Big Head Disease In Horses: Causes, Signs, and Bran Disease

Big head disease in horses is a bone disease caused by a diet with too little calcium or too much phosphorus. Vets call it nutritional secondary hyperparathyroidism, shortened to NSH.

The Merck Veterinary Manual lists the names “big head” and “bran disease” for NSH, and a 2010 case study in the Australian Equine Veterinarian writes “bighead.” The disorder is commonly known as “Horse Big Head Disease,” and the bone change behind the swollen face is called osteodystrophia fibrosa. Read more about hypocalcemia.

What Happens In The Body

Merck explains that NSH starts when the body keeps releasing parathyroid hormone to pull calcium out of the bones, because the diet does not supply enough. The bones of the head are most often visibly affected: osteoid and fibrous tissue take the place of the mineral in the facial bones and the lower jaw. That is where the name “big head” comes from. Merck adds that the changes show on both sides of the head but are not necessarily even.

As the disease progresses, chewing becomes difficult and painful, and Merck says this can lead to eating less and losing weight. Fractures might occur, and wounds in general heal poorly.

Merck says NSH is not common today. A 2025 paper in Research in Veterinary Science, by authors at Advanced Equine Diagnostics in Hawai'i and the Texas Tech University School of Veterinary Medicine, says the disease has largely disappeared where fortified feeds and legume forage are widely available, and that it remains common in regions without consistent access to fortified feeds and with high oxalate grasses.

Bran, Grain, and Big Head Disease

Merck names the horses most likely to develop a calcium shortage: horses of all ages fed mature grass hay or pasture of any variety, plus large amounts of unfortified grain-based concentrates such as corn, oats, or barley, or wheat bran.

The LSU AgCenter explains the problem in feed numbers. All grains except alfalfa pellets are extremely low in calcium and high in phosphorus, and a total grain ration needs extra calcium to reach a workable ratio. Wheat bran has far more phosphorus than calcium, as the table shows.

A 2018 case report in the Open Veterinary Journal, from the University of Bari in Italy, describes a 5-year-old Shetland pony whose keeper said it ate only wheat bran. The pony lay down persistently, had pain when its hind limbs were flexed, and had slight enlargement of the facial bones.

The Calcium To Phosphorus Ratio

Merck says too much phosphorus and too little calcium, a calcium to phosphorus (Ca:P) ratio below 1, cause the same clinical signs. The LSU AgCenter says rations should never have less than a 1 to 1 ratio. It adds that young horses can tolerate up to 2 to 1 and mature horses up to 3 to 1, but that ratios over 1.5 to 1 are not recommended.

Hay differs too. Merck says mature warm-season grasses such as Bermuda grass and teff tend to have lower Ca:P ratios, sometimes below 1:1, and that cool-season grasses tend to sit near 2:1 while legumes such as alfalfa may be closer to 4:1, sometimes higher. The LSU AgCenter gives 2 to 1 for grass hays and 5 to 1 for legume hays, and says the calcium and phosphorus in hay depend on fertilization and handling.

Merck says testing a forage in a forage-testing laboratory is the only way to know its mineral content.

Oxalate Grasses And Big Head Disease

Oxalate is a chemical in some tropical grasses. A 1981 paper in the Journal of Agricultural Science, from the Queensland Department of Primary Industries, fed horses tropical grass hays with more than 0.5% total oxalate. These horses were in negative calcium balance of up to 41 mg per kg of live weight per day. The authors explain this by calcium oxalate crystals: a major fraction of the calcium in the grasses sat in that form, and calcium in that form was unavailable for absorption.

The same paper says NSH may occur in horses that graze only on pastures with a calcium to oxalate ratio below 0.5, and that such ratios usually go with grasses holding more than 0.5% total oxalate.

A case study in the Australian Equine Veterinarian says NSH was first diagnosed in Australian horses grazing subtropical pastures in 1974. Since then it has been widely recognized in horses on setaria, kikuyu, buffel, pangola, green panic, guinea, signal, and purple pigeon grasses, which are planted along the seaboards of Australia. The authors report that the condition kept occurring 36 years after it was first described there.

The 2025 Research in Veterinary Science paper studied seventeen adult horses brought from the continental United States to Hawai'i that grazed kikuyu grass pastures. Nine of the 17 were diagnosed with NSH. The authors note that legume forage is largely unavailable on the Hawaiian Islands and that pastures there have high oxalate concentrations.

