Blastomyces

Dr. Neha Sharma

Introduction

  • Blastomyces is a dimorphic fungus that causes a fungal infection called blastomycosis in humans and animals.
  • The infection mainly affects the lungs, but it can also spread to the skin, bones, genitourinary system, and central nervous system (CNS).
  • People become infected by inhaling fungal spores present in contaminated soil and decaying organic matter.
  • Blastomycosis is not contagious and does not spread from person to person.
  • Blastomyces exists in two forms: mold at 25°C (in the environment) and yeast at 37°C (inside the human body), making it a thermally dimorphic fungus.
  • The disease may be asymptomatic, cause mild lung infection, or progress to severe disseminated disease, especially in people with weakened immune systems.

Taxonomic Classification

Category Classification
Kingdom Fungi
Phylum Ascomycota
Class Eurotiomycetes
Order Onygenales
Family Ajellomycetaceae
Genus Blastomyces
Species Blastomyces dermatitidis (most common human pathogen)

Other pathogenic species include:

  • Blastomyces gilchristii
  • Blastomyces helicus
  • Blastomyces percursus

Historical Background

  • First described by Thomas Casper Gilchrist in 1894.
  • Initially mistaken for a protozoan infection.
  • Later recognized as a pathogenic dimorphic fungus causing systemic mycosis.

Geographic Distribution

Blastomycosis is mainly found in:

  • North America
  • Mississippi River Valley
  • Ohio River Valley
  • Great Lakes region
  • Canada

Cases have also been reported in:

  • Africa
  • India
  • Middle East
  • South America

Habitat

Blastomyces grows naturally in:

  • Moist soil
  • Decaying wood
  • Forest areas
  • Riverbanks
  • Areas rich in organic matter
  • Leaf litter

The fungus thrives in warm, humid environments.

Morphology

Mold Form (25°C)

Environmental phase.

Characteristics

  • Septate hyphae
  • Hyaline mycelium
  • Branched hyphae
  • Produces single pyriform (pear-shaped) conidia

Importance

Responsible for environmental survival and transmission.

Yeast Form (37°C)

Parasitic phase found in human tissues.

Characteristics

  • Large round yeast cells (8–20 µm)
  • Thick refractile double-contoured wall
  • Broad-based budding (diagnostic feature)

Importance

The presence of broad-based budding yeast is characteristic of blastomycosis.

Dimorphism

Blastomyces is a thermally dimorphic fungus.

Temperature Morphology
25°C Mold
37°C Yeast

This conversion is essential for pathogenicity.

Life Cycle

  • The life cycle of Blastomyces is simple and temperature-dependent (dimorphic).
  • The fungus exists as a mold in the environment (25°C) and converts into a yeast form in the human body (37°C) after inhalation of spores.
  • Humans become infected accidentally by breathing in airborne fungal spores.

Step 1: Growth in the Environment

  • Blastomyces grows in moist soil, decaying wood, leaf litter, and organic matter.
  • At 25°C, the fungus exists as a mold (mycelial form).
  • The mold consists of septate hyphae that produce infectious conidia (spores).

Importance: The mold is the environmental and infective form of the fungus.

Step 2: Formation and Release of Conidia

  • The mold produces small conidia (2–10 µm) on short conidiophores.
  • Environmental disturbances such as:
    • Construction work
    • Farming
    • Excavation
    • Forestry activities
  • cause the conidia to become airborne.

Importance: Airborne conidia are the infective stage of Blastomyces.

Step 3: Inhalation of Conidia

  • Humans become infected by inhaling airborne conidia.
  • The spores enter the respiratory tract and reach the alveoli of the lungs.

Note: Blastomycosis is not transmitted from person to person.

Step 4: Conversion to Yeast Form

  • Inside the lungs, at 37°C, the conidia transform into large yeast cells.
  • This temperature-dependent change is called thermal dimorphism.

Characteristics of the Yeast

  • Round to oval cells
  • Thick double-contoured cell wall
  • Reproduce by broad-based budding

Importance: The yeast form is responsible for causing disease in humans.

Step 5: Multiplication in the Lungs

  • The yeast cells multiply by broad-based budding.
  • They survive inside the lungs by resisting destruction by immune cells.
  • This leads to inflammation and the development of pulmonary blastomycosis.

