Nutritional Secondary Hyperparathyroidism (NSH) is most commonly referred to by the name “Bighead” or sometimes “Bran Disease” and is a condition that develops as a result of a calcium imbalance within the horse’s diet. Calcium (Ca) is the most abundant mineral element in the animal body with 99% of calcium having a structural role in bone and teeth. In addition, calcium is an essential component of living cells and tissue fluids and is necessary for the transmission of nerve impulses, the contraction of muscles and immune system activation.
During periods of calcium shortage, horses will mobilise calcium and phosphorus from their bones to keep blood calcium levels normal. When this state of calcium deficiency occurs for an extended period of time, horses mobilise so much calcium and phosphorus that their bones become fibrous and weak. Richards (2012) indicates that it is essential that a horse’s diet is balanced and the calcium to phosphorus ratio must be kept above one part calcium to one part phosphorus for all horses. For mature horses the ideal range is 1:1 to 6:1 and for young horses this range will be 1:1 to 3:1.
Cause
Bighead arises from a severe and long term calcium deficiency. The two major causes of the condition are:
- Grazing sub-tropical grasses that contain high levels of oxalate that bind most of the calcium in the grass making it unavailable for absorption when the horse eats it. High oxalate containing grasses are: Setaria, Buffel Grass, Kikuyu, Purple Pigeon, Green Panic, Pangola Grass, Para Grass and Signal grass. These grasses may contain plenty of calcium but the horses cannot access it due to the level of oxalates and long term grazing can result in a severe calcium deficiency. The higher the level of oxalate that grass contains, the more rapidly the horse will develop bighead. Setaria species contain the most dangerous levels of oxalate for horses, especially the Kazungula variety that contains a massive 70g/kg of oxalate. This grass should really be considered “toxic” for horses and avoided (Richards, 2012).
- An incorrect calcium to phosphorus ratio in the horse’s diet where there is more phosphorus than calcium. The higher levels of phosphorus can block the absorption of calcium and eventually leads to a severe calcium deficiency. Grains and bran are high in phosphorus, low in calcium and also contain chemicals called phytates, which bind to calcium in the gut, forming calcium-phytate compounds that cannot be absorbed by the horse. Calcium and phosphorus content of common feeds – values sourced from (Kohnke, 1999)
| Feed | Calcium | Phosphorus |
|---|---|---|
| Wheat Pollard (millrun) | 1.3 | 9.0 |
| Rice Pollard | 0.5 | 12.0 |
| Oats | 1.0 | 3.2 |
| Barley | 0.63 | 3.8 |
| Corn | 0.2 | 2.8 |
| Copra Meal | 2.0 | 6.5 |
| Full Fat Soybean Meal | 2.5 | 5.9 |
Signs can include, but are not limited to the following:
- Painful bones and joints
- Lameness varying from; shortening of the gait, ‘hopping rabbit gait’, reluctance to move and difficulty rising
- Enlarged or swollen facial bones, usually starting as a ridge over the nasal bone or swelling of the upper jaw
- Noisy breathing during exercise due to swelling of nasal bones
- Ill-thrift and a harsh coat
- Susceptibility to worm infestations
- Loose or shifting teeth and difficulty chewing
- Ligament and tendon injuries
- Early tiring and a low tolerance for work
- Serous (watery) nasal discharge
- Failure to reach expected height in growing horses
- Severely affected animals can die (Hungerford, 1990)
Low blood calcium triggers the release of the parathyroid hormone, which acts to resorb or ‘mine’ calcium from the non-weight bearing bones of the face and pelvic structure. Chronic demineralization of the facial bones results in the development of Bighead (Kohnke, 1999).
Treatment and Prevention
Fortunately, Bighead is a condition that is easily avoided if adequate nutrition is provided. Restoring the correct balance of calcium and phosphorus within the horse’s bloodstream is imperative in the prevention and recovery of NSH. If the disease is occurring due to an incorrect calcium to phosphorus ratio, adding additional calcium to the diet to balance this ratio will enable this problem to be avoided. Heavy sweating horses, growing horses, pregnant and lactating mares should be of particular concern as these classes of horses are more predisposed to calcium deficiency and require higher levels of supplementation.
If horses are grazing grasses that contain high levels of oxalate then enough calcium must be fed to keep the calcium to oxalate ratio above 0.5 parts calcium to 1 part oxalate. The more oxalate the pasture contains, the more mineral supplement is required. The biggest difference between supplementing horses with Bighead from grazing high oxalate pastures to that of Bighead caused by an incorrect Ca:P ratio, is that when adding calcium to the diet there is a need to add phosphorus as well, ensuring it well balanced around 2:1. McKenzie et al. (1981) found that to prevent bighead, supplements should provided at least 20mg of calcium per kg of bodyweight per day and 10mg of phosphorus per kg of bodyweight per day.
For horses recovering from Bighead, follow these steps:
- Seek veterinarian advice
- If possible, remove horses from subtropical pastures
- Horses recovering from Bighead shouldn’t be ridden
- Provide access to Lucerne Hay (0.5 to 1 kg per 100kg of bodyweight).
- The remainder of forage requirements should be fed as low oxalate hay (Rhodes, Qld Bluegrass or temperate grass hay etc). Remember equines require a minimum of 1.5% of their bodyweight daily as forage.
- Supplement with calcium at 200 to 400% of requirements for 12 months.
Richards (2012) suggests that horses suffering from Bighead are also likely to be deficient in magnesium as well and require a balanced diet with their requirements for salt, trace-mineral and vitamins being balanced.
Besides calcium and phosphorus, the following nutrients play important roles in bone health and may require supplementation.
Magnesium to assist with magnesium deficiency that may occur when horses are fed high grain, low roughage diets, whilst sweating heavily or when factors interfere with magnesium absorption such as the case in high oxalate grasses. Keep the calcium to magnesium ratio less than three parts calcium to one part magnesium.
Vitamin D is extremely important in the role of calcium and phosphorus absorption and has a vital role in bone health.
Vitamin K has been recognised as being important in the development of bone. Adequate amounts are synthesised by hindgut micro-flora to satisfy the horse’s daily requirements. In healthy horses, vitamin K supplementation is not required, however, if horse’s have had prolonged antibiotic treatment supplementation may be needed.
Summary
Bighead can be a dangerous and debilitating disease but fortunately it can be corrected and prevented by appropriate nutrition and supplementation. The provision of a balanced diet, with correct mineral ratios is imperative for the health of all horses. Remember horses must have access to fresh, drinking water at all times and free choice plain salt is highly recommended.