Big Head Disease in Horses: Signs, Causes and Treatment

Big Head Disease is preventable, but once bone has been lost, recovery can take months and some facial changes may not fully reverse.

By Dr Duncan Houston

Big Head Disease sounds like an old-fashioned stable term, but the condition behind it is very real.

The medical name is nutritional secondary hyperparathyroidism, often shortened to NSH. It happens when a horse’s diet does not provide enough available calcium, provides too much phosphorus, or contains high levels of oxalates that bind calcium in the gut and stop the horse absorbing it properly.

The body tries to keep blood calcium stable because calcium is essential for nerves, muscles, blood clotting, and survival. To do that, it releases parathyroid hormone, which pulls calcium from the skeleton. Over time, bone becomes demineralised and can be replaced by fibrous tissue, especially in the facial bones and jaw. That is where the “big head” appearance comes from.

The difficult part is that the early signs are often not a big head at all. The first clues may be shifting lameness, stiffness, poor performance, loose teeth, chewing difficulty, weight loss, or a horse that just looks sore and unwell.

Quick Answer

Big Head Disease in horses is nutritional secondary hyperparathyroidism, a bone disease caused by long-term imbalance in calcium, phosphorus, vitamin D, or calcium availability. It commonly occurs when horses eat diets low in calcium, high in phosphorus, or graze oxalate-rich pastures that bind calcium and prevent absorption. Early signs can include shifting lameness, stiffness, jaw or facial swelling, difficulty chewing, weight loss, loose teeth, and poor performance. Treatment requires correcting the diet, restricting high-oxalate forage, supplementing calcium appropriately, managing pain, and monitoring recovery over months. Merck Veterinary Manual notes that full recovery is possible, but advanced cases can take several months and bone deformities may not completely resolve. (Merck Veterinary Manual)

What Is Big Head Disease?

Big Head Disease is a nutritional bone disease caused by chronic calcium imbalance.

The body responds to low available calcium or excessive phosphorus by increasing parathyroid hormone. This hormone keeps blood calcium within a safe range by mobilising calcium from bone. If the problem continues, the skeleton loses mineral strength, and bone can be replaced by fibrous tissue. Merck describes nutritional secondary hyperparathyroidism as a persistent imbalance of dietary calcium and phosphorus that triggers increased parathyroid hormone, bone resorption, and fibrous osteodystrophy. (Merck Veterinary Manual)

The facial bones and mandible are often the most visible areas affected. MSD Veterinary Manual explains that osteoid and fibrous tissue can replace mineral content in the facial bones and mandible, which creates the classic “big head” appearance. (MSD Veterinary Manual)

But do not wait for the horse’s face to change before taking this seriously. By the time facial swelling is obvious, the mineral imbalance may have been present for a long time.

Why Calcium Balance Matters

Calcium is essential for:

• Bone strength

• Muscle contraction

• Nerve function

• Blood clotting

• Normal movement

• Teeth and jaw support

• Growth in young horses

• Lactation in mares

• Repair after injury

Phosphorus is also essential, but the balance between calcium and phosphorus matters. Oklahoma State University Extension states that a two parts calcium to one part phosphorus ratio is ideal, with 1:1 to 6:1 generally acceptable, and that total diets with more phosphorus than calcium should be avoided because phosphorus competes with calcium absorption. (extension.okstate.edu)

The mistake owners make is looking at one ingredient instead of the whole diet.

A hay may be acceptable on its own. A grain may be acceptable in small amounts. But when the full ration is calculated, the total calcium-to-phosphorus ratio may be wrong, especially if the horse is getting lots of cereal grain, wheat bran, mature grass hay, or high-oxalate pasture without enough available calcium.

What Causes Big Head Disease?

Big Head Disease is usually caused by one of three major diet problems.

1. Too Little Calcium

A horse may not receive enough calcium if the diet relies heavily on low-calcium grass hay, poor-quality pasture, unbalanced home-mixed feeds, or grain without proper mineral balancing.

MSD notes that mature warm-season grasses can have lower calcium-to-phosphorus ratios, while legumes such as alfalfa are usually much higher in calcium. It also states that forage testing is the only way to know the mineral content of a forage. (MSD Veterinary Manual)

2. Too Much Phosphorus

High-phosphorus diets can interfere with calcium balance. The classic old name “bran disease” came from horses being fed large amounts of wheat bran, which is high in phosphorus.

