Also known as ‘’Millers Disease’’ and ‘’Big Head’’ this condition is a Fibrous Osteodystrophy caused by secondary hyperparathyroidism and is caused by either a deficiency of Calcium or adverse balance between Calcium & Phosphorous.
In classical Bran Disease, millers fed their draught horses on wheat bran because it was cheap and readily available. Bran is very high in phosphorous and very low in calcium, so the diet was imbalanced and calcium deficient. As blood calcium levels fall, the parathyroid gland secretes Parathyroid Hormone which mobilizes bone calcium re-modelling and releases calcium from skeletal stores into the blood to maintain critical blood calcium levels, essential for muscle and nerve function. Slowly the calcium in the skeleton is leached out leaving the fibrous component of bone which proliferates in an effort to shore up the skeletal structure.
The loss of mineral causes bones to soften and pathological fractures may result. Surface bone may flake and tear off where muscles and ligaments attach and pain along with shifting lameness is a feature. Lameness can be subtle and difficult to diagnose. A ‘’bunny hopping’’ gait due to hip pain may be noticed along with muscle wastage over the rump especially. The bones of the head, being non weight bearing are sacrificed first and enlarge due to fibrous tissue proliferation causing the rounded facial contours and thickened lower jaw typical of Bighead. An old test to confirm the disease was to push an ice pick quite easily through the jawbone. Drooping of the lower lip may be observed, possibly from compression of the nerve supply from the mental nerve where it passes through the lower jaw.
Correcting the calcium deficiency will reverse the disease process but may be too late to repair more serious skeletal and joint damage. Supplementation of the diet with ground limestone (Calcium Carbonate) fed at the rate of 60g, and – 120g per day has been found effective. Use of Bran should be restricted care taken to properly compensate the calcium balance of the ration. Improved strains of tropical pasture grasses containing oxalate may reduce calcium absorbtion and cause the condition. Grasses such as Rhodes, Para grass, Napier, Signal and Kikuyu all contain oxalic acid and particular attention needs to be paid to calcium nutrition where these are fed.