Parathyroid Gland, Calcium, and Phosphorus Regulation in Horses

Ramiro E. Toribio

Calcium

Calcium is the fifth most abundant element in the body, representing ≈1.5% of the body weight. Physiologic functions such as signal transduction, muscle contraction, hormone secre­tion, enzyme activation, gene transcription, cell division, cell membrane stability, cell migration, neuromuscular excitability, and blood coagulation are calcium dependent.^1-3 Processes that result in cell injury and death, such as free radical production, cytokine release, protease activation, excitotoxicity, vasoconstric­tion, and apoptosis, also require calcium.^2,3

Calcium has structural and nonstructural functions, and it is found in three main compartments: the skeleton, soft tissues, and the extracellular fluid.

Calcium and phosphorus requirements in horses depend on age, physiologic status, and amount of work or exercise performed (Table 41.2). Serum Ca²⁺ concentrations are not a reliable indicator of dietary calcium intake. A balanced diet for horses must have 0.15% to 1.5% of calcium and 0.15% to 0.6% of phosphorus in feed DM (Box 41.1). A

Distribution of Calcium in the Body

FIG. 41.15 Calcium distribution in the body. Approximately 99% of the total body calcium is in the skeleton. The remaining calcium is present in the cell membrane, mitochondria, endoplasmic reticulum, and extracellular fluid. In blood, calcium exists in a free or ionized form (Ca²⁺), bound to proteins, and complexed to anions such as citrate, bicarbonate, phosphate, and lactate. In horses, serum Ca²⁺ represents 50% to 55% of the total serum calcium concentration.

■ TABLE 41.2

■ TABLE 41.4

Calcium and Phosphorous Requirements in the Horse

Modified from NRC: Nutrient requirements, feedstuff composition, and other tables.

^aThese plants have an oxalate content higher than 0.5% of dry matter or a calcium-to-oxalate ratio of less than 0.5.

decrease intestinal absorption of calcium, decrease bone resorp­tion, and increase urinary excretion of Ca²⁺ in horses.^20,21

Calcium is eliminated through feces, urine, milk, sweat, and the fetus. In the kidney, approximately 60% calcium is reabsorbed in the proximal tubules by passive mechanisms and 35% is reabsorbed in the thick ascending loop of Henle and distal tubules by active mechanisms.²² The rest (≈5%) repre­sents the urinary fractional excretion of calcium. Endogenous losses of calcium in horses have been estimated around 20 to 25 mg/kg/body weight/day. Assuming a 50% calcium digest­ibility, a 500-kg horse would require 20 g of calcium to replace losses, or 40 mg/kg/day; growing and lactating horses can double these requirements. Interpretation of the urinary excretion of calcium could be difficult because horses eliminate large amounts of calcium (primarily calcium carbonate) in urine.^2,3,23

Phosphorus

Phosphorus represents ≈1% of body weight, with most (85%) located in the skeleton as hydroxyapatite crystals, 15% in blood and soft tissues, and less than 0.1% in the extracellular fluid. In blood, phosphorus exists as organic (intracellular) and inorganic (extracellular) phosphates. Organic phosphate consists of phosphate esters (phospholipids) bound to proteins and blood cells and represents most of the phosphorus in circulation; however, only inorganic phosphate (PO₄) is measured. PO₄ is found as ionized phosphate (≈50%), complexed with cations (Na⁺, Ca²⁺, Mg²⁺; ≈35%), and bound to proteins (≈15%). At pH 7.4, PO₄ exists as divalent (HPO₄^2-) and monovalent (H₂PO₄^-) anions in a 4:1 ratio. In acidosis this ratio is 1:1 and can be as high as 9:1 during alkalosis.²⁴ In soft tissues, most of the phosphate is organic and intracellular.

Phosphorus requirements depend on age, physiologic status, and amount of work or exercise performed (Table 41.2). In horses, phosphorus absorption ranges from 20% to 55% and occurs in the small and large intestines.^17,25 Excess dietary aluminum may reduce PO₄ absorption^26-28; however, magnesium does not seem to interfere with PO₄ absorption in horses.²⁹ In the kidneys, most of the PO₄ is reabsorbed in the proximal tubules by a Na⁺-dependent mechanism and the urinary fractional excretion of PO₄ in horses is low (is a common cause of hypophosphatemia in critically ill humans and small animals and occasionally occurs in horses with starvation, refeeding syndrome, and parenteral nutrition (hyperglycemia, hyperinsulinemia).

Chronic phosphate deficiency can be manifested as weight loss, weakness, pica (depraved appetite), lameness, and devel­opmental orthopedic disease (DOD). Rickets, as described in other species with PO₄ or vitamin D deficiency, is not a rec­ognized condition in foals. Serum PO₄ concentration is more indicative of dietary phosphorus intake and status than serum Ca²⁺ because PO₄ homeostasis is not as precise as that of Ca²⁺.

Normal calcium, magnesium, phosphorus, and regulatory hormone concentrations are presented in Table 41.6. Ionized calcium concentrations are lower in foals and pregnant mares

■ TABLE 41.6

Normal Calcium, Magnesium, Phosphorous, and Regulatory Hormone Concentrations in Horses

^aMeasured in ethylenediaminetetraacetic acid (EDTA) plasma.

FEc_a, Urinary fractional excretion of calcium; FEp urinary fraction excretion of phosphorus; PTH, parathyroid hormone; PTHrP, parathyroid hormone-related protein.

Conversion units: Calcium (mmol/L = mg/dL ? 0.25; mg/dL = mmol/L ? 4); magnesium (mmol/L = mg/dL ? 0.4; mg/dL = mmol/L ? 2.43); phosphorus (mmol/L = mg/dL ? 0.323; mg/dL = mmol/L ? 3.1)

Data from the College of Veterinary Medicine, Ohio State University, unless otherwise noted. Foal calcium and magnesium data from recent publications.^5,6

than adult horses.^5,6,23 Of interest, serum total and ionized magnesium concentrations tend to be higher in foals than adult horses.^5,6,23 Serum PO₄ concentrations and alkaline phosphatase activity are greater in foals compared with adult horses due to growth, increased intestinal absorption, bone osteoblastic activity, and bone formation.^2,6

Calcium and Phosphorus Homeostasis

Extracellular ionized calcium (Ca²⁺) concentrations are regulated by a homeostatic system that includes three hormones: para­thyroid hormone (PTH), calcitonin, and 1,25-dihydroxyvitamin D₃ (1,25[OH]₂D₃, calcitriol); three body systems (kidney, intestine, and bone); and a calcium-sensing receptor (CaR).^2,3,23 PTH increases during hypocalcemia and hyperphosphatemia while calcitonin increases during hypercalcemia (Fig. 41.16). Under physiologic conditions parathyroid hormone-related protein (PTHrP), which also activates the PTH-1 receptor, has little effect on Ca²⁺ homeostasis.³¹ PO₄ regulation is closely associated with Ca²⁺ homeostasis. Extracellular ionized mag­nesium (Mg²⁺) has permissive effects on calcium homeostasis by facilitating PTH secretion and action.

