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When a horse ingests grasses with too many oxalates, they bind to calcium in the gut to form calcium oxalate. This prevents the horse from absorbing calcium and causes a calcium deficiency. The decrease in calcium causes secondary hyperparathyroidism, when the parathyroid gland releases excessive amounts of the parathyroid hormone to aid in calcium absorption. If oxalates are preventing absorption from the gut, the hormone will then dissolve existing calcium from the bones to release into the blood stream to help support healthy muscle and heart function. The bones become weakened and brittle, and fibrous tissue then replaces the bone. In the facial bones, this appears as swelling, which gives this condition its name.
Big head disease, also called bran disease, Miller’s disease, osteitis fibrosa, equine osteoporosis, and swollen face disease, is an endocrine condition known medically as nutritional secondary hyperparathyroidism. It is caused by inadequate calcium absorption, which gradually leeches calcium from the bones to equalize blood calcium levels. Seen more often in Australia, it has been caused by the consumption of oxalate containing tropical grasses, or an incorrect calcium to phosphorus ratio in feed.
Signs are due to the demineralization of the bones, and often affects the head and limbs, causing swelling and lameness. While often horses will adapt to the gradual changes, eventually obstruction in the airway and jaw and tooth problems can cause weight loss and difficulty breathing. Symptoms generally appear within 2 to 8 months after initial grazing of grasses high in oxalates. An acute poisoning can occur with the ingestion of grasses containing high amounts of soluble oxalates, which produces muscle tremors and interferes with kidney function. Symptoms include:
The cause of big head disease in horses is due to an inadequate amount of calcium being absorbed into the body. This occurs most commonly through:
Horse more prone to big head disease include:
A diagnosis begins with a history of symptoms, current symptoms, diet analysis, and testing. X-rays may be taken to both rule out other diseases and to see changes in bone density, as well as fractures and tendon ruptures. Symptoms can precede bone changes by 14 weeks, as the changes in bone are progressive over a period of time. Blood, fecal, and urine tests can confirm the diagnosis by measuring the levels of minerals present.
Treatment includes eliminating the source of oxalates or correcting the diet, and adding supplements to the diet to correct the calcium and phosphorus imbalances. This can be achieved with agricultural lime (calcium carbonate), dicalcium phosphate, supplements containing magnesium, Lucerne or alfalfa hay, dolomite, chelated calcium, and sometimes molasses to increase palatability for your horse. Supplementation can sometimes be achieved with a mineral block, but can be difficult to adjust for your horse’s particular needs. Daily supplementation is preferred to weekly, and your veterinarian will run tests throughout treatment to monitor progress and adjust mineral supplement levels as needed.
Your horse may be treated for 6 to 12 months, depending on the severity of the deficiencies and the rate of recovery seen in testing. If pain is present, your veterinarian may also prescribe non-steroidal anti-inflammatory drugs and confinement.
Recovery is good for horses that can be successfully treated with a change of diet and mineral supplementation. In some cases, physical changes and bone density can resolve, although facial bone swelling may never disappear completely. It can take up to a year for bones to undergo remineralization, and your horse should be kept at rest during this period and not ridden. Up to 15% of cases of horses severely affected by big head disease are fatal.
Prevent big head disease in your horse by maintaining a correct calcium to phosphorus ratio in your horse’s diet, and by avoiding grazing of oxalate containing grasses for longer than a month. Encourage the growth of calcium rich plants such as alfalfa.
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