Hypocalcemic Disorders in Horses

Several symptoms are common to horses with hypocalcemia regardless of the cause of the hypocalcemia. They are, however, somewhat non-specific and further tests required for a definitive diagnosis:

• Abnormal gait
• Demineralization of the bone
• Diaphragm flutter
• Fever
• Hyperactivity
• Intestinal obstructions
• Loss of coordination
• Muscle weakness
• Rapid breathing
• Rapid heart rate
• Seizures

Urgency is particularly important if the patient is a lactating mare, as hypocalcemia can rapidly progress to death if untreated in this population. 

Types

Calcium can enter the bloodstream of a horse in several different ways.

• Absorption - Most calcium absorption occurs in the ileum, the last and longest portion of the intestines; unlike most mammals, the intestinal absorption of calcium is mostly independent of vitamin D intake 

• Excretion from kidneys - Both anion-bound and ionized calcium are filtered through an area of the kidney called the glomerulus, a group of tiny, intertwined blood vessels located within the kidneys; horses excrete a much larger percentage of absorbed dietary calcium in this manner than most other species

• Release from the bone - Calcium and phosphate are contained within the bones, and are released from the bone by stimulation from parathyroid hormone

Hypocalcemic disorders, caused by an overabundance of calcium contained in the bloodstream, also known as hypocalcemia, is usually diagnosed during comprehensive blood testing including a biochemical profile and complete blood count. Many factors can cause an inaccurate calcium test, however, and these should be taken into account when reading the results. Certain anticoagulants that contain citrate, EDTA, fluoride chelate calcium, or oxalate, for instance, can cause calcium levels appear to drop. Other disorders and diseases can cause erroneous results as well; abnormalities in either the albumin or protein concentrations can have an effect on the amount of calcium, erroneously increasing or decreasing the amount of calcium registered in the circulatory system, although there is less correlation in horses than in most other species. High levels of fat in the blood or the rupture of red blood cells may also falsely raise total calcium levels. Acidosis is another condition known to interfere with blood tests that involving calcium; although overall levels of calcium will be accurate, the ionized calcium is increased due to a decrease in the amount of calcium that is bound to protein. 

The diagnosis of hypocalcemia will need to be followed by additional testing in order to precisely determine the underlying condition and what other effects it may have caused.

If your horse is in distress when it is examined by the veterinarian, supportive treatments are likely to begin as soon as possible. Fluids and electrolytes will be administered intravenously in order to prevent dehydration, and if the underlying disorder is making it difficult to breathe as well, oxygen will usually be offered. In many cases, supplemental calcium will be added to the intravenous fluid as well.

There is usually an underlying cause to hypercalcemia that will require additional measures to treat, such as sepsis or hypoparathyroidism, before the calcium levels normalize. Generally, the first course of action is to administer oral calcium tablets, which will often be joined by additional supplementation of Vitamin D. Although added vitamin D can be quite helpful, it is only useful in those cases where calcium and phosphate are also increased. In cases of renal failure, a vitamin D analog is used instead as they do not require any metabolic alteration in the kidneys.