Oklahoma State University Extension notes that prolonged excessive phosphorus can lead to nutritional secondary hyperparathyroidism, especially when feeds are dominated by cereal grains such as wheat bran or oats without enough calcium-rich mineral supplementation. (extension.okstate.edu)

3. Oxalate-Rich Pastures

Oxalates bind calcium in the digestive tract, making calcium unavailable for absorption. The pasture may contain calcium on paper, but the horse cannot use enough of it.

This is especially important in tropical and subtropical regions. High-risk grasses can include buffel grass, setaria, kikuyu, pangola, green panic, guinea, signal grass, purple pigeon grass, and related tropical grasses. Published Australian reports describe Big Head Disease in horses grazing subtropical pastures containing oxalates that interfere with mineral utilisation. (ResearchGate)

A Queensland study of horses grazing buffel grass found the pasture was hazardous when total oxalate content was above 0.5% and the calcium-to-oxalate ratio was below 0.5. Supplementation with ground limestone and dicalcium phosphate prevented new cases during the initial study period and improved many clinical cases. (PubMed)

Is Vitamin D Involved?

Vitamin D helps regulate calcium absorption and calcium metabolism. True vitamin D deficiency is not usually the main cause of classic Big Head Disease in grazing horses, but it can contribute to calcium problems in some situations.

Merck notes that vitamin D deficiency can contribute to nutritional secondary hyperparathyroidism by reducing calcium and phosphorus concentrations and stimulating parathyroid hormone release. (Merck Veterinary Manual)

In practice, I would not jump straight to vitamin D supplements. The first step is usually a proper diet, forage, and mineral assessment. Random supplementation without knowing the diet can create new problems while pretending to solve old ones.

Early Signs of Big Head Disease

Early signs can be subtle and easy to mistake for lameness, dental disease, arthritis, poor conditioning, or training issues.

Watch for:

• Shifting lameness

• Stiff gait

• Reluctance to move

• Poor performance

• Soreness in multiple limbs

• Weight loss

• Dull coat

• Reduced appetite

• Difficulty chewing

• Dropping feed

• Loose teeth

• Jaw or facial swelling

• Enlarged mandible

• Poor body condition

• Muscle tremors in some cases

• Fractures or bone pain in severe cases

MSD lists swelling and softening of facial bones, alternating limb lameness, difficulty chewing, decreased feed intake, weight loss, increased alkaline phosphatase, prolonged clotting time, fractures, and poor wound healing as reported findings. (MSD Veterinary Manual)

Merck also notes that affected animals may show pain, unthriftiness, reluctance to walk, intermittent or shifting lameness, skull deformity, chewing problems, anorexia, body condition loss, and in some cases neurological signs if vertebral fractures or bone swelling affect nervous tissue. (Merck Veterinary Manual)

Why the Head Gets Bigger

The head appears enlarged because bone mineral is lost and replaced with fibrous tissue. The changes often affect the facial bones and jaw, but the disease is not limited to the head.

The skeleton more broadly is being affected. The face is simply where owners may notice it.

The real concern is not just facial appearance. It is that the horse’s bones are weakening, teeth may loosen, chewing may become painful, and the horse may develop lameness or fractures.

How Worried Should You Be?

Low Concern

This is more likely when:

• The horse is eating normally.

• There is no lameness.

• There is no facial swelling.

• The horse is on a balanced commercial feed or ration balancer.

• Forage has been tested or is known to be suitable.

• The horse is not grazing high-oxalate pasture.

Action: keep the diet balanced, avoid excessive bran or unfortified grain, and review forage and mineral intake at least annually.

Moderate Concern

This is more likely when:

• The horse grazes tropical or subtropical pasture.

• Buffel, setaria, kikuyu, pangola, panic, or similar grasses are present.

• The horse is on mostly grass forage with little calcium-rich supplementation.

• The horse receives grain or bran without proper mineral balancing.

• Mild stiffness, shifting lameness, or chewing changes appear.

