a ‘tight compartment’, sandwiched between bone (the sternum) and the large breast fillet. The minor pectoral muscle is also encased in a rigid fibrous sheath which restricts increases in muscle volume.
Therefore, when intramuscular pressure increases to levels above circulating blood pressure, the blood supply flowing into the muscle stops and, with continued muscle activity, oxygen deficiency rapidly develops and lack of oxygen (ischaemic necrosis) of the muscle fibres occurs. There is also an additive effect as the muscle pH falls. Typically, the middle third of the muscle is involved. In experimental studies, relatively short periods of wing flap are enough to induce these degenerative changes.
Recognition and identification of the development stages in DPM
In response to complaints of DPM from the processing plant and/or customers, an investigation should be organised. This should include the identification of the category of DPM (fresh or old) at the processing plant. This information can then be correlated to husbandry management practices.
Category 1. The acute inflammatory lesion in which the deep pectoral muscle is very red and haemorrhagic. Haemorrhages also appear on the fibrous sheath (Figure 2). There is an obvious suffusion of serous fluid in the area of the damage making it appear wet. This stage is likely to be associated with a handling event (e.g. catching) and will be present for about 48 hours.
Category 2. At this stage the lesion in the inner fillet has become well defined and is sometimes circumscribed by a haemorrhagic ring (Figure 3). The affected areas are pale pink to plumb coloured and there are clear changes consistent with early coagulative necrosis of the muscle, when the tissue texture becomes fibrous. This is sometimes described as ‘fish flesh’. This stage will continue for a few days after the initial event or incident.
Category 3. The progressive degeneration and greening of the damaged tissue (Figure 4). Often, only the middle part of the fillet is involved and the progressive greening is in parallel with the loss of cellular structure, so that a ‘putty like’ consistency develops within the lesion. This green, necrotic area will persist and through time will gradually reduce in size as it is reabsorbed, so that the symmetry of the breast is lost in some older birds. The green colour is produced by the breakdown of haemoglobin and myoglobin to bile salts.
Factors Affecting the Occurrence of DPM
The pectoral muscles make up nearly a quarter of the total liveweight in current-day meat chickens. Rearing broiler chickens to heavy market weights can increase the probability for occurrence of DPM. Incidence is dependant on management and husbandry systems and not simply bodyweight and birds at any age or weight can be affected.
DPM is associated with the following factors:
- Excessive wing flapping
- Heavy market bodyweight
- Sex: incidence can be higher in males compared to females
- High white meat yield
- Rapid growth rate
The desirable efficiency in growth and anatomy of today’s broiler brings with it the possibility of DPM development.
Commercially raised broiler chickens are kept relatively comfortable and inactive during the growing period. Consequently, the pectoral muscles are not exercised enough to increase efficiency of the circulatory supply to the muscles and to allow the expansion of the surrounding fibrous sheath. It is doubtful that even a subtle amount of wing activity would help improve circulation or develop the sheath adequately.
Few, if any, processing plants actually track or document the incidence of DPM on a regular basis. Detection of DPM on whole carcases and parts is extremely difficult as lesions are not visible during carcase inspection or sorting. As birds also exhibit no symptoms, finding affected live birds in a flock and treating them is not possible.
The key to avoiding the DPM lies with preventative management. Controlling the incidence of DPM hinges upon identifying and eliminating certain flock management issues that contribute to the development of the condition.
The key to reducing the incidence of DPM lies in management of the broiler flock and minimising wing flapping.
To avoid the occurrence of DPM, the following flock management guidelines (Table 1) are suggested as starting points to investigate and minimise any unnecessary wing activity. 
Conclusion
Reducing DPM is a broiler management responsibility.
