Most people have heard the term "big head" applied to an individual with an inflated or pampered ego. Of course, in this context the term is figurative but pampered horses may literally exhibit big head syndrome - a condition where the head coarsens and increases in size. The scientific term for this affliction is "Secondary Hyperparathyroidism," a twenty-eight letter description which explains why most folks stick to simple names like Big Head Syndrome, Miller's Disease or Bran Disease.
Calcium is the culprit that is responsible for the deformity of the equine head. Calcium is one of the most important minerals in the body. Aside from its well known contribution to the structure of bones, calcium is also necessary for muscle contraction, proper heart activity and the transport of nerve impulses. Because of its indispensable nature, the body has developed mechanisms by which calcium can be mobilized from one site to another area where the need is greater. This happens when there isn't enough calcium in the body - a condition known as hypocalcemia. When the body experiences prolonged periods of hypocalcemia, big head syndrome occurs. There are several ways that a horse can find himself in this deficient condition. Consuming a diet with inadequate amounts of calcium is the most common. A horse obtains calcium from the grasses and grains that he eats. Grazing on grasses growing on calcium deficient soils or eating feedstuffs grown in deficient areas limits the amount of dietary calcium. Fortunately this condition is not prevalent throughout the entire United States. In several areas of the country, a limestone base (calcium carbonate) exists immediately below the topsoil. This geological formation enriches the soil with calcium which produces agricultural produces rich in this dietary mineral. It is interesting to note that these endowed areas - Lexington, Kentucky, Ocala, Florida, North Central Texas and Hartford County, Maryland have traditionally been some of the leading horse producing regions.
Another circumstances that allows the development of Big Head involves
the relationship between calcium and its sister mineral, phosphorus.
Many body functions require the binding of these two minerals. For
example, the bone ratio of calcium to phosphorus is approximately two to
one; this means that bone is composed of two parts calcium to one part
phosphorus. Just like mixing water and cement, this proportion is
critical to the strength of the mortar or, in this case, the hardness of
Because bone is not the only place in the body which demands the binding of these minerals, the dietary ratio of calcium to phosphorus is closer to one to one. However, it is known that this ratio is not extremely critical as long as the horse has adequate levels of both minerals. For example, adult horses have been fed a dietary ratio of six to one calcium-phosphorus ratio without the occurrence of any problems.
(But, make a note: young horses cannot tolerate these imbalances and the calcium-phosphorus ratio in their diet should never exceed three to one. ) The key to the prevention of big head syndrome is the consumption of adequate phosphorus levels; then any excess of calcium does not normally create any complications. It is diets that provide either too little calcium or too much phosphorus that predispose horses to hypocalcemia. This sequence of events often prevailed in the past when millers used horse power to grind cereal grains into flour. The practice of the time was to give the by-products of the milling plus a percentage of the grain to the miller in payment for services rendered. The miller, in turn, used these resources to feed the horses that provided the power - a practice that caused his horses to receive a disproportionate amount of grain and bran. Today we know that these cereal products are high in phosphorus and contain little calcium - the very combination that sets up the opportunity for hypocalcemia to occur.
The added consumption of bran supplied the trigger. Bran contains phytic acid which chelates calcium further reducing the level of dietary calcium. These factors in the ration fed to the miller's horses predisposed his horses to big heads even though they were fat and slick and presumed to be the pampered horses of the era. Although millers' horses are a thing of the past, today's race horses and show horses are receiving rations, low in roughage and high in grains to boost energy intake without causing large gut fill. This type of management can produce the same results - big head.
A third cause of hypocalcemia involves inadequate amounts of the Vitamin D 3 which is necessary for the transport of calcium across the gut wall. Horses have the ability to manufacture Vitamin D 3 from sunlight and sun cured forages such as hay. Again, those pampered individuals which spend much of their life in stalls continually wearing sheets and blankets are most susceptible to low levels of Vitamin D 3. Fortunately this deficiency is not a very high risk problem for normal mature horses but Vitamin D 3 synthesis may be hampered in the old, the young or the sick.
copyright, Dr Jim and Lynda McCall
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