9-year-old, spayed female Persian cat, Felis cats.The cat developed a soft dermal nodule in the right thoracic region; the overlying skin was unremarkable. The cat was brought to the referring veterinarian due to rapid (2-week) enlargement of the nodule with ulceration (Fig. 1) 8 months after initial presentation. Cytology of the material was characterized by abundant necrotic debris, foamy reactive macrophages, degenerated neutrophils and occasional fragments of fungal hyphae. The nodule was surgically excised and processed for histology and electron microscopy. The histopathology revealed a pyogranulomatos inflammation centered on PAS positive fungal hyphae. After 3 years, the cat is still free of disease without any additional therapy. An electron micrograph at 12,000x magnification is provided.
Nodular lesion in dorso-lateral thorax with draining tracts oozing sero-sanguineous material
Macrophagic (granulomatous) dermatitis with fungal hyphae and Splendore-Hoeppli formation consistent with pseudomycetoma.
No fungal isolation was attempted by the referring veterinarian. CBC and urine analysis were unremarkable.
Electron Microscopy Description: Dermis, 12,000 magnification. On the electron micrograph, there are longitudinal and transverse sections of fungal hyphae within the cytoplasm of a reactive macrophage. On the right-hand top corner, two nuclei with slightly irregularly infolded nuclear envelopes, a prominent nucleolus, randomly scattered dispersed chromatin and multifocal aggregates of marginated chromatin are evident. The cytoplasm is abundant and contains mitochondria, stacks of rough endoplasmic reticulum and free ribosomes (binucleated macrophage).
The fungal hyphae are septate and characterized by a thick homogeneously moderately electrondense fibrillar cell wall with an underlying thin, highly electron-dense plasma membrane enclosing moderate amount of granular, moderately electron-dense cytoplasm. The latter contains few elongated to round mitochondria, moderate numbers of round single-membrane bound electron lucent vacuoles, scattered ribosomes and rare myelin figures. Two nuclei with central to paracentral prominent nucleoli and a nuclear envelope are present. Fungal structures are surrounded by a variably thick, moderately to highly electron-dense, fibrillar material (accumulation of immunoglobulins Splendore-Hoeppli reaction), multifocally radiating from the hyphal surface.
Fungal organisms are eukaryotes with structurally defined cell, cytoplasm and nucleus. According to ultrastructural descriptions, most fungal cells contain free ribosomes, mitochondria, vesicles involved in endocytosis and exocytosis and lipid. The fungal vacuole is similar to the plant vacuole has enzymatic functions. Nuclei are smaller than those of vertebrate cells with a defined nuclear membrane and a nucleolus. DNA is largely located in the nucleolus.(6)
Specifically, electron microscopy of M. can is has been well described.(17) M. canis hyphae have a cell wall consisting of an outer thin, electron-dense layer and an inner and broader fibrillar electron-lucid layer. Septa are produced from the inner and broader fibrillar electron-lucid layer and are characterized by septal pores. The plasma membrane is a thin, electron-dense delimiting membrane contiguous with the inner surface of the cell wall. In the cytoplasm, a large central vacuole surrounded by an electron-dense tonoplast enclosing electron-lucent flocculent material is visible. Mitochondria having no polarity are scattered throughout the cytoplasm of the hyphae and can be filamentous or spherical. Mitochondria have double membrane and extensive cristae that might extend across the organelles. Hyphae have sparse narrow tubular endoplasmic reticulum and a large tonoplast. Glycogen is abundant and often clumped and lipid inclusions are also present. Single membrane bound vesicles with central bodies are also present at the margins of the hyphae. Hyphae are often bi- to multinucleated (as in the image provided). The nuclear membrane has a double envelope with intervening nuclear pores. Additional features are the presence of single membrane bound vesicles with central bodies with high electron opacity.
Clinical, gross and microscopic findings were representative of a deep dermatophyte infection consistent with feline dermatophytic pseudomycetoma. The disease associated with Microsporum canis has been described also in dogs,(1,8) horses(12) and humans.(2,7,10,14) Dermatophytic pseudomycetomas are uncommon to rare, deep cutaneous to subcutaneous fungal infections that produce tissue grains or granules. These tissue grains are composed of fungal aggregates embedded in amorphous eosinophilic material representing antigenantibody complexes (SplendoreHoeppli reaction).
Feline pseudomycetomas have been reported mostly in Persian cats.(3,4,5,8,9,11,15) Sporadic cases have been reported in Himalayan, Domestic Shorthair, and in Maine Coon cats.(8,11,13,16) Genetic predisposition has been hypothesized to play a role in the development of these lesions since the Persian breed seems predisposed also to conventional dermatophytosis.(8) Age and gender predilections have not been observed.