Action: arrange a veterinary and nutrition review. Test forage where possible, assess the full diet, and correct calcium and phosphorus before obvious facial changes develop.

High Concern

This is more likely when:

• Jaw or facial swelling is visible.

• The horse has shifting lameness.

• The horse is losing weight or struggling to chew.

• Teeth feel loose or chewing is painful.

• The horse is grazing known high-oxalate pasture.

• Bloodwork shows high phosphorus, high alkaline phosphatase, or abnormal bone markers.

• Radiographs suggest bone demineralisation.

Action: call your vet promptly. The horse needs diagnosis, pain management, diet correction, pasture restriction, and monitoring.

Critical

Treat this as urgent if:

• The horse is severely lame.

• The horse is not eating.

• There are loose teeth or severe jaw pain.

• The horse has fractures or severe bone pain.

• The horse shows neurological signs such as weakness, ataxia, or paresis.

• There are muscle tremors or tetany.

• The horse is rapidly losing condition.

Action: this needs immediate veterinary assessment. Advanced disease can affect welfare significantly, and some complications may not fully reverse.

When Is Big Head Disease an Emergency?

Call your vet urgently if your horse has:

• Sudden severe lameness

• Shifting lameness affecting multiple limbs

• Facial or jaw swelling

• Difficulty chewing

• Dropping feed

• Loose teeth

• Rapid weight loss

• Severe reluctance to walk

• Suspected fracture

• Muscle tremors

• Weakness, ataxia, or neurological signs

• Pain that is worsening over days

• A foal, young horse, pregnant mare, or lactating mare with signs

Merck notes that nutritional secondary hyperparathyroidism can cause pain, reluctance to walk, shifting lameness, skull deformation, masticatory problems, body condition loss, fractures, and neurological signs in severe cases. (Merck Veterinary Manual)

Do not wait for the horse’s face to become obviously enlarged. Early nutritional correction is much easier than trying to reverse months of bone loss.

What Else Can Look Like Big Head Disease?

Big Head Disease should not be diagnosed from facial swelling alone.

Important rule-outs include:

Dental Disease

Tooth root infections, dental abscesses, periodontal disease, fractured teeth, and wave mouth can cause facial swelling, chewing problems, weight loss, and pain.

Sinus Disease

Sinusitis, sinus cysts, dental sinus disease, or masses can cause facial asymmetry, nasal discharge, and swelling.

Facial Trauma

Kicks, falls, or collisions can cause jaw or skull swelling and pain.

Tumours or Cysts

Bone tumours, soft tissue tumours, or cyst-like lesions can distort the face or jaw.

Primary Hyperparathyroidism

This is caused by disease of the parathyroid glands rather than diet. It is uncommon but part of the broader differential list.

Renal Secondary Hyperparathyroidism

Chronic kidney disease can alter calcium and phosphorus balance and cause secondary hyperparathyroidism. Merck describes renal secondary hyperparathyroidism as a complication of chronic renal failure associated with mineral imbalance and increased parathyroid hormone. (Merck Veterinary Manual)

Laminitis or Hoof Pain

Shifting or repeated lameness may be from laminitis, abscesses, bruising, or hoof imbalance rather than bone demineralisation.

Arthritis or Orthopaedic Disease

Stiffness and lameness may be caused by joint disease, tendon injury, ligament injury, or developmental orthopaedic disease.

Selenium Toxicity or Other Nutritional Disorders

Some nutritional problems can affect hooves, bones, and overall condition, so the full diet matters.

The point is not to make every swollen jaw a calcium crisis. The point is to recognise when diet, pasture, lameness, and facial changes fit the Big Head Disease pattern.

How Do Vets Diagnose Big Head Disease?

Diagnosis usually combines history, physical exam, diet analysis, bloodwork, urine testing, and imaging.

Your vet may assess:

• Full diet history

• Forage type and pasture species

• Grain and bran intake

• Calcium and phosphorus intake

• Oxalate exposure

• Body condition

• Facial bone swelling

• Jaw and dental health

• Lameness pattern

• Bone pain

• Blood calcium and phosphorus

• Alkaline phosphatase and bone markers

• Parathyroid hormone where available

• Urinary fractional excretion of calcium and phosphorus

• Radiographs of skull, limbs, or painful regions

• Kidney function to rule out renal causes

Merck lists dietary analysis, elevated parathyroid hormone concentration, and urinary fractional excretion of calcium and phosphorus as diagnostic tools. It also notes that blood calcium may be low or normal because the body maintains calcium by breaking down bone, while blood phosphorus is often mildly to moderately elevated and alkaline phosphatase is usually increased. (Merck Veterinary Manual)

This is a key clinical point: normal blood calcium does not rule out Big Head Disease.

