Clinical Exposures: Canine dermatophyte infection

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Dec 01, 2007

A 3-month-old intact male boxer was presented for a preadoption evaluation at the Foster Hospital for Small Animals at Tufts University after being rescued in North Carolina. The physical examination results were unremarkable except for mild, patchy alopecia on the frontal aspect of the head and a 3-x-2-cm dermal, nodular lesion on the left lateral thigh. These skin lesions were nonpruritic and nonulcerated.

CYTOLOGIC EXAMINATION FINDINGS

Demodex and Sarcoptes species mites were not found on microscopic examination of samples obtained by deep skin scraping of the alopecic area on the head. Cytologic examination of the skin scraping revealed low numbers of erythrocytes, multiple broken hair shafts and anucleate squamous epithelial cells, and small to moderate amounts of keratin debris.

Samples from the thigh lesion were obtained by fine-needle aspiration, and the slides were stained with Wright's-Giemsa. Cytologic examination revealed moderate numbers of erythrocytes and intact nucleated cells. Moderate numbers of inflammatory cells were found, with a predominance of nondegenerate neutrophils and lesser numbers of activated macrophages. A few small lymphocytes and eosinophils were also noted. Some macrophages contained phagocytosed cellular debris and rare hematoidin crystals.

The samples from the thigh lesion also contained a small population of superficial squamous epithelial cells—both individual cells and variably sized aggregates—mixed with varying amounts of keratin debris. A few squamous epithelial cells contained cytoplasmic melanin granules, and others had multiple adherent bacteria.



Fragmented fungal hyphae and small, solitary, irregularly shaped structures were also identified in the samples (Figure 1A), all with a basophilic interior and a prominent nonstaining cell wall. The hyphae were septate and exhibited occasional branching (45 to 60 degrees) (Figure 1B). The solitary structures were about 3-μm-x-3-to-5-μm. Most of the structures were round to oval, but some were cuboidal or cigar-shaped or hand-mirror-shaped with a clubbed end (Figure 1C). These structures appeared to be either detached globoid ends of hyphae or spores.

DIFFERENTIAL DIAGNOSES

The presumptive diagnosis was mixed inflammation secondary to infection with a nonpigmented fungus. Differential diagnoses for mycotic infections that form hyphal elements in tissues include dermatophytosis (e.g. Microsporum species, Trichophyton mentagrophytes) and infection with Aspergillus or Penicillium species or other nonpigmented fungi (e.g. Acremonium, Fusarium, Geotrichum, Paecilomyces, Pseudallescheria, and Scedosporium species). Zygomycosis (e.g. Basidiobolus and Conidiobolus species) and pythiosis were also considered because of the puppy's geographic origin.1

HISTOLOGIC EXAMINATION AND DEFINITIVE DIAGNOSIS



Soon after the preadoption examination, the puppy was neutered, and the thigh lesion was excised with wide margins for histologic examination. The histologic examination findings included a multifocal mixed inflammatory cell (primarily epithelioid macrophages and fewer neutrophils) infiltration of the dermis centered on the hair follicles. Moderate hyperplasia and hyperkeratosis were noted in the epidermis, consistent with self-trauma. Additionally, fungal arthrospores and hyphae were found within the hair shafts and within foci of inflammation (Figures 2A & 2B).

Granulomatous folliculitis and furunculosis with intralesional dermatophyte arthrospores and hyphae were diagnosed. The fungal elements were consistent with dermatophytosis, specifically Microsporum canis infection. The nodular form of dermatophytosis was suspected, but fungal culture to confirm the genus and species of the dermatophyte was not done.