Nutritional Secondary Hyperparathyroidism (NSH) – also known as Big Head Disease – is an uncommon problem in horses that stems from a mineral imbalance.

This skeletal condition develops due to a calcium deficiency or an imbalanced ratio of dietary calcium to phosphorus. These minerals play an important role in bone formation and growth.

In horses with chronically low levels of calcium, the parathyroid glands respond by releasing excessive levels of parathyroid hormone.

This hormone causes calcium to be released from bones into the blood, potentially leading to brittle bones over time. The condition can also result in a swollen face, hence the name Big Head Disease.

Studies report that Quarter Horses, Appaloosas, and pony breeds aged two to eight have a higher incidence of NSH. [1] Pregnant or lactating mares and young colts are also most commonly affected.

Nutritional secondary hyperparathyroidism is a serious disease that can be fatal if left untreated. It is not the disease itself that is typically fatal, but the resulting inability to consume feed can lead to death in some horses. [2]

The good news is that NSH is easily preventable with appropriate diet balancing. Knowing the causes, symptoms, and treatment for NSH is important for all horse owners.

What is Nutritional Secondary Hyperparathyoroidism?

Big head disease in horses is induced by a diet with a persistent lack of calcium, excess phosphorus levels, and/or imbalanced calcium to phosphorus ratio.

It can also be caused by problems with calcium absorption from the digestive system, including diets with hay that is high in oxalates.

Secondary hyperparathyroidism can also develop due to renal failure or tumour growth, though this is rare in horses. [6]

Calcium is essential for maintaining many physiological functions including muscle contractions, nerve function, blood clotting, and bone health. When horses have low levels of calcium in their blood (hypocalcemia), the body pulls this mineral from stores in bone tissue. [5]

Horses with NSH have increased parathyroid hormone levels. The parathyroid glands are two pairs of glands located in the neck behind the thyroid gland. These glands are responsible for regulating calcium levels in the blood.

As calcium levels in the blood fall, the parathyroid glands secrete more parathyroid hormone to resorb calcium from the bones and maintain homeostasis (stable levels) in the blood. [5]

In horses with hyperparathyroidism, excessive bone mineral loss leads attachment of fibrous tissue to the bone, known as fibrous osteodystrophy. This results in weak and misshapen bones. This is most noticeable in the horse’s head – hence the name Big head disease.

The horse’s limbs can also be affected which results in pain and the reluctance to walk or even stand in some cases. [1]

History of Big Head Disease

First described in 400 AD, NSH has also been referred to as bran disease, big head, and Miller’s disease. [5] Though rare in most parts of the world today, this condition was more common in the past when diet balancing and mineral requirements of horses were not well understood.

In South Africa between 1905 and 1909, there were approximately 400 reports of horses suffering from big head disease. In 1931, more than a quarter of the horses at the army hospital in the Philippines presented with NSH symptoms. [1]

Horses were sometimes fed large quantities of wheat bran which is high in phosphorus and low in calcium.

The condition was often noticed by distillery workers who fed their horses wheat bran and other grain by-products from the brewing process. The distillery workers noticed abnormal bony growths on their horses’ heads and observed that the bridge of the horses’ noses were abnormally wide and misshapen.

Researchers now know that these bony distortions occur in response to bone mineral loss and fibrous growths on the bone. [5]

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Dietary Causes of NSH

There are a few different dietary scenarios that can result in NSH. Ultimately, a poor dietary supply of calcium causes hyperactivity of the parathyroid glands which induces secondary hyperparathyroidism.

Horses should ideally consume a diet with 1.5 – 2 grams of calcium (Ca) for every 1 gram of phosphorus (P). This 1.5 – 2:1 calcium to phosphorus ratio of corresponds to the ratio of calcium to phosphorus found in bones.

For example, if a 500 kg (1100 lb) horse at maint