Infectious Disease and Microbiology – Exophiala Species

Overview

Exophiala species are dematiaceous (darkly pigmented) filamentous fungi that are distributed worldwide. They are uncommon causes of human disease but can produce cutaneous, subcutaneous, ocular, pulmonary, and invasive systemic infections.

Because of their melanin-containing cell walls and pigmented hyphae, infections caused by these organisms are generally classified among the phaeohyphomycoses.

Important Species

Clinically recognized species include:

• Exophiala dermatitidis

• Exophiala jeanselmei

• Exophiala moniliae

• Exophiala pisciphila

• Exophiala spinifera

Different species vary in their clinical associations and potential for invasive disease.

Microbiologic Characteristics

Exophiala species are:

• Dematiaceous fungi

• Filamentous molds

• Characterized by dark pigmentation related to melanin in the fungal cell wall

In infected tissue, they may appear as:

• Yeast-like forms

• Pseudohyphae

• Septate pigmented hyphae

Dematiaceous Fungi

The term dematiaceous refers to fungi with brown-to-black pigmentation in their cell walls.

When these pigmented fungi produce infection characterized by yeast-like cells, pseudohyphae, or irregular pigmented hyphae in tissue, the disease is generally termed:

Phaeohyphomycosis

Epidemiology

Exophiala species have a worldwide distribution.

Despite their environmental distribution, human infections are rare.

Disease may remain localized or, particularly in susceptible hosts, become invasive.

Clinical Infections

Reported infections include:

• Corneal infection

• Cutaneous and subcutaneous infection

• Prosthetic valve endocarditis

• Septic arthritis

• Brain abscess

• Pneumonia

• Other systemic infections

The clinical spectrum therefore ranges from localized superficial disease to potentially severe invasive infection.

Corneal Infection

Exophiala species can occasionally cause fungal keratitis.

Clinical manifestations may include:

• Eye pain

• Redness

• Photophobia

• Decreased vision

• Corneal inflammation or ulceration

A history of ocular trauma or environmental exposure may provide a clue to fungal infection.

Subcutaneous Infection

One of the better-recognized manifestations is subcutaneous phaeohyphomycosis.

Patients may develop:

• Slowly enlarging nodules

• Cysts

• Abscess-like lesions

• Chronic localized swelling

Traumatic implantation of environmental fungal material into the skin or subcutaneous tissue may initiate infection.

Pneumonia

Pulmonary infection due to Exophiala species is uncommon but has been reported.

Manifestations can range from chronic respiratory infection to invasive pneumonia, depending on the species and host factors.

Prosthetic Valve Endocarditis

Rare cases of Exophiala-associated prosthetic valve endocarditis have been reported.

This represents a serious invasive infection and may be difficult to eradicate because both fungal infection and infected prosthetic material are involved.

Septic Arthritis

Exophiala species have occasionally been associated with fungal arthritis.

Diagnosis generally requires demonstration of the fungus in synovial fluid or tissue together with compatible clinical findings.

Brain Abscess

One of the most serious manifestations is cerebral infection with brain abscess formation.

Certain dematiaceous fungi have a notable ability to cause central nervous system disease.

Neurologic manifestations may include:

• Headache

• Altered mental status

• Focal neurologic deficits

• Seizures

Systemic Infection

Although uncommon, disseminated or systemic Exophiala infection can occur and may involve multiple organs.

Invasive disease is potentially life-threatening and requires accurate species identification and antifungal management.

Diagnosis

Diagnosis is based on:

Culture of the fungus

and

Histopathologic examination of infected tissue

Using both methods can help establish that the recovered fungus represents true tissue infection.

Histopathology

Histopathologic examination may demonstrate:

Pigmented yeast-like cells

• ●

Pseudohyphae

• ●

Septate, darkly pigmented hyphae

These findings are characteristic of phaeohyphomycosis caused by dematiaceous fungi.

Culture

Fungal culture allows isolation and identification of Exophiala species.

Because treatment response may vary between species and isolates, accurate identification and, in severe disease, consideration of antifungal susceptibility information can be valuable.

Treatment

The source describes treatment with:

Amphotericin B

with or without:

Flucytosine

The optimal treatment depends on the site and severity of infection.

Additional Treatment

The source also lists:

Itraconazole

as an alternative antifungal agent.

Localized disease and invasive systemic infection may require substantially different therapeutic approaches.

Surgical Management

For localized subcutaneous lesions, abscesses, infected prosthetic material, or cerebral lesions, surgical management may sometimes be required in addition to antifungal therapy.

Management of invasive disease should therefore consider both antifungal treatment and appropriate source control.

High-Yield Clinical Pattern

Chronic subcutaneous cyst or nodule

• ●

Dematiaceous fungus

• ●

Pigmented septate hyphae in tissue

→ Think phaeohyphomycosis due to Exophiala species

Invasive Disease Pattern

Pigmented fungus

• ●

Pneumonia, prosthetic valve endocarditis, arthritis, or brain abscess

→ Consider invasive Exophiala infection

Exam Essentials

Genus: Exophiala

Type: Dematiaceous filamentous fungus

Pigment: Melanin-containing/dark fungal elements

Tissue morphology: Yeasts, pseudohyphae, and septate pigmented hyphae

Distribution: Worldwide

Frequency: Rare human pathogen

Disease category: Phaeohyphomycosis

Localized infections: Corneal and subcutaneous disease

Invasive infections: Pneumonia, arthritis, endocarditis, brain abscess

Diagnosis: Fungal culture + histopathology

Treatment in source: Amphotericin B ± flucytosine

Additional treatment: Itraconazole

Management consideration: Surgical intervention/source control may be necessary for selected localized or invasive lesions

Key clinical pearl: Exophiala species are dematiaceous fungi causing phaeohyphomycosis, recognized by pigmented septate hyphae, yeast-like forms, or pseudohyphae in tissue; although infection is usually rare and localized, these organisms can occasionally produce severe disease such as prosthetic valve endocarditis, pneumonia, and brain abscess.