The frequent localization of the lesions in the dorsal trunk, most commonly in outdoor cats, suggests a traumatic implantation of organisms from hair follicles with dermatophytic colonization by biting or fighting.(8) Some authors hypothesize that mycelial elements reach the dermis from spontaneously ruptured hair follicles in association with dermatophyte infection. Once in the dermis, fungi aggregate and induce a granulomatous reaction. Positive dermatophyte cultures from normal-appearing areas distant from the dermatophytic pseudomycetoma indicate that affected cats may previously have been inapparent carriers. Inapparent carriage of Microsporum canis is common in Persian and Himalayan cats.(5,8)
Grossly, dermatophytic pseudomycetomas are characterized by one or more subcutaneous nodules that occur most commonly over the dorsal trunk or tail base.(5,8,9,11) Lesions are firm, irregularly shaped nodules that gradually enlarge and coalesce in the dermis or the underlying subcutaneous tissue. Lesions may fistulate and discharge a seropurulent to necrotic material. Systemic clinical signs are uncommon, but lymphadenomegaly may be present. Intra-abdominal dermatophytic granulomatous peritonitis sharing many features with pseudomycetoma has been reported in Persian cats.(3,15,18)
Although dermatophytic pseudomycetoma fulfils most of the criteria for true mycetomas (nodular inflammation with fibrosis, fistulae draining from deep tissue, presence of tissue grains), fewer hyphal elements are present, and the lesions apparently lack the cement substance that holds true mycetoma grains together.(8)
Microscopically, lesions are located mostly in the dermis where aggregates of grey, refractile and highly pleomorphic fungal hyphae are characteristically found. These are tangled and delicate, and contain numerous large, clear, bulbous, thick-walled dilatations, resembling spores. Smaller swellings within the hyphae create a vacuolated or bubbly appearance to these structures. The fungal aggregates are imbedded in amorphous eosinophilic material to form large tissue grains, or granules that are also visible grossly. SplendoreHoeppli reaction is brightly eosinophilic and locates around the periphery of organized aggregates of organisms. Granules are cuffed by and intermingled with large macrophages, giant cells, and variable, sometimes numerous neutrophils. Macrophages have abundant, granular cytoplasm. In some cases, fragments of hyphae are present within individual macrophages beyond the boundaries of tissue grains or granules. Reactive fibroblasts and collagen may surround or dissect the lesions often creating lobules composed of multiple granules and their attendant inflammation.(8,11)
Cytology has proven useful in the diagnosis of pseudomycetoma.(9,13,19) However, histopathology and fungal culture of biopsy specimens are required for a definitive and specific diagnosis. Organisms can be stained with periodic acid-Schiff, Gomori methenamine silver, Grocott stains and Fonata-Masson.(8,10) Microsporum canis has been isolated in typical cases of dermatophytic pseudomycetomas where fungal culture has been performed.(8) PCR from paraffin embedded tissues is also useful for M. canis identification and has been utilized in cases of feline, canine and equine lesions.(12) Immunohistochemistry using rabbit anti-Microsporum canis antiserum identified this agent also in two canine cases.(1)
Dermatophyte pseudomycetomas are considered difficult to manage clinically and the prognosis is considered poor in cats.(3,4,8) The lesions often recur after surgical excision alone(5) although in this case no recurrence was observed. There are contrasting reports regarding poor(4) or successful response of feline pseudomycetomas following terbinafine treatment.(13) Little response to griseofulvin, ketoconazole or itraconazole has also been reported.(5,13,18) However, a combination of surgical excision with adjunctive long term medical therapy has recently been reported to be successful.(5,18)
Clinical differential diagnoses should include cryptococcosis and other systemic mycoses, sporotrichosis, cutaneous infections of other opportunistic fungi, and neoplasia. The marked breed predilection for Persian cats is helpful in increasing the index of suspicion for dermatophytic pseudomycetoma. Histologically, most of the systemic and opportunistic fungi affecting cats and dogs are smaller and more uniform in appearance and do not form granules or grains in tissue.(8) Trichophyton mentagrophytes has been reported to cause dermatophytic granulomatous inflammation in cats but the lesions do not have the typical histologic features of dermatophytic pseudomycetoma, and are characterized by a more diffuse tissue reaction with associated heavy colonization of the keratin of hair follicles and epidermis.
Haired skin: Dermatitis and panniculitis, granulomatous, focally extensive, severe, with Splendore-Hoeppli and numerous fungal hyphae.
The presentation of this case provides a challenging perspective on an otherwise routine histopathologic diagnosis. Provided only the EM image, many participants were able to recognize a fungal hyphae within the cytoplasm of a macrophage; however, when given the accompanied glass slide during the conference, the characteristic granules of a pseudomycetoma due to Microsporum can is infection were readily identified.
The contributor provides an eloquent discussion on this entity, highlighting the characteristic ultrastructural, histopathologic and gross findings while adeptly discussing clinical presentation, management and appropriate differentials worthy of consideration.
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