Bighead & Osteoporosis In Horses

The condition known as 'bighead' was first identified in Australian horses grazing on subtropical pastures in 1974. Since then, it has become widely recognised in horses grazing on buffel, pangola, setaria, kikuyu, green panic, guinea, and signal grasses. These tropical grasses, commonly planted along Australia's seaboards, contain oxalate - a chemical that significantly interferes with mineral utilisation by horses.

Historical Overview

Bighead disease, also known as nutritional secondary hyperparathyroidism (NSH) or osteodystrophia fibrosa (OF), has been recognised since ancient times. Vegetius described it as 'animal osteomalacia' in 400 AD. While it can affect various animals such as dogs, monkeys, cats, rats, rabbits, pigs, and goats, it is primarily a disease of horses.

In the early 1900s, bighead disease was prevalent among horses fed large amounts of bran by-products, particularly mill wheat. This led to the condition being referred to as 'bran-disease' or 'millers disease.'

The first reliable description of bighead disease came from Switzerland in 1851. A two-year-old horse exhibited symptoms such as slow, troubled chewing and uniform swelling extending backward from the last molars. The teeth were freely movable, and the bone appeared like pumice stone. By 1854, 'pica' or dirt eating was identified as a symptom of bighead. In 1860, Varnell noted that bran constituted a major portion of the diet in horses affected by this new disease of osseous tissue.

Bighead in Modern Horses

No longer considered an ancient disease or a disease of stabled horses, bighead has been a continuing problem over the last 30 years in horses grazing high-oxalate pastures in Ceylon, Germany, Japan, the Philippines, Switzerland, the United States, and Australia.

Case Study: Queensland, 1972

In the early summer of 1972, four young Thoroughbreds on a property in Queensland developed stiff, stilted gaits and swelling of the head. Analysis of the pasture showed adequate calcium levels but high levels of oxalates.

The Role of Oxalates in Bighead Disease

In bighead-producing grasses, the major portion of calcium is present as crystalline calcium-oxalate, which is insoluble, making the calcium unavailable for absorption in the gut. Grasses with more than 0.5% oxalate are capable of inducing bighead. The oxalate content of grasses is highest under conditions of rapid growth, with concentrations as high as 6% or more commonly occurring. Although the dietary calcium intake may meet requirements, the calcium cannot be absorbed and passes straight out into the manure, causing body calcium levels to fall.

The Body's Response to Low Calcium Levels

In response to falling blood calcium levels, the parathyroid gland releases parathyroid hormone (PTH). The primary function of PTH is to prevent blood calcium from reaching dangerously low levels. It does this by:

1. Increasing phosphorus excretion and resorbing calcium from the urine.

2. Acting on the gut to increase the uptake of calcium from the feed. Unfortunately, for horses grazing high-oxalate pastures, there is no available calcium in the diet.

3. Stimulating the release of small amounts of calcium from the bones to increase blood levels back to the normal healthy range.

As the low calcium intake continues, the demand for calcium removal from the skeleton becomes greater, and the release of calcium from the bones increases to maintain blood levels. Affected horses also enter a state of phosphorus deficiency as urinary losses increase under the influence of PTH. As calcium is progressively removed from the bones, they weaken and are replaced by fibrous tissue, leading to the appearance of clinical signs.

SYMPTOMS

There are three clinical manifestations of bighead described in Australia: ill-thrift, lameness, and swelling of the facial bones in the skull.

Ill-Thrift: Horses suffering ill-thrift have harsh coats and lose condition, even while grazing on adequate pasture. The appetite is often normal but affected horses are often seen eating dirt.

Lameness: Typically insidious and shifting, with general tenderness of the joints or a stiff, stilted gait. Horses may only be mildly lame, and in many cases, no physical deformity is found. Other horses may creak badly in the joints when mobilised. These signs are thought to result from the separation of tendon and ligament attachments and pain due to defective mineralisation and osteoporosis, which result in weakening and bending of the bones. Some horses prefer to canter rather than trot, while others are reluctant to move.

Swelling of Facial Bones:

The most severe changes occur in the skull bones, notably the jaws, maxilla, mandible, and nasal bones, which often swell, giving the disease its expressive name, 'bighead.' Swelling is usually bilateral, first seen as a ridge over the nasal bones above the facial crest or as swelling of the maxillae over the roots of the anterior cheek teeth. Affected horses may present with upper airway breathing difficulties or noise, a watery nasal discharge, or mild swelling of the distal patterns.

Chewing problems usually occur late in the course of the disease due to extensive resorption of bone around the teeth. However, difficulty chewing and dental pain may be the first signs. Reduced feed intake can result in weight loss and poor body condition.

Radiographic Changes

At least 30% of bone mineral must be removed to see radiographic changes, which appear late in the course of the disease. The changes are insidious and progressive, appearing sooner and progressing faster in the maxilla and mandible than in the cannon bones. Radiographically, there is extreme porosity of the entire skeleton.