MSD also states that blood calcium concentrations do not reflect intake because of homeostatic mechanisms, while blood inorganic phosphorus may increase because bone mineral is being mobilised. (MSD Veterinary Manual)

Why Feed and Forage Testing Matter

You cannot diagnose or prevent this properly by guessing.

Forage and feed testing can help assess:

• Calcium content

• Phosphorus content

• Calcium-to-phosphorus ratio

• Magnesium

• Protein and energy

• Pasture species

• Oxalate risk where available

• Whether supplementation is actually adequate

MSD states that the only way to know the mineral content of forage is to have it tested by a forage-testing laboratory. (MSD Veterinary Manual)

This is especially important when horses are grazing tropical or subtropical pastures. A paddock can look lush and still create calcium deficiency if the calcium is bound by oxalates.

Green does not always mean balanced. Horses, sadly, cannot read pasture chemistry.

Treatment for Big Head Disease

Treatment is based on correcting the mineral imbalance and removing the cause.

1. Remove or Restrict the Cause

If high-oxalate pasture is involved, the horse may need to be removed from that pasture or managed with a carefully formulated supplement program.

Merck states that horses should have restricted access to plants containing large quantities of oxalates. (Merck Veterinary Manual)

2. Correct the Calcium and Phosphorus Balance

Treatment requires adjusting calcium, phosphorus, and vitamin D to appropriate recommendations for the horse’s age, work level, and physiological status. Merck states that calcium can be supplemented as ground limestone and that dietary correction is the core treatment. (Merck Veterinary Manual)

Common tools may include:

• Alfalfa or lucerne hay

• Calcium carbonate, also called ground limestone

• Dicalcium phosphate when phosphorus also needs attention

• Properly formulated mineral supplements

• Commercial ration balancers

• Removing excessive wheat bran or unfortified grain

• Feed analysis and nutritionist-guided ration balancing

Do not just throw calcium into the feed and hope. The total diet matters, and oxalate pasture may require a specific strategy.

3. Manage Pain

Pain control may be needed for lameness, bone pain, jaw pain, or secondary injuries. Merck notes that NSAIDs are advisable for pain control in nutritional secondary hyperparathyroidism. (Merck Veterinary Manual)

This should be vet-directed, especially if the horse is dehydrated, not eating, losing weight, or has kidney concerns.

4. Dental and Chewing Support

Horses with jaw pain, loose teeth, or chewing difficulty may need:

• Dental exam

• Softer feed

• Soaked hay cubes or pellets

• Chopped forage

• Diet changes that maintain fibre intake

• Monitoring for weight loss

• Pain control

The aim is to correct the mineral problem while keeping the horse eating enough.

5. Rest and Controlled Movement

A horse with bone demineralisation, shifting lameness, or fracture risk should not stay in normal work.

Depending on severity, the horse may need:

• Reduced work

• Small paddock rest

• Controlled turnout

• No jumping, racing, hard turns, or heavy training

• Gradual return only after comfort and bone health improve

6. Monitor Over Months

Recovery is slow because the skeleton must remineralise and the diet must remain corrected long enough for the body to stabilise.

Merck states that full recovery is possible, but advanced cases can take several months, bone deformities may not fully resolve, and recovery is unlikely when nerve or spinal cord trauma has occurred from healed vertebral fractures. (Merck Veterinary Manual)

How Long Does Recovery Take?

Mild cases may improve clinically within weeks after proper dietary correction, especially if the horse is removed from the cause early.

More advanced cases may take 6 to 12 months or longer to show meaningful skeletal recovery. Facial deformity may reduce but not always disappear completely.

