Diet-related diseases caused by deficiencies, toxicities, or imbalances of nutrients usually occur during periods of greatest nutritional stress. This usually occurs during rapid growth of the foal. The common diseases of growing foals include nutritional secondary hyperparathyroidism, developmental orthopedic disease, and contracted tendons.
Nutritional secondary hyperparathyroidism is caused by an imbalance of the amounts of calcium and phosphorus in the diet. Diets rich in organic phosphorus and low in calcium can cause release of the parathyroid hormone from the parathyroid gland in the neck of horses. This hormone is released to stimulate removal of calcium from the bones of the animal in an attempt to maintain normal blood levels of calcium. The calcium lost from the bone is replaced by connective tissue resulting in enlargement of the bones of the head. In fact, the disease is often called "Big Head" because of this abnormality. It also is sometimes called Miller’s Disease, because historically it was seen commonly in the horses owned by wheat millers who fed their horses large amounts of bran.
Insuring that growing animals consume appropriate amounts of calcium and phosphorus easily prevents this disease. Because horses, like other animals, cannot choose a balanced diet if individual nutrients are provided separately, it is important that these nutrients be balanced in the diet fed to the animals and that consumption be monitored to ensure appropriate intake.
Developmental orthopedic disease describes a variety of bone abnormalities that occasionally occur in young, rapidly growing horses (and other species). Various causes for these diseases have been reported, including environmental, genetic, and nutritional. It is likely that there is not one single cause of all cases, and many factors may interact.
It has been suggested that providing excessive amounts of feed to foals to attempt to achieve rapid growth rates may promote the disease. It also has been suggested that deficiencies of trace minerals, including copper, zinc, and manganese may play a role in development of this disease.
Whatever the cause, there is sufficient evidence available to conclude that providing moderate quantities of a nutritionally balanced diet is the best insurance against diet-induced causes of developmental orthopedic diseases. Moreover, there is no evidence to suggest that foals must be fed large amounts of feed and maintain rapid rates of growth to achieve their maximal final adult size. Young horses that are fed so they maintain a lean body condition through appropriate amounts of feed and exercise appear to have the best chance of avoiding nutritional causes of this disease.
Contracted flexor tendons in foals have also been associated with rapid growth rates and high planes of nutrition. Contracted tendons can occur quite rapidly in foals; a concavity can develop in the hoof wall and the heel will begin to rise within 24 hours. The name contracted tendons comes from the appearance that the flexor tendons of the foot have contracted. However, it is more likely that development of the flexor apparatus of affected foals does not develop at the same rate as the rest of the limb, which results in the pointed-toe appearance.
As with developmental orthopedic disease, contracted tendons are best prevented with moderate amounts of feed and exercise. It is also important to recognize that when mares and foals are confined to stalls for long periods of time, their food intake should be monitored closely to avoid overfeeding.