JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-T03

 

Signalment (JPC #2019268): A mature, female New Zealand white rabbit

HISTORY: This rabbit became depressed and emaciated.

HISTOPATHOLOGIC DESCRIPTION: Bone: Diffusely thickening the epiphyseal and metaphyseal cortical bone as well as the few remaining medullary trabeculae are variably thick bands of lamellar bone (osteosclerosis) lined by wide seams of deeply basophilic matrix with numerous embedded osteoblasts. Layers of lamellar bone are separated by prominent, irregular, basophilic lines (cementing lines) that are frequently smoothly contoured (resting lines) or rarely are scalloped (reversal lines). There is a generalized absence of osteoclastic activity. The articular cartilage is multifocally eosinophilic (loss of proteoglycans) and is either thin (erosion) and irregular with occasional irregularly clustered chondrocytes (chondrones) or thickened. Focally, the thickened cartilage contains variably sized clefts. There are multifocal areas along the margin of the bone where the periosteum and adjacent bone is thickened, undulant, irregular, and punctuated by remodeling and areas of basophilic matrix.

MORPHOLOGIC DIAGNOSIS: 1. Bone: Osteosclerosis, diffuse, moderate, with osteoblast hyperplasia and abundant basophilic osteoid matrix, New Zealand White rabbit, lagomorph.

2. Articular cartilage: Degeneration, diffuse, moderate, with multifocal erosions and clefts.

CONDITION: Hypervitaminosis D

GENERAL DISCUSSION:

• In rabbits, intestinal uptake of calcium is not influenced by Vitamin D and is proportional to their diet; rabbits rely on renal excretion to regulate serum calcium levels
  • Hypercalcemia in rabbits leads to hypercalcemia arteriosclerosis
• In general, Vitamin D functions to maintain serum calcium via 3 mechanisms:
  • Increasing intestinal absorption of calcium (and phosphorus);
  • Enhancing parathyroid hormone (PTH)-dependent renal tubular resorption of calcium (and excretion of phosphorus);
  • Enhancing parathyroid hormone (PTH)-dependent mobilization of calcium (and phosphorus) from bone
• Vitamin D is also required for mineralizing osteoid to form bone
• Normal sources of vitamin D include:
  • Provitamin D2 (ergosterol), from plants > converted to vitamin D2 (ergocalciferol) by exposure to UV light
  • Provitamin D3 (7-dehydrocholesterol) from animal tissues > converted to vitamin D3 (cholecalciferol) in skin by exposure to UV light
  • Vitamin D3 is hydroxylated in the liver > forms 25-hydroxyvitamin D > renal hydroxylation in the proximal tubular epithelium via 1-alpha-hydroxylase > forms 1,25-dihydroxyvitamin D (active form of vitamin D and the toxic principle)
• Sources of intoxication:
  • Acute intoxication; most common in dogs and cats:
    • Cholecalciferol rodenticide intoxication
  • Chronic intoxication; more common in rabbits; cattle, horses and sheep:
    • Iatrogenic: Excessive dietary supplementation
    • Ingestion of plants containing the toxic principle 1,25-dihydroxycholecalciferol-glycoside (most active form of vitamin D3): Solanum glaucophyllum (formerly malacoxylon), Cestrum diurnum, and Trisetum flavescens, and alfalfa
  • Excess vitamin D is stored mainly in muscle and fat and is released as fat is metabolized
  • Vitamin D3 is considered more toxic than vitamin D2
    • D2 = ergocalciferol which is regarded as the plant form of vitamin D
  • Lesions in poisoned animals are indistinguishable from those of excess vitamin D

PATHOGENESIS:

• Soft tissue changes: Most common in the acute toxicosis
  • Increased vitamin D2, D3, or 1, 25- dihydroxycholecalciferol-glycoside > increased osteoclastic activity in bone; decreased urinary excretion and increased intestinal absorption of calcium > hypercalcemia and hyperphosphatemia > metastatic mineralization (kidneys, gastric mucosa, lungs, endocardium, arterial walls)
  • Tissues with an alkaline compartment are predisposed to mineralization (intima and media of arteries, endocardium, myocardium, gastric mucosa, lung and kidney)
  • If Ca x P > 70 (mg/dL), metastatic mineralization may occur (though it is dependent on other factors as well)
  • Death usually occurs due to renal failure before changes in the skeleton
• Bone changes: Most common in chronic toxicosis
  • Sclerosis or rarefaction of bone depending on dietary calcium and pattern of exposure
  • Early response is widespread, intense osteoclastic activity – removes most of primary spongiosa
  • With continued administration: Abnormal matrices produced by osteoblasts continue to accumulate and obliterate the cancellous spaces, forming a mosaic of basophilic matrix, eosinophilic matrix, and newly formed, coarse, woven bone separated by resting lines
  • Intermittent administration causes surges of osteosynthetic activity with abnormal matrix deposition, maturation, and mineralization, which produce multiple layers separated by broad, basophilic resting lines

TYPICAL CLINICAL FINDINGS:

