Histoplasmosis is caused by infection with the dimorphic soil fungus Histoplasma capsulatum. Histoplasmosis is in any parts of the world with warm humid soil and large migratory bird populations. It is the most common pulmonary and systemic mycosis of humans. Even under the most adverse conditions, ordinary soil teems with life. One square meter of plain dirt can contain more than 10,000 varieties of bacteria and fungi and up to 1,000 species of protozoa, nematodes and earthworms. Most soil organisms are highly beneficial, but a few can be dangerous, and some can even be deadly. One of these is the fungus Histoplasma capsulatum, which in humans and other animals causes a lung disease called histoplasmosis.
It's transmitted through airborne spores that you breathe into your lungs when you work in or around soil that contains the fungus. Farmers, landscapers, construction workers and people who have contact with birds or bats are especially at risk.
Most people with histoplasmosis never develop signs and symptoms and aren't aware they have the disease. But for some people primarily infants and those with compromised immune systems histoplasmosis can be much more serious.
Fortunately, effective treatments are available for even the most severe forms of histoplasmosis. But these therapies often involve extensive hospital stays and can cause serious side effects, so it's best for people with compromised immune systems to do everything possible to avoid infection.
Signs and symptoms
Several types of histoplasmosis exist, ranging from mild to life-threatening. The most benign form produces no signs or symptoms, but severe infections may cause serious problems throughout your body. When signs and symptoms do occur, they usually appear three to 17 days after exposure.
Common types of histoplasmosis include:
Histoplasma capsulatum is primarily found in the temperate regions of the world.
The fungus thrives in damp soil that's rich in organic material, especially the droppings from birds and bats. For that reason, it's particularly common in chicken and pigeon coops, old barns, caves and parks.
Birds themselves aren't infected with histoplasmosis their body temperature is too high but they can carry H. capsulatum on their wings, and their droppings support the growth of the fungus. Birds commonly kept as pets, such as canaries and parakeets, aren't affected.
Although bats, which have a lower body temperature, can be infected, you can't contract histoplasmosis from a bat or from another person.
Instead, you develop histoplasmosis when you inhale the reproductive cells (spores) of the fungus. The spores are extremely light and float into the air when dirt or other contaminated material is disturbed. That's why a high number of cases occur in farmers, landscapers, construction workers and spelunkers. In one instance, an entire Boy Scout troop became infected after cleaning up an old park filled with roosting starlings.
Histoplasmosis and your lungs
Because the spores of H. capsulatum are no more than two-millionths of a meter in diameter, they can easily enter your lungs and settle in the small air sacs. There, the spores are trapped by macrophages immune system cells that attack foreign organisms. The macrophages carry the spores to lymph nodes in your chest, where they continue to multiply. This may lead to inflammation, scarring and calcium deposits. In cases of heavy infection, the lymph nodes may become so enlarged that they obstruct your esophagus or your lungs' airways.
Most often, however, you're not likely to have noticeable signs and symptoms, and the infection clears on its own without treatment. But if your immune system isn't able to eliminate the spores, they can enter your bloodstream and travel to other parts of your body. In that case, you may develop a variety of severe problems that can be fatal if not diagnosed and treated quickly.
Anyone exposed to H. capsulatum is likely to become infected. People who inhale a huge number of spores those who work with heavily infected soil or have close contact with bats, for example are more likely to develop signs and symptoms.
Most at risk of infection are:
Because their immune systems are weakened, the following people are most likely to develop disseminated histoplasmosis, the potentially fatal form of the disease:
Histoplasmosis > 1 > 2 > 3 > 4