Endemic mycosis caused by Histoplasma capsulatum, classically associated with Ohio and Mississippi River valleys and bird/bat guano exposure.
Also known as: Histoplasma capsulatum, Ohio Valley disease, spelunker's lung, cave disease
Overview
Systemic infection caused by the dimorphic fungus Histoplasma capsulatum, acquired by inhalation of microconidia from soil enriched with bird or bat guano. Clinical presentations range from asymptomatic infection to acute pulmonary, chronic cavitary pulmonary, and progressive disseminated disease.
Epidemiology
Endemic to the Ohio and Mississippi River valleys in the United States, parts of Central and South America, Africa, and Asia. Outbreaks linked to cave exploration, demolition of bird roosts, and excavation in contaminated soil. Severe disease is most common in immunocompromised patients (HIV with CD4 <150, solid organ transplant, TNF-alpha inhibitor therapy) and at the extremes of age.
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Question 1PulmonaryMedium
A 35-year-old man in Ohio has had fever, fatigue, and dry cough for 6 weeks after cleaning a chicken coop. He is not hypoxemic and is hemodynamically stable. A chest radiograph shows patchy infiltrates with mediastinal lymphadenopathy, and urine and serum Histoplasma antigens are positive. Which of the following is the most appropriate treatment?
AItraconazole × 12 weeks
BObservation without antifungals
CLiposomal amphotericin B × 12 weeks
DVoriconazole × 12 weeks
EFluconazole × 6 months
Reveal answer & full explanation
Correct answer: A — Itraconazole × 12 weeks
AItraconazole × 12 weeks✓
BObservation without antifungals
CLiposomal amphotericin B × 12 weeks
DVoriconazole × 12 weeks
EFluconazole × 6 months
Why Itraconazole × 12 weeks is correct
Histoplasmosis (Histoplasma capsulatum) is endemic to the Ohio and Mississippi River valleys and is acquired from bird and bat droppings, as in this chicken-coop exposure
Urine and serum Histoplasma antigen testing is the most sensitive diagnostic approach, and patchy infiltrates with mediastinal lymphadenopathy are typical
Per IDSA guidance, symptomatic pulmonary histoplasmosis persisting beyond about a month without hypoxemia or hemodynamic instability is treated with itraconazole, typically for 6-12 weeks
Itraconazole is the azole of choice for non-severe histoplasmosis and is also the step-down agent after amphotericin in severe disease
Why the others are wrong
Voriconazole × 12 weeks — first-line for invasive aspergillosis, not histoplasmosis, for which itraconazole is preferred (confused-with-aspergillosis trap)
Liposomal amphotericin B × 12 weeks — reserved for severe or disseminated disease and given only 1-2 weeks before stepping down to itraconazole, so both the indication and the duration are wrong here (right-class-wrong-setting)
Fluconazole × 6 months — has poor activity against Histoplasma and is not recommended (buzzword-matching "antifungal")
Observation without antifungals — most acute pulmonary histoplasmosis in an immunocompetent host resolves without treatment, so watchful waiting is tempting in a stable, non-hypoxemic patient, but IDSA recommends itraconazole once symptoms persist beyond about a month, and his have lasted 6 weeks (self-limited-disease trap)
Question 2PulmonaryMedium
A 47-year-old man from the Ohio River Valley presents with 3 weeks of nonproductive cough, low-grade fevers, fatigue, and a 4-kg weight loss. He recently spent a weekend cleaning out an old barn that contained extensive bat guano. He has no significant past medical history. Temperature is 37.8 C, RR 18, SpO2 96% on room air. Exam reveals scattered crackles bilaterally and small painful red nodules over both shins. Chest radiograph shows patchy bilateral mid-lung infiltrates with mediastinal and hilar lymphadenopathy. Which of the following is the most appropriate test to confirm the diagnosis?
ASputum acid-fast bacilli smear
BSerum Aspergillus galactomannan
CCryptococcal serum antigen
DUrine Histoplasma antigen
ESerum Coccidioides antibodies
Reveal answer & full explanation
Correct answer: D — Urine Histoplasma antigen
ASputum acid-fast bacilli smear
BSerum Aspergillus galactomannan
CCryptococcal serum antigen
DUrine Histoplasma antigen✓
ESerum Coccidioides antibodies
Why urine Histoplasma antigen is correct
This patient has subacute pulmonary histoplasmosis: Ohio River Valley exposure, disturbance of bat guano (which aerosolizes Histoplasma capsulatum microconidia), mediastinal and hilar lymphadenopathy, mid-lung infiltrates, and erythema nodosum as an immunologic reaction.
