Author: Robert Brooks

Histoplasma capsulatum is a dimorphic fungus that is responsible for the clinical disease Histoplasmosis and is the most common cause of fungal respiratory infections around the world. It is commonly found in soils and is primarily associated with areas that have been contaminated by bird or bat droppings such as caves, poultry farms, warehouses and vacant buildings. This organism can infect humans, dogs, cats as well as equines. Histoplasma capsulatum var. capsulatum is the North American variety whereas H. capsulatum var. dubosii is the African variety. The variant H. capsulatum var. farciminosum is responsible for lymphangitis in horses. All three of these organisms behave in a similar manner, undergoing a phase change once the spores or microconidia have been inhaled. Once inhaled, the organism quickly converts to its virulent form. This organism is endemic to the Mississippi and Ohio River valleys in the United States.

Dimorphic fungi, such as Histoplasma, undergo a temperature induced transition between growth phases, which means that it has two distinct phenotypes. This dimorphism makes these organisms difficult to correctly identify using traditional microbiological methods. At ambient temperatures below 30°C in contaminated soils, Histoplasma grows in its mycelial form with the appearance of typical molds. Once the spores or microconidia have been inhaled, the organism then undergoes a metamorphosis to its yeast or parasitic form. It is at this stage that Histoplasma becomes virulent. At normal body temperatures, around 37°C, the organism looks and behaves as yeast with the typical budding off of cells. This phase change, induced by the elevated temperatures of the body, is required for virulence.

Once infection occurs, the disease forms lesions within the host’s lungs or lymphatic system and mimics the symptoms seen in tuberculosis. Histoplasma can coexist with other chronic respiratory diseases such as tuberculosis, actinomycosis, and sarcoidosis. Isolates of Histoplasma may also resemble other infectious fungi, primarily Chrysosporium and Sepedonium. This variation and resemblance to other types of diseases and disease-causing fungi makes the diagnosis of Histoplasma not only critical, but adds to the difficulty of positive identification. The yeast-like form of the disease is not transmissible from person to person, but latent infections can be re-activated. Primarily, this disease causes mild infections in healthy individuals, but in some cases (5-10%), chronic symptoms occur or complications arise that can become life threatening. Individuals with weak or suppressed immune systems are especially at risk as are small children under the age of 2 years old.

Microbiological methods currently available for the detection of Histoplasma, require someone with a specific knowledge of their morphology and growth behavior, and require someone who uses media that may not be readily available. These methods are expensive, time consuming and rely upon growth of the organism for analysis. DNA-based PCR technology, which uses probes specific to these organisms, is a quicker, more accurate and cost-effective way of determining their presence or absence. Sequences that are found only in the genome of Histoplasma are targeted by the probes, allowing a precise, definitive result and do not require a viable organism for detection.

Microbac Laboratories Knoxville Division has the capability to reliably analyze environmental samples for Histoplasma capsulatum var. capsulatum by DNA-based methods. Using a semi-nested PCR approach with primers specific for H. capsulatum, Microbac can assess the danger posed by Histoplasma to workers at contaminated sites. This allows remediation companies and clean up crews to plan for the use of appropriate personal protective equipment to eliminate the chance of workers becoming infected by this organism. Quick identification of H. capsulatum allows clean up efforts to proceed in a safe and timely manner and minimizes costs from delays or from worker illness.