Pulmonary Idiopathic Alveolar Ossification in a Raccoon (Procyon lotor)

Abstract

Here we describe gross and histopathologic findings in a laboratory-confined adult male raccoon (Procyon lotor) with microscopic ossified areas in pulmonary alveoli. At the time of necropsy, gross lesions were present in the kidneys and in one thyroid gland. Noteworthy microscopic findings included multifocal foci of osseous tissue within the alveoli of the lungs, bilateral thyroid adenomas, pancreatic islet cell amyloidosis, cortical kidney infarcts, cystic adenomatous hyperplasia of urinary bladder, and mineralizations (psommama bodies) of small blood vessels of meninges and choroid plexus. Pulmonary ossification in raccoons has not been reported previously. The other histopathologic lesions have been documented to occur as incidental findings in raccoons and do not appear to have any apparent association with the formation of osseous foci in the lungs of the animal described.

Pulmonary ossification is the presence of osseous tissue in alveolar spaces of lungs. It is a relatively rare condition in humans and is often asymptomatic. Several predisposing conditions are associated with pulmonary parenchymal calcification with or without ossification. These include hypercalcemia, hyperphosphatemia, alkalosis, and lung injury in the presence or absence of conditions that result in angiogenesis and increased pulmonary blood flow causing elevated vessel wall shear stress.³ In animals only 3 cases of this condition have been documented.^2,10,11 We describe here a case of idiopathic pulmonary microlithiasis in a captive raccoon (Procyon lotor).

The raccoon was obtained by live trapping in Pennsylvania and was used in a study to assess the effects of lead administration to raccoons.⁶ The subject, an adult (approximately 2 y) male, was part of a group of raccoons that received 4 mg/kg lead acetate PO daily for 8 wk (5 d each week) as described previously.⁶ At the end of the experiment, the raccoons were sedated with ketamine hydrochloride and euthanized with intravenous sodium pentobarbital, and underwent complete necropsy. The whole brain and representative samples of striated muscles (heart, diaphragm, tongue, masseter, psoas major, triceps, biceps femoris), lung, liver, kidney, skin, spleen, eye, mesenteric lymph node, palatine tonsil, salivary glands, rectal mucosa, and urinary bladder were immersion-fixed in 10% neutral buffered formalin for histopathology. The fixed brain was cut in serial coronal sections of 2 to 4 mm in width, and sections of medulla, cerebellum, superior colliculus, and rostral cerebrum were processed for routine histopathology, stained with hematoxylin and eosin, and examined under light microscopy. Selected sections of kidney underwent modified Ziehl–Neelsen staining to demonstrate the presence of lead inclusions and Warthin–Starry and Stiner silver staining to demonstrate leptospiral organisms.

At necropsy, noteworthy gross lesions consisted of unilateral enlargement of the thyroid gland and presence of multifocal pale, white foci on the cortical surfaces of both kidneys.

Microscopic lesions were seen in the lungs, pancreas, brain, kidneys, urinary bladder, and thyroid. Lung lesions consisted of multifocal foci of trabecular bone within the alveoli (Figure 1). These foci revealed Haversian system structures and the presence of normal osteocytes within the lacunae (Figure 2). Very often the osseous tissue foci were larger than the unaffected alveolar spaces and thus had distended the affected alveolar spaces, giving an appearance of emphysematous areas (Figure 1). Only a mild mononuclear inflammatory infiltrate was present around rare foci of osseous tissue. The thyroid lesion was diagnosed as follicular adenoma. In addition, a similar but much smaller adenoma was present in the contralateral gland. Both kidneys revealed cortical wedge-shaped areas of fibrosis with infiltration of some mononuclear cells. Stiner- and Warthin–Starry-stained kidney sections did not reveal leptospiral-like organisms. Modified Ziehl–Neelsen stained sections revealed many intranuclear and intracytoplasmic lead inclusions in the tubular epithelial cells. The urinary bladder showed several hyperplastic areas with cystic adenomatous hyperplasia of the mucosa. Islet cells in the pancreas revealed the presence of eosinophilic homogeneous material (amyloid); this material fluoresced apple-green when sections stained with Congo red were examined under polarized light. Several sections of brain revealed concentric mineralized foci (psammoma bodies) in small-caliber blood vessels in the choroid plexus and meninges.