Recovery depends on:

• Severity of bone loss

• Duration of the diet imbalance

• Age of the horse

• Whether teeth are affected

• Whether fractures occurred

• Whether oxalate pasture exposure continues

• Accuracy of the corrected diet

• Owner compliance

• Pain control and rest

• Overall health and body condition

The most important owner expectation is this: the diet can be corrected quickly, but bones recover slowly.

Can Big Head Disease Be Reversed?

Often, yes, especially when caught early.

Early cases with lameness, stiffness, and mild facial changes may improve significantly once the diet is corrected and the horse receives enough available calcium.

Advanced cases are more guarded. Bone deformity may not fully reverse. Loose teeth may not return to normal. Fractures, spinal complications, and severe jaw changes can have long-term consequences.

Merck’s prognosis is balanced: full recovery is possible, but advanced disease may take months and bone deformities may not resolve entirely. (Merck Veterinary Manual)

What Should You Do Right Now?

If you suspect Big Head Disease:

1. Stop Guessing the Diet

Write down everything the horse eats:

• Pasture type

• Hay type

• Grain

• Bran

• Supplements

• Treats

• Salt

• Mineral blocks

• Commercial feeds

• Amounts fed per day

2. Identify the Pasture

Find out whether the paddock contains high-oxalate tropical or subtropical grasses such as buffel, setaria, kikuyu, pangola, panic, signal, guinea, or purple pigeon grass.

3. Call Your Vet

Call promptly if there is lameness, facial swelling, chewing difficulty, weight loss, or suspected bone pain.

4. Test the Forage and Review the Ration

Forage testing is the safest way to understand calcium and phosphorus intake. Blood calcium alone is not enough.

5. Do Not Add Random Supplements

Too much of the wrong mineral can create new problems. Calcium, phosphorus, magnesium, vitamin D, and total diet balance all matter.

6. Remove High-Risk Feed Patterns

Avoid large amounts of wheat bran, unfortified cereal grain, and pasture-only diets on high-risk grasses unless the ration has been properly balanced.

7. Reduce Work if the Horse Is Sore

A horse with bone demineralisation should not be pushed through lameness.

8. Monitor Teeth, Weight, and Movement

Track appetite, chewing, body condition, lameness, and facial swelling every week.

Common Mistakes Owners Make

Waiting for the Head to Become Big

Early disease may look like stiffness, shifting lameness, or poor performance. Waiting for facial swelling means you may miss the best window for simple correction.

Relying on Blood Calcium Alone

Blood calcium may remain normal because the horse is pulling calcium from bone. Normal blood calcium does not prove the diet is safe.

Feeding Bran as a “Healthy Mash” Too Often

Wheat bran is high in phosphorus. Occasional use may not be a problem in a balanced diet, but heavy regular bran feeding without calcium balance is a classic risk.

Assuming Green Pasture Is Safe

High-oxalate pasture can look lush and still cause calcium deficiency.

Adding Calcium Without Checking the Whole Diet

The calcium-to-phosphorus ratio, total calcium intake, oxalate exposure, magnesium, vitamin D, and horse’s life stage all matter.

Ignoring Young Horses and Mares

Growing horses, pregnant mares, lactating mares, and performance horses may have higher mineral demands and less room for error.

Keeping the Horse in Work

A horse with shifting lameness and bone demineralisation needs diagnosis and rest, not more work to “see if it loosens up.”

How Can Big Head Disease Be Prevented?

Prevention is far easier than treatment.

Practical prevention includes:

• Test forage where possible

• Identify pasture species

• Avoid long-term grazing on high-oxalate grasses without a formulated supplement plan

• Keep the total calcium-to-phosphorus ratio appropriate

• Avoid diets where phosphorus exceeds calcium

• Do not feed large amounts of wheat bran or unfortified grain

• Use a balanced commercial feed or ration balancer when needed

• Provide calcium-rich forage such as alfalfa or lucerne where appropriate

• Review diets for growing horses, pregnant mares, lactating mares, and performance horses

• Recheck the ration when changing hay, pasture, grain, or supplements

• Monitor for shifting lameness, chewing changes, and jaw swelling

• Work with a vet or equine nutritionist in high-risk regions

Oklahoma State University Extension warns that random mineral supplementation without considering the full diet should be avoided, because mineral interactions are complex and excesses or deficiencies can affect absorption and metabolism. (extension.okstate.edu)

That is the whole prevention message in one line: balance the diet, do not guess.