The mechanism involves inhalation of conidia that convert to yeast within macrophages, producing granulomatous inflammation.
Urine Histoplasma polysaccharide antigen is the test of choice for acute and subacute pulmonary disease because it is highly sensitive in moderate-to-severe and disseminated disease, yields rapid results, and sensitivity is augmented by simultaneous serum antigen and serology.
Why the others are wrong
Sputum acid-fast bacilli smear — tests for mycobacterial disease; while tuberculosis can produce a similar radiographic pattern, the strong epidemiologic exposure to bat guano and erythema nodosum point to histoplasmosis.
Serum Aspergillus galactomannan — used in invasive aspergillosis, which typically occurs in profoundly neutropenic or transplant patients with angioinvasive nodules and halo signs, not in this immunocompetent host.
Cryptococcal serum antigen — evaluates Cryptococcus, which more often causes meningitis in immunocompromised hosts and a single pulmonary nodule rather than this exposure-linked syndrome.
Serum Coccidioides antibodies — erythema nodosum with hilar adenopathy and subacute pneumonia is a classic presentation of coccidioidomycosis (valley fever), but Coccidioides is endemic to the arid southwestern United States; Ohio River Valley residence and heavy bat guano exposure point to Histoplasma, so Coccidioides serology would not confirm this diagnosis.
Additional high-yield points
Fungal culture and tissue biopsy remain confirmatory but are slower than antigen testing.
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Pre-existing structural lung disease (predisposes to chronic cavitary form)
Age <2 years or >55 years
Pathophysiology
Microconidia are inhaled and reach the alveoli, where they convert to the yeast phase at body temperature and are phagocytosed by alveolar macrophages. Inside macrophages, yeast replicate and disseminate via the lymphohematogenous route. Cell-mediated immunity (Th1, IFN-gamma) is required to contain infection; granulomas form within 2-3 weeks and typically calcify. Defective cell-mediated immunity allows uncontrolled replication and progressive disseminated disease.
Clinical presentation
Symptoms
Acute pulmonary: fever, dry cough, fatigue, chest pain, myalgias 1-3 weeks after heavy exposure; most immunocompetent infections are asymptomatic
Chronic cavitary pulmonary: productive cough, weight loss, night sweats, hemoptysis in patients with underlying COPD or emphysema
Mediastinal lymphadenopathy may cause cough, dysphagia, or superior vena cava syndrome
Signs / physical exam
Hepatosplenomegaly in disseminated disease
Oral or oropharyngeal ulcers (disseminated form)
Skin papules, nodules, or molluscum-like lesions in advanced HIV
Crackles or diminished breath sounds depending on form
Adrenal enlargement or evidence of adrenal insufficiency
Classic findings
Pancytopenia, hepatosplenomegaly, and oral ulcers in a patient with HIV/AIDS and exposure to the Ohio/Mississippi River valley. Calcified granulomas in lungs, spleen, or liver in patients with prior asymptomatic infection.
Differential diagnosis
Tuberculosis — Chronic cough, weight loss, upper-lobe cavitary disease, exposure history; AFB smear and culture, IGRA. Histoplasmosis and TB radiographically indistinguishable on chronic cavitary form
Sarcoidosis — Bilateral hilar adenopathy, non-caseating granulomas, elevated ACE; ethnic and geographic risk; biopsy and exclusion of fungi
Coccidioidomycosis — Southwestern US and parts of Central/South America; erythema nodosum, eosinophilia; serology and spherules on biopsy
Blastomycosis — Overlapping endemic areas; skin and bone disease more common; broad-based budding yeast on KOH or histopathology
Lung cancer — Solitary pulmonary nodule or mass; biopsy required to distinguish from histoplasmoma
Lymphoma — Mediastinal adenopathy, B symptoms; biopsy distinguishes
Community-acquired pneumonia — Acute onset, lobar consolidation; bacterial etiologies more common but exposure history should prompt fungal testing in endemic regions
Diagnostic workup
Diagnostic criteria
Definitive diagnosis by culture or histopathologic identification of yeast forms (2-4 microm narrow-based budding yeast within macrophages). Strong presumptive diagnosis from positive antigen testing in an appropriate clinical setting.
