Conference 09 - 2009 Case: 02 20091202

Signalment:

6-year-old male green iguana (Iguana iguana).Presented to the UF VMC as an emergency. He had been profoundly and progressively lethargic for approximately 10 days. It was reported that the animal was anorexic and not drinking for at least 7 days. The animal was housed in an outdoor glass screened enclosure with access to sunlight. Diet consisted of a commercial iguana pelleted diet and lettuce. On physical exam, this iguana was cachectic, critically dehydrated (estimated 12% dehydrated) and unresponsive. The animal exhibited muscle stiffness and fasciculations. Therapy (fluids, calcium gluconate) was initiated, but this animal spontaneously died approximately 10 hours post-admission.

Gross Description:

The iguana is in poor nutritional condition, with depleted fat bodies. The kidneys are pale tan with numerous pin-point white foci visible beneath the capsule and on cut section within the parenchyma (Fig. 1). There is a thin, tan-white streak immediately subtending the epicardium of the ventricle (Fig 2). The aorta is expanded by white-yellow, hard, gritty plaques which elevate the intima. There are small fragments of wood in the stomach.

Histopathologic Description:

Multifocally, there is extensive mineralization of the basement membranes, mesangium and visceral and parietal epithelium of glomeruli. Diffusely, there is marked mineralization of the tubular basement membranes. Multifocally, the mineralized basement membranes are markedly expanded, and the tubular epithelium is also multifocally mineralized. Multifocally, there is mild to moderate degeneration and necrosis of tubular epithelial cells, and occasional tubular regeneration. Tubules sometimes contain small amounts of amorphous eosinophilic material and sloughed necrotic cellular debris, and occasionally, tubules are expanded by basophilic, radiating, acicular material (urate tophi). The epithelial cells surrounding the urate tophi are often partially effaced and/or appear necrotic. Tubules also occasionally contain amorphous, basophilic, mineralized debris. There is multifocal mineralization of arteries and veins which expands the intima, elevating the endothelium, and extensively extends into the media. There are often aggregates of mineralized material luminally within vessels; this material sometimes has a radiating appearance.

Multifocally and extensively, there is mineralization of cardiac myofibers. The mineralization is evident as a linear band in the outer myocardium and also as scattered myofibers throughout the myocardium. There is extensive mineralization of the vessels in the myocardium and epicardium. There are moderate amounts of fibrous connective tissue associated with the mineralized myofibers in the linear band region. Surrounding myofibers are frequently markedly vacuolated and occasionally necrotic and infiltrated with moderate numbers of macrophages.

Multifocally, there is often extensive mineralization of the gastric mucosa. The mineralization affects all levels of the mucosa including the luminal and crypt epithelium and the connective tissue of the lamina propria. In more severely mineralized foci, there is often fibrosis in the lamina propria and loss of glands. There is multifocal moderate mineralization of vessels in the submucosa and lamina propria. There is mild mineralization of the outer aspects of the tunica mucosa and the serosa.

Morphologic Diagnosis:

1. Metastatic mineralization, multifocal, chronic, moderate-severe, with:
    a. Glomerular, tubular, and vascular mineralization, chronic, multifocal, severe, kidneys.
    b. Myocardial and vascular mineralization, chronic, multifocal, marked, with mild myofiber degeneration and necrosis and mild myocardial fibrosis, heart.
    c. Gastric mucosal and vascular mineralization, chronic, multifocal, moderate, with fibrosis and loss of glands.
    d. Mineralization and fibrosis, chronic, multifocal, severe, aorta (not included in slide set).
2. Renal tubular urate tophus deposition, acute, multifocal, minimal to mild, with tubular necrosis.

Lab Results:

Calcium 9.8 mg/dL; Ionized calcium 0.68 mmol/L (normal 1.22-1.62 mmol/L); Blood pH 6.8; Phosphorus 30.4 mg/dL; CK 133,460 U/L; Uric acid 258 mg/dL; AST 788 U/L; GGT 125 U/L; Potassium 7.6 mEq/L; WBC 21,200/μL; Heterophils 17,384/μL with mild degranulation and +2 toxicity

Condition:

Metastatic calcification

Contributor Comment:

On microscopic examination, there was extensive mineralization of the tunica intima and tunica media of vessels in nearly all of the tissues examined as well as extensive mineralization of the renal tubules (predominantly basement membranes), glomeruli, myocardium, pulmonary interstitium, and other tissues. The mineralized tissue largely appeared relatively normal, consistent with metastatic mineralization.

Causes of metastatic soft tissue mineralization include hypervitaminosis D (due to excess supplementation or ingestion of vitamin D-containing rodenticides or plants [Cestrum spp. and Solanum spp.]) and elevated calcium and/or phosphorus (nutritional or renal secondary hyperparathyroidism, primary hyperparathyroidism, hypercalcemia of malignancy).(4) Specifically, animals are considered at risk for soft tissue mineralization when the calcium-phosphorus product is greater than 60-70.(4)

Green iguanas require UV-B light to convert provitamin D3 to active vitamin D3, and oral supplementation of vitamin D3 (instead of a UV-B source) has been determined to be ineffectual.(7) A study on iguanas at the National Zoological Park presenting for necropsy revealed widespread mineralization of soft tissues, especially of vessels and basement membranes, cardiac and skeletal muscle degeneration and necrosis with mineralization, and occasional mild fibrous osteodystrophy. In these animals, circulating levels of vitamin D3 were 7-36 ng/ml (normal is >400ng/ml).(7,8)