Step 6: Dissemination to Other Organs

  • In some patients, especially those with weakened immunity, the yeast enters the bloodstream and lymphatic system.
  • It can spread to:
    • Skin
    • Bones
    • Genitourinary tract
    • Central nervous system (CNS)

This stage is known as disseminated blastomycosis.

Step 7: Continuation of the Life Cycle

  • The fungus does not return to the environment from infected humans.
  • In nature, the yeast form converts back to the mold form only under suitable environmental conditions.
  • Humans are considered accidental hosts and do not play a role in maintaining the natural life cycle.

Mode of Transmission

1. Inhalation of Airborne Conidia (Most Common Route)

The primary mode of transmission is the inhalation of airborne fungal spores (conidia).

Process

  • Blastomyces grows as a mold in moist soil and decaying organic matter.
  • It produces infectious conidia (spores).
  • When the soil is disturbed, these spores become airborne.
  • People inhale the spores, which enter the lungs and cause infection.

This is the most common and natural route of transmission.

2. Environmental Exposure

People become infected after exposure to environments where the fungus naturally grows.

High-risk environments include:

  • Moist soil
  • Decaying wood
  • Forest areas
  • Riverbanks
  • Leaf litter
  • Areas rich in organic matter

Individuals living or working in endemic areas are at greater risk of exposure.

3. Occupational Exposure

Certain occupations increase the risk of inhaling fungal spores.

High-risk occupations include:

  • Farmers
  • Forestry workers
  • Construction workers
  • Gardeners
  • Landscapers
  • Archaeologists
  • Outdoor laborers

Activities such as digging, excavation, tree cutting, and land clearing can release spores into the air.

4. Recreational Exposure

Outdoor recreational activities may also expose individuals to airborne spores.

Examples include:

  • Camping
  • Hiking
  • Hunting
  • Fishing
  • Gardening

These activities increase contact with contaminated soil and decaying vegetation.

5. No Person-to-Person Transmission

Blastomycosis does not spread from one person to another.

  • An infected person cannot transmit the disease through coughing, sneezing, or physical contact.
  • Isolation of patients is not required.

6. Rare Transmission

Rarely, infection may occur through:

  • Accidental laboratory exposure.
  • Direct inoculation of the fungus into the skin through trauma (extremely uncommon).

These routes are very rare compared with inhalation.

Pathogenesis

  • Inhaled conidia reach the alveoli.
  • Conversion to yeast occurs in the lungs.
  • Yeast cells evade phagocytosis due to their thick cell wall.
  • Multiplication by broad-based budding.
  • Granulomatous inflammation develops.
  • Hematogenous dissemination may occur.

Common organs involved:

  • Lungs
  • Skin
  • Bones
  • Prostate
  • Central nervous system

Clinical Manifestations

Pulmonary Blastomycosis

Symptoms include:

  • Fever
  • Cough
  • Chest pain
  • Shortness of breath
  • Weight loss
  • Night sweats

Cutaneous Blastomycosis

Features include:

  • Verrucous skin lesions
  • Ulcerative plaques
  • Painless nodules
  • Draining sinuses

Osteoarticular Blastomycosis

  • Bone pain
  • Osteomyelitis
  • Joint swelling

Genitourinary Blastomycosis

  • Prostatitis
  • Epididymitis
  • Testicular swelling

Central Nervous System Blastomycosis

  • Headache
  • Meningitis
  • Brain abscess
  • Neurological deficits

Laboratory Diagnosis

1. Specimen Collection

The type of specimen depends on the site of infection.

Common Clinical Specimens

  • Sputum
  • Bronchoalveolar lavage (BAL)
  • Tracheal aspirate
  • Lung biopsy
  • Skin lesion biopsy
  • Pus from abscesses
  • Bone biopsy
  • Cerebrospinal fluid (CSF) in CNS infection
  • Blood (rarely)

Proper collection and prompt transportation improve the chances of detecting the organism.

2. Direct Microscopic Examination

Direct microscopy provides a rapid presumptive diagnosis.

Specimens Used

  • Sputum
  • BAL fluid
  • Tissue biopsy
  • Pus

Stains Used

  • 10% Potassium Hydroxide (KOH) Mount
  • Calcofluor White Stain
  • Gram Stain
  • Giemsa Stain

Microscopic Findings

The characteristic finding is:

  • Large round to oval yeast cells
  • Thick double-contoured cell wall
  • Broad-based budding yeast (diagnostic feature)

Clinical Significance

Identification of broad-based budding yeast strongly suggests Blastomyces infection.