• Depression, loss of appetite, dehydration
• Cessation of growth or weight loss, progressive emaciation and weakness
• Impaired mobility
• Polydipsia, polyuria, and reduced urine specific gravity
• Serum chemistry: Hypercalcemia, hyperphosphatemia early; return to normal with chronicity

TYPICAL GROSS FINDINGS:

• Acute toxicosis:
  • Gastric and small intestinal hemorrhage, areas of myocardial discoloration
  • Kidneys, other organs: Multifocal tan to gray mineralized areas in the cortices
  • Heart, endocardium and arteries (generally aorta and large arteries): Raised mineralized irregularities on both intimal and adventitial surfaces
• Chronic toxicosis:
  • Bone (long bones especially): Epiphyses and growth plates are normal; thickened, sclerotic metaphyses; loss of metaphyseal spongiosa extending into diaphysis; sparse marrow, replaced by dilated veins and loose connective tissue

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Pathognomonic bone lesions:
  • Acute toxicosis: Severe osteopenia with abundant osteoclasts in Howship’s lacunae
  • Chronic toxicosis: Osteoblastic production of abundant fibrillar, highly basophilic matrix that irregularly coats the surface of trabeculae, endosteum, and the periosteal cortex
    • Tangled fibrillary arrangement, somewhat mucoid, floccular and intensely basophilic
    • Lots of osteoblasts
  • Maturation of matrix is initially local, irregular in distribution and unrelated to normal patterns of osteogenesis
  • Necrosis of osteocytes occurs with groups of empty lacunae in cortical bone and in center of trabeculae
  • Rarefaction or osteosclerosis of bone depending on level of dietary calcium and exposure to excess vitamin D
  • Soft tissue mineralization, especially fibroelastic tissues and basement membranes of renal tubules and glomerular capsule, tunica intima and media of aorta and large vessels
  • Kidney: Mineralization of tubular and glomerular basement membranes

DIFFERENTIAL DIAGNOSIS:

Other toxic osteodystrophies:

• Vitamin A: Excessive retinoids found in plants (e.g. sweet potatoes) and meat (liver) or drug toxicity; causes multifocal premature closures of growth plates with growth deformities, osteoporosis, and pathologic fractures; retinoids cause degeneration and necrosis of chondrocytes and osteoblasts in growing animals; stimulate osteoblasts in adult animals
• Lead: Ingestion of lead-based paints; causes characteristic “lead line”, an osteosclerotic band of bone parallel to the growth plate; acid-fast lead inclusions within the osteoclasts; lead is toxic to osteoclasts and bone mineral containing lead is somewhat resistant to osteoclasis
• Fluoride: Ingestion of rock phosphates naturally high in fluoride; grazing pastures with high fluoride (nature or contamination); causes osteosclerosis at chronic low toxic levels; chronic higher levels cause osteopenia and periosteal bone formation; low levels of fluoride are stimulatory to osteoblasts; the mechanism of higher levels of chronic fluoride are unclear; may be a direct toxic injury to osteoblasts
• Spontaneous atherosclerosis of New Zealand white rabbits; inherited disorder of aortic calcification; lesions seen in rabbits as young as 6 weeks of age

COMPARATIVE PATHOLOGY:

Hypervitaminosis D in other species:

• Most species: Iatrogenic overdoses, accidental dietary over-supplementation, or cholecalciferol rodenticide exposure can cause significant disease
• Grazing animals:
  • Vitamin D glycoside-containing plant toxicity is the most frequent cause: Solanum glaucophyllum (formerly malacoxylon), Cestrum diurnum, and Trisetum flavescens
  • "Hyena disease" in calves: Has been associated with Vitamin A toxicity; experimentally induced by injecting newborn calves with a combination of high doses of vitamins A and D on day 1 after birth, and daily oral feeding of exceedingly high doses of Vitamin A mixed with milk substitute for up to 8 weeks; "hyena disease" results from relative underdevelopment of the caudal body structures due to premature growth-plate closure in long bones of the pelvic limbs and lumbosacral vertebrae and, to a lesser extent, the pectoral limbs; the role of Vitamin D is not clear, but is thought to be a factor
• Carnivores:
  • Cats appear to be more sensitive to vitamin D toxicity than dogs
  • Cats: Chronic vitamin D toxicity and FeLV infection can cause osteopetrosis

Other causes of vessel mineralization:

• Cattle: Chronic infection by Mycobacterium avium paratuberculosis (Johne’s disease) can lead to mineralization of vessels and heart

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley Blackwell; 2016:313-315.
2. Rosol T, Grone, A. Endocrine glands. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier Limited; 2016:301-302.
3. Stockham SL, Scott MA. Fundamentals of Veterinary Clinical Pathology. 2nd ed. Ames, IA: Blackwell publishing; 2008:596, 600-601.
4. Sula MM, Lane LV. The urinary system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:737.
5. Thompson K: Bones and joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier Limited,; 2016: 89-90.
6. Olson EJ, Dykstra JA, Armstrong AR, Carlson CS. Bones, joints, tendons, and ligaments. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1068.

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