Granular Cell Tumor in an Endangered Puerto Rican Amazon Parrot (Amazona vittata)

C.F. Quist, Antonio Rivera, K.S. Latimer, S. Goldade, and F.J. Dein

Southeastern Cooperative Wildlife Disease Study (Quist) and Department of Pathology (Latimer), College of Veterinary Medicine, University of Georgia, Athens, Georgia 30602; U.S. Fish and Wildlife Service, Rio Grande Field Office, Rio Grande, Puerto Rico (Rivera, Goldade); National Wildlife Health Center, U.S. Geological Survey, Madison, Wisconsin 53711 (Dein).

Abstract. A subcutaneous granular cell tumor was surgically excised from the left wing of a Puerto Rican Amazon Parrot (Amazona vittata). The microscopic and ultrastructural features of the neoplasm are described. The pertinent literature on granular cell tumors also is reviewed.

Introduction

The Puerto Rican Amazon Parrot (Amazona vittata) (Figure 1) is endemic only to the island of Puerto Rico. When Columbus discovered the island in 1493, it was estimated that there were over one million parrots on the island. But by 1967, the Puerto Rican parrot was listed as an endangered species, and recovery efforts were begun in 1968. In 1972, the population was estimated at 16 individuals, and captive breeding efforts were begun by the U.S. Fish and Wildlife Service at the Luquillo Aviary located in the Caribbean National Forest. Prior to Hurricane Hugo, there were 53 parrots in captivity and an estimated 47 wild individuals. After Hugo, it was estimated that 1/2 of the wild population was lost. New techniques were developed to better manage the wild and captive populations. In 1993, 12 Puerto Rican Amazon Parrots were transferred to the Jose L. Vivaldi Aviary in the Rio Abajo state forest. This facility is managed by the Department of Natural and Environmental Resources. As of February 1998, there are approximately 40 Puerto Rican Amazon Parrots in the wild, and each aviary has approximately 50 parrots each. This makes the Puerto Rican Amazon Parrot one of the 10 most endangered birds in the world.

Fig. 1. Puerto Rican Amazon Parrots. Two Puerto Rican Amazon parrots appear in their natural habitat.

Case History

Annual physical examination on a 12-year-old male Puerto Rican Amazon Parrot (Amazona vittata) revealed a round, 3 x 3 cm diameter, firm, subcutaneous mass between the 6th and 7th primary flight feathers in the left metacarpal region. The mass was restricted to soft tissues between the feather follicles; radiographs revealed no bony involvement. No other major abnormalities were found. No treatment was attempted prior to excisional biopsy.

Prior to surgery, the bird was preoxygenated for 5 minutes. Anesthesia was induced with 3% isoflurane and 3 liters oxygen, and maintained with 2% isoflurane and 1 liter oxygen. The surgical area was prepped routinely with betadine scrub and alcohol. The mass was approached via a craniocaudal incision of the skin, extending from 0.5 cm caudal to the metacarpophalangeal area of the wing to the skin between the 6th and 7th feather follicles. The growth was dissected free of the surrounding soft tissues. The subcutaneous tissues were closed with a continuous suture pattern using 4-0 vicryl, and the skin was closed with a simple interrupted suture pattern using 4-0 vicryl. An acrylic bandage was applied to the wound. Recovery from anesthesia occurred in about 15 minutes. Postoperatively, the bird was held for seven days in the hospital. Recovery was uneventful.

The excised tissue was placed in 10% neutral buffered formalin and submitted to the Southeastern Cooperative Wildlife Disease Study for examination. Tissues were routinely processed and 4 um sections were initially stained with hematoxylin and eosin (H&E).

Histologically, sections of the mass revealed a well-demarcated and partially circumscribed nodule composed of sheets of large round to polygonal cells with abundant pale basophilic cytoplasm containing numerous faintly eosinophilic granules (Figure 2). Cell nuclei were round to pleomorphic, with coarsely clumped to vacuolated cytoplasm and occasional prominent nucleoli.

Fig. 2. Puerto Rican Amazon Parrot, granular cell tumor, H&E stain. Pleomorphic neoplastic cells with granular eosinophilic cytoplasm.

Occasionally, neoplastic cells had nucleolar vacuoles, irregularly indented nuclei, or were multinucleated. Anisokaryosis was common. The mitotic rate was very low; <1 per high power field. In some portions of the mass, sheets of neoplastic cells were very loose, giving the tumor a lacy appearance. Neoplastic cells were supported on a fine fibrous connective tissue stroma.

A variety of histochemical and intermediate filaments stains were applied to replicate sections. Gram and acid-fast stains were negative. Giemsa and periodic-acid Schiff (PAS) stains were positive, and PAS staining was resistant to diastase digestion (Figure 3). Luxol-fast blue was negative, and Grimelius stain was equivocal. Intermediate filament staining was positive for vimentin and desmin, and negative for S100 and cytokeratin.

Fig. 3. Puerto Rican amazon Parrot, granular cell tumor, PAS-diastase technique with hematoxylin counterstain. Prominent PAS staining is resistant to diastase treatment.

Several 2 mm³ samples from the formalin-fixed tissue were processed for electron microscopy. Ultrastructurally, formalin fixation-associated artifact was present. Cell nuclei were round to elongate to irregular (Figure 4). The nuclear membrane occasionally had irregular indentations and folds. The euchromatin was dispersed. Occasionally, nuclei contained an eccentrically-placed, prominent nucleolus. The cytoplasm contained innumerable, round to elongate, membrane bound structures (Fig. 5). Some of these were swollen or distorted (formalin fixation artifact) mitochondria (especially the elongate structures). Other round structures appeared granular internally and were bound by a single membrane, indicating tertiary lysosomes.

