Meghan Ellis

Calcium and Phosphorus Metabolism in Reptiles

Clinical Importance

• More than 3.9 million US households with 1+ reptile pets

• Ca and P derangements are common in pet reptiles

• Preventable with proper husbandry

• Treatable with early intervention

Calcium Importance

• Cell signaling

• Neural transmission

• Muscle function

• Blood coagulation

• Enzymatic co-factor

• Membrane and cytoskeletal functions

• Bone mineralization

Calcium Regulation

Parathyroid Gland

PTH Regulation

• Decreased serum ionized calcium increased synthesis and secretion of PTH from chief cells

• Increased serum ionized calcium negative feedback loop

• Increased phosphorus decreased serum Ca stimulation of PTH increased renal excretion of phosphorus

Metabolic Functions of PTH

• Activate osteoclasts mobilizes calcium from bone

• Increase renal tubular reabsorption of calcium

• Increases conversion of vitamin D to active dihydroxy form in kidneys

• Augments GI calcium absorption

Metabolic Functions of Calcitonin

• Promotes absorption of calcium by skeletal system

• Inhibits resorption of bone by osteoclasts

Calcitonin Regulation

• Increased serum ionized calcium increased synthesis and secretion of calcitonin by C cells of thyroid gland

• Decreased serum ionized calcium negative feedback loop

Thyroid Gland

Phosphorus Importance

• Bone mineralization

• Phospholipid in cellular membranes

• Cellular respiration

• Component of DNA and RNA

• Post-transcriptional modification of proteins

Phosphorus Regulation

Vitamin D3

• Stimulates calcium and phosphorus absorption by intestinal mucosa

• Maintains calcium and phosphorus balance for optimal bone mineralization

Calcium Clinical Pathology

• Normal total serum calcium: 8-11mg/dL in most reptiles

• Normal ionized calcium in Green Iguana: 1.47 +/- 0.104 mmol/L

Hypocalcemia (<8.0 mg/dL)

• Dietary deficiency

• Excessive dietary phosphorus

• Secondary nutritional hyperparathyroidism

• Vitamin D3 deficiency (dietary or lack of UV light)

• Hypoablbuminemia

• Hypoparathyroidism

Hypercalcemia (>20mg/dL)

• Excessive dietary calcium and vitamin D3

• Primary hyperparathyroidism

• Pseudohyperparathyroidism

• Osteolytic bone disease

• Vitellogenesis (physiologic)

Phosphorus Clinical Pathology

• Normal serum phosphorus: 1-5mg/dL

Hypophosphatemia

• Starvation

• Nutritional deficiency

Hyperphosphatemia (>5mg/dL)

• Excessive dietary phosphorus

• Hypervitamintosis D3

• Renal disease

• Severe tissue trauma

• Osteolytic bone disease

• Sample handling error

Nutritional Secondary Hyperparathyroidism

• Cause: diet low in calcium or vitamin D3, diet high in phosphorus, lack of exposure to UV-B light

• Pathogenesis: Low serum Ca PTH secretion increased bone resorption, increased renal absorption of Ca and increased excretion of PO4, stimulation of 1,25-dihydroxycholecalciferol which increases GI absorption of Ca increase serum calcium and decrease serum PO4

NSHP Effects

• Hypocalcemia neural hyperexcitability twitching digits, hyperreflexia, ataxia, disorientation, cloacal prolapse

• Increased osteoclastic osteolysis demineralization of bone fractures of long bones and spine, thickening and swelling of long bones and mandibles, “rubber jaw”

NSHP Effects continued

• Effects on reproduction: decreased calcification of egg shells weak rubbery eggs

• Clinical Pathology: low normal serum calcium; increased PTH

NSHP

NSHP Treatment

• Correct hypocalcemia

• Rehydrate

• Correct Husbandry

Renal Secondary Hyperparathyroidism

• Cause: chronic renal disease- degenerative nephroses due to inadequate husbandry, poor nutrition, NSHP

• Pathogenesis:failure of kidneys to excrete excess phosphorus decreased serum Ca due to Ca X P rule PTH secretion

• Hyperphosphatemia renal tubular cells cannot hydroxylate 25-hydroxycholecalciferol to active vitamin D3 increased PTH secretion

• Decreased vitamin D3 no GI absorption of Ca

RSHP Effects

• Crispy critters: Ca x P >9 mineralization of kidneys; >12 mineralization of other soft tissues especially GI and cardiorespiratory systems

• Ischemic necrosis of tail, vomiting, passing poorly digested food, congestion of peripheral blood vessels especially obvious in sclera

• All the signs of hypocalcemia

RSHP

RSHP Clinical Pathology

• Hyperphosphatemia

• Low or low normal serum calcium

• Dehydration

• Nonregenerative anemia

• Inverted Ca:P ratio (should be 1.2-2:1)

• Increased PTH

• Iohexol clearance study for GFR estimate

Other Methods • Renal biopsy

• Ultrasound

• Radiograph

RSHP Treatment

• Usually present in end-stage renal failure• Reduce dietary protein• Correct hyperphosphatemia • Correct hypocalcemia• Correct and prevent dehydration• Supplement vitamin B complex• Omega-6: Omega-3 = 3:1 • Correct husbandry

The End

References

• Robbins and Cotran Pathologic Basis of Disease 7th ed

• Nelson and Couto Small Animal Internal Medicine 3rd ed

• Mader Reptile Medicine and Surgery

• Hernandez-Divers, Stephen et al “Renal evaluation in healthy green iguanas (Iguana iguana): assessment of plasma biochemistry, glomerular filtration rate, and endoscopic biopsy.” Journal of Zoo and Wildlife Medicine June 2005; 36(2):155-168