Possible causes for paradoxical soft tissue mineralization in these animals which were proposed were chronically low vitamin D levels leading to hypocalcemia, resulting in an exaggerated PTH response and excessive calcium mobilization from bone with resultant mineral deposition in soft tissues; or metabolic derangements altering the calcium-phosphorus ratio, which could result in an elevated calcium-phosphorus product which could predispose to mineralization.(7,8)

In this case, [Ca] = 9.8 mg/dL (normal 10.9-14.4), [P] = 30.4 mg/dL (normal 2.8-7.8), and [Ca] x [P] = 297.92. Therefore, in this case, we may have an explanation for mineralization based on elevated [Ca] x [P] product. Vitamin D3 levels were not determined. Given the sub-optimal husbandry reported by the clinician in this case, calcium and phosphorus abnormalities could have been nutritional and/or related to lack of adequate vitamin D3. Renal dysfunction was likely secondary to mineralization. While a few tophi were identified in renal tubules, these were acute lesions without associated inflammatory response, and likely developed terminally, possibly secondary to dehydration.

This animal did not have any evidence of fibrous osteodystrophy.

JPC Diagnosis:

1. Kidney: Glomerular, vascular, and tubular basement membrane mineralization, diffuse, with tubular epithelial necrosis, interstitial fibrosis and few gout tophi.
2. Stomach: Mucosal and vascular mineralization, multifocal, with epithelial degeneration and necrosis and interstitial fibrosis.
3. Heart, ventricle: Myocardial and vascular degeneration, necrosis, and mineralization, multifocally extensive, moderate, with fibrosis.

Conference Comment:

Conference participants reviewed several unique reptilian anatomical structures that were demonstrated in this case. Because lizards and snakes have a sexually dimorphic kidney, participants could surmise that this was a male iguana. Males possess a sexual segment of the nephron located between the distal segment and the collecting tubules, characterized by prominent hypereosinophilic intracytoplasmic granules. The secretory product that the sexual segment produces is incorporated into the seminal fluid.(2) With regard to the histologic anatomy of the heart, the ventricular myocardium consists of an outer stratum compactum and an inner stratum spongiosum. In most slides myocardial mineralization centered at the junction between the two muscular strata, and participants theorized the finding might be due to the abrupt change in myofiber orientation at this location, resulting in greater susceptibility to injury during contraction.

This case is an excellent example of metastatic mineralization of soft tissues, and the contributor provides a succinct review of the pathophysiology of the condition, including hypovitaminosis D in iguanas. Conference participants briefly reviewed the following causes of metastatic mineralization in animals:(1,3,5)
    1. Kidney failure → phosphate retention → renal secondary hyperparathyroidism
    2. Hypervitaminosis D (e.g. ingestion of calcinogenic plants [Solanum malacoxylon, Cestrum diurnum, Trisetum flavescens] or cholecalciferol-containing rodenticides) → secondary hyperparathyroidism
    3. Hyperparathyroidism (primary or secondary) or pseudohyperparathyroidism (due to PTH-related protein production associated with canine lymphoma or adenocarcinoma of the anal sac)
    4. Granulomatous disease (e.g. canine blastomycosis, bovine paratuberculosis)
    5. Osteolytic bone lesions (e.g. primary or metastatic neoplasia)

In general, tissues predisposed to metastatic mineralization include the gastric mucosa, renal tubules, alveolar septa, and subpleural intercostal connective tissue.(5,6) In cattle with Johnes disease, intimal mineralization may occur in the thoracic aorta.(3) In dogs with blastomycosis, mineralization is thought to be caused by 1,25-dihydroxycholecalciferal production by activated macrophages.(1)

References:

1. Ferguson DC, Hoenig M: Endocrine system. In: Duncan & Prasse's Veterinary Laboratory Medicine Clinical Pathology, eds. Latimer KS, Mahaffey EA and Prasse KW, 4th ed., pp. 278-280. Blackwell, Ames, IA, 2003
2. Jacobsen ER: Overview of reptile biology, anatomy, and histology. In: Infectious Diseases and Pathology of Reptiles, ed. Jacobsen ER, pp. 13-19, CRC Press, Boca Raton, FL, 2007
3. Maxie MG, Robinson WF: Cardiovascular system. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 3, p. 61. Elsevier Saunders, Philadelphia, PA, 2007
4. Morrow CK, Volmer PA: Hypercalcemia, hyperphosphatemia and soft tissue mineralization. Comp Cont Ed Pract Vet 24:380-387, 2002
5. Myers RK, McGavin MD: Cellular and tissue responses to injury. In: Pathologic Basis of Veterinary Disease, ed. McGavin MD, Zachary JF, 4th ed., pp. 48-49. Mosby Elsevier, St. Louis, MO, 2007
6. Newman SJ, Confer AW, Panciera RJ: Urinary system. In: Pathologic Basis of Veterinary Disease, ed. McGavin MD, Zachary JF, 4th ed., pp. 641-644. Mosby Elsevier, St. Louis, MO, 2007
7. Richman LK, Montali RJ, Allen ME, Oftedal, OT: Paradoxical pathologic changes in vitamin D deficient green iguanas (Iguana iguana). In:Proceedings of the American Association of Zoo Veterinarians, pp. 203-204. East Lansing, MI, 1995
8. Richman L, Montali R, Allen M, Oftedal, O: Widespread metastatic soft tissue mineralization in vitamin D deficient green iguanas (Iguana iguana). In: Proceedings of the Conference of the American College of Veterinary Pathology, p. 57. Atlanta, GA, 1995