Morphologic Criteria for the Preliminary Identification of Fusarium, Paecilomyces, and Acremonium Species by Histopathology

Nontraditional human pathogenic fungi, including Fusarium, Paecilomyces, and Acremonium species, have been increasingly documented as agents of infection in immunocompromised patients and, occasionally, in normal hosts. Although definitive identification of these fungi requires culture, they often can be identified provisionally in tissue sections by a combination of histologic features, including hyaline septate hyphae and characteristic reproductive structures known as phialides and phialoconidia. These morphologic characteristics, although familiar to mycologists, are easily overlooked by histopathologists; as a result, Fusarium species and Paecilomyces lilacinus are frequently misidentified in tissue sections as Aspergillus or Candida species. We identified 19 culture-proved cases of infection with species of Fusarium, Paecilomyces, or Acremonium; retrospectively reviewed histologic specimens stained by routine hematoxylin and eosin, Gomori methenamine silver, and/or periodic acid-Schiff stains; and delineated morphologic criteria that will help pathologists make a preliminary identification of these fungi by histopathology. Adventitious sporulation was found in 9 of 9 infections caused by Paecilomyces species, 7 of 10 infections caused by Fusarium species, and in the single case of infection caused by Acremonium strictum. Histologic recognition of these morphologies may help clinicians select appropriate initial antifungal treatment and manage the infection.