Hypocalcemia refers to a low level of calcium in the blood. Calcium is extremely important for muscle and nerve function. Muscles are bathed in blood and rely on the presence of calcium for contraction. When the concentration of calcium in the blood becomes too low, muscle function declines. Muscles that are affected include the heart, skeletal muscles (the muscles we associate with movement of the skeleton that are mostly under voluntary control), and smooth muscle (the muscle that lines the gastrointestinal system, the blood vessels, and the respiratory system, and are not under voluntary control).
If the calcium level is only moderately decreased, muscle may become hyper-irritable (this may manifest as a high heart rate or muscle twitching). When the calcium level is extremely low, the muscle will become flaccid, and the horse may be unable to rise. Seizures and death may occur if severe hypocalcemia is prolonged.
Hypocalcemia is seen most commonly in endurance horses or other horses that are undergoing strenuous, lengthy exercise, mares who have been lactating one to two weeks, and in horses who have been stressed by being transported a long distance.
What to Watch For
Diagnostic tests are necessary to confirm hypocalcemia and rule out other causes of arrhythmias, muscle tetany, colic, weakness, twitching, or flaccidity of the muscles. Tests may include:
The most critical treatment for hypocalcemia is intravenous infusion of calcium. Because this is a potentially very dangerous thing to do, it must be done under veterinary supervision only. During administration, it is essential to monitor heart rate and rhythm very carefully.
Home Care and Prevention
Monitor your horse for any recurrence of signs. Some horses require multiple treatments over hours to days for full resolution of signs.
Provide a diet balanced for calcium and phosphorous. The recommended ratio of these minerals is from 1.2 to 2 parts calcium to 1 part phosphorous.
Hypocalcemia can be complicated by both low and high calcium diets. The body develops a wasteful attitude toward calcium when it is in high supply. Then, when the horse has an unusual demand (such as during an endurance ride), the mechanisms for capturing calcium from the body's stores can't respond quickly enough.
One successful approach is to reduce dietary calcium for 3 to 4 days prior to a strenuous ride (take away alfalfa hay, for instance, if this is what you feed). At the same time, you can make sure that your horse is well supplemented with other electrolytes during a long ride. Water and electrolyte losses during prolonged strenuous activity can cause your horse to be alkalotic, which in turn can precipitate hypocalcemia.
During the ride, and two hours before the ride, make sure that your horse receives small amounts (2 oz) of a supplement containing sodium, potassium, chloride, calcium, and magnesium.
Above all, make sure that your horse is in top shape for the competition that he faces. If your horse is going to be transported a long distance, make sure that he receives plenty of water, electrolytes and rest along the way.
Sweating should alert you to the possibility of major electrolyte losses, alkalosis, and hypocalcemia.
Calcium absorption, which takes place in your horse's small intestines, can be enhanced by vitamin D activity, and is inhibited by the presence of phosphorous. This is one reason why you don't want to feed your horse too much bran or other sources with excessive phosphorous. In general, grains are much higher in phosphorous than is roughage. Once the calcium has been absorbed into the bloodstream, it is further regulated in the kidney.
Horses ordinarily lose large amounts of calcium in their kidneys, but their diets are so high in calcium that it doesn't usually matter. Calcium can be filtered and re-absorbed in the kidneys – this is dependent on the presence of Vitamin D and another hormone called parathyroid hormone (PTH).
The biggest store of calcium is in the bones, where it is in constant flux. Cells called osteoclasts, under the influence of PTH and vitamin D, break down bone to supply calcium to the body, and cells called osteoblasts build bone back up, storing calcium in the process. This constant breakdown and repair of bone is completely normal and necessary for proper function.
Even though most blood tests measure total calcium, all calcium is available for use by muscles and nerves. Some is stored in the blood in a useless form. There is a big functional difference between free and bound calcium, otherwise known as ionized and non-ionized calcium. Only ionized calcium can fulfill the physiologic processes that are needed for muscles, nerves, and cellular messenger systems to work. This is why the body's acidity level, or pH, is important. When the pH is low, meaning that the body is acidic, there is more ionized calcium. When the body's pH is high, or alkalotic, there is less ionized calcium. So, the endurance horse who is alkalotic at the end of a long ride because of water and electrolyte losses is also more likely to suffer from hypocalcemia.