coccidioidomycosis
Summary
Coccidioidomycosis ('Valley Fever') is a fungal infection caused by Coccidioides immitis/posadasii, endemic to the southwestern US (San Joaquin Valley, Arizona) and Central/South America. It is acquired by inhaling arthroconidia from disturbed soil, causing primarily pulmonary disease that is often self-limited but can disseminate in immunocompromised hosts. Classic exam associations include desert travel, erythema nodosum ('desert bump'/desert rheumatism), and spherules on histopathology.
Detail
Coccidioidomycosis is caused by the dimorphic fungus Coccidioides immitis or C. posadasii, found in soil of arid regions (San Joaquin Valley in California, Arizona, parts of Mexico, Central/South America). Infection occurs via inhalation of arthroconidia, especially after soil disruption (construction, dust storms, earthquakes). In the lungs, arthroconidia transform into spherules (the tissue form), which rupture to release endospores that form new spherules—this is the diagnostic histologic finding.
Most infections (60%) are asymptomatic or mild. Symptomatic primary pulmonary coccidioidomycosis presents with fever, cough, chest pain, and fatigue, sometimes with erythema nodosum or erythema multiforme and arthralgias (classically called 'desert rheumatism'). Chest imaging may show infiltrates, hilar adenopathy, or nodules; some patients develop residual pulmonary nodules or thin-walled cavities.
Dissemination occurs in <1% of cases but is more common in immunocompromised patients (HIV/AIDS, transplant recipients, those on TNF-alpha inhibitors), pregnant women (especially third trimester), and certain ethnic groups (Filipino, African American) due to genetic susceptibility. Disseminated disease can involve skin (verrucous plaques, ulcers), bones/joints (osteomyelitis), and meninges (coccidioidal meningitis, which is often fatal without treatment and requires lifelong fluconazole).
Diagnosis: Direct visualization of spherules on KOH prep, histopathology (H&E, silver stains), or culture (BSL-3 precautions needed due to infectivity of mold phase). Serology (EIA, immunodiffusion, complement fixation) is useful for diagnosis and monitoring; rising complement fixation titers suggest dissemination. Coccidioidin/spherulin skin tests indicate past exposure but are not diagnostic of active infection.
Treatment: Mild pulmonary disease often does not require antifungal therapy (self-limited). Antifungals (fluconazole or itraconazole) are indicated for severe pulmonary disease, immunocompromised patients, pregnancy, or disseminated disease. Amphotericin B is used for severe/life-threatening disease. Coccidioidal meningitis requires high-dose fluconazole, often lifelong.
High-yield boards points: Think Coccidioides in a patient with pneumonia + erythema nodosum + recent travel to southwestern US/California; spherules containing endospores on pathology; increased risk of dissemination in Filipino/Black patients and pregnant women (3rd trimester); San Joaquin Valley fever synonym.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- CDC Coccidioidomycosis (Valley Fever) Clinical Overview
- UpToDate: Coccidioidomycosis: Clinical manifestations and diagnosis
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