What are the typical histopathological findings in a neonate suspected of having aspergillosis?

Histopathological Findings in Neonatal Aspergillosis

The characteristic histopathological finding in neonatal aspergillosis is acute-angle branching septate hyphae visible on special fungal stains, particularly Gomori methenamine silver (GMS) and periodic acid-Schiff (PAS) stains. 1

Key Histopathological Features

Microscopic Morphology

• Acute-angle branching septate hyphae are the hallmark microscopic feature that distinguishes Aspergillus from other filamentous fungi 1
• The hyphae demonstrate septation (internal divisions within the fungal filaments) and branch at characteristic acute angles (typically 45 degrees or less) 1
• Tissue invasion by hyphae indicates invasive disease rather than colonization 1

Optimal Staining Techniques

• Gomori methenamine silver (GMS) stain (also called Grocott-Gomori) is the primary special stain for fixed tissue 1
• Periodic acid-Schiff (PAS) stains provide complementary visualization of fungal elements 1
• Optical brightener methods (Calcofluor or Blankophor) are rapid stains with high sensitivity and specificity for detecting Aspergillus-like features in direct examination 1
• Standard H&E staining alone is insufficient; special fungal stains must be performed simultaneously when fungal infection is suspected 1

Tissue Pathology Patterns

• Tissue necrosis with angioinvasion is characteristic of invasive aspergillosis 1
• Hyphae invading lung tissue or other organs confirm invasive disease 1
• In neonatal cutaneous aspergillosis, histopathology shows fungal elements within skin tissue with associated necrosis 2, 3

Critical Diagnostic Limitations

Histopathology Cannot Definitively Identify Species

• No histopathologic finding can definitively diagnose Aspergillus species specifically—confirmation by culture or molecular methods is necessary to distinguish Aspergillus from other filamentous fungi such as Fusarium and Scedosporium 1
• All three genera (Aspergillus, Fusarium, Scedosporium) can appear morphologically similar on histopathology 1

Antifungal Therapy Effects

• Atypical appearances of the organism may be seen in tissue, particularly in patients receiving antifungal therapy, making morphological identification even more challenging 1
• This emphasizes the importance of obtaining tissue before initiating treatment when feasible

Specimen Processing Requirements

Optimal Specimen Handling

• Specimens should be submitted in adequate quantities for both histopathologic/cytologic testing and culture simultaneously 1
• Timely delivery of fresh specimen to the laboratory or refrigeration if delay is anticipated (though refrigeration may reduce recovery of some organisms) 1
• Include a brief clinical history to aid the pathologist and microbiologist in interpretation 1
• Communicate suspicion for fungal infection directly with pathology and microbiology laboratory personnel 1

Culture Requirements

• Cultures should be incubated for at least 5 days (and up to 3 weeks for other fungal pathogens) 1
• Fungal-specific media should be included in culture methods 1
• Despite optimal processing, culture yield remains low and a negative culture does not exclude invasive aspergillosis 1

Adjunctive Diagnostic Methods

Immunohistochemistry

• Monoclonal antibody Mab-WF-AF-1 demonstrates strong immunoreactivity in pediatric tissues with invasive aspergillosis, staining the Aspergillus cell wall, septa, and cytoplasm 4
• Optimal immunoreactivity requires microwave antigen retrieval using high pH 4
• This technique shows minimal background with no cross-reactivity to Candida albicans 4

Molecular Methods

• Molecular assays targeting ribosomal DNA sequences can be used for detection of Aspergillus in tissues, though not FDA-cleared for clinical use 1
• Aspergillus-specific nested PCR from the site of infection shows superior performance compared to blood samples, particularly in patients on antifungal prophylaxis or therapy 5
• DNA sequencing in reference laboratories can identify "cryptic" species that are misidentified by microscopic appearance, some of which are more resistant to azole antifungals 1

Common Pitfalls to Avoid

• Never rely on histopathology alone for species identification—always attempt culture or molecular confirmation 1
• Do not assume negative culture excludes aspergillosis—histopathologic evidence of fungal invasion remains diagnostic even without positive culture 1
• Recognize that diagnostic accuracy of histopathology is suboptimal despite being crucial for determining significance of Aspergillus growing in culture 1
• In neonates specifically, be aware that bifidobacteria in the gut microflora can cause false-positive galactomannan tests, making histopathology particularly important in this age group 1