This is an uncommon nutritional imbalance seen more frequently in growing horses. It is caused by excessive phosphorus in the diet without adequate calcium. Calcium and phosphorus together make up the inorganic part of bone. If this calcium/phosphorus imbalance exists, it disrupts the normal balance of bone resorption and deposition. Bone density is lost and bones become thicker to compensate for weakness.

Signs of this disease include shifting leg lameness (thought to occur because of abnormal bone turnover and tiny microfractures within the bone), and symmetrical enlargement of the facial bones - hence the name "Big Head". It can also cause difficulty eating because of changes in the bone of the tooth sockets. Bone fractures and heart failure may result.

Cereal grains like oats, corn and red bran are high in phosphorus and not calcium.

Veterinarians diagnose this condition with a careful history, specifically noting diet/feed, appearance and physical exam. Blood and urine levels of calcium and phosphorus are also helpful in diagnosing this condition.

Treatment involves diet change to normal calcium to phosphorus ratio. It may take months for any change in appearance of the skull bones.