Does your horse have a "big head"?

A swollen-face, dull coat, shifting lameness, bunny-hop canter…are any of these signs familiar? If so your horse may suffer from a disorder described as ‘Big Head’. ‘Big Head’ is a calcium deficiency disease induced by a diet with a persistent lack in calcium, excess in phosphorus and/or imbalanced calcium to phosphorus ratio. This disorder has severe effects on horses including difficulty breathing, painful movement and lameness. Being a nutritional related disease, it can easily be prevented and treated by correcting the imbalance in your horse’s diet.

What is Big Head?

Nutritional Secondary Hyperparathyroidism, commonly known as ‘Big Head’, is a severe calcium deficiency in horses caused by a diet low in calcium, excessive in phosphorus, or with a calcium to phosphorus ratio less than 1:1. In order for horses to maintain their blood calcium levels, they will mobilize mineral from their bones in cases where the calcium intake via the diet is limited. It is important to note that calcium plays a huge role in the structural integrity of the horse’s skeleton, with 99% of the total calcium within the horse’s body found in the bones and teeth. During prolonged calcium deficiency horses mobilize large amounts of bone mineral primarily from their facial and pelvic bones which become fragile and fibrous connective tissue develops. This fibrous tissue causes their facial bones to swell, giving them a ‘Big Head’ appearance. Young horses are most prone to this facial swelling as their bones have not completely formed and hardened.

Symptoms

Horses that have a severe calcium deficiency can exhibit a number of symptoms which may increase as the disease progresses, including:

• Enlargement of facial features (swelling of the jaw as the bones enlarge)
• Lameness: horses appear stiff and have a shortened gait
• Ill-thrift: loss of condition even though they have access to ample feed
• Noisy breathing during exercise (upper airways can become obstructed due to swollen bones)
• Loose and shifting of teeth

Causes

Horses require a specific amount of calcium in relation to phosphorus in their diet, as increased amounts of phosphorus compared to calcium interferes with the absorption of calcium. A ratio of 2 parts calcium to 1 part phosphorus is considered the ideal dietary ratio and coincides with the concentration of calcium to phosphorus found in bones. The dietary ratio should not be lower than 1:1. Horses develop ‘Big Head’ due to two main reasons, both of which are related to their diet:

• Imbalanced diet: Horses obtain their nutritional intake of calcium and phosphorus, amongst other nutrients, from their diet. High amounts of phosphorus in the diet depress calcium and magnesium absorption. Certain feeds such as wheat pollard and bran (grain by-products) are low in calcium or too high in phosphorus in comparison (inverted calcium to phosphorus ratio). Diets that contain such feeds and are not balanced in a complete feed with added calcium can result in ‘Big Head’.
• Oxalated Pastures: An oxalate-rich pasture contains particular grass species that are high in oxalates. Oxalates are specific molecules found in the grass blades that bind calcium in the plant and the digestive tract. Oxalates prevent horses from absorbing the calcium in the plant once digested. Therefore, horses grazing exclusively on pastures high in oxalates, including tropical grasses such as buffel, setaria, green panic and kikuyu grass commonly found in Queensland, may develop ‘Big Head’.

Diagnosis

This calcium deficiency disease can be detected through a physical examination of your horse by a veterinarian in combination with a diet analysis.

Prevention and Treatment

Horses affected can be treated by correcting their dietary imbalance. The strategy to prevent or treat ‘Big Head’ depends on the cause:

• Imbalanced diet: The diet should be balanced to meet your horse’s calcium and phosphorus requirements and have a calcium to phosphorus ratio of at least 1:1, ideally 2:1. Phosphorus should not be supplied in excess. Di-calcium phosphate and limestone can be added to increase the calcium content of the diet. Feeds and supplements like HYGAIN BALANCED^® and HYGAIN^® SPORTHORSE^®, respectively, provide an optimal Ca:P ratio of 2:1.
• Oxalated Pastures: Horses should graze grasses that are poor in oxalates where possible. If they graze on high-oxalate grasses, the growth of a legume component in the pasture, such as lucerne, is highly recommended due to their high calcium content. Horses grazing oxalate-rich grass should be supplemented with additional calcium and phosphorus. Those with high calcium requirements such as gestating or lactating mares and growing horses require even higher levels of calcium to be added to their diet.

Key points

• “Big Head” is a calcium deficiency disorder.
• Common symptoms include enlarged head, lameness and ill-thrift.
• It can arise from an imbalanced diet or from grazing oxalate-rich pastures.
• The diet should be balanced by providing adequate calcium and phosphorus and in a ratio of at least 1:1, ideally 2:1.

The table below shows the calcium and phosphorus daily requirements of horses of different physiological activities. Table1: Daily requirements of Calcium and Phosphorus for different types of average (500kg) horses

Horse (500kg)	Calcium (g)	Phosphorus (g)
Maintenance	20 - 25	14 - 18
Lactating mare right after birth (Highest requirement)	70 - 85	39 - 41
Young growing horse	36 - 40	20 - 22