Big Head Disease, scientifically known as Secondary Nutritional Hyperparathyroidism (SNH), is a serious metabolic and nutritional disorder in horses. It occurs when the horse’s diet is chronically deficient in bioavailable calcium or contains excess phosphorus, causing the body to leach calcium from the bones, particularly in the skull and limbs. Over time, this results in facial swelling, bone weakness, and deformity, hence the name “Big Head.”
What Causes Big Head in Horses?
The condition stems from a long-term imbalance in the calcium-to-phosphorus ratio, typically when diets are:
- Too high in phosphorus (e.g. grain-based or bran-rich feeds)
- Too low in calcium
- Contain calcium in a form that cannot be absorbed (e.g. calcium bound by oxalates or phytic acid)
The ideal calcium-to-phosphorus ratio in a horse’s diet is around 2:1. When this ratio is off for extended periods, the body triggers the parathyroid glands to maintain blood calcium by extracting it from the bones.
Common Causes of Big Head
Grain and Bran-Based Diets
- Grains and wheat or rice bran are high in phosphorus and low in calcium.
- Bran also contains phytic acid, which binds calcium and prevents its absorption, often leading to the classic “bran disease.”
Oxalate-Rich Pastures
Some tropical and subtropical grasses contain oxalates, which bind calcium in the digestive tract and make it unavailable for absorption.
High-risk pasture grasses include:
- Setaria (especially Kazungula cultivar): can contain up to 7% oxalic acid
- Buffel Grass – Ca:oxalate ratio around 0.22:1
- Kikuyu – Ca:oxalate ratio around 0.23:1
Grazing these pastures over several months, especially during rapid growth after rain, can significantly increase risk.
Signs and Symptoms of Big Head Disease
Symptoms often appear gradually and may go unnoticed in the early stages.
Early signs:
- Intermittent or shifting lameness
- Stiffness or reluctance to move
- Poor performance
- Weight loss and dull coat
- Changes in behaviour or appetite
Advanced signs:
- Swelling or distortion of the jaw and facial bones
- Loose or displaced teeth
- Difficulty chewing and eating
- Fragile bones, lameness, or spontaneous fractures
- Enlarged parathyroid glands
Without intervention, the condition can become irreversible and lead to severe pain, disability, or even euthanasia.
How Is Big Head Diagnosed?
Diagnosis is based on:
- Clinical signs (especially facial swelling and lameness)
- Diet history (grain-heavy or oxalate-rich feeding)
- Radiographs showing bone demineralisation or abnormal thickening
- Blood calcium may appear normal due to internal calcium mobilisation
Treatment and Recovery
Correcting the horse’s diet is the cornerstone of recovery. Simply adding calcium is not sufficient, a complete nutritional review is required. Use Truguide your trusted guide as your suppment selector
Recommended steps:
- Remove access to oxalate-rich pastures.
- Provide calcium-rich roughage such as lucerne hay.
- Supplement with calcium carbonate or dolomite (if magnesium is also needed).
- Use a balanced multi-mineral supplement such as Equilibrium Mineral Mix designed for horses grazing tropical or subtropical pastures.
- Allow rest and gentle paddock movement; avoid forced work or training.
- Involve your vet to monitor bone recovery, dental complications, and long-term outcomes.
Recovery may take up to 12 months, and some skeletal damage may be permanent if left untreated for too long.
Prevention – The Best Approach
Pasture Management:
- Avoid grazing horses on high-oxalate grasses like Setaria, Kikuyu, or Buffel.
- Re-seed with non-oxalate species like Rhodes grass, cocksfoot, or native pasture.
- Avoid using poultry manure or superphosphate fertiliser, which increase oxalate levels.
- Rotate pastures, and avoid grazing during rapid growth phases following rain.
Dietary Management:
- Avoid feeding large amounts of bran or cereal grains unless balanced with additional calcium and minerals.
- Feed calcium-rich roughage like lucerne or clover.
- Use formulated supplements tailored to horses on tropical pastures such as Equilibrium Mineral Mix
- Closely monitor young, growing, pregnant, or lactating horses—their calcium needs are higher.
- Regularly review feed plans with a qualified equine nutritionist or veterinarian.
Conclusion
Big Head or Secondary Nutritional Hyperparathyroidism is a painful and preventable disease caused by long-term dietary imbalance. With proper nutrition, pasture management, and early intervention, horses can recover and go on to lead healthy lives. Prevention is always better than cure, if your horse grazes on tropical grasses or eats large amounts of grain, ensure their diet is properly balanced for calcium and phosphorus using balanced minerla supplemnts such as Equilibrium Miineral Mix