FIG. 41.16 Calcium and phosphate homeostasis. A decrease in serum Ca²⁺ or increase in serum PO4 concentrations increases PTH secretion. PTH increases renal Ca²⁺ reabsorption and vitamin D synthesis, decreases renal PO4 reabsorption, and increases osteoclastic bone resorption. In turn, vitamin D increases intestinal absorption and renal reabsorption of Ca²⁺ and PO4 and facilitates bone resorption. On the contrary, hypercalcemia decreases PTH secretion and stimulates calcitonin secretion to inhibit osteoclastic bone resorption.

Organs involved in PO₄ homeostasis include the intestine (absorption), kidneys (excretion), and bone (storage). Extracel­lular PO₄ concentrations are under the control of various endocrine factors including PTH, 1,25(OH)₂D₃, calcitonin, insulin, and the fibroblast growth factor-23 (FGF-23)/klotho axis.³ In healthy animals, endocrine control of intestinal absorp­tion of PO₄ is minimal, but hormones such as 1,25(OH)₂D₃ can increase absorption in PO₄-deficient states. PTH and the FGF-23/klotho axis are considered the main regulators of PO₄ concentrations.^32-34 Information on phosphatonins (FGF-23), which inhibit renal PO₄ reabsorption and 1,25(OH)₂D₃ synthesis in a number of species,^32-35 is limited in the horse.³⁰

PTH is secreted by the chief cells of the parathyroid gland in response to hypocalcemia and hyperphosphatemia. Para­thyroid chief cells detect changes in Ca²⁺ concentrations by a calcium-sensing receptor (CaR).^16,36 Through the PTH receptor, PTH increases renal Ca²⁺ reabsorption (distal nephron), decreases renal PO₄ reabsorption (proximal tubules), stimulates renal calcitriol synthesis (proximal tubules), and stimulates osteo­clastic bone resorption (see Fig. 41.16). Calcitriol (1,25[OH]₂D₃) increases intestinal absorption and renal reabsorption of Ca²⁺ and PO₄ and inhibits PTH synthesis and secretion.^2,3 PTH secretion is under the influence of Ca²⁺, PO₄, and vitamin D. Biologically active intact PTH is measured with immunometric assays.

Vitamin D plays an important role in Ca²⁺ and PO₄ homeostasis and, to a lesser extent, in magnesium metabolism. Vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol) are secosterols derived from photolytic cleavage of the B rings of ergosterol (plants and yeasts) and 7-dehydrocholesterol (animals), respectively. Vitamin D₃ is transported to the liver by a vitamin D-binding protein (GC-globulin), where it is hydroxylated (25-hydroxylase) to 2 5-hydr oxyvitamin D₃ (25[OH]D₃, calcidiol), which is then transported to the kidney to produce the active metabolite, 1,25(OH)₂D₃ (calcitriol), by 1α-hydroxylase. In mammalians, 25(OH)D₃ is considered the main circulating form of vitamin D. However, recent studies found that 25(OH)D₂ was the main vitamin D metabolite in equine blood.^37,38 Hypocalcemia, hypophosphatemia, and PTH stimulate 1,25(OH)₂D₃ synthesis by inducing renal 1α-hydroxylase activity, whereas hypercalcemia, hyperphos­phatemia, FGF-23, and 1,25(OH)₂D₃ inhibit 1α-hydroxylase.

Vitamin D stimulates intestinal absorption and renal reab­sorption of Ca²⁺ and PO₄ and, to a lesser extent, of Mg²⁺. 1,25(OH)₂D₃ increases the expression and activity of proteins important for transcellular Ca²⁺ transport, including epithelial Ca²⁺ channels, Ca²⁺ binding proteins (calbindin D_9k, calbindin D_28k), Na⁺∕Ca²⁺ exchangers, and Ca²⁺-ATPases.^2,3,39 The effects of 1,25(OH)₂D₃ on intestinal absorption and renal reabsorption of PO₄ are mediated by Na⁺∕PO₄ cotransporters.³ In addition, 1,25(OH)₂D₃ increases Mg²⁺ renal reabsorption.⁴⁰ In bone, 1,25(OH)₂D₃ increases bone matrix synthesis and mineralization and stimulates osteoclastic activity and bone resorption. In the parathyroid gland, 1,25(OH)₂D₃ inhibits chief cell proliferation, PTH synthesis, and secretion.⁴¹ Vitamin D deficiency results in rickets in young animals and osteomalacia in adults; however, the existence of rickets in the horse is not well documented.

Calcitonin is a 32-amino acid peptide secreted by the parafol­licular cells (C cells) of the thyroid gland in response to hypercalcemia in a number of species, including the horse.^39,42 Calcitonin inhibits osteoclast function and bone resorption, and it decreases renal reabsorption of Ca²⁺ and PO₄ in most 23

species.^2,3

Parathyroid hormone-related protein (PTHrP) is produced by almost every tissue in the body and has a broad range of functions that have little to do with Ca²⁺ homeostasis.³¹ Under physiologic conditions, PTHrP functions (morphogenesis, cell differentiation) are considered to be paracrine, autocrine, and intracrine. For the most part, the endocrine functions of PTHrP are considered pathologic (humoral hypercalcemia of malignancy; HHM). HHM is a paraneoplastic syndrome that results from excessive secretion of PTHrP by some tumors. By interacting with PTH receptors, PTHrP promotes bone resorp­tion and inhibits renal Ca²⁺ excretion, causing hypercalcemia in humans and domestic animals, including the horse.^43-48

