In chronic vitamin D deficiency, marked skeletal distortions become apparent (Scott et al., 1982) in which the spinal column may bend downward in the sacral and coccygeal region. The sternum usually shows both a lateral bend and an acute dent near the middle of the breast. These changes reduced the size of the thorax with consequent crowding of the vital organs.
A disease condition known as endochondral ossification defects (EOD) produces bone deformations, fractures and lameness in broiler chickens throughout the world within the first few weeks after hatching. Flocks with a high incidence of EOD have significantly lower bone ash and 1,25-(OH)2D3 compared with mildly affected flocks, and it seems probable that higher systemic concentrations of 1,25-(OH)2D3 between seven to 14 days of age will enhance the ability of broiler chickens to effectively mineralize the cartilaginous growth plates in the appendicular skeleton during early bone maturation (Vaiano et al., 1994).
Tibial dyschondroplasia (TD) is a bone disease characterized by nonvascularized, cartilaginous plugs in the epiphyseal growth plates of long bones in young birds. Though frequently found in the tibia, hence the name, TD can also occur in the femur and tarsus. Derangements in vitamin D metabolism have long been suspected to be intimately involved in TD (Xu and Henry, 1997). Indeed, recent research supports a role for vitamin D3 metabolism in clearance of the lesion. The hormonal form of vitamin D3, 1,25-(OH)2D3 (Elliot and Edwards, 1997) and the 1,24,25-(OH)3D3 metabolite (Edwards, 1990) have been shown effective in reducing the incidence and severity of the disease. It appears that vitamin D3 metabolites with a hydroxyl group at the 1-alpha position are most effective. The effects of 1,25-(OH)D3 on TD have been variable under laboratory conditions (Edwards, 1989; Rennie and Whitehead, 1996).
As in many other nutritional diseases of poultry, the feathers soon become ruffled. In red or buff color breeds of chickens, a deficiency of vitamin D causes an abnormal black pigmentation of some of the feathers, especially those of the wings. If the deficiency is very marked, the blackening becomes pronounced and nearly all the feathers may be affected (NRC, 1994). When vitamin D is supplied in adequate quantity, the new feathers and newer part of older feathers are normal in color, although the discolored portion remains black.
Signs of vitamin D deficiency begin to occur in laying hens in confinement within four weeks of the onset of vitamin D deficiency (Tsang et al., 1990). When laying chickens are fed a diet deficient in vitamin D, the first sign of deficiency is a thinning of the eggshells. Commercial layers will continue to lay eggs with reduced shell quantity for weeks. If the diet is also completely devoid of vitamin D3, egg production decreases rapidly and eggs with very thin shells or no shell will be produced.
Nys et al. (1992) observed that concentrations of circulating 1,25-(OH)2D3 and ionized calcium were inversely proportional during eggshell formation. As eggshell formation progresses, levels of ionized calcium decrease, resulting in a short-term hypocalcemia and thus levels of 1,25-(OH)2D3 increase in the plasma. In laying hens, eggshell strength tends to decrease as the hen ages. The decline in shell strength may be due to a decrease in the hen's ability to synthesize 1,25-(OH)2D3. The effect of dietary 1,25-(OH)2D3 on eggshell strength in older hens was evaluated; within three weeks the percentage of cracked or broken eggs was lower for 1,25-(OH)2D3 supplemented hens (Tsang, 1992).
Vitamin D nutriture of the hen also influences its content in egg yolk and the subsequent need for this vitamin by the chick (Stevens and Blair, 1985). Hatchability is markedly reduced, with embryos frequently dying at 18 to 19 days of age. Malpositions are increased dramatically, apparently as a result of reduced embryonic bone and muscular development (Narbaitz and Tsang, 1989). These embryos often show a short upper mandible or incomplete formation at the base of the beak. Eventually breast bones become noticeably less rigid and there may be beading at the ends of the ribs. Individual hens may show temporary loss of use of the legs, with recovery after laying an egg (usually shell-less) (Scott et al., 1982). During the periods of extreme leg weakness, hens show a characteristic posture that has been described as a "penguin-like squat."