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- General description:
- Darkly pigmented (dematiaceous) filamentous fungus that lives in soil and grasses, especially with warm and humid conditions.
- E. rostratum is by far the most common infecting species.
- E. longirostratum and E. mcginnisii have also been reported, but these may actually be the same organism as E. rostratum.
- Under direct microscopy with KOH, Exserohilum spp. appear as brown septate hyphae.
- In selective culture media, Exserohilum spp. tend to grow fast.
- Conidia may take up to 3 weeks to grow.
- Colonies of Exserohilum spp. have a dark (brown-black or “olivaceous”) color.
- Appearance of Conidia:
- E. rostratum: long (25-90 by 9-22 μm), straight or slightly curved with 4 to 9 internal septations.
- Dark bands at both ends and one prominent septum at the base.
- Color is brown to olivaceous.
- Protruding hilum that looks like the beak of a bird at site where the conidia attaches at a sharp angle to the conidiophore (conidia bearing filament).
- E. longirostratum: similar in appearance to E. rostratum, but with larger conidia (up to 228 by 12-19 μm).
- E. mcginnisii: 44 to 76 by 11 to 18 μm, with 4 to 8 internal septations, warty projections on their outer walls and without dark bands at the ends
- Non-culture-based diagnostics:
- Polymerase chain reaction (PCR) has facilitated the process of prompt identification of Exserohilum, but is extremely susceptible to contamination.
- Results should be used only in conjunction with clinical and epidemiological data.
- CSF (1-3)-β-D-glucan (BDG) level measurements in outbreak associated E. rostratum CNS infections:
- Cutoff of 138 pg/mL provided 100% sensitivity and 98% specificity for diagnosis of fungal meningitis.
- Decline in CSF BDG correlated with clinical improvement.
- Biopsies from tissue invasive disease with Exserohilum spp. will show broad septate hyphae with acute angle branching. These organisms can be angioinvasive and hyphal elements can be seen in and around blood vessels.
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