The FGF-23/klotho axis plays central functions in calcium, phosphorus, and vitamin D regulation. FGF-23 is considered the main PO₄-regulating hormone and is secreted by osteocytes in response to 1,25(OH)₂D and PTH.^49-51 It promotes renal PO₄ excretion by decreasing renal Na⁺/PO₄ cotransporter numbers and suppressing 1α-hydroxylase activity, therefore reducing 1,25(OH)₂D₃ synthesis.^49,51-54 FGF-23 also inhibits PTH synthesis and secretion.^49,51-54 Klotho is a protein that functions as a coreceptor for FGF-23 in the kidney and parathyroid gland.^50,51,55 Abnormalities in the FGF-23/klotho axis were recently documented in critically ill foals.³⁰ Foals with high FGF-23 and low klotho serum concentrations had more severe disease and were more likely to die.³⁰

Organs involved in Ca²⁺ homeostasis (parathyroid gland, thyroid gland, and kidney), known as the calcium-sensing system, express a calcium-sensing receptor (CaR) that is activated by extracellular Ca²⁺ and to lesser affinity by Mg²⁺.³⁶ CaR activation inhibits PTH secretion and stimulates CT secretion in various species, including the horse.^15,16,39 In the kidney, CaR regulates Ca²⁺ and Mg²⁺ reabsorption independently of PTH. Renal CaR activation inhibits the furosemide-sensitive Na⁺∕K⁺∕2Cl^- cotransporter in the distal nephron, resulting in diuresis and urinary waste of Ca²⁺ and Mg²⁺, in humans and horses.^36,56

Calcium Disorders in the Horse

Calcium dysregulation in the horse is associated with hypocalcemic and hypercalcemic disorders. Equine pathologic conditions charac­terized by abnormal calcium homeostasis include hypoparathyroid- ism,^57-59 primary hyperparathyroidism,^60-62 nutritional secondary hyperparathyroidism,⁶³ renal failure,⁶⁴ humoral hypercalcemia of malignancy,^43-47 vitamin D toxicity,^65,66 exercise-induced hypo­calcemia,^67,68 idiopathic hypocalcemia of foals,⁶⁹ cantharidiasis,⁷⁰ gastrointestinal disease-²³-^70-74 and sepsis.⁶-⁸-²³-⁷¹-⁷³-⁷⁴ Low Mg²⁺ concentrations can contribute to calcium dysregulation. Other conditions that may have abnormal calcium and phosphorus regulation include calcinosis-⁷⁵ idiopathic systemic granulomatous disease (sarcoidosis)-³ and equine bone fragility syndrome.⁷⁶

Hypocalcemic Disorders

Conditions associated with hypocalcemia in the horse are presented in Box 41.2. Calcium deficiency can be acute or chronic. Horses with acute calcium deficiency develop clinical signs associated with neuromuscular excitability and decreased smooth muscle cell contractility (Box 41.3). A decrease in extracellular Ca²⁺ concentrations increases cell membrane Na⁺ permeability- decreasing the resting membrane potential- thus making muscle cells and nerve fibers more excitable. This results in spontaneous and continuous discharges- muscle

■ BOX 41.2

Equine Clinical Conditions in Which Hypocalcemia Has Been Reported

Acute renal failure Bicarbonate administration Cantharidin toxicosis (blister beetles) Chronic renal failure

Colic

During lactation (lactation tetany) During transport (transit tetany) Dystocia

Endotoxemia Endurance exercise Enterocolitis

Furosemide administration Heat stroke

Hypomagnesemia Late pregnancy Liver disease

Magnesium toxicosis Malignant hyperthermia Oxalate ingestion

Pancreatitis Pleuropneumonia Postoperative myopathy Primary hypoparathyroidism Retained placenta Rhabdomyolysis

Sepsis

■ BOX 41.3

Clinical Signs Reported in Horses With Hypocalcemia

fasciculations- tremors- tetany- and seizures. Tachycardia and cardiac arrhythmias may be present- although bradycardia may develop during severe hypocalcemia. Low serum Mg²⁺ concentrations are a frequent finding in horses with hypocal­cemia and can further aggravate neuromuscular excitability. Chronic calcium deficiency in general is manifested as abnormal cartilage and bone development (DOD) and lameness. When calcium deficiency is suspected- feed analysis is recommended to determine if dietary calcium and phosphorus content is adequate.

Synchronous diaphragmatic flutter (SDF) or “thumps” may occur in horses with hypocalcemia associated with gastro- 2370 77

intestinal disease-^23-70 endurance exercise- hypoparathyroid-

• 59 78 69 79

ism- ^- idiopathic hypocalcemia-⁶⁹ tetany (lactation- transport)-⁷⁹ 23 7080 238182

sepsis-²³ blister beetle toxicosis-^70-80 retained placenta-^23-81-82 and alkalosis.^2-3 SDF develops when depolarization of the right atrium stimulates action potentials in the phrenic nerve as it crosses over the heart. Clinically- these animals have a rhythmic movement on the flank from diaphragmatic contractions that are synchronous with the heartbeat.

During alkalosis there is increased Ca²⁺ binding to albumin resulting in ionized hypocalcemia. Alkalosis also increases Mg²⁺ binding to proteins. Exercising horses may develop alkalosis from hyperventilation (respiratory alkalosis) and chloride losses in the sweat (metabolic hypochloremic alkalosis). Hypochlo­remic metabolic alkalosis may develop in horses with proximal gastrointestinal disease due to luminal chloride retention or losses in nasogastric reflux. Hypomagnesemia is common in horses with SDF and hypocalcemia.

Hypocalcemic tetany is the development of sustained skeletal muscular contractions in horses with hypocalcemia. Although hypocalcemic tetany can occur in any horse with hypocalcemia­lactating mares and horses transported for long distances are at greatest risk. Lactation tetany in mares may occur anytime immediately before foaling up to the end of the lactation period. In particular- mares producing large amounts of milk and eating diets low in calcium- grazing lush pastures- or performing physical work (draft mares) are at risk. Clinical signs may include anxiety- depression- ataxia- muscle fasciculations and tremors- stiff gait- tachypnea- dyspnea- dysphagia- hypersaliva­tion- and hyperhidrosis (Box 41.3).^2-3

Hypocalcemic seizures, seen in foals and adult horses- are caused by decreased CNS extracellular Ca²⁺ concentrations leading to increased neuronal excitability. Clinical signs usually improve with calcium treatment- although some animals may require repeated treatments. In general- horses and foals with refractory hypocalcemic seizures have a poor prognosis for recovery.

