Histoplasmosis: Fungal Infection Mimicking Tuberculosis

Histoplasmosis stands out among fungal infections for its ability to closely mimic tuberculosis, creating diagnostic challenges that healthcare professionals must navigate carefully. Caused by the fungus Histoplasma capsulatum, this infection primarily targets the respiratory system but can spread throughout the body, particularly affecting organs of the reticuloendothelial system. Understanding how this pathogen behaves as an intracellular parasite helps explain both its clinical presentations and why it’s so frequently confused with other diseases.

What makes Histoplasma capsulatum unique 🔗

Histoplasma capsulatum is a dimorphic fungus with a remarkable ability to survive and multiply inside the very cells designed to destroy it. Unlike most pathogens that remain outside human cells, H. capsulatum lives almost exclusively as an intracellular pathogen within macrophages-the immune cells meant to engulf and eliminate invaders. In the environment, it exists as a mold in soil enriched with bird or bat droppings. When these spores are inhaled, they transform into a yeast form at body temperature and establish infection within host cells.

The fungus specifically targets the reticuloendothelial system, which includes organs like the spleen, liver, lymph nodes, and bone marrow. During the first couple of weeks, Histoplasma yeasts multiply inside alveolar macrophages and spread throughout the reticuloendothelial system. This ability to parasitize macrophages and travel within them throughout the body explains the wide range of clinical manifestations seen in histoplasmosis.

How infection occurs and who’s at risk 🔗

Histoplasmosis is typically acquired via inhalation of airborne microconidia, often after disturbance of contaminated material. Activities that increase risk include spelunking (cave exploration), cleaning chicken coops, construction work, demolition, and landscaping. The fungus thrives in environments containing large amounts of bird or bat droppings, where soil becomes heavily contaminated.

In the United States, Histoplasma mainly lives in soil in the central and eastern states, particularly areas around the Ohio and Mississippi River Valleys. However, the disease occurs worldwide, with significant endemic areas in Central and South America, Africa, and parts of Asia.

While most infections remain asymptomatic, certain populations face higher risks for severe disease. Disseminated histoplasmosis is more likely to occur in immunosuppressed persons including people who have HIV/AIDS, organ transplant recipients, those using immunosuppressive medications, infants, and adults age 55 years and older.

The tuberculosis masquerade 🔗

The resemblance between histoplasmosis and tuberculosis represents one of the most significant diagnostic challenges in infectious disease medicine. Chronic pulmonary histoplasmosis can present with features similar to pulmonary tuberculosis and may be misdiagnosed as smear-negative pulmonary TB in endemic areas. Both diseases can cause chronic cough, weight loss, fever, and chest pain. Radiologically, both may show upper lobe cavitation, making them nearly indistinguishable without specific laboratory testing.

This similarity creates real-world consequences for patients. The non-specific nature of clinical, biologic, histologic, and radiologic findings makes differential diagnosis difficult in disease-endemic areas, with numerous publications reporting cases of histoplasmosis mimicking tuberculosis, often because diagnostic facilities were absent or the diagnosis wasn’t considered. In countries with high TB burdens, patients may receive anti-tuberculosis treatment for months without improvement because their actual infection is histoplasmosis.

The confusion works both ways-histoplasmosis and tuberculosis can occur simultaneously in immunocompromised patients, particularly those with HIV/AIDS. Distinguishing between the two, or recognizing when both are present, requires specific fungal and mycobacterial testing rather than relying solely on clinical presentation.

Clinical manifestations of histoplasmosis 🔗

Acute pulmonary disease 🔗

Most people infected with H. capsulatum never develop symptoms. Symptomatic infections usually present 3 to 17 days after exposure, with symptoms including fever, malaise, cough, headache, chest pain, chills, and myalgias. These flu-like symptoms typically resolve on their own in immunocompetent individuals within a few weeks, though fatigue may persist longer.

