JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1596989): Adult female domestic shorthair cat.
Slide A: Haired skin and subcutis: Diffusely expanding the deep dermis and subcutis are multiple coalescing nodules of pyogranulomatous inflammation, composed of numerous viable and degenerate neutrophils, epithelioid macrophages, scattered multinucleated giant cells, and rare lymphocytes and plasma cells admixed with moderate numbers of reactive fibroblasts (fibrosis), eosinophilic cellular and karyorrhectic debris, fibrin, hemorrhage, and edema. Scattered throughout the nodules are many golden-brown pigmented fungal yeast and hyphae. Yeast are ovoid, 7 to 20 um in diameter, with 2-3 um-thick dark brown cell walls, clear to pale brown cytoplasm with a central basophilic nucleus. Hyphae are 5-10 um wide, septate with irregular, dichotomous and non-dichotomous, acute angle to right angle branching and thin, pigmented, nonparallel walls. Multifocally there are intact and degenerate fungal elements present within the cytoplasm of macrophages and multi-nucleated giant cells.
Slide B (Periodic acid-Schiff stain): Haired skin and subcutis: The fungal elements are strongly PAS positive.
MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Dermatitis and panniculitis, pyogranulomatous, diffuse, severe, with many dematiaceous yeast and few hyphae, domestic shorthair, feline.
ETIOLOGIC DIAGNOSIS: Cutaneous phaeohyphomycosis
CAUSE: Pigmented (dematiaceous) fungi
- Heterogenous group of opportunistic mycotic infections caused by pigmented (dematiacious) fungi (hyphae and/or yeast) of many genera, including Alternaria, Bipolaris, Cladosporium, Curvularia, Exophiala, and Ochroconis
- Worldwide in distribution and widespread in the soil, on wood, and in vegetation
- Uncommon opportunistic infection which usually affects the skin and/or subcutis, but central nervous system and disseminated infections may also occur (often associated with immune compromise)
- May pose a public health risk from exposure during necropsy or when handling the organism
- Infection occurs primarily by traumatic implantation or wound contamination
- Disseminated and central nervous system lesions thought to be from hematogenous dissemination of inhaled spores; relatively neurotropic
- Melanin synthesis in cell walls thought to be the main virulence factor
- Contributes to ability to elude host immune response and protect the organism from oxidative burst
- Intact granulocyte function is important in host defense against these fungi
- May be associated with immunosuppression
TYPICAL CLINICAL FINDINGS:
- Slow-growing, firm to fluctuant, dermal and subcutaneous nodules or plaques that coalesce, enlarge, ulcerate and form fistulous tracts
- In cats, lesions are primarily on the face and distal extremities and disseminated infections are rare
- Often non-painful and non-pruritic
- Disseminated infections often have a neutrophilic pleocytosis in cerebrospinal fluid
TYPICAL GROSS FINDINGS:
- Dermal or subcutaneous nodules (single or multiple) that are ulcerated or have fistulous tracts, and may form abscesses or thick-walled cysts
- Disseminated disease causes multifocal granuloma-like masses
- Lesion may be blue-gray or dark in color due to pigmented fungi – may be mistaken grossly for melanoma
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Diffuse or nodular granulomatous, pyogranulomatous, or lymphocyte-rich granulomatous dermatitis and panniculitis; diagnosis confirmed by presence of pigmented hyphae within tissue
- Fungi grow in tissue as yeast, hyphae; septate hyphae, irregular width, branched and unbranched
- Curvularia produce characteristic C-shaped scrolls in tissue section
- While fungi always form pigment on culture, this may not be apparent in tissue sections (i.e. with Alternaria) and the special stains (Fontana-Masson) are necessary for differentiation
- Hyphae are septate, irregular in width (2-8 um), branched or unbranched with non-parallel pigmented walls and random dilated segments up to 25 um in diameter, resembling chlamydospores
ADDITIONAL DIAGNOSTIC TESTS:
- Stain positive with Gomori methenamine silver (GMS) or periodic acid Schiff (PAS) (Note: fungal stains mask the natural color)
- Fontana-Masson staining method highlights melanin pigment that might not be evident on H&E sections, thereby differentiating it from nonpigmented fungi
- Culture necessary for a definitive diagnosis
- Dematiaceous eumycotic “black grained” mycetomas have macroscopic tissue grains composed of entangled mass of pigmented hyphae embedded in cement-like material; may be surrounded by Splendore-Hoeppli material; forms discrete extracellular organized granules whereas phaeohomycosis is in small aggregates and individual hyphae throughout the lesion that can be intracellular within macrophages or multinucleate giant cells
- Chromoblastomycosis-causing fungi are pigmented, but don't form hyphae in tissue and their division by septation is distinctive; form large (4-15 um), round, thick and dark-walled yeast-like cells (sclerotic bodies, muriform cells)
- Emerging disease in domestic and wild animals and invertebrates; More common in cats than other domestic mammals, but also occurs in horses (multiple lesions at different locations), cattle, goats, dogs (multiple extensive lesions, German shepherds may be predisposed), as well as fish, reptiles, amphibians, and birds (and humans)
- Ochroconis (Dactylaria) gallopava causes cerebral phaeohyphomycosis in chickens and turkeys
- Scolecobasidium causes phaeohyphomycosis in poikilotherms
- Oidiodendron causes phaeohyphomycosis in green iguanas
- Exophiala causes phaeohyphomycosis in Aldabra and Galapagos tortoises and leafy and weedy seadragons
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