Ileus and retained placenta in mares are both believed to result in part from decreased smooth muscle tone and contractility secondary to hypocalcemia.^2-3 While in skeletal muscle most of the Ca²⁺ required for contraction comes from the sarcoplasmic reticulum- in smooth muscle Ca²⁺ comes from the extracellular space.² For this reason- any condition that results in hypocal­cemia can decrease smooth muscle contractility. Retained placenta in mares occurs in up to 10% of foalings-⁸¹ and it has been associated with hypocalcemia.⁸¹

Hypoparathyroidism in horses is characterized by hypocal­cemia- hyperphosphatemia- hypomagnesemia- and decreased serum PTH concentrations. Primary hypoparathyroidism results from decreased secretion of PTH- while secondary hypopara­thyroidism results from magnesium depletion and sepsis. There are few documented cases of primary hypothyroidism in horses.^58-78 Hypoparathyroidism should be suspected in any horse or foal with refractory hypocalcemia. Clinical signs include ataxia- seizures- hyperexcitability- SDF- tachycardia- tachypnea­muscle fasciculations- bruxism- stiff gait- recumbency- ileus- and colic.^2-3-58-83 Laboratory findings include hypocalcemia­hyperphosphatemia- and low or normal serum intact PTH concentrations. Hypomagnesemia may be present in some horses and foals.^6,23 Secondary (functional or acquired) hypo­parathyroidism is poorly documented in the horse; however, some critically ill foals and horses with hypocalcemia have impaired parathyroid gland function,^6,8,23 which could be a form of hypoparathyroidism secondary to inflammatory cytokines and/or hypomagnesemia.

A syndrome consistent with hypoparathyroidism, character­ized by abnormally low PTH concentrations, hypocalcemia, SDF, and muscle fasciculations, has been recognized in young Standardbred and Thoroughbred horses by the author. The condition develops progressively, initially associated with poor performance and SDF. In some animals it has been reported after a surgical procedure (e.g., castration), while in others it has been linked to excessive calcium supplementation. The underlying cause of this disorder remains unknown. Suppression of mechanisms responsible for calcium homeostasis from excessive calcium administration, immune-mediated parathy­roiditis, cytokines, and mutations in the calcium-regulatory system are potential explanations.

A number of critically ill foals develop a form of hypocal­cemia that is refractory to calcium treatment.⁶⁹ These foals have low or normal PTH concentrations despite hypocalcemia, suggesting hypoparathyroidism. This condition has been called neonatal idiopathic hypocalcemia.⁶⁹ It is believed that abnormal parathyroid gland function may result from increased inflamma­tory cytokines.^6,69 Prognosis for survival in foals with refractory hypocalcemia is poor.

Sepsis and endotoxemia are the most common causes of hypocalcemia in equine patients admitted to veterinary hos­pitals.^6,8,23 Clinical observations also indicate that hypocalcemia 23 71 73 is common in horses with severe gastrointestinal disease.’’ Hypocalcemia in septic patients often results from parathyroid gland dysfunction (insufficient PTH secretion), as well as intracellular calcium sequestration.^2,3,23 Inflammatory mediators known to be increased in horses and foals with sepsis and endotoxemia such as TNF-, IL-1, and IL-6 have been shown to increase CaR activation and decrease PTH secretion by equine parathyroid cells.¹⁶

Horses under intense exercise develop electrolyte and acid-base abnormalities. Exercise-induced hypocalcemia may result from Ca²⁺ losses in the sweat; intracellular movement of Ca²⁺; increased Ca²⁺ binding to albumin, lactate, phosphate, and bicarbonate during alkalosis; and parathyroid gland dysfunction.^2,3,67,68

Oxalate toxicity causes hypocalcemia by interfering with calcium absorption; a diet consisting of 1% of oxalate or higher can reduce most intestinal calcium absorption.⁸⁴ It is important that the equine diet contains less than 0.5% of oxalate or a calcium-to-oxalate ratio over 1:0. Clinical signs associated with oxalate excess are those of phosphate excess, calcium deficiency, and nutritional hyperparathyroidism. Signs of acute hypocal­cemia may develop.⁸⁵ Neurologic signs from excessive bone loss and vertebral fractures have been documented.⁸⁵ Oxalates are present in several grasses and toxic plants (Table 41.5).

Equine cantharidiasis (blister beetle toxicosis) is a condition reported in the southwestern and midwestern United States and results from the ingestion of alfalfa contaminated with beetles (Epicauta spp.) that produce cantharidin (cantharidic acid).^70,80 Clinical signs develop from the irritant effects of cantharidin on mucosal surfaces (gastrointestinal and urinary tracts). Cantharidin often causes acute hypocalcemia and hypomagnesemia. Thus, clinical signs of severe hypocalcemia (muscle fasciculations, SDF, ataxia, dyspnea, laryngeal spasm, and cardiac arrhythmias) may be present.^70,80 It is unclear why these horses develop hypocalcemia; however, it may be a combination of severe gastrointestinal disease associated with acute renal tubular necrosis and parathyroid gland dysfunction.

Hypocalcemia and hypomagnesemia are common findings in horses with acute renal failure.^2,3 Reabsorption of Ca²⁺ and Mg²⁺ in the kidney is highly dependent on functional tubular cells,² and these cells are susceptible to various insults (hypoxia, ischemia, toxins).²²

The pathogenesis of hypocalcemia in exertional rhabdomyolysis is unknown.⁸⁶ It is speculated that damage to muscle fibers during intense exercise results in Ca²⁺ influx and sequestration in the sarcoplasmic reticulum.

