Hyperparathyroidism & Big Head

The parathyroid glands are two pairs of glands, one on each side of the neck near to the thyroid glands. They secrete the hormone Parathormone which regulates blood calcium. When the blood calcium level falls (hypocalcaemia), more parathormone is secreted and this stimulates bone remodeling, releasing extra skeletal calcium into the blood to raise the blood calcium level back to normal. Calcium is also retained at the kidney by resorption instead of being lost in the urine. Blood calcium level is strictly maintained within narrow limits and when the level drops, for example by being excreted into the milk of a lactating mare, skeletal calcium reserves are mobilized to maintain blood levels. Parathormone controls this ebb and flow of calcium in conjunction with another hormone, calcitonin.

If dietary calcium supply is inadequate, or if the diet contains more phosphorous than calcium for a prolonged period, then there is constant demand for skeletal calcium to maintain blood levels and the parathyroid glands have to work overtime to produce parathormone continuously. Long term the glands will enlarge in response to extra demand giving rise to the name hyperparathyroidism. The skeletal effects of this continuous drain on calcium weakens the bones which respond by adding more fibrous tissue. This results in demineralized bone being replaced by fibrous tissue in the condition called fibrous osteodystrophy. Bones become soft and prone to fracture. Bone surfaces may tear away where ligaments attach and exert a pulling force. The bones of the face and jaw enlarge and develop rounded contours giving rise to the name ‘’Big Head’’. Other old names for the same condition are Bran Disease and Millers Disease which relate to the habit of millers to feed bran to their own draft horses as it was a cheap by-product. Bran has very low calcium and high phosphorous content and caused the disease when other calcium supplies were lacking. We now know that equine diets should have a Calcium: Phosphorous ratio in the range 1.8: 1 to 2.2: 1.

One of the common causes for the problem is the presence of oxalic acid in several strains of tropical pasture grasses like Rhodes, Para grass, Kikuyu, Napier, Signal, and others. This chemical combines with dietary calcium to form insoluble calcium oxalate which renders the calcium unavailable for absorbtion from the horse’s gut. The solution to this problem is to raise dietary calcium intake by means of calcium supplementation, usually ground limestone or other form of calcium carbonate. This can be added as a supplement or included in a high calcium manufactured feed to prevent problems as well as to treat the condition.

in    0