Histoplasmosis (cont.)

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Are there different types of histoplasmosis?

Histoplasmosis has three major types of disease, and these three have other subtypes included in them. They are summarized with their subtypes as follows:

  • acute pulmonary histoplasmosis; asymptomatic and symptomatic;
  • chronic pulmonary histoplasmosis; chronic lung symptoms and occasionally ocular involvement termed ocular histoplasmosis syndrome; and
  • progressive disseminated histoplasmosis: chronic progressive disseminated histoplasmosis with oropharyngeal lesions or ulcers; subacute progressive disseminated histoplasmosis with intestinal, adrenal, cardiac or central nervous system (CNS) involvement; and acute progressive disseminated histoplasmosis with encephalopathy, meningitis, mass lesions and cutaneous (skin) lesions.

Drawings and pictures of some of the different types of histoplasmosis are available at the last Web site listed below.

How is histoplasmosis transmitted?

Histoplasmosis is not transmitted person to person except for a few rare instances when a transplant patient has contracted histoplasmosis from a transplanted organ. The large majority of cases occur when people inhale fungal mycelia and spores, usually from a source where the fungus growth is enhanced. Such sources or areas are in caves containing bat or bird droppings, chicken coops, birdhouses, bird roosts, or soil contaminated with such droppings. Unfortunately, H. capsulatum can survive in soil for years, and if the soil becomes airborne (dust), inhalation of H. capsulatum-contaminated dust may lead to histoplasmosis.

How is histoplasmosis diagnosed?

Definitive diagnosis of histoplasmosis can be complicated, especially if the patient gives no history of exposure to areas contaminated with H. capsulatum. Many bacterial and fungal infections can produce clinical findings that are found with H. capsulatum infections (for example, granulomatous nodules found in sarcoidosis and tuberculosis, or lung infiltrates seen on X-ray in coccidiodomycosis, blastomycosis, aspergillosis, and other lung infections). In fact, before 1932, many patients with chronic pulmonary histoplasmosis were diagnosed as having tuberculosis. Differentiating histoplasmosis from other lung infections can still be a diagnostic challenge for clinicians.

Cultures of blood, sputum, or tissue biopsy samples can be cultured on media that supports H. capsulatum growth. If H. capsulatum is cultured, the diagnosis is confirmed. However, at best, cultures are positive in only about 60% of patients with chronic pulmonary infections and are positive in only about 15% of acute cases. Furthermore, it may take from two to 12 weeks for the fungus to grow enough to be identified in culture, which could delay treatment especially in progressive disseminated cases. Blood cultures range from 50%-90% positive in progressive disseminated cases. Positive cultures for H. capsulatum definitively diagnose histoplasmosis. Unfortunately, acute progressive disseminated histoplasmosis, if not treated quickly and appropriately, can lead to death in a few weeks. In suspected cases, treatment should begin immediately without waiting on cultures to grow positive for H. capsulatum.

There are several types of serology tests (tests for antigens and antibodies) that are used on blood, urine, and cerebral spinal fluid (CSF). They can give fairly rapid results especially with symptomatic, chronic, or progressive disseminated disease (75%-95% positive after six weeks) but take about three weeks to be positive in a small number (about 15%) of acute cases. Unfortunately, the reagents share some cross-reactivity with other fungal genera (for example, Aspergillus, Blastomyces, and Coccidioides), so false-positive results can be obtained. Further, these tests for H. capsulatum can be positive in people that live in areas with endemic histoplasmosis and also can be positive in patients that have inactive disease.

Stains (Giemsa stain or methenamine silver stain) of tissue samples, blood, sputum, bone marrow, lymph node aspirates, and other fluids can allow microscopic visualization of H. capsulatum. However, the accuracy of identification depends on the experience of the observer as other organisms may resemble H. capsulatum. Researchers suggest this method be backed up by other tests such as serology and cultures.

Many other tests may be used to help the physician determine the extent of infection by H. capsulatum. Examples are listed below:

  • CBC or complete blood count: Low white blood cell counts can occur in chronic progressive histoplasmosis.
  • Chest X-rays can show lung changes (infiltrates, cavitations, and enlarged lymph nodes) that may occur in chronic and acute progressive disseminated histoplasmosis.
  • CT scan may show bilateral adrenal gland involvement in subacute progressive disseminated histoplasmosis.
  • Echocardiography helps determine if heart valves are infected or if pericarditis is present in acute pulmonary or progressive disseminated histoplasmosis.
  • Alkaline phosphatase levels in the blood are increased in chronic pulmonary and acute progressive disseminated histoplasmosis.

Other tests and procedures may be done to help determine the diagnosis or extent of infection; those listed above are the most common. Occasionally, a surgeon may need to be consulted to obtain lung tissue, pleural fluid, or lymph node biopsy for culture and microscopic evaluation to aid diagnosis.

Medically Reviewed by a Doctor on 4/11/2014

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