Treatment of Hypocalcemia

Calcium deficit, maintenance, losses, and sequestration should be considered when treating hypocalcemia. If parathyroid gland function is normal, minimal to no calcium supplementation may be required. Most horses with hypocalcemia do not show overt signs of hypocalcemia. Lack of therapy, however, can result in development of additional complications, in particular ileus. Horses with functional kidneys can rapidly eliminate large amounts of calcium, and hypercalcemia from excessive calcium administration is rare, particularly if the horse is receiving fluid therapy. When possible, calculate the calcium deficit on the basis of Ca²⁺ concentrations (mg/dL = mmol/L ? 4). From a practical standpoint, the use of standard formulas for electrolyte deficit can be used (multiplied by 10 as calcium is expressed as mg/dL). Total calcium can be used to estimate calcium deficit, keeping in mind that total calcium has more variability than Ca²⁺ concentration as it is dependent on plasma protein concentration. A horse can have total hypocalcemia, but serum Ca²⁺ concentrations may be within the normal range (pseudohypocalcemia), and calcium administration may not be necessary. It is also important to remember that 9.3% of calcium gluconate is elemental calcium, which means that a 23% solution of calcium gluconate contains 2.14% of elemental calcium or 21.4 mg/mL. Avoid the use of injectable calcium chloride as it can cause subcutaneous irritation.

Frequent monitoring of Ca²⁺ concentration and adjustment of dosage is important. Rapid administration of calcium may result in cardiovascular complications, especially in septic horses, which may be more vulnerable to the toxic effects of calcium. In our experience, horses can handle high calcium dosages. For an average-size horse with mild ionized hypocalcemia (5.0 mg/dL), we administer 50 mL of 23% calcium gluconate per 5 L of crystalloid fluids, at a fluid rate twice mainte­nance. Oral treatment with calcium salts is feasible in some horses with non-life-threatening, refractory hypocalcemia. Dicalcium phosphate and calcium carbonate (limestone, 200 to 300 g/day) can be used safely (Table 41.3). In horses with refractory hypocalcemia, measuring Mg²⁺ concentrations is highly recommended.

Hypercalcemic Disorders

Hypercalcemic disorders in horses are divided into two groups: (1) parathyroid gland-dependent hypercalcemia (develops due to parathyroid gland hyperfunction) and (2) parathyroid gland­independent hypercalcemia (develops despite parathyroid gland suppression). This distinction is clinically relevant in the differential diagnosis and interpretation of specific diagnostic tests, including intact PTH, PTHrP, Ca²⁺, PO4, and vitamin D concentrations. Parathyroid gland-dependent hypercalcemia in the horse is limited to primary hyperparathyroidism, while parathyroid-independent hypercalcemia results from various conditions (secondary hyperparathyroidism, chronic renal failure, hypercalcemia of malignancy, hypervitaminosis D, calcinosis, and idiopathic systemic granulomatous disease).

Primary hyperparathyroidism results from an excessive and autonomous synthesis and secretion of PTH by the parathyroid gland that is not responsive to the negative feedback of Ca²⁺. Primary hyperparathyroidism has been reported in ponies and 5860628788

horses,^58,60-62,87^,88 and results from parathyroid adenomas or parathyroid hyperplasia. The elevated PTH concentrations increase renal Ca²⁺ reabsorption, decrease PO₄ reabsorption, increase 1,25(OH)₂D₃ synthesis, and increase bone resorp­tion (osteodystrophia fibrosa). Laboratory findings include hypercalcemia, hypophosphatemia, hypocalciuria, and hyper- phosphaturia. PTHrP concentrations are within normal limits (low or undetectable). Clinical findings include facial bone enlargement, lameness, and a poor body condition. Radiographic findings include decreased long and facial bone density, fibrous proliferation of the maxilla and mandible, and loss of the lamina dura surrounding the molars.⁶⁰ Endoscopic examination may reveal narrowing of the nasal passages.

Nuclear scintigraphy and ultrasonography were successfully used to detect and remove a functional parathyroid adenoma in a pony.⁸⁷ Ultrasonography was useful to identify parathyroid tissue within the thyroid gland of a horse with signs and labora­tory findings consistent with primary hyperparathyroidism.⁸⁸

Postmortem findings include enlargement of the maxilla and mandible, stenosis of the nasal passages, and loosening of premolars and molars. Histologic evaluation of the parathyroid gland is important to confirm the diagnosis of primary hyper­parathyroidism; however, finding the parathyroid glands in the horse is a challenge due to their small size and variable location.^2,16

Secondary hyperparathyroidism is characterized by excessive secretion of PTH in response to hyperphosphatemia and hypovitaminosis D from chronic renal failure (renal secondary hyperparathyroidism) or hyperphosphatemia and/or hypocal­cemia from nutritional imbalances (nutritional secondary hyperparathyroidism). Renal secondary hyperparathyroidism is not a well-recognized disease in the horse. Unlike humans and small animals in which chronic renal failure results in hyper­phosphatemia, horses with chronic renal failure (CRF) often have hypophosphatemia. Moreover, the hypercalcemia in horses with CRF is the result of renal Ca²⁺ retention rather than increased PTH concentrations.^2,3,89 The increased Ca²⁺ con­centrations in turn decrease PTH secretion; thus, PTH concentrations in horses with CRF frequently are below or within the normal range.^3,57 In contrast, nutritional secondary hyperparathyroidism is a well-documented pathologic condition of the horse² and is described in more detail as follows.

Nutritional Secondary Hyperparathyroidism

Definition and Etiology

Horses fed diets low in calcium, high in phosphorus, high in oxalates, or with a phosphorus-to-calcium ratio of greater than or equal to 3 : 1 may develop nutritional secondary hyperparathy­roidism, also known as bran disease, miller's disease, big head, osteodystrophia fibrosa, osteitis fibrosa, and equine osteoporo- sis.^2,3,63,90-94 Pastures and toxic plants with high content of oxalates (Table 41.6) predispose to secondary hyperparathyroidism. The condition remains endemic in some areas in Latin America, North America, Africa, Asia, Australia, and Europe.⁶³,⁸⁵,⁹⁰,⁹²,^95-99

Pathogenesis

Excessive dietary PO₄ reduces intestinal calcium absorption and results in hyperphosphatemia. With advances in animal nutrition, this condition is rarely associated with excessive grain feeding, but more commonly linked to the ingestion of oxalate-rich plants. Large amounts of oxalate (oxalic acid) bind dietary calcium to form insoluble calcium oxalate (CaC₂O₄), reducing calcium absorption. Oxalate also binds magnesium, but the complex (MgC₂O₄) is more soluble than CaC₂O₄ and more likely to be absorbed. Both high-phosphorus and low-calcium diets induce parathyroid cell hyperplasia and stimulate PTH secretion in the horse.^3,100 Hyperphosphatemia directly stimu­lates PTH secretion and inhibits renal 1,25(OH)₂D₃ synthesis. Because 1,25(OH)₂D₃ inhibits parathyroid cell proliferation, low 1,25(OH)₂D₃ concentrations contribute to parathyroid cell hyperplasia and PTH secretion. Hyperphosphatemia also results in the formation of calcium phosphate precipitates, further reducing blood Ca²⁺ and inducing additional PTH secretion. PTH increases osteoclastic activity, bone resorp­tion, and bone loss (Fig. 41.17).¹⁰¹ There is facial bone loss

FIG. 41.17 Pathogenesis of nutritional secondary hyperparathyroidism in horses. Excessive dietary phosphorus reduces intestinal absorption of calcium and induces hyperphosphatemia. In addition, phosphate, oxalate, and phytate bind dietary calcium to reduce absorption. Both high-phosphorus and low-calcium diets induce parathyroid cell hyperplasia and stimulate PTH secretion. PTH increases osteoclastic activity and bone resorption, resulting in bone loss (osteodystrophia fibrosa).

