- Definition
- Pathophysiology
- Systems Affected
- Genetics
- Incidence/Prevalence
- Geographic Distribution
- Signalment
- Signs
- Causes
- Risk Factors
BASICS
- Calcium, a major component of bone, also is necessary for blood coagulation, muscle contraction and neuromuscular excitability, hormone secretion, and enzyme activation
- Fractions of total serum calcium concentration occur as protein bound (50%), ionized (40%), or complexed with other anions (10%)
- Ionized calcium is the physiologically active fraction
- Of the protein-bound fraction, ≅50% is complexed with albumin. Serum albumin has a direct effect on total serum calcium concentration. Ionized calcium usually is not affected by albumin concentrations
- Acidosis increases ionized calcium by decreasing protein binding. Total calcium concentration usually remains within the reference interval
- Hypocalcemia can be seen with dietary deficiency or imbalance, sepsis, GI disease, hypocalcemic tetany (e.g. lactation, transport, idiopathic, eclampsia), hypoalbuminemia or hypoproteinemia, toxicosis (cantharidin [blister beetle], oxalate), administration of certain drugs (tetracycline, furosemide, bicarbonate), excessive sweating, rhabdomyolysis, renal disease, or pancreatic disease
- In response to hypocalcemia, calcium is mobilized from bone to maintain other metabolic functions
- Consequences include too little or abnormal bone formation, bone demineralization, and a skeleton more prone to injury
- Most acute cases manifest as tetany rather than paresis
- Hypocalcemia may lead to increased neuroexcitability and seizures
Retained placenta and acute endometritis are seen in mares with hypocalcemia and may occur as a result of decreased uterine tone and contractility, possibly resulting from a mechanism similar to that seen with ileus.
- Lactation tetany most frequently occurs in mares ≅10 days post foaling or 1–2 days post weaning
- Draft breeds are more susceptible
Owners may describe lethargy, colic, anorexia, or depression after ingestion of alfalfa; lameness, swollen painful joints, or poor growth; diets of high grain content and low-quality roughage or bran supplement added to grain.
- Tetany, increased muscle tone or weakness, stiffness, muscle fasciculations, SDF, tachypnea, cardiac arrhythmias, ileus, colic, hyperthermia, sweating, excitation
- In severe cases, incoordination, recumbency, convulsions, death
Hypoalbuminemia or Hypoproteinemia
- The protein-bound fraction of calcium is directly affected by serum protein concentration; low serum protein concentrations may mask hypercalcemia
- Correction formulas determined for dogs have not been validated in horses
Dietary Calcium Deficiency or Imbalance
- Occurs from lack of dietary calcium, or factors limiting calcium utilization—excess phosphorus (in the form of inorganic phosphorus, phytate phosphate); oxalic acid
- In young animals, skeletal mass does not keep up with increasing body size; the skeleton is more prone to injury
- NHP occurs from low calcium and excess phosphorus intake. Parathyroid hormone increases as compensatory mechanism
- Rickets occurs from combined calcium and vitamin D deficiency
- Growing animals with vitamin D deficiency may be hypocalcemic, hypophosphatemic, and have elevated ALP. Vitamin D deficiency causes defective mineralization of new bone, resulting in painful swelling of the physis and metaphysis of long bones and costochondral junctions, bowed limbs, and stiff gait
- Natural cases of rickets in foals are not well documented
Ingestion of alfalfa hay or alfalfa-containing products contaminated with blister beetles (Epicauta spp.).
Hypocalcemic Tetany (Lactation, Transport)
- Lactation tetany occurs ≅10 days post foaling or 1–2 days post weaning
- Draft mares, mares that produce large amounts of milk, and mares on pasture only or on a marginal plane of nutrition are at greatest risk
- Prolonged transportation or strenuous activity can predispose to hypocalcemia and tetany
- Concurrent hypomagnesemia is common in hypocalcemic tetany
- Sepsis and GI disease are common causes of hypocalcemia
- Endotoxemia may be the underlying stimulus that triggers several mechanisms leading to hypocalcemia
- Plasma calcium concentrations may decline during the postoperative period after abdominal surgery and while receiving IV fluid therapy
- Hypocalcemia may contribute to ileus
Excessive Sweating and Exertional Rhabdomyolysis
- Horses lose calcium, chloride, and potassium in sweat
- Endurance horses are especially prone to electrolyte imbalances and acid–base disturbances (e.g. alkalosis) after prolonged activity, and may develop SDF. In addition to calcium loss in sweat, the ionized calcium fraction is further reduced in alkalosis
- Hypocalcemia has been reported in neonatal foals with severe rhabdomyolysis. The pathogenesis of hypocalcemia in exertional rhabdomyolysis is not understood
Horses with acute kidney injury may be hypo-, hyper-, or normocalcemic. Although hypercalcemia is more common, some horses in chronic renal failure may have normocalcemia early in the disease process or if not on an alfalfa or high calcium diet.
Several plant species contain oxalates, which, in excess, reduce calcium absorption.
- Oxytetracycline—acute signs of hypocalcemia may occur immediately following drug administration
- Furosemide—promotes diuresis and inhibits calcium reabsorption at the level of the distal tubules in the kidney; horses may not exhibit clinical signs
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS