JPC SYSTEMIC PATHOLOGY
Signalment (JPC 1597220): Young alligator.
HISTORY: This alligator died unexpectedly. At necropsy, a necrotic skin lesion was present in the flank area in the region where the skin of the leg meets the abdominal skin.
HISTOPATHOLOGIC DESCRIPTION: Scaled skin: Focally there is ulceration with replacement by abundant eosinophilic cellular and karyorrhectic debris (necrosis), large numbers of macrophages, fewer heterophils and lymphocytes, and colonies of 2 x 4 um bacilli. The inflammatory infiltrate extends into the deep dermis. The adjacent epidermis is moderately hyperplastic and keratinocytes are shrunken and hypereosinophilic with pyknotic nuclei (necrotic), or are swollen and rounded with clear cytoplasm (hydropic degeneration). There is mild ortho- and parakeratotic hyperkeratosis.
Scaled skin, periodic acid-Schiff (PAS) stain: Within the epidermis and extending into the deep dermis, there are numerous PAS positive yeast and septate hyphae with parallel walls 3 to 5 um in diameter, acute angle, dichotomous branching and terminal bulbous swellings.
MORPHOLOGIC DIAGNOSIS: Scaled skin: Dermatitis, necrotizing, subacute, focally extensive, moderate, with ulceration, superficial bacilli and many yeast and hyphae, etiology consistent with Aspergillus spp., alligator, reptile.
ETIOLOGIC DIAGNOSIS: Cutaneous aspergillosis
CAUSE: Aspergillus spp.
- Primary route of infection is through inhalation of spores or through the gastrointestinal tract
- Aspergillus species contain numerous lipid compounds, including aflatoxins, gliotoxins, ochratoxins and complement activation inhibitors, which play a role in evasion of host defenses
- Proteinases play a role in the ability of the organism to invade tissues; hematogenous dissemination can occur
- Gastrointestinal infections are associated with damage to mucous membranes, viral erosive diseases, stress, and metabolic disturbances
- flavus and A. niger can also produce large quantities of oxalates in feed
- Protection is mediated by macrophages that phagocytize spores and by neutrophils that attach to hyphae
- Resistance or recovery from a systemic fungal infection requires strong T‑cell mediated immunity; humoral immunity is not protective
TYPICAL CLINICAL FINDINGS:
- Variable depending on species and route of infection
- Pulmonary and intestinal lesions; chronic rhinitis; vertebral osteomyelitis; discospondylitis; uveitis; CNS signs
TYPICAL GROSS FINDINGS:
- Multifocal to coalescing, necrohemorrhagic foci; epithelialized plaques on mucosa
- Multifocal to coalescing, raised, irregular, crusty plaques on skin
- Non-invasive pulmonary form is characterized by formation of aspergilloma, consisting of a tangled mycelial mass, usually located within a preformed pulmonary cavity; may be mistaken for a neoplasm
- Aborted fetus lesions in cattle resemble ichthyosis or extensive ringworm
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- In tissues, 3 to 8 um wide, regularly septate hyphae with thin parallel walls and dichotomous, acute angle branching
- Primary lesion in invasive aspergillosis is usually pulmonary, with hematogenous spread
- Multiple septate branching hyphae can radiate from the center of vessels
- Well-aerated necrotic cavities in lungs, air sacs of birds, pleura, or nasal mucosa may develop conidiophores (fruiting bodies); extensive deposition of birefringent calcium oxalate crystals may be present in these areas
- Characteristic conidia of Aspergillus may provide a definitive diagnosis but are rarely seen in tissue sections
- Conidiophore stalks are enlarged at the uppermost point to form a globose, hemispherical, flask-shaped vesicle; peg-like sterigmata form in a single layer (uniseriate) or two layers (biseriate) with the second row arising from the first; unbranched chains of conidia form from the distal ends
- Superficial infections mainly in the epidermis with infrequent invasion of the hair follicles and/or dermis
- Fibrinohemorrhagic granulomas extending through all layers of gastrointestinal tract
ADDITIONAL DIAGNOSTIC TESTS:
- Isolation by culture
- Immunohistochemical staining using monoclonal and polyclonal antibodies
- ELISA, AGDD
DIFFERENTIAL DIAGNOSIS (for fungal hyphae in tissue):
- Zygomycetes – hyphae (up to 15 um in diameter) are infrequently septate, have nonparallel walls, often appear collapsed or twisted; stain less uniformly
- Candida – produce hyphae, pseudohyphae and budding yeast in tissue
- Pseudallescheria boydii – tissue phase is similar; must culture to differentiate
- Cattle – rumenitis and abomasitis; gastroenteritis; mastitis; abortions; bronchopneumonia and hyperkeratotic dermatitis (fetal lesions)
- Horses – dissemination often results from primary intestinal lesion; guttural pouch and pulmonary mycosis; keratitis; abortions; endocarditis; persistent diarrhea in foals
- Dairy sheep – mastitis in ewes; respiratory, gastrointestinal, and systemic lesions in lambs
- Dogs – granulomatous rhinitis ( fumigatus), infection of kidneys and other viscera (A. terreus commonly involved, especially in German Shepherd Dogs)
- Cats – pulmonary and cutaneous infections
- Birds – respiratory, digestive, or CNS; chronic infections more common in older birds, but acute, often fatal infections more common in young birds
- Goat – pyogranulomatous rhinitis and dermatitis
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