Big Head Disease in Horses: Osteomalacia and Nutritional Secondary Hyperparathyroidism
- Big head disease is a diet-related bone disorder in horses caused by long-term calcium deficiency, excess phosphorus, or a reversed calcium-to-phosphorus ratio.
- Common signs include swelling of the face or jaw, pain while chewing, weight loss, shifting leg lameness, and in severe cases, fractures.
- See your vet promptly if your horse has facial swelling, trouble eating, or unexplained lameness. Early diet correction can improve comfort and limit further bone loss.
- Diagnosis usually involves a physical exam, diet review, and bloodwork. Your vet may also recommend skull or limb radiographs to look for bone changes.
- Treatment focuses on correcting the diet, reducing phosphorus-heavy feeds such as large amounts of bran or unfortified grain, and supporting the horse while bone remodeling occurs.
What Is Big Head Disease in Horses?
Big head disease is the common name for nutritional secondary hyperparathyroidism, a bone disorder caused by a long-term mineral imbalance in the diet. It happens when a horse does not get enough usable calcium, gets too much phosphorus, or eats a ration with a calcium-to-phosphorus ratio that is too low. In response, the body releases more parathyroid hormone to keep blood calcium normal by pulling calcium out of bone.
Over time, normal bone is replaced with softer osteoid and fibrous tissue. The facial bones and jaw are often the most visible areas affected, which is why some horses develop a broad, swollen appearance to the face. This process is also called fibrous osteodystrophy. The changes are often bilateral, though they may not look perfectly even from side to side.
As the disease progresses, chewing can become painful and less effective. Some horses lose weight because eating hurts. Others show shifting lameness, poor performance, or fractures because the whole skeleton can be weakened, not only the head.
The good news is that this condition is often preventable, and some horses improve when the diet is corrected early. Recovery can take time, and the degree of improvement depends on how advanced the bone changes are when your vet first evaluates your horse.
Symptoms of Big Head Disease in Horses
- Firm or soft swelling of the face, upper jaw, or lower jaw
- Pain or difficulty chewing hay and feed
- Weight loss or poor body condition
- Shifting or alternating limb lameness
- Reluctance to work, stiffness, or reduced performance
- Loose teeth or abnormal wear from painful chewing
- Bone pain or sensitivity around the skull and jaw
- Pathologic fractures or sudden worsening lameness
Call your vet sooner rather than later if your horse has facial swelling, trouble chewing, dropping feed, or unexplained lameness. These signs can overlap with dental disease, sinus disease, trauma, and other bone disorders, so your vet needs to sort out the cause.
See your vet immediately if your horse cannot eat comfortably, seems painful when handling the head, or develops a suspected fracture. Severe cases can involve fragile bones, and forcing exercise or transport without guidance may make things worse.
What Causes Big Head Disease in Horses?
The underlying cause is a chronic imbalance between calcium and phosphorus in the diet. Horses are at highest risk when they eat rations that are low in calcium, high in phosphorus, or both. A calcium-to-phosphorus ratio below 1:1 is a classic setup for disease, especially when that pattern continues for weeks to months.
Common diet patterns linked to big head disease include mature grass hay or pasture with naturally lower calcium, plus large amounts of unfortified grain-based concentrates such as oats, corn, or barley. Heavy use of wheat bran has also been associated with this condition, which is why older references sometimes call it bran disease. Some warm-season grasses can have less favorable calcium-to-phosphorus ratios than cool-season grasses, and forage mineral content can vary with plant maturity, soil, and growing conditions.
This is not always about a horse eating too little overall. A horse may be getting enough calories but still have a mineral imbalance that slowly weakens bone. Blood calcium often stays in the normal range because the body works hard to maintain it, even while calcium is being removed from the skeleton.
Less commonly, your vet may consider other causes of bone loss or secondary hyperparathyroidism, including kidney disease or other metabolic problems. That is one reason a full exam and diet history matter so much before making assumptions.
How Is Big Head Disease in Horses Diagnosed?
