Homeostatic imbalances of bone

Osteomalacia and rickets

Osteomalacia is a condition in which the bones are insufficiently mineralized. Rickets is a name for the same condition when it is present in prepubertal animals.

Parturient paresis (milk fever)

As dairy cattle begin milk production, the first milk produced (called colostrum) contains high concentra­tions of Ca²⁺. Colostrum requires approximately 3g of calcium per hour to produce. When a cow cannot mobilize this amount of calcium, she can develop milk fever within 72 hours following parturition. Symptoms include loss of appetite, followed by muscle weakness, decreased body temperature, labored breath­ing, and paralysis of hind legs. If left untreated, the cow can collapse into a coma and die.

To prevent milk fever, cows should be given suffi­cient vitamin D in the diet prior to parturition. If milk fever develops, cows are given oral or intravenous calcium, according to individual severity.

Egg-Iaying fatigue in birds

Similar to milk fever in cows, high-producing egg­laying hens can develop weak and brittle bones. A hen must deposit as much as 8-10% of her total calcium into the eggshell each day. Since the eggshell is depos­ited during the night, the hen must draw upon the calcium reserves located in a specialized type of bone called medullary bone. Under the influence of estro­gens and androgens secreted from the developing ovarian follicles, medullary bone is produced in hens 2 weeks prior to commencement of egg laying. As blood calcium levels decrease during eggshell forma­tion, the hen releases PTH, which mobilizes bone calcium. If insufficient stores of calcium are present in the bones, the bones become weaker as they become demineralized.