Labs
Histoplasma urine and serum antigen (sensitivity ~90% in disseminated disease, lower in localized pulmonary)
Histoplasma serology by immunodiffusion and complement fixation (limited utility in immunocompromised due to impaired antibody response)
Fungal blood cultures (positive in disseminated disease; may take 2-4 weeks)
Bone marrow biopsy with fungal stains and culture in suspected disseminated disease
Chest x-ray: patchy infiltrates, hilar/mediastinal adenopathy, or calcified granulomas; cavitary upper-lobe disease in chronic form
Chest CT for better characterization of nodules, adenopathy, and cavities
Abdominal imaging for hepatosplenomegaly or adrenal enlargement
Diagnostic algorithm
flowchart TD
A[Inhaled microconidia<br/>bird/bat guano exposure] --> B[Alveolar deposition]
B --> C[Yeast conversion in macrophages]
C --> D{Immune status}
D -->|Immunocompetent| E[Self-limited<br/>or asymptomatic]
D -->|Heavy inoculum| F[Acute pulmonary<br/>fever, cough, CXR infiltrates]
D -->|Structural lung dz| G[Chronic cavitary<br/>upper lobe]
D -->|HIV CD4 <150,<br/>transplant, TNF-i| H[Progressive disseminated<br/>HSM, pancytopenia, oral ulcers]
F --> I[Itraconazole if persistent]
G --> I
H --> J[Liposomal amphotericin B<br/>then itraconazole >=12 mo]
Histoplasmosis pathogenesis and treatment based on host immune status.
Treatment
First-line
Mild to moderate acute pulmonary in immunocompetent: often self-limited, observation reasonable; itraconazole 200 mg PO TID x 3 days then BID for 6-12 weeks if symptoms persist >1 month
Moderately severe to severe acute pulmonary (diffuse infiltrates, hypoxemia): lipid amphotericin B 3-5 mg/kg/day IV for 1-2 weeks, then itraconazole 200 mg PO TID x 3 days then BID to complete 12 weeks total; add IV methylprednisolone for 1-2 weeks if hypoxemia or significant respiratory distress
Progressive disseminated disease (moderately severe to severe): liposomal amphotericin B 3 mg/kg/day IV for 1-2 weeks, then itraconazole 200 mg PO TID x 3 days then BID for at least 12 months (longer, or lifelong suppression, if immunosuppression persists)
Chronic cavitary pulmonary: itraconazole 200 mg PO TID x 3 days then once or twice daily for at least 12 months
CNS histoplasmosis: liposomal amphotericin B 5 mg/kg/day for 4-6 weeks, then itraconazole for at least 12 months and until CSF abnormalities resolve
Second-line / adjunct
Alternative azole antifungal — voriconazole, posaconazole, fluconazole — for itraconazole intolerance or interaction
Therapeutic drug monitoring of itraconazole trough (target >1 microg/mL) — absorption is variable and pH-dependent
Lifelong suppressive itraconazole 200 mg daily in advanced HIV until immune reconstitution (CD4 >150 for >6 months on ART)
Complications
Progressive disseminated histoplasmosis with multiorgan failure
Adrenal insufficiency from bilateral adrenal involvement
Fibrosing mediastinitis (rare, late complication: invasive, often calcified fibrosis encasing mediastinal structures, causing SVC syndrome, pulmonary vessel or airway obstruction; distinct from mediastinal granuloma, a discrete caseous nodal mass that is usually asymptomatic)
Broncholithiasis from calcified lymph nodes eroding into airways
CNS infection with chronic meningitis or focal lesions
Pericarditis from adjacent mediastinal inflammation
PANCE pearls
Think histoplasmosis in any HIV patient with CD4 <150, fever, pancytopenia, and exposure to Ohio/Mississippi River valley.
Urine antigen is the most useful single test for disseminated disease and is sensitive even when other tests fail.
Itraconazole capsule absorption requires gastric acidity — give with cola or food; solution is taken on empty stomach.
Avoid TNF-alpha inhibitor therapy in patients with active histoplasmosis; there is no routine screening test for latent histoplasmosis comparable to TB testing; histoplasmosis within the past 2 years may warrant itraconazole prophylaxis and urine antigen monitoring during immunosuppression.
Fibrosing mediastinitis is a feared late complication that is not improved by antifungal therapy and may require stenting or surgery.
References
IDSA 2007 — Clinical Practice Guidelines for the Management of Patients with Histoplasmosis (Wheat et al., Clin Infect Dis 2007)
CDC — CDC — Histoplasmosis: epidemiology, diagnosis, and treatment
NIH/CDC/HIVMA-IDSA (clinicalinfo.hiv.gov) — Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV — Histoplasmosis section
IDSA 2025 — Arnold SR, Spec A, Baddley JW, et al. 2025 Clinical Practice Guideline Update by the Infectious Diseases Society of America on Histoplasmosis: Treatment of Asymptomatic Histoplasma Pulmonary Nodules (Histoplasmomas) and Mild or Moderate Acute Pulmonary Histoplasmosis in Adults, Children, and Pregnant People. Clin Infect Dis 2026;81(Suppl 3):i27-i32
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