Welfare implications of avian osteoporosis

Cage layer fatigue was first noticed after laying hens began to be housed in cages in the mid-20th century. Hens producing eggs at a high rate were most susceptible to the disease. Early research revealed that cage layer fatigue was associated with osteoporosis and bone brittleness. Severe osteoporosis leads to spontaneous bone fractures commonly in the costochondral junctions of the ribs, the keel, and the thoracic vertebrae. Vertebral fracture may damage the spinal cord and cause paralysis. Osteoporosis appears to be inevitable in highly productive caged laying hens. The condition can be made worse by metabolic deficiency of calcium, phosphorus, or vitamin D. Hens in housing systems that promote physical activity tend to have less osteoporosis and rarely manifest cage layer fatigue. Genetic selection may produce laying hens that are less prone to bone weakness. The welfare implications of osteoporosis stem from pain, debility, and mortality associated with bone fracture. The chicken has well-developed neural and psychological systems specialized to respond to pain associated with trauma and inflammation. Although studies on the chicken have not focused on pain due to bone fracture, physiological and behavioral similarities to other species allow inference that a hen experiences both acute and chronic pain from bone fracture. There is little information on osteoporosis in commercial caged layer flocks, however, evidence suggests that it may be widespread and severe. If true, most caged laying hens suffer osteoporosis-related bone fracture during the first laying cycle. Osteoporosis also makes bone breakage a serious problem during catching and transport of hens prior to slaughter. Estimates of mortality due to osteoporosis in commercial caged layer flocks are few, but range up to a third of total mortality. Many of these deaths would be lingering and attended by emaciation and possibly pain. Osteoporosis-related bone breakage during processing has reduced the marketability of spent caged laying hens, contributing to the need to develop humane on-farm killing methods to support alternative means of spent hen disposition. Overall, the evidence indicates that cage layer osteoporosis is a serious animal welfare problem. A determined effort must be made to make the laying hen no longer susceptible to the harmful effects of excessive bone loss.