Histopathologic diagnosis of fungal infections in the 21st century

Abstract

Fungal infections are becoming more frequent because of expansion of at-risk populations and the use of treatment modalities that permit longer survival of these patients. Because histopathologic examination of tissues detects fungal invasion of tissues and vessels as well as the host reaction to the fungus, it is and will remain an important tool to define the diagnostic significance of positive culture isolates or results from PCR testing. However, there are very few instances where the morphological characteristics of fungi are specific. Therefore, histopathologic diagnosis should be primarily descriptive of the fungus and should include the presence or absence of tissue invasion and the host reaction to the infection. The pathology report should also include a comment stating the most frequent fungi associated with that morphology as well as other possible fungi and parasites that should be considered in the differential diagnosis. Alternate techniques have been used to determine the specific agent present in the histopathologic specimen, including immunohistochemistry, in situ hybridization, and PCR. In addition, techniques such as laser microdissection will be useful to detect the now more frequently recognized dual fungal infections and the local environment in which this phenomenon occurs.

Fig. 1.

Morphology, description, diagnosis, and comment for endemic fungal infections that present as yeasts in tissues. All photographs are of Grocott methenamine silver (GMS)-stained specimens except for the inset in the second row, which is a mucicarmine stain. For each type of infection, alternative testing and correlation with culture, epidemiologic, and clinical features are necessary. (The photographs of Cryptococcus spp., Histoplasma capsulatum, and Coccidioides spp. are reprinted from the CDC Public Health Image Library collection.)

Fig. 2.

Morphology, description, diagnosis, and comment for fungal infections that present with hyphae or pseudohyphae in tissues. All photographs are of Grocott methenamine silver (GMS)-stained specimens except for the inset in the fourth row, which is a Fontana-Masson stain. The brown color observed in the fungal element is melanin. For each type of infection, alternative testing and correlation with culture, epidemiologic, and clinical features are necessary. (The photograph in the fourth row is reprinted from the CDC Public Health Image Library collection.)

Fig. 3.

Morphology, description, diagnosis, and comment for fungal infections that show characteristic yeast morphology in tissues. Except for the last row, which shows an H&E-stained asteroid body, all photographs are of Grocott methenamine silver (GMS)-stained specimens (including the inset of S. schenckii, which is counterstained with H&E). For each type of infection, alternative testing and correlation with culture, epidemiologic, and clinical features are necessary. (The photographs of Pneumocystis, Penicillium marneffei, and Paracoccidioides brasiliensis are reprinted from the CDC Public Health Image Library collection.)

Fig. 4.

Diagnostic pitfalls. When using GMS stains, normal tissue structures can appear as yeasts or hyphae. (A) Neurosecretory granules (arrow). (B) Collagen fibers, with one even showing a “pseudoseptum” (arrow). (C) In specimens with few organisms, hyphae cut transversally can appear as yeasts that may have “pseudobudding” (arrows).

Fig. 5.

Sequential specimens stained with GMS (magnification, ×120) showing mold infections in a neutropenic patient with chronic lymphocytic leukemia. (A and B) Hyaline septated hyphae in the lung. The culture was positive for Aspergillus fumigatus. (C) Hyphae (by culture a Fusarium sp.) in a nasal debridement sample obtained in the same patient 3 days after the lung biopsy. The morphology of fungal elements could be confused with mucormycosis, since there are few septations and the hyphae twist and turn. (D) Hyaline pauciseptated hyphae in a lung specimen obtained at autopsy 22 days after the nasal debridement. The specimen stained positive using immunohistochemistry for mucormycosis. The morphology of the hyphae is distorted, probably due to previous antifungal treatment.