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Nutritional secondary hyperparathyroidism (NSH), also known as miller’s disease, bran disease, and big-head disease, is caused by excessive mobilization of calcium from the skeleton under the influence of parathyroid hormone.

The maintenance of blood calcium homeostasis is critical to the function of the muscular and nervous systems and is therefore under close regulation by the hormones parathormone and calcitonin. When blood calcium levels fall below normal, calcium is resorbed from the bone under influence of parathormone in order to re-establish normal blood levels. If this occurs over an extended period of time, the bones become depleted of calcium and lose their structural integrity, resulting in lameness. The facial bones, depleted of calcium, become fibrotic and enlarged, hence the name big-head disease.

The condition occurs due to a dietary calcium deficiency, excess levels of phosphorus, an inverted calcium-to-phosphorus ratio in the total diet, or high levels of oxalates in forages which interfere with digestion and absorption of calcium.

In earlier times, millers fed the horses they used to work their mills the wheat bran that was a by-product of the mill. Bran is very high in phosphorus and low in calcium and many of the millers’ horses developed this condition, thus the name miller’s disease or bran disease. In modern times, the problem is most prevalent in instances when high levels of grain are being fed without calcium supplementation or adequate intake of good-quality forage or when horses are grazing pastures high in oxalate content.

Due to the strong homeostatic mechanism controlling blood calcium, analysis of the blood for calcium is not very valuable as a diagnostic tool. Instead, the most accurate method of determining if this problem exists is by evaluation of the ration. The calcium intake of the horse should be determined and compared with the requirement of the specific class of horse in question. Additionally, phosphorus level in the feed should be determined to make sure that its level is not greater than that of calcium. Provided that an inverted calcium-to-phosphorus ratio is not being fed and that calcium levels are adequate, this problem should then be limited to areas where there are high levels of oxalates.

Horses respond well to supplemental calcium in the form of calcium carbonate. As such, the problem of NSH should not be of concern on the well-managed stud farm where horses are fed a properly formulated diet.

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