A fairly rare nutritional disease in horses is nicknamed big head disease. The actual name is nutritional secondary hyperparathyroidism, so the reason it is referred to as big head disease is because one of the major clinical signs is that the head bones grow toobig. This enlargement occurs because the horse's feed has an improper calcium to phosphorus ratio causing high phosphorus levels in the horse's blood. This enlargement occurs due to a diet low in calcium and either high in phosphorus or low in vitamin D. Also, some grasses like Dallis grass, buffel grass, panic grass and Setaria contain high levels of oxalates, and these grasses decrease absorption of calcium. High phosphorus in the blood triggers a release of hormones that causes calcium and phosphorus to be resorbed from the bone. As calcium is absorbed from the bone, it is replaced by fibrous tissue that exceeds the amount of bone resorbed. Since the bones of the skull are affected, the head enlarges and causes the big head appearance.
In some cases, the head enlarges so much that horses cannot eat and their teeth loosen and fall out. Early signs also involve a shifting leg lameness due to weakened bones that can lead to fracture. Treatment involves changing the feed to one with a correct calcium to phosphorus ratio. Alfalfa is a good choice as it has a high calcium level. You can also add limestone to the alfalfa to increase the amount of calcium. It will usually take up to a year for these horses to improve, if they survive. So it is important to realize nutrition is important and feeding a balanced diet is critical for all animals. For this reason, the calcium to phosphorus ratio of all supplements you are giving your horse should be reviewed as some supplements could lead to problems.