Microbiology · Dimorphic fungi
Coccidioides immitis !
- Fungus
- Exam priority: medium
A soil fungus of the deserts of the Americas; inhaled with dust, it causes pneumonia, sometimes with skin nodules and joint pain.
Updated 2026-11-01
Also called: Coccidioides posadasii, Valley fever, coccidioidomycosis
At a glance
- Shape
- Barrel-shaped arthroconidia in soil; large spherules full of endospores in tissue
- Family
- Onygenaceae (Onygenales)
- Type
- Dimorphic fungus: mold in soil, spherules in tissue
- Stain
- H&E, PAS and silver (GMS) in tissue; KOH on sputum
- Size
- Large spherules, about 20–80 µm
Morphology
- In soil: hyphae that break into barrel-shaped arthroconidia, alternating with empty cells
- In tissue: thick-walled spherules
- Mature spherules burst and release endospores
Transmission
- Inhaling arthroconidia with dust (wind, construction, digging, farming)
- Laboratory infection from cultures (highly infectious in the lab)
- Not spread from person to person
Reservoir: Soil of arid and semi-arid areas
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- In dry soil it grows as a mold and forms arthroconidia
- Wind or digging lifts the arthroconidia, which are inhaled
- In the lung each arthroconidium turns into a spherule
- The mature spherule fills with endospores and bursts
- Each endospore forms a new spherule and the infection spreads
Virulence factors
- Spherules are too large for a phagocyte to engulf
- Glycoprotein-rich wall that modulates the immune response
- Spread to skin, bone and meninges if cellular immunity fails
Diseases it causes
- Coccidioidomycosis: acute pneumonia, chronic lung disease and disseminated disease (skin, bone, meninges) Endemic mycoses: blastomycosis and coccidioidomycosis →
Clinical picture
- Fever, cough, chest pain, fatigue and joint pain weeks after dust exposure
- Erythema nodosum or erythema multiforme
- Dissemination risk: pregnancy, HIV, transplant, diabetes and people of African or Filipino ancestry
- Chronic meningitis in disseminated disease
Diagnosis
- Serology (IgM and IgG by immunoassay, immunodiffusion or complement fixation)
- Spherules with endospores on biopsy or sputum
- Culture only in a biosafety level 3 laboratory
- PCR on respiratory samples
Treatment
Many mild pneumonias resolve on their own. Severe or disseminated disease or high-risk groups: azoles (fluconazole, itraconazole); amphotericin B in severe disease or in pregnancy.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Reduce dust exposure in endemic areas (mask, wetting the soil)
- Warn the laboratory when it is suspected, so the culture is handled with biosafety
Test yourself
Think of the answer, then tap the question to see it.
Which Coccidioides structure is seen in tissue instead of yeasts?
Large thick-walled spherules full of endospores.
Which form of the fungus is inhaled?
The arthroconidia that form in dry soil and are lifted with dust.
Why does Coccidioides culture require a biosafety level 3 laboratory?
The mold in culture produces arthroconidia that disperse easily and infect staff.
Related
Related conditions
← Back to the group: Dimorphic fungi
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: About Valley Fever (CDC, 2024); Clinical Overview of Valley Fever (CDC, 2024).
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Representative model, for educational purposes only.