A 54-year-old woman with pancytopenia

3. Which of the following tests would establish a definitive diagnosis in this patient?

• Methenamine silver stain of the marrow
• Serum antibody testing
• Fungal culture
• Peripheral blood smear
• Carbolfuchsin stain of marrow
• Urine histoplasma antigen

A prompt diagnosis is critical in patients with acute pulmonary histoplasmosis or progressive disseminated histoplasmosis because early treatment may shorten the clinical course and length of treatment and, in cases of disseminated histoplasmosis, prevent death.^8–10

Histopathologic examination of the bone marrow gives the most rapid results, although biopsy to obtain the tissue is invasive. It can give a definitive diagnosis if it reveals the typical 2- to 4-μm yeast structures of H capsulatum. These are observed on an aspirate smear of the patient’s bone marrow biopsy (Figure 1) and can be confirmed by methenamine silver or periodic acid-Schiff staining of the tissue.

Antibody detection is less practical because the antibodies take 2 to 6 weeks after infection to form.¹¹ Also, it is less useful in cases of disseminated infection because many of these patients are immunosuppressed.

Fungal culture remains the gold standard diagnostic test for histoplasmosis. However, results may take up to 1 month and may be falsely negative in less severe cases.

Histoplasma antigen testing is of greater utility in patients with severe disease, including cases of disseminated histoplasmosis. Rates of antigen detection approach 90% in urine specimens from non-AIDS patients with disseminated infection.¹² The urine assay has a greater sensitivity and specificity than the serum assay. The rate of detection is lower (ie, around 82%) in patients with acute pulmonary histoplasmosis when both the serum and urine specimens are tested.¹³

The immunoassay for histoplasma antigen is particularly useful for monitoring the response to therapy. Antigen levels should be measured before treatment is started and at 2 weeks, 1 month, and then approximately every 3 months during therapy.¹⁴ If the treatment is effective, antigens should decline by at least 20% in the first month of treatment and by another 20% in each of the following 3-month intervals. Antigen testing should be done every 3 months until a negative antigen level is achieved. The antigen level should also be followed for at least 6 months after treatment has stopped.¹⁴

HISTOPLASMA IS INHALED

H capsulatum is the cause of one of the most common pulmonary and systemic mycotic infections in the world, with hundreds of thousands of new cases annually. In areas where the soil is contaminated by bird or bat guano, the fungus is inhaled, resulting in an asymptomatic or a self-limiting influenza-like syndrome in an immunocompetent individual.¹⁵

An antigen-specific CD4⁺ T lymphocytemediated immunity occurs. The immune response of the host is thought to be fungistatic rather than fungicidal, resulting in a persistent inactive infection capable of reactivation in the presence of a host-pathogen imbalance.¹⁶

Most infections are asymptomatic or self-limited. For every 2,000 acute infections there is one that results in severe and progressive dissemination, usually in an immunocompromised host.^17,18

TREATMENT OF HISTOPLASMOSIS

4. What is the appropriate initial choice of treatment for a severe case of disseminated histoplasmosis?

• Amphotericin B in a lipid complex formulation (Abelcet)
• Itraconazole (Sporanox)
• Fluconazole (Diflucan)
• Ketoconazole (Nizoral)

Untreated, acute disseminated histoplasmosis can progress over a period of 2 to 12 weeks, ultimately killing the patient.^17,19

The leading therapies include amphotericin B in a lipid formulation and azole drugs, in particular itraconazole. Fluconazole and ketoconazole are not first-line options in severe cases because they are less predictably effective, and ketoconazole has a higher rate of side effects.^20–23 The current recommendation is to treat severely ill hospitalized patients with one of the liposomal formulations or the lipid complex formulation of amphotericin B. Itraconazole is used for patients who have mild to moderate symptoms and as a step-down therapy in patients who improve after initial use of amphotericin B.

CASE CONCLUDED: THE PATIENT RECOVERS

At the time of the initial patient encounter, there was no history of or obvious cause of immunosuppression in this patient. She was found to be HIV-negative and was subsequently diagnosed with “profound immunosuppression of unknown etiology” resulting in a low CD4 count.

The patient receives trimethoprim-sulfamethoxazole (Bactrim, Septra) and azithromycin (Zithromax) for prophylaxis against Pneumocystis carinii pneumonia and Mycobacterium avium intracellulare infection. Two months after the hospitalization, she recalls being at a corn maze 1 month before becoming ill.