Histoplasmosis is a lung infection caused by breathing in spores from a fungus called Histoplasma, which lives in soil enriched by bird or bat droppings. Most people who inhale the spores never get sick, and only about 1% of infections cause noticeable symptoms. But for certain groups, particularly people with weakened immune systems or chronic lung disease, histoplasmosis can become serious or even life-threatening.
How People Get Histoplasmosis
Histoplasma spores live in soil, especially soil contaminated with bird or bat droppings. When that soil gets disturbed, whether by construction, demolition, cleaning out an old chicken coop, or even gardening, microscopic spores become airborne. They’re far too small to see. You get infected simply by breathing them in.
Once inside the lungs, body heat triggers the spores to transform into a yeast form, which is what actually causes infection. Histoplasmosis is not contagious. You can’t catch it from another person or from touching contaminated soil directly. The only route is inhaling the airborne spores.
Where the Fungus Lives
In the United States, Histoplasma is most concentrated in the central and eastern states, particularly around the Ohio and Mississippi River valleys. Statistical modeling from the CDC suggests the fungus thrives in areas with specific combinations of soil acidity, land cover, and proximity to water. That said, cases have been identified well outside these traditional hotspots, and the fungus exists on every continent.
The average annual incidence in states that track histoplasmosis runs between 1 and 2 cases per 100,000 people, though some states report up to 7 cases per 100,000. Those numbers only reflect diagnosed, reported cases. Many mild infections are never identified.
Symptoms and Timeline
When histoplasmosis does cause symptoms, they typically appear 3 to 17 days after exposure. The acute pulmonary form, by far the most common, feels a lot like the flu: fever, chills, cough, chest pain, headache, muscle aches, and general fatigue. Most healthy adults recover on their own within a few weeks without treatment.
People with pre-existing lung conditions, such as emphysema, can develop chronic pulmonary histoplasmosis. This form lingers, with symptoms like a persistent cough and worsening shortness of breath that may persist for months. It can gradually damage lung tissue if untreated.
Disseminated Histoplasmosis
The most dangerous form occurs when the infection spreads beyond the lungs to other organs, including the liver, spleen, bone marrow, and brain. This disseminated form primarily affects people with severely weakened immune systems. In people living with HIV, disseminated histoplasmosis is closely linked to advanced immune suppression, typically when a specific type of immune cell (CD4 count) drops below 50 cells per cubic millimeter, a level that signals profound vulnerability to opportunistic infections.
Among people hospitalized with histoplasmosis of any type, death rates generally range from 5% to 7%. With proper antifungal treatment, however, outcomes improve significantly. More than 80% of AIDS patients with disseminated histoplasmosis respond well to appropriate therapy when it’s started promptly.
How It’s Diagnosed
Diagnosing histoplasmosis can be tricky because its symptoms overlap with pneumonia, tuberculosis, and other lung infections. Doctors rely on two main types of lab tests.
Antigen tests look for pieces of the fungus itself in your urine or blood. Urine antigen testing is especially useful for disseminated disease, with a sensitivity of about 95%, meaning it correctly identifies the infection in nearly all cases. Blood-based antigen testing is slightly less sensitive at around 86%.
Antibody tests check whether your immune system has mounted a response to the fungus. These work better for less severe or chronic infections, where the body has had time to produce antibodies. Combining both antigen and antibody testing pushes overall sensitivity up to about 96%, giving doctors the clearest picture.
Treatment
Mild cases in otherwise healthy people often resolve without medication. When treatment is needed, the standard approach uses antifungal drugs, with the choice depending on how severe the infection is.
For mild to moderate cases, treatment typically involves an oral antifungal taken for 6 to 12 weeks. This medication can cause side effects like nausea, vomiting, rash, and swelling in the feet or ankles. It also interacts with many other medications, so your doctor will need to review everything you’re taking. Rare but more serious side effects include liver damage and heart problems, which require monitoring. For severe or disseminated infections, treatment starts with a stronger intravenous antifungal in a hospital setting before transitioning to oral medication.
Treatment timelines vary. A straightforward acute case might need only a few weeks of medication, while disseminated disease can require a year or more of antifungal therapy.
Fibrosing Mediastinitis: A Rare Complication
In very rare cases, fewer than 1 in 20,000 people with histoplasmosis, the body overreacts to the infection and produces excessive scar tissue in the center of the chest. This condition, called fibrosing mediastinitis, creates a dense mass of collagen that can compress the airways, major blood vessels, and the esophagus.
Symptoms include chest pain, cough, difficulty breathing, and sometimes pain with swallowing. About 6% to 11% of patients develop a blockage of the large vein that returns blood from the upper body to the heart, which causes swelling in the face, neck, and arms. Prognosis depends heavily on the location and extent of the scarring. When fibrosis is limited to one side of the chest, survival rates are comparable to the general population. Bilateral involvement carries higher risk.
Who Faces the Highest Risk
Certain groups are more likely to develop severe histoplasmosis:
- People with weakened immune systems, including those with HIV/AIDS, organ transplant recipients on immunosuppressive drugs, and people receiving certain cancer treatments or biologic therapies
- People with chronic lung disease, particularly emphysema, who are prone to the chronic pulmonary form
- Infants and older adults, whose immune systems may not mount an effective response
- Workers who disturb contaminated soil, such as construction crews, farmers, landscapers, and anyone cleaning buildings with heavy bird or bat droppings
Protecting Yourself
There is no vaccine for histoplasmosis. Prevention comes down to reducing your exposure to contaminated soil and droppings, and wearing proper respiratory protection when avoidance isn’t possible.
For medium-risk activities like outdoor work that involves digging or disturbing soil, the CDC recommends at least an N95 filtering facepiece respirator. For high-risk activities like cleaning up large accumulations of bird or bat droppings, stronger protection is recommended: a powered air-purifying respirator or a full facepiece respirator with high-efficiency filters. Standard N95 masks offer a protection factor of only 10, which may not be sufficient for heavy exposure.
Beyond respirators, wear gloves whenever your hands will contact contaminated material. Safety goggles protect your eyes, especially in areas with ventilation that could blow spores around. Disposable coveralls and shoe covers prevent you from carrying spores home on your clothing. Wetting soil before disturbing it can also help keep spores from becoming airborne, though it doesn’t eliminate the risk entirely.