Filamentous Forms of Ajellomyces (Blastomyces) dermatitidis in a Dog

Abstract

The diagnosis of blastomycosis often is based on Ajellomyces (Blastomyces) dermatitidis in sections prepared from formalin-fixed tissue. The tissue form of this fungus is a thick walled, round to oval yeast cell, from 5 to 15 micrometers in diameter. The cells produce a single bud that is attached to the mother cell by a broad base. A 2-year-old German Shepherd dog had right foreleg lameness. A radiograph showed an extensive radio-dense area in the medullary cavity and periosteal osseous proliferation in the midshaft of the humerus. A diagnosis of eosinophilic panosteitis was made and treatment with phenylbutazone and aspirin initiated. A radiograph 2 months later showed medullary cavitation and cortical rarefaction in addition to the previously described changes. The dog also had a head tilt. Treatment with corticosteroids and sulfadimethoxine had no effect, and the dog died 2 weeks later with terminal coma and convulsive seizures. Formalin-fixed parts of brain, liver and spleen were examined. Other than several 2-to-3centimeter pale, firm nodules in the spleen, there were no gross lesions. The splenic nodules were diffuse areas of necrosis and hemorrhage with peripheral aggregates of neutrophils. Numerous filamentous and yeast forms of A. dermatitidis were seen in sections stained with periodic acid-Schiff and Gomori's methenamine silver. Multiple, randomly distributed foci of acute necrosis with a neutrophilic infiltrate were in the liver. The necrotic foci contained numerous fungal forms similar to those in the spleen. The meninges overlying the cerebral cortex were diffusely infiltrated with a few neutrophils; only a few fungal forms were seen. The filamentous forms were short septate hyphae with occasional branches (fig. 1,2). Their width vaned, from about 1.5 to 5 micrometers. Small yeast forms, typically 3 to 5 micrometers in diameter, were mixed with the filaments. Cells with single broad based buds typical of A. dermatitidis were frequent (fig. 2). Tissue sections were treated with fluorescein-conjugated antibody specific for A. dermatitidis, Candida albicans, Cryptococcus neoformans and Sporothrix schenckii [5]. The filamentous and yeast forms stained brightly with A. dermatitidis conjugate but did not bind the other conjugates. Filamentous forms of A. dermatitidis have been reported in a naturally-occurring human blastomycosis [3] and in experimentally infected mice [I]. Whether these filamentous forms are true hyphae or pseudohyphae has not been resolved [I]. Filaments occur in infections caused by C. albicans [7, 81, Coccidioides immitis 181, C. neoformans [6] and Histoplasma capsulatum [2,4, 91. When tissues are not available for culture, the occurrence of filaments or other atypical forms may lead to misidentification or a misdiagnosis of a mixed fungal infection. Immunofluorescence is useful for diagnosis.