Diagnosis starts with a hands-on exam and a careful diet review. Your vet will want to know exactly what your horse eats each day, including hay type, pasture access, grain, bran, supplements, and any ration balancer. This history is often the key that connects the clinical signs to a mineral imbalance.
Your vet will usually recommend bloodwork to look at calcium, phosphorus, and markers such as alkaline phosphatase. In horses with nutritional secondary hyperparathyroidism, blood calcium may still look normal because of the body’s homeostatic controls, while phosphorus may be increased and alkaline phosphatase may be elevated. That means normal calcium does not rule the disease out.
If facial swelling, lameness, or fracture risk is present, your vet may also suggest radiographs of the skull or affected limbs. Imaging can help show bone demineralization, fibrous change, or fractures, and it can help rule out dental root disease, sinus disease, trauma, or other bone conditions.
In some cases, your vet may recommend forage testing or a full ration analysis. That can be especially helpful when the diet looks reasonable on the surface but the hay or pasture mineral profile tells a different story.
Treatment Options for Big Head Disease in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call or outpatient exam
- Detailed diet history and ration correction
- Removal or major reduction of high-phosphorus feeds such as large bran portions or unfortified grain-heavy meals
- Addition of a calcium-appropriate forage or ration balancer based on your vet's guidance
- Basic bloodwork focused on calcium, phosphorus, and alkaline phosphatase
- Exercise restriction if bone pain or fracture risk is suspected
Recommended Standard Treatment
- Complete veterinary exam and full diet review
- CBC and chemistry panel with calcium, phosphorus, and alkaline phosphatase
- Targeted skull or limb radiographs when indicated
- Structured nutrition plan using tested forage, fortified feed, or ration balancer to restore a safer calcium-to-phosphorus balance
- Pain control and supportive care if your vet feels it is appropriate
- Dental evaluation if chewing is painful or inefficient
- Recheck exam and repeat bloodwork to monitor response
Advanced / Critical Care
- Equine hospital evaluation or urgent referral
- Expanded imaging for severe skull changes, marked lameness, or suspected fractures
- Hospitalization for pain control, assisted feeding, or close monitoring when eating is difficult
- Advanced dental or oral assessment if jaw pain or tooth instability is present
- Management of complications such as pathologic fractures or severe weight loss
- Serial rechecks and tailored nutrition support during recovery
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Big Head Disease in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Does my horse's diet have a calcium-to-phosphorus imbalance, and which feeds are the biggest concern?
- Should we test my hay or do a full ration analysis before changing supplements?
- Does my horse need bloodwork only, or would radiographs help us understand how advanced this is?
- Is my horse safe to ride, lunge, or trailer right now, or should activity be restricted?
- Could dental disease, sinus disease, or another bone problem be causing similar signs?
- What changes should I make first if I need a more conservative care plan?
- How long should it take before I see improvement in chewing, weight, or lameness?
- When should we recheck bloodwork or repeat imaging to track recovery?
How to Prevent Big Head Disease in Horses
Prevention centers on balanced nutrition over time. Horses need an appropriate calcium-to-phosphorus ratio in the total diet, not only in one feed. In general, avoid building a ration around large amounts of wheat bran or unfortified grain without veterinary or nutrition guidance. If your horse eats mostly grass hay, especially mature warm-season hay, ask your vet whether a ration balancer or fortified feed would help fill mineral gaps.
One of the most practical prevention steps is to test your forage. Hay mineral content can vary widely by species, maturity, soil, and harvest conditions. A forage test gives your vet or equine nutrition professional something concrete to work with instead of guessing.
Routine wellness care also matters. Horses with subtle chewing pain, weight loss, or shifting lameness may be showing early signs of a nutrition or dental problem. Regular exams and dental care can help catch issues before bone changes become advanced.
If you are changing feeds, adding supplements, or trying to support a growing, pregnant, lactating, or hard-working horse, involve your vet early. Thoughtful, individualized diet planning is the safest way to prevent this condition.