FIG. 41.18 Big head. Two-year-old Belgium horse presented to The Ohio State University Veterinary Medical Center with clinical signs consistent with nutritional secondary hyperparathyroidism, including facial bone enlargement and upper respiratory noise. The horse was fed excessive amounts of grain. There was narrowing of the nasal passages, loss of bone mass, and excessive accumulation of unmineralized bone matrix (osteodystrophia fibrosa) in maxillary and mandibular bones.

with excessive accumulation of subperiosteal unmineralized connective tissue (osteodystrophia fibrosa) resulting in facial enlargement (big head) (Fig. 41.18). Because this is a condition of slow progression, the homeostatic mechanisms that regulate extracellular Ca²⁺ concentrations (PTH, vitamin D, calcitonin) in general are effective in maintaining Ca²⁺ within the normal range. These horses preserve normocalcemia at the expense of the skeletal reserves and rarely develop clinical signs of acute hypocalcemia.

Clinical Signs

Clinical signs result from increased bone resorption and include unthriftiness; intermittent, shifting lameness; and a stiff gait. Younger animals may develop physitis and limb deformities. There is a typical and symmetric swelling of the facial bones; however, facial bone enlargement may not be evident in old horses (see Fig. 41.18). The facial changes and increased bone resorption around molars and premolars may result in masticatory problems. These horses are physically weak and may be in poor body condition from the pain associated with lameness and mastication. In severe cases, teeth may become loosened and spontaneous fractures of long bones may occur. Upper airways obstruction, dyspnea, and epiphora may be present.^3,63,90-92 Vertebral demineralization and compressive neurologic signs may occur.⁸⁵

Acute hypocalcemia may develop from high dietary oxalate content combined with a low-calcium diet.⁸⁵ In addition, absorption of soluble oxalate can chelate calcium in circulation to worsen signs of hypocalcemia. Soft tissue mineralization was described in a foal suspected of having nutritional secondary 102

hyperparathyroidism.¹⁰²

Laboratory Findings

Typical laboratory changes in horses with nutritional secondary hyperparathyroidism include hyperphosphatemia, hypocalcemia or normocalcemia (hypercalcemia is unusual), and increased intact PTH concentrations, especially if the animal is eating a low-calcium or high-phosphorus or high-oxalate diet at the time of evaluation. The urinary fractional excretion of calcium is low (hypocalciuria) while the excretion of phosphorus is increased (hyperphosphaturia). Urinary excretion of phosphorus greater than 2% is common in horses ingesting oxalate-rich plants.⁹⁵ Serum alkaline phosphatase activity and collagen degradation products may be increased. Laboratory findings may be within normal limits if the animal is eating a balanced diet.

Radiologic Findings

Decreased bone density is frequent; however, bone mass must be decreased by 30% before it can be detected radiographically.³ Decreased facial bone density along with fibrous proliferation is a consistent finding. Resorption of alveolar sockets and loss of the dental lamina dura may be present before other radiographic changes are present, and long bones are affected only in advanced cases. Radiolucency and osteopenia may be evident in long bones, vertebrae, and ribs early on; however, a patchy osteoporotic pattern may be present, in particular with chronic oxalate toxicity.⁸⁵

Necropsy

There is increased bone resorption, bone fragility, accumulation of fibrous tissue around facial bones, obstruction of nasal passages, and parathyroid gland hyperplasia (see Fig. 41.18).

Treatment

Diet evaluation is indicated. Eliminate or reduce any grain-based diet and avoid high-containing oxalate feeds. The addition of alfalfa to the diet may be helpful. Supplementation with calcium carbonate (limestone; CaCO₃) or dicalcium phosphate may result in improvement.⁶³ Ground limestone, which con­tains no phosphorus, is recommended as a good source of calcium (35%). An affected animal may require a total of 100 to 300 g/day. The diet should have a Ca:P ratio of 3 to 4:1. Limestone may decrease feed palatability, and adding molasses should be considered. Supplementation with vitamin D has been proposed. Horses may require 9 to 12 months for com­plete recovery, although some bone changes may not regress. Confinement of severely affected horses is advised. The use of NSAIDs may be indicated in some animals. Bisphosphonates have not been evaluated with this condition.

Hypervitaminosis D

Definition and Etiology

The ingestion or administration of ergocalciferol (vitamin D₂) or cholecalciferol (vitamin D₃) results in disturbances of the calcium and phosphorus metabolism in horses.^{65,66,103-105} Ingestion of plants containing 1,25(OH)₂D-like compounds results in typical clinical signs of vitamin intoxication.^103-109 The ingestion of Solanum glaucophyllum (S. malacoxylon) results in a condition known as “enteque seco” in Argentina and “espichamento” in Brazil.^{105,106,108,109} In Hawaii, Solanum sodomaeum, and the southern United States, jessamine (Cestrum diurnum) may cause hypervitaminosis D.^103,104,107^,109 In Europe, the ingestion of golden oat (Trisetum flavescens) results in enzootic calcinosis.^110-112

Pathogenesis

Hypervitaminosis D increases the intestinal absorption and renal reabsorption of calcium and phosphorus. Hyperphosphatemia is the most consistent and early laboratory finding in horses with vitamin D intoxication.^65,66,102 Serum calcium concentra­tions may be increased or within the normal range.^{65,66,102,103,106} Hypervitaminosis D results in parathyroid cell atrophy and decreased PTH secretion. In addition, hypercalcemia con­tributes to decrease PTH secretion, lowering bone turnover. Azotemia and hyposthenuria may be present.^65,66

Clinical Signs

Most clinical findings in horses with hypervitaminosis D are the result of hyperphosphatemia. These horses often have weight loss, poor appetite, lameness, and painful stiffness, and they are reluctant to move. Acute death from severe cardio­vascular mineralization has been reported.^65,66 Polyuria and polydipsia are frequent findings. In cases with hypercalcemia, mineral deposition in the kidneys may precede mineralization elsewhere, resulting in renal failure. Lameness is probably due to calcification of ligaments and tendons.

