JPC SYSTEMIC PATHOLOGY
Signalment (JPC# 1960546): Persian cat
HISTORY: This cat had a nodular skin mass.
HISTOPATHOLOGIC DESCRIPTION: Haired skin and subcutis: Expanding the deep dermis and subcutis, extending to the deep border, and elevating the overlying epidermis are multiple focally extensive pyogranulomas composed of numerous epithelioid macrophages, surrounded by fewer degenerate neutrophils, lymphocytes, plasma cells, occasional multinucleated giant cells, centered on necrotic debris with multiple embedded, occasionally fragmented aggregates of densely packed, 5-10 um wide, septate fungal hyphae with thick, nonparallel walls, rare irregular non-dichotomous branching, and up to 25 um diameter bulbous swellings. Hyphae are often surrounded by radiating, club-shaped amorphous eosinophilic material (Splendore-Hoeppli reaction). Surrounding and separating pyogranulomas are numerous reactive fibroblasts, small caliber blood vessels, and loose fibrous connective tissue admixed with mild hemorrhage and edema (granulation tissue), progressing to more mature fibrous connective tissue (fibrosis). Multifocally, infiltrating the superficial dermis are low numbers of periadnexal to perivascular lymphocytes, plasma cells and fewer macrophages. There is diffuse mild epidermal and multifocal follicular orthokeratotic hyperkeratosis.
MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Pyogranulomas (pseudomycetomas), multiple, with granulation tissue, fibrosis, and numerous fungal hyphae embedded in Splendore-Hoeppli material, Persian, feline.
ETIOLOGIC DIAGNOSIS: Dermal and subcuticular dermatophytosis
CAUSE: Microsporum canis
CONDITION: Feline dermatophytic pseudomycetoma
- Dermatophyte pseudomycetoma is an atypical rare form of dermatophytosis that involves the deep dermal and subcutaneous tissues and is typically caused by M. canis
- Feline dermatophytic pseudomycetomas occur almost exclusively in Persian cats, but may be seen in other longhaired breeds (Persian cats are prone to dermaophytes)
- There is one report of a cat with an intra-abdominal dermatophytic pseudomycetoma
- Hyphae within the deep dermis and subcutis cause the formation of mycetoma-like granulomas:
- These contain fewer hyphal filaments than true eumycotic granules and lack cementing substance with less Splendore-Hoeppli reaction
- Follicular dermatophyte colonization > traumatic implantation of colonized hairs into dermis and subcutis or spontaneous rupture > mycelial elements escape from hair follicle into surrounding deep dermis and subcutis > aggregate and induce an immune response (humoral and cell-mediated) > pyogranulomatous/granulomatous inflammation
- Possible specific genetic deficit in innate or adaptive immunity in Persians predisposing them to pseudomycetoma formation
TYPICAL CLINICAL FINDINGS:
- Longhaired cat (especially Persian) with one or more subcutaneous nodules
- Over dorsal tail or trunk
- May be hyperpigmented and/or ulcerated
- Alopecia if typical dermatophytosis is occurring
- Young cats <1yr: Classic circular foci of alopecia and crusting, usually non-pruritic
- Adult cats: Subtle lesions, patchy mild alopecia or broken hairs with little skin change
- Lesions may be hyperpigmented
- May resemble common ailments: Recurring chin folliculitis (feline acne), dermatitis of the dorsal tail (stud tail), or focal pruritic lesions (eosinophilic plaques)
- Rarely otitis externa, onychomycosis
TYPICAL GROSS FINDINGS:
- One or more subcutaneous nodules; sometimes ulcerated and draining
- Occur most often over dorsal trunk or tail base
- May have more typical, superficial dermatophyte lesions on other areas of body
- May have lymphadenopathy
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Nodular to diffuse, granulomatous to pyogranulomatous dermatitis and panniculitis with spherical to multilobular fungal aggregates ("granules") surrounded by Splendore-Hoeppli reaction
- Fungal forms include nonpigmented (hyaline), broad, septate, irregularly branched hyphae; chainlike pseudohyphae; and large, thick-walled chlamydospore-like cells
- May be overlying follicular dermatophytosis with or without inflammation
- Proliferating fibroblastic tissue surrounding outer aspect of lesions or coursing amongst granules
- Peripheral lymphoid nodules
ADDITIONAL DIAGNOSTIC TESTS:
- PAS, GMS
- Immunohistochemistry for canis (limited availability)
- Trichogram (microscopic hair examination)
For histopathologic findings:
- Eumycetoma: Numerous variably pigmented tissue grains composed of masses of opportunistic, septate, branching, 2-4 um wide, fungal hyphae often embedded in a cementing substance and surrounded by a less intense Splendore-Hoeppli reaction; most fungi isolated are saprophytes (Curvularia geniculata, Madurella grisea, Acremonium hyalinum, Pseudoallescheria boydii, and Phaeococcus)
- Actinomycetoma: Center of granuloma contains filamentous gram-positive bacteria (Actinomyces, Nocardia spp.); Actinomyces spp. often surrounded by Splendore-Hoeppli reaction ("sulfur granules"); Nocardia spp. are weakly acid-fast (Fite-Faraco)
- Botryomycosis (Staphylococcus): Gram-positive cocci; uncommon in the cat
- Foreign body granulomas
Pseudomycetomas in other species:
- Dogs: Rare; possibly Yorkshire terriers are predisposed; most cases caused by M. canis; one report of dermatophytic granuloma in a Manchester terrier caused by Trichophyton mentagrophytes
- Horses: Rare; caused by Trichophyton equinum
- Humans: Most commonly occur on the scalp and are caused by Microsporum and Trichophyton spp.
- Mauldin EA, Peters-Kennedy J. Integumentary. In: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol. 1. 6th Philadelphia, PA: Elsevier Saunders; 2016:649-653.
- Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Skin Diseases of the Dog and Cat Clinical and Histopathologic Diagnosis. 2nd ed. Ames, IA: Blackwell Publishing Professional; 2005:272, 288-291, 302.
- Hargis AM, Ginn P. The integument. In: McGavin MD, Zachary JF eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO:Mosby Elsevier; 2012:1040
- Miller RI. Nodular granulomatous fungal skin diseases of cats in the United Kingdom: a retrospective review. Veterinary Dermatology. 2010;21:130-135.
- Scott DW, Miller WH. Equine Dermatology. 2nd St. Louis, MO:WB Saunders Co;2011:187-189.