3. Histopathological Examination

Histopathology is one of the most reliable methods for diagnosing blastomycosis.

Specimens

  • Lung biopsy
  • Skin biopsy
  • Bone biopsy
  • Lymph node biopsy

Special Stains

Hematoxylin and Eosin (H&E)

Shows:

  • Granulomatous inflammation
  • Suppurative lesions
  • Broad-based budding yeast

Periodic Acid–Schiff (PAS)

Highlights the fungal cell wall in bright magenta.

Gomori Methenamine Silver (GMS) Stain

  • Most commonly used fungal stain.
  • Demonstrates yeast cells as black organisms against a pale green background.

Histopathological Features

  • Thick-walled yeast cells
  • Broad-based budding
  • Granulomatous inflammation
  • Giant cells
  • Neutrophilic infiltration

4. Fungal Culture

Culture is considered the gold standard for confirming blastomycosis.

Culture Media

  • Sabouraud Dextrose Agar (SDA)
  • Brain Heart Infusion (BHI) Agar

Incubation

  • At 25°C, the fungus grows as a mold.
  • At 37°C, it converts into the yeast form.

Colony Characteristics

Mold Form

  • White to tan colonies
  • Cottony or fluffy appearance
  • Septate hyphae with pear-shaped conidia

Yeast Form

  • Cream-colored
  • Soft, yeast-like colonies

Limitation

Culture may require 2–6 weeks for fungal growth.

5. Antigen Detection Tests

Detection of fungal antigens helps in early diagnosis.

Specimens

  • Urine
  • Serum

Method

  • Enzyme Immunoassay (EIA)

Advantages

  • Rapid diagnosis
  • High sensitivity
  • Useful in disseminated blastomycosis

Limitation

Cross-reactivity may occur with other endemic fungi such as Histoplasma capsulatum.

6. Serological Tests

Serological tests detect antibodies against Blastomyces.

Common Tests

  • Enzyme-Linked Immunosorbent Assay (ELISA)
  • Complement Fixation Test (CFT)
  • Immunodiffusion Test (ID)

Limitations

  • Lower sensitivity than antigen detection.
  • Antibody levels may be low in immunocompromised patients.
  • Cross-reactivity with other fungal infections may occur.

7. Molecular Diagnosis

Molecular methods provide rapid and highly accurate identification.

Techniques

  • Polymerase Chain Reaction (PCR)
  • Real-Time PCR
  • DNA Sequencing

Advantages

  • High sensitivity
  • High specificity
  • Rapid identification
  • Useful for species differentiation

Clinical Importance

PCR is particularly valuable when culture results are negative or delayed.

8. Imaging Studies

Imaging supports the diagnosis by identifying the extent of organ involvement.

Chest X-ray

May reveal:

  • Pulmonary infiltrates
  • Lung nodules
  • Cavitary lesions
  • Consolidation

CT Scan

Useful for detecting:

  • Pulmonary lesions
  • Bone involvement
  • CNS infection
  • Disseminated disease

Treatment

Treatment of blastomycosis depends on the severity of the infection and the patient’s immune status.

1. Mild to Moderate Blastomycosis

  • Drug of choice: Itraconazole
  • Duration: 6–12 months
  • Used for most patients with mild pulmonary or localized disease.

2. Severe or Disseminated Blastomycosis

  • Drug of choice: Liposomal Amphotericin B
  • After clinical improvement, patients are switched to oral itraconazole.
  • Treatment duration: At least 6–12 months.

3. Central Nervous System (CNS) Blastomycosis

  • Initial treatment with Liposomal Amphotericin B.
  • Followed by Voriconazole, Fluconazole, or Itraconazole for long-term therapy.

4. Immunocompromised Patients

  • Require prolonged antifungal therapy.
  • Close monitoring is necessary because of the increased risk of severe disease and relapse.

5. Supportive Care

  • Adequate hydration
  • Oxygen therapy if needed
  • Nutritional support
  • Regular monitoring of liver and kidney function

Prevention

  • Avoid exposure to contaminated soil in endemic areas.
  • Wear N95 respirators during excavation or forestry work.
  • Control dust generation at construction sites.
  • Early diagnosis and treatment of infected individuals.
  • Educate high-risk workers about occupational exposure.

Complications

  • Respiratory failure
  • Disseminated blastomycosis
  • Osteomyelitis
  • Meningitis
  • Brain abscess
  • Permanent lung damage
  • Death if untreated