Seasonal and Age Factors

Post-clinical cases occur in horses in spring and summer, with prevalence ranging from 1-100%. The time of onset after grazing high-oxalate pasture is 2-9 months, and horses from weaning to 7 years of age are most commonly affected.

Treatment and Prevention

Subtropical grasses like kikuyu, buffel, and setaria are excellent sources of protein and energy, making them great for growing cattle (which is what they were introduced for). Luckily, with the correct calcium and trace mineral supplementation, they can also be made safer and great for horses too. So, what are the options for supplementation, and how do they stack up?

A supplement for grazing horses must be inexpensive, high in calcium, palatable, consumed in sufficient quantity to promote storage when supplied regularly, able to correct phosphorus deficiency, and safe in the long term.

Many studies have been done on increasing calcium intake, and sources of calcium for inclusion in drinking water have been found not to fulfil these criteria.

Calcium Supplementation

Several feeding trials have shown that horses do not consume enough calcium in the form of lime or dicalcium phosphate (DCP) - either alone or mixed with salt - to overcome a calcium deficit in their diet. Lime, which is around 36% calcium, requires horses to consume 1 kilogram per week to restore calcium levels. However, lime supplementation alone will still leave the horse phosphorus deficient. A kilogram of lime fed with 1.5 kg of molasses once weekly provided calcium storage in horses fed high-oxalate grass, but the negative phosphorus balance was not corrected. Dolomite, which is around 21% calcium, would require horses to consume 1.5 kg per week, but again, this does not address the phosphorus deficiency.

Practical Supplementation Limits

There is a limit of 20 mg/kg/day for calcium and 10 mg/kg/day for phosphorus that can be provided in a once-weekly supplement. Practically, 1 kilogram of rock phosphorus or a mixture of rock phosphorus and lime with 1.5 kilograms of molasses given once a week may be adequate to prevent bighead in horses grazing high-oxalate grasses and losing 20 milligrams of calcium and 10 milligrams of phosphorous per kilogram bodyweight per day. However, the negative calcium balances with buffel, kikuyu, and setaria are often double these losses, and 1 kilogram of the above supplements will only delay the onset of clinical bighead. Greater amounts and daily feeding are necessary to prevent and cure bighead in horses grazing these pastures.

Frequency and Efficiency of Supplementation

Research shows that horses retain only 35% of the minerals from supplements when fed weekly. Once-weekly supplementation with lime exacerbated the negative phosphorus balance, an effect not seen with daily supplementation. Increasing the frequency of supplementation or extending the time of consumption is more efficient than increasing the actual dose and daily feeding of supplements results in better mineral retention. Increasing the intake of magnesium also increases calcium absorption, as do sodium and chloride. Additionally, any supplement should contain essential bone trace and micro-minerals commonly deficient in Australian soils and pastures, such as iodine and selenium.

Correcting Imbalances

Affected animals can only be treated by correcting the imbalance of calcium and phosphorus. The good news is that even severe lesions throughout the horse’s body may disappear with correct treatment. It can take 4 to 12 months to remineralise the bones, but there is always the possibility that the 'big head' appearance may remain unchanged. For horses at pasture, the most effective way to increase calcium and phosphorus intake is to provide a correctly formulated mineral block. Ordinary salt and mineral blocks do not usually contain adequate minerals. A survey of six blocks marketed for horses found that only one met the recommended calcium-phosphorus ratio of 3:1 and provided significant amounts of the minerals commonly deficient in Australian soils and pasture. Of the other blocks, one had a calcium-phosphorus ratio known to cause bighead, and the others were phosphorus deficient. Four blocks had extreme excesses of salt, selenium, iodine, and copper, while one had such low levels as to be insignificant. Always check the analysis!

Even horses grazing non-hazardous pastures need a mineral supplement. Alternatively, for horses that receive some hand feeding, a correctly formulated powder can be used to balance the mineral deficiencies in the feed and pasture.

Jenquine Calsorb Forte® includes Bone Formula for horses that are not regularly hand-fed. Weighing the blocks weekly is a useful way to monitor intake.

Jenquine Bone Formula Forte® can be fed with 60–120g of finely powdered calcium carbonate (lime) to provide extra calcium for 'mopping' up free oxalate. The amount of Jenquine Bone Formula Forte® and lime can be adjusted based on your treating veterinarian’s clinical assessment.

Further published articles:

Serum parathyroid hormone and faecal calcium levels in horses on oxalate-containing grasses.
Bighead in horses - Vet Clinic Newsletter
Bighead in Horses - Not an Ancient Disease - Dr Jen Stewart, Dr Oliver Liyou, Dr Garry Wilson
Bran Disease, Bighead, Millers Disease and Nutritional Secondary Hyperparathyroidism

Dr Jennifer Stewart
BVSc BSc PhD Equine Veterinarian and Consultant Nutritionist