Description

When a horse feeds exclusively on certain grasses or is fed grain diets which are high in phosphorous and low in calcium, the horse's body's need for calcium causes calcium and minerals to be mobilized from his bones.

As the calcium is stripped from the bones in the horse's head, fibrous connective tissue develops, creating enlargements of the facial features and leading to an enlargement of the head. Known as Nutritional Secondary Hyperparathyroidism, it is a bone disorder found in horses.

Horses develop enlarged facial bones above and behind the facial crests because of the fibrous tissue that develops.

Some or all horses in a pasture may develop this big head. The disease comes on rather rapidly, often within two months of being put on pastures of tropical grass species, such as buffel grass, green panic, setaria, kikuyu, guinea grass, para grass, and numerous others.

The hazard is greatest when these grasses provide all, or almost all, the feed available. Native grasses such as rye and the sorghums do not cause Big Head.

Symptoms

• Lameness, with a shortened gait
• Appearance of stiffness
• Swollen facial bones, thickened jaw bones, and difficulty chewing
• Loose and shifting teeth
• Obstruction of nasal passages which may cause noise and/or discharge
• Ruptured tendons and increased risk of fractures

Causes

Nutritional Secondary Hyperparathyroidism is seen mostly in horses grazing exclusively on grasses containing high levels of oxalate that locks up calcium, making it unabsorbable by the horse's intestines. As calcium and minerals are taken from the bones, they are replaced by fibrous connective tissue, which causes the bones in the head to enlarge.

The low calcium in the diet leads to low blood calcium which, in turn, causes the horse to begin mining calcium and phosphorus from the bones to maintain normal blood calcium levels.

Grazing hazardous pastures planted with tropical grasses causes a calcium deficiency because of the crystals of calcium oxalate in the grass blades that prevent the horse from absorbing calcium from the grass during digestion.

Nutritional Secondary Hyperparathyroidism is also caused by a diet high in grains that are high in phosphorous and low in calcium. The phytates in bran also bind up calcium and prevent absorption during the digestive process.

Prevention

Horses should feed on native grasses where possible. If they do graze on introduced tropical species of grass, the grazing period should be limited to no more than a month without supplementary feedings. If only hazardous grasses are available, the growth of a legume component in the pasture is highly recommended to provide a source of feed free of oxalate.

A calcium and phosphorus supplement should also be given the horse on a daily basis. Mineral and supplement mixtures that will provide the required amount of calcium and phosphorus for horses for a week include 1 kg rock phosphate mixed with 1.5 kg molasses plus 1 kg of a mixture of 1/3 ground limestone and 2/3 dicalcium phosphate mixed with 1.5 kg molasses.

If good quality lucerne is available, 20 kg would need to be fed to each horse weekly to maintain a healthy calcium/phosphorous level adn preventNutritional Secondary Hyperparathyroidism.

Treatment

Veterinary diagnosis using X-rays may be required to rule out other possible causes and to determine the severity and degree of involvement of the bone structure. Blood, urine, and fecal tests may be necessary to confirm the diagnosis and assess response to treatment.

The diet must be corrected to reverse the calcium deficiency. If horses are showing enlarged head bones and lameness, the veterinarian should be consulted and the proper diet with necessary mineral supplements should be developed and fed for at least six months. It can take up to twelve months for remineralization of bone to occur and horses should not be regarded as sound and safe for riding until this has been confirmed.

If grazing tropical grass pastures, the grasses should be supplemented with lucerne hay or chaff, which are good sources of calcium. If horses are being fed bran mashes, do not add calcium supplements, as bran phytates will bind calcium, making it unavailable to the horse.

Growing horses, working horses, heavy sweaters, pregnant mares, and older horses are particularly susceptible to calcium deficiency and require higher levels of calcium supplementation.

Lameness usually disappears in four to six weeks after the diet has been corrected. The enlarged facial bones may or may not return to normal. Ordinarily, nine to twelve months of calcium supplementation may be needed for bones to return to their full strength.