Figure 1.

Lung of a raccoon with alveolar ossification. Note the distended alveolar spaces and absence of inflammatory cellular infiltrate around the foci of ossification. Hematoxylin and eosin stain; magnification, ×25.

Figure 2.

Lung of a raccoon with alveolar ossification. Higher magnification of section in Figure 1, showing trabecular bone with osteocytes in lacunae in the alveolar spaces. Hematoxylin and eosin stain; magnification, ×400.

Idiopathic pulmonary ossification is a rare condition that usually occurs in men 40 to 60 y of age.⁹ This condition has not been described in current veterinary textbooks, and we know of only 3 cases—1 of a slaughter-house specimen from a bovine¹⁰ and 2 in dogs^2,11—that have been described. Idiopathic pulmonary ossification has not been documented in wild animals. In humans, the pathogenesis of pulmonary ossification is unknown, and serum calcium and phosphorus levels in such cases have been within the normal range.³ Reported cases occurred in 9-y-old dogs,^2,11 which in addition to the alveolar lesions had foci of microlithiasis with a laminated appearance in the pulmonary alveolar septa, thus suggesting possible prior insult to the lung tissue. In the current case, the condition occurred in a wild-caught, laboratory-confined animal that received oral lead acetate for 8 wk. Only 1 of 8 raccoons given lead acetate developed alveolar lithiasis, and this condition had not previously been reported in animals experimentally or naturally exposed to lead, suggesting that the experimental treatment did not lead to the osseous tissue within the alveoli of lungs of this raccoon. The exact cause of pulmonary ossification in this raccoon remains undetermined, but the mature pulmonary osseous foci suggest that the condition developed over many months. Therefore, we speculate that the condition existed before the raccoon was captured and recruited to the experimental study.

Deposition of amyloid in pancreatic islet cells is a common finding in free-ranging raccoons and has been reported to occur in laboratory-confined raccoons.^1,5 Because pancreatic amyloidosis appears to be more prevalent in older raccoons,⁵ we were not surprised to find it in this animal.

The fibrotic cortical lesions in the kidneys of this raccoon were indicative of partially resolved areas of infarcts. The cystic hyperplasia in the uroepithelium of the bladder was suggestive of infection by Capillaria spp., which were present in the bladders of other raccoons in this group.⁶ Vascular mineralization (psammoma bodies) have previously been observed in many free-ranging and captive raccoons and are considered incidental findings of unknown etiology.⁵

Neoplastic lesions have been documented infrequently in raccoons. A retrospective survey of more than 400 raccoon necropsies⁴ revealed only 2 cases of neoplasia—an astocytoma of the brain and a fibroma of the skin. A review of medical records for neoplasms of raccoons¹² identified cases of skin fibroma, pancreatic adenoma, thyroid adenoma and adenocarcinoma, brain astrocytoma, nasal adenocarcinoma, and intestinal adenocarcinoma. Since then, other neoplasms, including lymphosarcoma, adrenal gland adenomas, papillomas on the legs, and transitional cell tumors in the bladder, have been documented to occur in raccoons.⁵ With the exception of thyroid adenomas⁸ and adrenal adenomas,⁷ the reported cases of neoplasia have been individual case reports of neoplasms.

The raccoon described here had ossified foci in the alveolar tissue and other incidental lesions, including bilateral thyroid adenomas, pancreatic amyloidosis, renal infarcts, hyperplasia of bladder uroepithelium, and cerebrovascular mineralization. Because all of these conditions have occurred previously in raccoons in the absence of pulmonary ossification, an association between these lesions and pulmonary ossification is unlikely. In humans, pulmonary ossification is usually idiopathic but can be associated with conditions such as hypercalcemia, hyperphosphatemia, alkalosis, and lung injury in the presence or absence of conditions that result in angiogenesis and increased pulmonary blood flow, causing increased vessel wall shear stress.³ This raccoon had no lung injury or abnormal pulmonary blood supply, but blood calcium, phosphate, and pH levels were not determined, thus preventing us from ruling out potential metabolic conditions.