Endemic Dimorphic Fungal Infections
On this page
Direct answer
Endemic dimorphic fungal infections are systemic mycoses caused by fungi that grow as moulds in soil at room temperature and as yeasts (or spherules, in Coccidioides) in tissue at 37 degrees Celsius. The classic three are blastomycosis from Blastomyces dermatitidis in the North American river valleys, coccidioidomycosis from Coccidioides species in the American deserts, and paracoccidioidomycosis from Paracoccidioides brasiliensis in Latin America.
What you must remember
- Dimorphism: these fungi switch from mould to yeast form (spherules in Coccidioides), and the tissue form is the diagnostic one on histology.
- Blastomycosis: Blastomyces dermatitidis causes pulmonary disease and verrucous skin lesions, with thick-walled broad-based budding yeasts on histology.
- Coccidioidomycosis: arthroconidia inhaled from desert soil cause valley fever, often with erythema nodosum, and tissue shows spherules containing endospores.
- Paracoccidioidomycosis: Paracoccidioides brasiliensis causes ulcerated oral and mucosal lesions, lymphadenopathy and adrenal involvement, with multiple budding yeasts in a mariner's wheel pattern.
- Histoplasmosis: Histoplasma capsulatum is a further classic dimorphic fungus from the Ohio and Mississippi valleys, and disseminated disease in advanced HIV is AIDS-defining.
- Serology and antigen: antibody tests support coccidioidomycosis diagnosis, while urinary antigen testing is useful in histoplasmosis.
- Treatment: amphotericin B is reserved for severe or disseminated disease, while itraconazole or fluconazole treats milder forms over weeks to months; coccidioidal meningitis is the exception and is treated with high-dose fluconazole.
- Indian relevance: these American mycoses are uncommon in India (histoplasmosis is reported, mainly in the Gangetic plain), so questions are framed around American geography; the Indian dimorphic pathogen is Talaromyces marneffei, seen in advanced HIV in the north-east.
Matching geography to tissue findings
All three fungi share the mould-to-yeast switch, so the geography and the tissue stain together usually identify the species. The table sets out the features that examiners expect a student to link.
| Infection | Core geography | Tissue hallmark | Usual treatment |
|---|---|---|---|
| Blastomycosis | Ohio and Mississippi valleys, Great Lakes | Broad-based budding yeast | Itraconazole; amphotericin B if severe |
| Coccidioidomycosis | San Joaquin Valley, Arizona, parts of Latin America | Spherules with endospores | Fluconazole or itraconazole; fluconazole for meningitis; amphotericin B for severe disease |
| Paracoccidioidomycosis | Brazil and much of South America | Multiple budding yeast, mariner's wheel | Itraconazole or cotrimoxazole; amphotericin B if severe |
The three infections are separated mainly by where the patient lived and the shape of the organism in tissue. Treatment follows severity, with the oral azoles covering most patients and amphotericin B held for the sickest.
Blastomycosis and histoplasmosis share the Ohio and Mississippi valleys, so biopsy findings decide between them. Blastomyces shows broad-based budding yeast, while Histoplasma appears as small yeasts inside macrophages, and this distinction is a favourite viva question.
Where the exam tries to catch you
Examiners use the tissue hallmark and the geography to separate fungi that look alike on a clinical stem. The traps below are the errors that cost marks most often here.
- Trap: amphotericin B for every case of endemic mycosis Correct: itraconazole or fluconazole treats most mild to moderate disease, and amphotericin B is for severe or disseminated illness (coccidioidal meningitis is treated with fluconazole).
- Trap: coccidioidomycosis as a Mississippi valley disease Correct: Coccidioides belongs to arid soils of the Southwest and Latin America, while the Ohio and Mississippi valleys are home to Blastomyces and Histoplasma.
- Trap: spherules with endospores read as broad-based budding yeast Correct: spherules mark Coccidioides, whereas broad-based budding yeast is the Blastomyces hallmark.
Frequently asked questions
Which fungus causes valley fever?
Coccidioides species cause valley fever, acquired by inhaling arthroconidia from desert soil in the Southwest United States and parts of Latin America. Coccidioidomycosis often causes mild pneumonia with erythema nodosum, but it can disseminate in some patients.
What is the tissue hallmark of blastomycosis?
Broad-based budding yeasts are the tissue hallmark of Blastomyces dermatitidis, seen in sputum or tissue specimens. Blastomycosis is diagnosed by recognising these yeasts, and cultures confirm the species.
Which fungus shows a mariner's wheel pattern in tissue?
Paracoccidioides brasiliensis shows multiple budding yeasts arranged around a central point, the mariner's wheel pattern, in tissue. Paracoccidioidomycosis causes ulcerated mucosal lesions and lymphadenopathy in Latin America.
Which endemic fungus causes disseminated disease in advanced HIV?
Histoplasma capsulatum causes disseminated histoplasmosis in advanced HIV, and the illness is AIDS-defining. Urinary antigen testing supports diagnosis, and liposomal amphotericin B is used for severe disease.
When is amphotericin B preferred over an oral azole in endemic mycoses?
Amphotericin B is preferred in endemic dimorphic mycoses when the disease is severe or disseminated. Milder disease is usually treated with an oral azole such as itraconazole or fluconazole.