Radiologic Findings

These horses often have increased bone density, decreased size of the medullary cavity, and increased calcification of soft tissues.

Treatment

Reducing dietary calcium intake and the use of calcium- binding agents such as sodium phytate has been proposed.¹⁰⁵ Glucocorticoids are used in humans with hypervitaminosis D because they may inhibit the vitamin D-mediated intestinal calcium absorption. In equids, glucocorticoids decrease intestinal absorption of calcium, increase urinary excretion of calcium, and decrease bone resorption.^20,21 Dexamethasone has been administered to horses with hypervitaminosis D with vari­able results. Drugs that suppress osteoclastic bone resorption (bisphosphonates) have not been evaluated in horses with hypervitaminosis D. The prognosis for horses with hypervi- taminosis D is poor.

Necropsy Findings

Postmortem examination may reveal mineralization of soft tissues. Mineralization of the endothelium of the aorta and pulmonary vessels, as well as of the endocardium, is frequent. Mineralization may be found in the kidney, liver, lymph nodes, lungs, ligaments, and tendons. Osteopetrosis of epiphyses and metaphyses may be present. Atrophy of the parathyroid gland

Hypercalcemia of Malignancy

Humoral hypercalcemia of malignancy (HHM, pseudohyper­parathyroidism) is a paraneoplastic condition in which humans and animals develop hypercalcemia associated with various types of tumors. These malignancies secrete parathyroid hormone-related protein (PTHrP), which interacts with PTH receptors to increase renal reabsorption of Ca²⁺ and bone resorption.¹¹³ In horses, HHM has been associated with squamous cell carcinoma, adrenocortical carcinoma, lympho­sarcoma, multiple myeloma, and ameloblastoma.^2,3 Laboratory findings include hypercalcemia, hypocalciuria, hypophospha­temia, hyperphosphaturia, normal or low PTH concentrations, and increased PTHrP concentrations. HHM should be sus­pected in any horse with hypercalcemia, no evidence of renal disease, and normal PTH concentrations.

Neonatal Hypercalcemia and Asphyxia

Clinical observations indicate that a number of critically ill newborn foals develop hypercalcemia associated with peripartum asphyxia (neonatal encephalopathy). The mechanisms underlying this problem remain unknown, although it is speculated to be associated with placental insufficiency.

Idiopathic Systemic Granulomatous Disease

This is an immune disorder documented in horses and ponies (equivalent to human sarcoidosis), in which crusty skin lesions develop along the coronary bands and mucocutaneous junctions. Granuloma formation in multiple organs is the hallmark.^114-116 Some animals may develop hypercalcemia.¹¹⁴ Lesion distribution may resemble those of multisystemic eosinophilic epitheliotropic disease of horses. It has been proposed that excessive production of vitamin D or PTHrP by granulomatous cells leads to hypercalcemia in these animals.^3,114 Similar lesions have been documented in horses and cattle grazing hairy vetch (Vicia villosa).^ιvι,lls

Calcinosis

Soft tissue calcification (liver, heart, kidney, lungs, vessels, muscle), hyperphosphatemia, and elevation of the calcium ? phosphorus product were documented in horses with a number of pathologic conditions.⁷⁵ This disorder is different than enzootic calcinosis from the ingestion of plants with vitamin D-like activity. The mechanism by which these animals develop calcinosis is unclear, although it is likely that hyperphosphatemia plays a role.

Equine Bone Fragility Syndrome

The equine bone fragility syndrome (BFS) or silica-associated osteoporosis (SAO) is a chronic and progressive disorder of horses characterized by increased respiratory efforts, exercise intolerance, skeletal deformation, lameness, stiff­ness, and fractures.^119,120 (Also see the Equine Bone Fragility section in Chapter 38.) Horses of any age are affected.^119,121 Bowing of appendicular bones (scapula in particular), lordosis, and neurologic signs from vertebral deformation and spinal cord compression have been documented.^119,120 The pathogenesis remains unclear, but it has been associated with pulmonary silicosis. Radiographically, there is osteopenia and a respiratory pattern consistent with silicosis. Intracellular silica crystals (silica dioxide) in bronchoalveolar cells and pulmonary mac­rophages, as well as evidence of dystrophic calcification, are consistent with SAO. A high concentration of cristobalite, a fibrogenic form of silicate, is found in regions of Califor­nia where the condition is more common (Monterey shale soil).^{76,119,120,122} Histologically, there is excessive osteoclastic activation and bone resorption. In the respiratory tract there is pulmonary granuloma formation, fibrosis, and lymphadenitis.⁷⁶ The diagnosis of BFS is based on clinical history, clinical find­ings, geography, and imaging (ultrasonography, radiography, and scintigraphy). Nuclear scintigraphy is the best diagnostic modality, but scapular ultrasonography can be diagnostic in some animals.¹²¹ Management of these horses depends on clinical signs and prevention. Nonsteroidal antiinflamma­tory drugs should be considered in animals with orthopedic pain considering that their use can increase the risk of frac­tures. Glucocorticoids may be necessary in severe pulmonary disease, but they also enhance bone loss. This is one of the few equine conditions in which bisphosphonates may be indicated.¹²³ Zoledronate has reduced the severity of clinical signs and scintigraphic findings in horses with BFS.¹²³ Decreased physical activity and dietary manipulation are recommended.

