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What is Histoplasmosis? - Infections Caused by Histoplasma capsulatum

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  • 0:05 Urban Farming
  • 1:07 Histoplasma capsulatum
  • 3:08 Histoplasmosis
  • 4:07 Diagnosis and Treatment
  • 5:03 Epidemiology
  • 5:52 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Raising backyard chickens can be a fun and rewarding experience, but you might end up raising more than just chicks. In this lesson, we will explore the world of Histoplasma capsulatum, a pathogenic fungus that can grow on chicken feces.

Urban Farming

People in the United States today are shifting the way they look at farming. More and more people are planting personal gardens in smaller and smaller spaces. Some have taken to roofs, while others are filling apartment window boxes with edibles. Growing vegetables is not the only farm activity gaining in popularity. I live in a major American city, and I could take a 20-minute walk and pass at least five houses with farm animals in the yard.

It seems that urban chickens and chicken coops have been springing up just as fast as urban gardens. Access to homegrown eggs and meat is not the only thing that can result from an increase in chickens. Due to the high concentration of bird feces, chicken coops are a major source of the infectious fungus Histoplasma capsulatum. However, these are not the only places that this fungus can develop. In addition to chicken coops, bat caves are also a common source of Histoplasma capsulatum.

Histoplasma Capsulatum

Histoplasma capsulatum, or simply Histoplasma for short, is a dimorphic fungus capable of causing respiratory disease in humans. A dimorphic fungus can live in two different forms depending on environmental conditions. Normally, Histoplasma is found in soil where it grows as a filamentous, mold-like fungus. It forms many branching filaments, similar to bread mold. When Histoplasma infects a human host, it switches and starts growing as a single-celled yeast.

The switch between growth forms depends on the temperature. Remember, the human body maintains a constant temperature of 37 degrees Celsius. Below 35 degrees, like in soils, Histoplasma is filamentous. Above 35 degrees, like in your body, it switches to the single-celled form.

I mentioned that Histoplasma is normally found in soil, growing as a filamentous fungus. This normal inhabitant of soil is found worldwide, yet poses little threat to people. People are really only in danger of infection when the population of the fungus gets concentrated. So, how does that happen?

Histoplasma forms spores in dry, dusty soil that can get stirred up and dispersed by wind. The spores are able to live happily if they land in soil again, but there is one substrate that the spores grow very well in: bird and bat feces. Sheltered areas with high concentrations of these feces provide an excellent growth media. Chicken coops and bat caves definitely fit this description. And, not coincidentally, these two environments serve as the major source of infections for humans.

A person exploring a cave covered in guano or tending to chickens housed in a coop, can inhale Histoplasma spores. This is the only documented method of Histoplasma exposure. The disease cannot be transmitted between humans or between humans and animals.

Histoplasmosis

For 99% of people that inhale Histoplasma spores, absolutely nothing will happen. The fungus won't cause any disease, or the body will eliminate the infection before any symptoms develop.

The unlucky 1% will develop histoplasmosis, which is a respiratory disease characterized by fever, headaches, chest pains, and a dry, nonproductive cough as a result of the parasite invading the lungs. These symptoms are usually mild and most healthy people can easily fight off the disease without treatment within a month or so.

There are a few high-risk groups in which histoplasmosis can become more severe. People with compromised immune systems, like AIDS patients, young children, and people with chronic lung diseases can develop painful lesions in the lungs, which can spread to other organs if left untreated. This, combined with the respiratory symptoms, results in a chronic pneumonia-like disease.

Diagnosis and Treatment

The good news is that Histoplasma is easy to diagnose. There is a very accurate urine test that detects antigens from recent infections. This is usually combined with a culture. A sample of lung tissue or respiratory secretions is obtained from a patient and spread on a specific growth medium. If Histoplasma is present, it will grow and form visible, identifiable colonies.

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