Osteopenia refers to a reduction in bone mineral density, whereas osteoporosis refers to clinically evident disease in which bone fractures result from osteopenia. Osteopenia/osteoporosis affects laying hens raised in cages toward the end of the laying cycle and has been termed cage layer fatigue. Slow chronic loss of calcium stored in the skeleton occurs during the laying period, leading to osteopenia. Clinical disease develops when the bone mass becomes inadequate to support the body weight.
Clinical signs are variable and include leg weakness, vertebral fractures, posterior paralysis, and sudden death. The sternum is often deformed, with the ribs infolded at the costochondral junction. Fractures occur in the long bones and vertebrae. Bone cortices are thin and brittle. Parathyroid glands are hypertrophic and hyperplastic. Birds that die suddenly often have a partially shelled egg in the oviduct, and death is presumed to be due to hypocalcemia.
The cause of osteopenia is due in part to a lack of mechanical stress on the bone in hens confined to cages (lack of exercise) and to selection for high egg production. Suboptimal nutrition, with inadequate calcium, phosphorus, and/or vitamin D, contributes to the condition. Excessive dietary calcium during the growing period may predispose to the development of osteoporosis during the laying period.
Although prevention of osteopenia and osteoporosis in layers with nutritional management has not been successful, sources of calcium that enable the slow release of mineral, such as oyster shell, appear to give the best results to reduce development of osteopenia and improve egg shell quality. In addition to being a problem during the egg production period, bone fractures that occur during transport and processing at the end of the flock production life are significant animal welfare and economic issues.