Case Example: Gelding on Buffel Grass With Jaw Swelling

A 9-year-old gelding is presented for lower jaw swelling, shifting forelimb lameness, and weight loss. The owner reports that he has been grazing buffel grass for several months and receiving cereal grain without a balanced mineral supplement.

On examination, the horse is sore through multiple limbs, has mild mandibular enlargement, and is dropping feed. Blood calcium is not dramatically low, but phosphorus and alkaline phosphatase are elevated. Diet history and pasture risk strongly support nutritional secondary hyperparathyroidism.

The plan includes removing him from the high-risk pasture, changing the forage base, adding a properly formulated calcium and phosphorus supplement, stopping the unbalanced grain, providing pain relief, and reducing work. Over several months, lameness improves and body condition returns, although some jaw shape change remains.

The important part of this case is not just the supplement. It is identifying and removing the reason the horse became calcium deficient in the first place.

Myth vs Reality

Myth	Reality
Big Head Disease only matters once the face swells.	Early signs may be shifting lameness, stiffness, chewing difficulty, or poor performance.
Normal blood calcium rules it out.	Blood calcium may stay normal because calcium is being pulled from bone.
Green pasture means good nutrition.	High-oxalate pasture can bind calcium and create deficiency.
Bran mash is always harmless.	Regular high-bran feeding can invert the calcium-to-phosphorus balance if the diet is not corrected.
Just add calcium and it is fixed.	The whole ration, phosphorus intake, oxalate exposure, vitamin D, and horse’s needs must be assessed.
Facial changes always fully reverse.	Early cases may recover well, but advanced bone deformity may not completely resolve.

FAQs About Big Head Disease in Horses

Is Big Head Disease painful?

Yes, it can be. Horses may develop bone pain, shifting lameness, jaw pain, difficulty chewing, loose teeth, and in severe cases fractures or neurological complications. (Merck Veterinary Manual)

Can Big Head Disease be cured?

Often, especially if caught early. The diet must be corrected and high-oxalate exposure removed or managed. Advanced cases can take several months, and some bone deformities may not fully reverse. (Merck Veterinary Manual)

Can horses get Big Head Disease on pasture alone?

Yes. Horses grazing high-oxalate tropical or subtropical grasses can develop nutritional secondary hyperparathyroidism because oxalates bind calcium and reduce its availability. (Merck Veterinary Manual)

What calcium-to-phosphorus ratio should horses have?

Oklahoma State University Extension describes two parts calcium to one part phosphorus as ideal, with a total diet range of 1:1 to 6:1 generally acceptable. Diets with more phosphorus than calcium should be avoided. (extension.okstate.edu)

Should I feed calcium carbonate for Big Head Disease?

Calcium carbonate, also called ground limestone, may be used as part of treatment, but it should be given as part of a properly balanced ration. Merck lists dietary correction and calcium supplementation as treatment, with calcium supplemented as ground limestone. (Merck Veterinary Manual)

The Bottom Line

Big Head Disease is a preventable nutritional bone disease.

The core problem is not the size of the head. The core problem is long-term calcium imbalance. That may come from too little calcium, too much phosphorus, or pasture oxalates that stop calcium being absorbed.

The earliest warning signs are often not dramatic. Shifting lameness, stiffness, chewing difficulty, weight loss, and subtle jaw changes should all raise suspicion in a horse on a risky diet or high-oxalate pasture.

The safest approach is practical:

• Identify the pasture.

• Test the forage.

• Calculate the full diet.

• Correct calcium and phosphorus balance.

• Restrict high-oxalate grasses where needed.

• Use supplements carefully.

• Manage pain.

• Give the skeleton time to recover.

If caught early, many horses improve well. If ignored, the disease can cause lasting jaw deformity, dental problems, fractures, lameness, and long-term welfare issues.

This is one of those conditions where nutrition is not a nice extra. It is the treatment.

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If your horse has shifting lameness, jaw swelling, chewing difficulty, or is grazing high-oxalate pasture, ASK A VET™ can help you organise the diet history, pasture risk, photos, and questions to discuss with your vet or equine nutritionist.