Treatment of Hypercalcemia

Hypercalcemia as an equine emergency is rarely presented; however, the differential diagnosis of hypercalcemia is important for its treatment. Few disorders in the horse are associated with hypercalcemia (hyperparathyroidism, CRF, HHM, hypervitaminosis D). Mild to moderate hypercalcemia in general is not life-threatening, and treatment should be directed to the primary cause. Parathyroidectomy will be treatment of choice for primary hyperparathyroidism.⁸⁷ Bisphosphonates and calcimimetics (cinacalcet) are also recommended in people with hyperparathyroidism^124,125; however, information on their use for this condition in horses is lacking. Bisphosphonates can cause renal injury in people and, anecdotally, in horses as well. In theory, calcitonin could reduce serum calcium con­centrations, but this effect is transient. Surgical removal of epithelial tumors can be a successful treatment for HHM. Some horses with lymphosarcoma may show improvement with chemotherapy. Bisphosphonates and gallium nitrate have shown benefits in reducing serum calcium and bone resorption in people with HHM.¹²⁶ Bisphosphonates improved signs in horses with BFS.¹²³ In severe cases of hypercalcemia that may require medical treatment, initial therapy should include the administration of 0.9% saline solution and loop diuretics. Furosemide is the diuretic of choice because it inhibits the Na⁺/K⁺/2Cl^- cotransporter in the distal tubules, increasing the urinary excretion of calcium. Thiazide diuretics are contrain­dicated because they stimulate calcium reabsorption. Gluco­corticoid administration should be considered, in particular for horses with hypervitaminosis D.

Phosphorus Disorders in the Horse

Disorders of phosphorus homeostasis can be acute or chronic, resulting in hypophosphatemia or hyperphosphatemia. In critically ill horses hypophosphatemia is more frequent, while in foals hyperphosphatemia is more common.^6,127

Hypophosphatemic Disorders

Hypophosphatemia develops from reduced intestinal PO₄ absorption, increased urinary PO₄ excretion, and intracellular shift of PO₄ (redistribution). Acute hypophosphatemia often reflects PO₄ redistribution from the extracellular to intracellular compartment and not necessarily imply total body PO₄ deple­tion. However, PO₄ depletion may occur in animals that have been sick for many days. From personal experience, hypo­phosphatemia seems to be more common in small equids, usually associated with hyperlipemia and parenteral nutrition. Intestinal PO₄ absorption is reduced by gastrointestinal dis­orders, hypovitaminosis D, PO₄-deficient diets, and interfering substances. Urinary excretion of PO₄ is increased with hyper­parathyroidism, hypovitaminosis D, malignancies that secrete phosphatonins or PTHrP, chronic renal failure, and drugs. Sepsis, carbohydrate-rich diets, hyperglycemia, insulin admin­istration, starvation, refeeding syndrome, hyperlipemia, and parenteral nutrition (hyperglycemia, hyperinsulinemia) decrease serum PO₄ mainly through insulin-mediated intracellular shift of PO₄ and enhanced glycolysis. Catecholamines act by a mechanism similar to insulin shifting PO₄ into the cell. Alkalosis induces hypophosphatemia by stimulating phosphofructokinase activity (glycolysis) and the need for PO₄ to synthesize ATP. In horses with a history of malnutrition, close monitoring of serum PO₄ concentrations is indicated once caloric intake resumes because they may develop refeeding syndrome. Clinical signs of acute hypophosphatemia in horses often go unnoticed. Signs are consequence of PO₄ regulatory functions on energy metabolism, cell membrane stability, and ion transport.³ These include muscle weakness, fasciculations, dysrhythmias, neuro­muscular excitability, ileus, and cell membrane fragility and lysis (hemolysis, rhabdomyolysis). Signs from mild hypophos­phatemia are difficult to document. Severe hypophosphatemia can result in tissue hypoxia from reduced erythrocyte content of 2,3-bisphosphoglycerate. Chronic hypophosphatemia is rare in horses and manifests as weight loss, weakness, depraved appetite (pica), reduced bone density, DOD, lameness, and hemolysis. Ideally, therapy for hypophosphatemia would be based on the extracellular deficit, which should be overestimated because PO₄ is a major intracellular anion and cellular depletion is likely to occur in animals with prolonged hypophosphatemia. Preferred parenteral solutions include potassium phosphate and sodium phosphate. Critically ill horses can be supplemented with oral potassium phosphate or sodium phosphate (chemical grade or with phosphate-based enema solutions). Oral supple­mentation with dicalcium phosphate is a good option in animals with chronic phosphate deficiency.

Hyperphosphatemic Disorders

Hyperphosphatemia develops from increased PO₄ absorption; acute renal injury; hypoparathyroidism; vitamin D toxicity; metabolic acidosis; cell lysis (hemolysis, rhabdomyolysis, tumor necrosis); or iatrogenic causes.³ Lactic acidosis may cause hyperphosphatemia by shifting PO₄ to the extracellular compartment. Metabolic acidosis and high intracellular ATP concentrations inhibit phosphofructokinase, a key enzyme in glycolysis that uses PO₄ for carbohydrate phosphorylation. Overuse of phosphate-based enemas can cause hyperphospha­temia in foals. Unlike other species, chronic renal failure is an uncommon cause of hyperphosphatemia in horses. Inappropriate blood sample handling can result in spurious hyperphospha­temia. Hemolysis, hyperbilirubinemia, hypertriglyceridemia (hyperlipidemia), and hyperproteinemia can falsely elevate PO₄ values (pseudohyperphosphatemia). Multiple myeloma can cause pseudohyperphosphatemia from high circulating immunoglobulin concentrations. Acute hyperphosphatemia may cause hypocalcemia from interactions between PO₄ and Ca²⁺ and reduced renal synthesis of 1,25(OH)₂D. Hyperphos­phatemia is common in critically ill foals.^6,127 The pathogenesis of chronic hyperphosphatemia is described in the Nutritional Secondary Hyperparathyroidism section. Clinical signs of acute hyperphosphatemia are similar to those of acute hypocalcemia and include tetany, hyperexcitability, muscle fasciculations, colic, and dysrhythmias. Signs of chronic hyperphosphatemia are those of calcium deficiency, including lameness, orthopedic pathologies, fractures, and osteodystrophia fibrosa. Treat­ment of hyperphosphatemia is based on primary pathology and duration, which in some instances may not be necessary. In acute hyperphosphatemia, fluid therapy and diuretics are indicated. Dietary PO₄ restriction must be implemented in chronic hyperphosphatemia. Dialysis and phosphate binders are impractical and expensive. Calcium carbonate is a cheap phosphate binder that also provides calcium supplementation. Phosphate-binding polymers (e.g., sevelamer) are the most effective way to treat chronic hyperphosphatemia in other species, but their use is not described in horses.