The severity of the welfare problems, the large number of animals involved globally and the fact that these welfare concerns have not been adequately addressed to date, highlight that the problem requires urgent attention (RSPCA, n.d.). As a consequence of genetic selection for rapid growth, energy is diverted primarily to muscle growth. This process can deprive other parts of the body of energy and oxygen and put pressure on the bird’s organs, particularly the heart and lungs (RSPCA, n.d.). As such, accelerated growth can increase the risk of two types of heart disease: ascites and sudden death syndrome. It also increases the risk of several muscle problems such as muscle tissue diseases.
- Sudden death syndrome
This condition is caused by acute heart failure, i.e. infarction, which can be triggered by stress; either environmental stress such as heat stress or a stressful event such as trapping and transportation. It is difficult to determine the true prevalence of death from heart failure, but it has been estimated that this condition accounts for just over a third of all on-farm mortality. Furthermore, although birds that arrive dead at the abattoir are not usually examined for the cause of death, one study suggests that the majority of these birds are likely to have died from sudden death syndrome (RSPCA, n.d.).
- Ascites
Ascites, commonly known as “water belly”, is an accumulation of fluid in the abdominal cavity (Julian, 1993). In a worldwide study, the incidence of ascites was estimated at 4.7%, making it one of the major causes of death in broiler chickens (RSPCA, n.d.). It is considered a multi-factorial disorder.
However, it is believed that the main contributor to the condition is the increased need for oxygen in the bloodstream, being driven by the increased metabolic demand of rapid growth. Thus, increased metabolic demand, coupled with exposure to environmental conditions such as temperature, lighting and ventilation, and nutritional factors, for example, the form or content of food, all appear to encourage the development of ascites (Druyan, 2012; Balog, 2003). These in turn create stress on the heart and lungs, leading to enlargement of the heart. As a result, fluid drains from the liver and collects in the bird’s abdomen.
- Increased blood pressure
In addition, increased blood pressure in the veins, liver and abdominal vessels forces plasma fluid out of the vessels, especially the hepatic vessels, into the abdominal space (Julian, 1998; Baghbanzadeh & Decuypere, 2008; Wideman et al., 2013). This pushes blood through the blood capillaries in the lung and the increased workload for the right side of the heart results in pulmonary hypertension and ventricular hypertrophy. Most commonly seen in male chickens, ascites mortality can range from 0 to 30% in broiler flocks (Pavlidis et al., 2007). Because ascites develops gradually, birds suffer a long period before death (Bessei, 2006). In addition to this condition causing mortality on farms, chickens are increasingly condemned to slaughter due to this disease. Each year, between 2011 and 2013, an average of 2.4 million chickens were rejected from the food chain due to this disease. In addition to the economic impact, ascites also has a major impact on the welfare of birds by causing them to suffer for a long period before death (RSPCA, n.d.).
- Muscular myopathies
Another health problem affecting the welfare of poultry is muscular myopathies or muscle tissue diseases. They are more common today than in previous years and are all associated with the intensive genetic selection of broilers to improve their growth rate, body weight and breast yield (Barbut, 2008; Dransfield & Sosnicki, 1999; Mudalal et al., 2015). Deep Pectoral Myopathy is caused by ischemic necrosis of the chest muscle (lack of blood supply to the muscle fibres causing them to die and become necrotic). It is known as green muscle disease because of the appearance of green flesh in muscle tissue (Bilgili, 2006).
- Wooden breast syndrome
Wooden breast is a muscle disorder characterised by necrotic muscle fibres and replacement of muscle by connective tissue, water, and fat, resulting in a palpably firm consistency of the breast muscles (Papah et al., 2017). This condition often occurs at the same time as the condition known as White Stripes (Sihvo, 2017). The condition is found on the outside of the pectoralis major muscle. It is visible as white striations parallel to the muscle fibres (Petracci et al., 2013). These striations turn out to be adipose (fatty) tissue (Bailey et al., 2015).
Even though the cause of both conditions is unclear, several studies have suggested that their pathogenesis is associated with several biological processes, such as localised hypoxia in muscle, oxidative stress, increased intracellular calcium accumulation (Zambonelli et al., 2016; Abasht et al., 2016; Mutryn et al., 2015). Although the incidence rate of ‘wood-breast disease’ in commercial chickens is unknown, it is becoming increasingly common (Abasht et al., 2016). Affected chicks are most likely to be the fastest growing and have the highest body weight (Mudalal et al., 2015). A recent study found that wood-breast disease is associated with poor walking ability and therefore may be partly related to the common gait abnormalities suffered by broiler chicks.
Broilers with larger breast dimensions appear to be more prone to be affected by this condition (Baldi et al., 2018). Although there is a clear meat quality issue, the pain or discomfort caused by these breast muscle myopathies has not yet been determined. While the wood breast condition is apparently asymptomatic, clinical signs such as lateral anterior breast swelling and decreased wing movement have been observed in severely affected birds (Papah et al., 2017). Likewise, the degenerative process leading to wood-breast is similar to Duchenne muscular dystrophy in humans, a painful and debilitating condition (Papah et al., 2017; Baldi et al., 2018). Thus, it is likely that Broiler chicks suffering from wood-breast disease may also experience similar pain and discomfort.
- Muscle abnormality called spaghetti meat
This condition often associated with white stripes is characterised by poor muscle growth due to immature intramuscular connective tissues. The affected muscle is so structurally weak that bundles of muscle fibres can be easily pulled apart with fingers, like spaghetti (Maiorano, 2017; Huang & Ahn, 2018).
References
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