December 2009

Case A

open slide in the AHA AAPSP Digital Slide Server


Cutaneous nodule from a horse

Example histopathological description

This section of haired skin (epidermis, dermis and subcutis is 13mm long and extends 20mm, through the epidermis and dermis, to the subcutis; one lateral margin is surgical, while the other, sharply linear and vertical to the skin surface, is due to post-surgical tissue trimming.

Expanding the deep dermis is a 14.5mm-diameter, moderately-circumscribed, non-encapsulated, multilobular, cellular mass (nodule), which causes protrusion of the overlying intact epithelium and dermis and compression and protrusion of the underlying panniculus carnosus (skeletal muscle) within the subcutis. The tumour cells of the nodule are arranged in sharply-circumscribed, confluent sheets and peripheral cords dissected by bundles of dermal collagen.  The cells are polygonal to oval with distinct cell borders, a moderate amount of faintly eosinophilic cytoplasm obscured by large numbers of blue-grey granules, and with a central nucleus, which is round to oval with coarsely stippled to vesicular chromatin and a single, often inconspicuous, nucleolus (well-differentiated mast cells). There is mild anisocytosis. Mitotic figures are not detected. All areas of the nodular mass are heavily infiltrated by eosinophils. There are large irregular necrotic areas (about 35% of the nodular mass) comprising a coagulum of degenerate eosinophils and cell debris, bordered in some areas by a palisade of macrophages, fibrosis and rare foci of lymphocytes. Overlying the tumour nodule and extending to the superficial dermis (1.6mm deep and containing normal hair follicles and adnexa), is a band of dermis (2 -3 mm deep) free of neoplastic cells but containing small numbers of eosinophils and mild perivascular infiltrates of lymphocytes.

Within the attenuated panniculus carnosus underlying the nodule there are small numbers of eosinophils, occasional individual mast cells and segmental contracture and disruption of the deepest skeletal muscle fibres (possibly sampling artifact), but no evidence of neoplastic mast cells in the underlying subcuticular fascia. Between the main cellular mass and the single lateral surgical margin, there is a 2mm wide zone of collagenous dermis and panniculus carnosus, and within 0.2mm of this surgical margin there are two discrete dermal nests of tumour cells.


Cutaneous mast cell tumour (well differentiated).

Comment on the significance of the lesions

In this case, although the main nodular tumour mass appears to have been excised (the tumour nodule in the specimen appears to have been bisected by post-surgical tissue trimming), the presence of two small nests of mast cells close to the lateral surgical margin, suggests that excision may not be complete. The excision site should be closely monitored for evidence of possible tumour recurrence.

Cutaneous mast cell tumours in horses commonly develop as solitary nodules on the head, trunk, neck and limbs. Multiple congenital tumours may regress spontaneously. Excision of solitary nodular tumours should be curative. Cutaneous mast cell tumours in the horse are not reported to metastasise.

Case B

open slide in the AHA AAPSP Digital Slide Server


Sulphur-crested cockatoo with feather loss and beak deformity.

Example histopathological description

There are three sections of skin; two large and one small. The second largest and the small section, contain transverse sections of five feather follicles (0.3 to 2 mm diameter). Within the follicular epidermis (1-6 cells deep) surrounding the developing feather, there are scattered necrotic epidermal cells and, in the basal areas, small numbers of macrophages with numerous intracytoplasmic, basophilic, botryoid inclusion bodies. There is moderate hyperkeratosis of the feather sheaths.  Within the epidermis of the developing feathers, the barbule epithelial cells surrounding the pulp include variable areas of hyperplasia and disorder with occasional necrosis; the hyperplastic barbule cells often contain a large vesicular nucleus with a prominent nucleolus. In the largest follicle, there are several large, naked nuclei with marginated chromatin, that are distended with amphophilic material (equivocal intranuclear viral inclusion bodies). This disordered epidermis of the developing feather contains macrophages with prominent intracytoplasmic, basophilic, botryoid inclusion bodies (as in the surrounding follicular epidermis), and these cells extend, often in large numbers, into the pulp which also contains many prominent small vessels and small to moderate numbers of small macrophages, lymphocytes, and heterophils. The smallest follicle contains only a hyperkeratotic feather sheath surrounding concentric keratin lamellae with a central coagulum of degenerate inflammatory cells.

The largest skin section is predominantly non-feathered, with a segment containing three follicles, each distended by a mature calamus, and a partial follicle containing concentric keratin lamellae with a central focal aggregate of heterophils and degenerate cells and at least one macrophage distended with intracytoplasmic, botryoid, inclusion bodies. In all three skin sections there is a diffuse, moderate orthokeratotic hyperkeratosis.

Morphological diagnosis

Folliculitis and pulpitis; necrogranulomatous, chronic-active, multifocal, moderate, with feather epithelial necrosis and dysplasia, and intrahistiocytic, intracytoplasmic, basophilic, botryoid inclusion bodies (consistent with circoviral infection).

Aetiological diagnosis

Psittacine beak and feather disease (PBFD).


The following references cover avian skin anatomy and describe and illustrate the histological changes of PBFD.

Pass DA, Perry RA. The pathology of psittacine beak and feather disease. Aust Vet J 1984;61(3):69-74.

Pass DA. The pathology of the avian integument: a review. Avian Pathol 1989; 18:1-72.

Randall CJ, Reece RL. Color atlas of avian histopathology.  Mosby-Wolfe 1996; pp44-45.