Chronic pulmonary histoplasmosis 🔗

Persons with underlying lung disease, particularly emphysema, can develop chronic pulmonary disease. This form progresses slowly over months, causing weight loss, productive cough, and sometimes hemoptysis (coughing up blood). The upper lobe cavitation that develops closely resembles reactivation tuberculosis both clinically and radiologically.

Disseminated disease with organomegaly 🔗

When the infection spreads beyond the lungs, it preferentially affects organs of the reticuloendothelial system. In disseminated disease, involvement of the reticuloendothelial system leads to enlargement of the spleen (splenomegaly), liver (hepatomegaly), and lymph nodes (lymphadenopathy). Patients may also develop fever, anemia, and changes in blood cell counts due to bone marrow involvement.

Without treatment, disseminated histoplasmosis proves fatal, particularly in immunocompromised individuals. The infection can also affect the central nervous system, gastrointestinal tract, adrenal glands, and skin.

Oral and mucocutaneous lesions 🔗

One particularly distinctive feature of disseminated histoplasmosis is the development of painful ulcers in the mouth and on mucous membranes. Oral lesions can occur at any oral mucosa, but are most commonly found on the tongue, palate, and buccal mucosa, presenting as painful ulcers that persist for several weeks. These lesions may have a malignant appearance with raised, irregular borders, prompting biopsies to rule out cancer.

The clinical spectrum of disseminated histoplasmosis includes painful mucocutaneous lesions that can occur anywhere along the gastrointestinal tract, from mouth to anus. In some cases, these oral ulcers represent the first or only visible manifestation of disseminated disease, making them important diagnostic clues.

Diagnosing histoplasmosis 🔗

Given the similarity to tuberculosis and other infections, accurate diagnosis requires specific testing. Histoplasma antigen detection in urine or serum is the most widely used and most sensitive method. This test can provide results within days and works even in immunocompromised patients who may not mount strong antibody responses.

Other diagnostic methods include antibody testing (though antibodies may take 2-6 weeks to develop), fungal culture (which can take up to 6 weeks), and microscopic examination of tissue samples. Biopsy of oral ulcers often proves diagnostic when these lesions are present, showing the characteristic small yeast forms within macrophages on special stains.

Treatment approaches 🔗

Mild to moderate cases of acute pulmonary histoplasmosis will often resolve without treatment, but treatment is indicated for moderate to severe acute pulmonary, chronic pulmonary, disseminated, and central nervous system histoplasmosis. For severe infections, liposomal amphotericin B is the initial therapy of choice, followed by step-down treatment with itraconazole for mild-to-moderate infections and long-term suppressive therapy.

The duration of treatment varies based on disease severity and immune status. Immunocompromised patients may require prolonged or even lifelong suppressive therapy to prevent relapse.

Prevention strategies 🔗

Preventing histoplasmosis focuses on avoiding exposure to contaminated environments. High-risk individuals, particularly those who are immunocompromised, should avoid activities involving soil disruption in endemic areas, especially cleaning bird roosts, bat-inhabited caves, or old chicken coops. When exposure is unavoidable, wearing N95 respirators provides protection against inhaling spores.

For construction or demolition projects in endemic areas, wetting down soil before disturbance helps reduce aerosolization of spores. In highly immunocompromised individuals living in endemic areas, prophylactic itraconazole may be considered.

What do you think? How might you distinguish between histoplasmosis and tuberculosis in a patient presenting with chronic cough and weight loss? What additional steps would you take if a patient with suspected tuberculosis doesn’t improve on anti-TB therapy?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC4827932/
2. https://www.atsjournals.org/doi/full/10.1513/pats.200907-069AL
3. https://www.cdc.gov/histoplasmosis/hcp/clinical-overview/index.html
4. https://www.cdc.gov/histoplasmosis/data-research/maps/index.html
5. https://academic.oup.com/ofid/article/7/5/ofaa119/5816327
6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3919221/
7. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/histoplasma-capsulatum
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC5677060/
9. https://www.acpjournals.org/doi/10.7326/aimcc.2022.0643