Signs, Diagnosis, and Treatment

Signs of big head disease in horses are the subject of a Mad Barn article. The Merck Veterinary Manual lists the signs as swelling and softening of the facial bones and alternating limb lameness. The University of Bari case report lists lameness, weakness, ataxia, and deformation of the bones of the skull as the clinical signs, and says that in more severe cases the teeth can be loose, which together with bigger upper and lower jaw bones leads to chewing problems, anorexia, and cachexia.

A horse with signs like these should have the vet out.

The University of Bari authors list four things a diagnosis rests on: history, clinical signs, X-ray findings, and laboratory tests. Merck says blood calcium does not reflect intake, because the body regulates it, while blood phosphorus may be raised as mineral leaves the bone. Serum alkaline phosphatase (ALP) activity is usually increased.

For treatment, the Bari authors write that horses with NSH need the calcium and phosphorus ratio in the diet corrected with calcium and phosphorus supplements, along with treatment of the skeletal problems. They say anti-inflammatory drugs are useful in horses with severe skeletal deformities, but that the prognosis is poor in any case. One of their two ponies improved clinically after a single dose of tiludronate, a bisphosphonate approved for horses, given along with a balanced diet, though its parathyroid hormone level and X-ray findings were not appreciably changed. The authors say this may not resolve the deformities.

In the Hawai'i study, two calcium supplements were given to two groups of horses for eight months. Of the horses diagnosed with NSH, 1 of 5 in the first group and 2 of 3 in the second group had resolved by the end. The authors write that calcium supplementation may be effective in some horses, but continued monitoring is required.

Prevention

The LSU AgCenter says rations that use large quantities of whole grains should be supplemented with calcium. The calcium can come from quality hay or from a mineral supplement, and dicalcium phosphate or limestone are normally available from feed distributors. LSU lists calcium carbonate at 38% calcium, limestone at 33%, and dicalcium phosphate at 22% calcium and 18.5% phosphorus.

LSU's table lists alfalfa pellets at 4.7 to 1, and its text says alfalfa pellets give a 5 to 1 calcium to phosphorus ratio. LSU says they are useful in grain rations as a calcium source. It warns that when alfalfa or lespedeza is the main forage, too much calcium would be fed and abnormal bone growth in colts may occur, so it recommends limiting alfalfa to one third of the hay.

Merck says warm-season grasses are best harvested before they reach maturity, because mature ones tend to have lower Ca:P ratios.

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FAQs

What is bran disease in horses?

Bran disease is another name for big head disease. Merck lists “Bran Disease” and “Big Head” in the title of its entry on nutritional secondary hyperparathyroidism in horses, and names wheat bran among the feeds that put horses at risk.

How to prevent big head disease in horses?

The LSU AgCenter says to keep rations at or above a 1 to 1 calcium to phosphorus ratio and to supplement calcium when large quantities of whole grains are fed. Merck says to test a forage in a forage-testing laboratory to know its mineral content. Read more about Pet disaster preparedness.

What causes big head disease in horses?

Merck says it occurs because the body releases parathyroid hormone to mobilize calcium from the bones when the diet falls short. The 2025 Hawai'i paper describes it as a consequence of diets with abnormal calcium to phosphorus ratios, and the Australian Equine Veterinarian case study links it to oxalate in tropical grasses.

Sources

• Merck Veterinary Manual. “Nutritional Diseases of Horses and Other Equids,” section “Nutritional Secondary Hyperparathyroidism (Big Head, Bran Disease) in Horses.” merckvetmanual.com.
• LSU AgCenter. Depew, Clinton G. “Minerals For Horses.” lsuagcenter.com.
• Blaney, B. J., Gartner, R. J. W., and McKenzie, R. A. “The effects of oxalate in some tropical grasses on the availability to horses of calcium, phosphorus and magnesium.” The Journal of Agricultural Science, 1981. cambridge.org.
• Stewart, J., Liyou, O., and Wilson, G. “Bighead in Horses - Not an Ancient Disease.” Australian Equine Veterinarian, volume 29, issue 1, February 2010. sciquest.org.nz.
• Hunyadi, L. M., and Sundman, E. A. “A cohort study of nutritional secondary hyperparathyroidism in horses in Hawai'i.” Research in Veterinary Science, November 2025. pubmed.ncbi.nlm.nih.gov.
• Lacitignola, L., and others. “Nutritional secondary hyperparathyroidism in two ponies.” Open Veterinary Journal, 2018. pmc.ncbi.nlm.nih.gov.