Histopathology of fungal diseases of the lung

Fungal pneumonias can be a diagnostic problem. However, their recognition is important as they can pose a significant health risk, especially in the immunocompromised host. While many of these infections are accompanied by necrotizing or non-necrotizing granulomas, some might be characterized by cellular interstitial pneumonia, intra-alveolar frothy material or only minimal inflammatory change. Much of the tissue reaction is dependent on the immune status of the patient and the type of fungal organism. While many of the fungi can be identified in tissue, especially if using histochemical stains such as Grocott's Methenamine Silver (GMS) stain and/or Periodic Acid Schiff (PAS) stain, in some cases, these stains are negative and the organisms can only be identified in cultures or using special techniques such as PCR or fungal serology. Some fungi can be accurately identified in tissue based on morphologic features; others require culture for exact classification. Knowledge about immune status, geographic region and social history of the patient are helpful in identifying the fungus and, therefore, detailed clinical and travel histories are important. In this manuscript we aim to describe the most common fungal infections that occur in the lung, their morphologic features, and differential diagnoses.