Hypocalcemic Disorders in Horses

Chronic renal failure, hypoparathyroidism, and even sepsis can cause a persistent deficiency of calcium in the blood that can lead to rapid breathing and heart rate, muscle weakness, and, in severe cases, seizures. Known as hypocalcemia, this disorder may also be related to insufficient amounts of vitamin D or even to severe gastrointestinal distress. The prognosis for horses that are diagnosed with this imbalance is dependent on the underlying cause and the speed of diagnosis.

Hypocalcemia, or a deficiency in the amount of calcium in the blood, can be an indicator of other conditions. Symptoms of hypercalcemia may be an indication of serious illness and should be evaluated by a veterinary professional.

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

• Diet - Either a lack of calcium in the diet or an overabundance of phosphorus can decrease levels of calcium in the blood; certain natural toxins, such as plants containing oxalates and black beetles, may also be inadvertently ingested, leading to hypocalcemia
• Exercise-induced (transport tetany) - Hypocalcemia can be the result of electrolyte and acid abnormalities caused by the loss of calcium and other minerals through sweat after heavy exercise
• Lactation (lactation tetany) - Creating milk requires large stores of calcium, and if sufficient amounts of calcium are not given at that time then the amount of calcium in the mother’s blood may be decreased
• Primary hypoparathyroidism - Along with the hypocalcemia caused by a decreased functionality of the parathyroid, decreased levels of phosphate and magnesium may also be present 
• Renal failure - The kidneys are particularly important for the excretion of calcium for horses; kidneys that are functioning poorly may not be excreting enough calcium, causing it to build up in the blood 
• Sepsis - The connection between sepsis and lowered concentrations of calcium, known as hypocalcemia, is well documented in humans; the role of sepsis in lowering the concentration of calcium is imperfectly understood in animals but is a common cause of hypocalcemia in horses with acute gastrointestinal diseases

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.

Initial treatments that increase the level of calcium in the blood should ease many of the symptoms. However, the overall prognosis for horses that have deficient levels of calcium in their blood depends on the underlying causes. The outcome will also be influenced by how advanced the condition is particularly in situations involving bone demineralization or kidney degeneration. Calcium rich feeds that are high in vitamin D are often added to the equine’s daily diet to prevent further deficiency in the levels of blood calcium, and corticosteroids should be avoided as they inhibit vitamin D activity.