Fig. 4. Puerto Rican Amazon Parrot, granular cell tumor, electron micrograph. Irregular, folded nuclear membrane and numerous cytoplasmic granules.	Fig. 5. Puerto Rican Amazon Parrot, granular cell tumor, electron micrograph. Cytoplasm of neoplastic cell contains numerous mitochondria and tertiary lysosomes.

Myelin figures with membrane whorls were observed infrequently. The cytoplasm also contained scattered free ribosomes, a few dense bodies, and widely scattered intermediate filaments. No apparent basal lamina was adjacent to the plasma membrane. Neither desmosomes nor hemidesmosomes were observed.

The histologic and ultrastructural features were consistent with a diagnosis of a granular cell tumor.

Discussion

Granular cell tumors are rare in humans and domestic animals, but have been reported in a variety of species including humans, dogs, cats, rats, horses, and one bird. In humans ⁴ and dogs,⁵ granular cell tumors have been reported in a wide variety of sites. However, in dogs, most described tumors occur on the tongue. ⁵ Patnaik's study of dog tumors suggests a histiocytic origin, as is suspected in similar tongue tumors in humans. ³ In human tumors from other locations, positive staining for myelin and S100 seen in many cases has lead to the hypothesis that these tumors are of Schwann cell origin. ^2,4 In horses, the tumors are generally reported in the lungs where they are thought to be of Schwann cell origin. ¹ In rats, granular cell tumors are most common in the central nervous system and neural crest origin is favored. ⁶ In cats, tumors have been reported on the digits, tongue, and vulva;³ a myogenic origin is favored in one cat tumor. There has been one previous report of a granular cell tumor in a bird; in that report, a granular cell tumor was diagnosed in the periorbital region of a cockatiel. ³ Granular cell tumors in most species are considered benign lesions, though occasional malignant examples can be found in humans⁴ and animals. ³ Malignancy is usually inferred by cellular pleomorphism and increased mitotic activity.

The previously reported tumor in a cockatiel was a small (0.5 cm) nodule near the eye. That tumor was positive for L-anti-trypsin, muscle common antigen, actin, and desmin, and negative for vimentin, S100, nonspecific enolase and lysozyme. No ultrastructural features were reported. In contrast, the tumor in this parrot was strongly positive for vimentin, indicating mesenchymal origin. Here, the concurrent positive staining for desmin staining indicates muscle or myofibroblast origin. The negative staining for myelin contrasts with the human and equine granular cell tumors that are generally positive for myelin. The ultrastructural features are consistent with other descriptions of granular cell tumors. Most such tumors have indented or convoluted nuclei and cytoplasmic lysosomes that impart the granularity seen in light microscopy. ⁴

While the light and electron microscopic appearance of granular cell tumors in most species is very similar, histochemical and immunohistochemical staining varies from tumor to tumor and species to species, suggesting no uniform cell or tissue of origin. Most granular cell tumors are PAS-positive and diastase resistant, leaving this as one of the few consistent findings among the various tumors and species. The inconsistency of staining for myelin raises doubts as to whether all such tumors are of Schwann cell or neural origin. However, the consistently positive reaction to PAS does indicate the presence of a glycogen moiety, leaving open the possibility that the stained material is a myelin breakdown product such as sphingomyelin. As ultrastructurally, multiple lysosomes are the primary component of the classically described granules, it appears that all granular cell tumors are engaged in some type of cellular degradative process. It is plausible that, as a group, these tumors have a similar pathogenesis but lack a uniform histogenesis.

Acknowledgments

This work was conducted through the sponsorship from the fish and wildlife agencies of Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, Missouri, North Carolina, Puerto Rico, South Carolina, Tennessee, Virginia, and West Virginia. Funds were provided by the Federal Aid to Wildlife Restoration Act (50 Stat. 917) and through Grant Agreement 1445-GT09-96-0002, U.S. Geological Survey, U.S. Department of the Interior. Support was also received through Cooperative Agreement 98-9613-0032-CA, Veterinary Services, Animal and Plant Health Inspection Service, U.S. Department of Agriculture.

References

1. Kelley LC, Hill JE, Hafner S, Wortham KJ: Spontaneous equine pulmonary granular cell tumors: morphologic, histochemical, and immunohistochemical characterization. Vet Pathol 32:101-106, 1995.

2. Mittal KR, True LD: Origin of granules in granular cell tumor. Arch Pathol Lab Med 112:302-303, 1986.

3. Patnaik AK: Histologic and immunohistochemical studies of granular cell tumors in seven dogs, three cats, one horse, and one bird. Vet Pathol 30:176-185, 1993.

4. Troncoso P, Ordonez NG, Raymond AK, Mackay B: Malignant granular cell tumor: immunocytochemical and ultrastructural observations. Ultrastruct Pathol 12:137-144, 1988.

5. Van der Gaag I, Walvoort HC: Granular cell myoblastoma in the tongue of a dog: a case report. Vet Quart 5:89-93, 1983.

6. Wright JA, Goonetilleke URP, Waghe M, Stewart M, Carlile A: Comparison of a human granular cell tumour (myoblastoma) with granular cell tumours (meningiomas) of the rat meninges - an immunohistological and ultrastructural study. J Comp Pathol 103:191-198, 1990.

This Page Last Updated May 15, 1998

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