Introductory Chapter: Histoplasmosis – Challenges and New Perspectives

1. Introduction

Clinicians continue to pay attention to fungus infections because they represent a problematic pathology from various perspectives. Due to their largely non-specific symptomatology, polymorphous imaging features, and unpredictable evolution from asymptomatic disease to one that endangers life, pulmonary mycoses enter the differential diagnosis with many other disorders, whether infectious, granulomatous, neoplastic, or systemic in nature [1, 2].

A practitioner in an area with an endemic fungal illness is more equipped to facilitate a rapid and accurate diagnosis because they have a greater understanding of these diseases.

Despite being worldwide spread, histoplasmosis is better known in the Central and Midwestern parts of the United States, Ohio, and the Mississippi River Valley regions receiving the greatest attention [3]. European or African nations, however, report considerably fewer cases.

2. Challenges for histoplasmosis diagnosis

Geographic location seems to be less important today in triggering suspicion of histoplasmosis, given the extraordinary population movements (tourism, migration, or other reasons) and the numerous risk factors for host immunosuppression.

The quantity of conidia inhaled, the host’s immunological status, comorbid conditions, chronic immunosuppressive medications, local access to cutting-edge research techniques, and available therapeutic choices are examples of the many additional aspects that must be considered [4].

For instance, Ocansey and colleagues pointed out in a recent article that the exact burden of this disease in African nations is unknown. The high incidence of HIV infection in the population, the sometimes-confusing diagnosis of tuberculosis, and the restricted access of the public to precise diagnostic procedures in specialized clinics all contribute to the risk of underdiagnosis of this illness. Although the rate of histoplasmosis cases has increased in the last decade and awareness among healthcare workers has improved, studies are needed on the diversity of species of the genus Histoplasma (including African species), the clinical-evolutionary features, the HIV–histoplasmosis–tuberculosis interaction, and, last but not least, the genetic predispositions of the population [5].

Histoplasmosis can be present in various ways, ranging from the localized pulmonary or cutaneous form, to affecting multiple organs in acute and chronic forms, and from self-limited to widespread. Consequently, many clinical presentations at the thoracic level, such as infiltrates, nodules, cavity lesions, and mediastinal damage, including lymphadenopathies or mediastinitis, raise the suspicion of histoplasmosis and must be distinguished from other granulomatosis like sarcoidosis, tuberculosis, or malignancy [6].

It is necessary to have an even higher suspicion of histoplasmosis (5–27%) in patients living with HIV with CD4 below 150 cells/microL [7]. The disseminated forms are predominating in the clinical presentation. The acute disseminated disease, typical in individuals with significant immunosuppression, including those living with HIV or with organ transplantation, is characterized by non-specific symptoms such as fever, sweats, fatigue, cough, dyspnea, and weight loss [8]. Additionally, extrapulmonary involvement such as hepatosplenomegaly, lymphadenopathy, injury to the skin or mucosa, digestive issues, diseases of the adrenal glands and bone marrow, or abnormalities of the central nervous system are more common [9].

The rapidity of diagnosis depends on the sensitivity and specificity of the laboratory instruments used in conjunction with the time after exposure, the organ affected, the acute or chronic course, and the extent of infection (localized or disseminated). A rapid diagnosis can be made by enzyme immunoassay (EIA) to detect histoplasma antigens in bronchioloalveolar lavage fluid, urine, blood, or cerebrospinal fluid (CSF).

In acute diffuse pulmonary forms, serologic and antigen tests have high sensitivity several weeks after exposure. In localized mediastinal, adenophatic, or pulmonary forms (nodules), these tests may be negative or have lower sensitivity. To confirm the diagnosis, the physician must request staining of tissues for fungi in combination with culture from blood, sputum, or other tissues (adenophatic, lung masses, or bone marrow) [9]. Microscopic detection of yeasts in clinical specimens and growth of molds in cultures are the gold standards for confirming the diagnosis.

Differential diagnosis can be challenging, especially with other granulomatosis, such as sarcoidosis. Therefore, histoplasmosis should be ruled out in patients with mediastinal lymphadenopathy, nodules, infiltrative lesions, and even elevated angiotensin-converting enzyme serum levels if histopathologic examination revealed a noncaseating granuloma before initiating immunosuppressive treatment for sarcoidosis.

Several conditions must be met to support the diagnosis of histoplasmosis. First, the physician should think of this pathology when the symptoms and radiological manifestations (focal or diffuse airspace disease, lymphadenopathy, cavitary upper lobes, thick-walled bullae, and fibrotic mediastinitis) are compatible, especially in cases of high local endemicity [1, 3, 4]. Secondly, access to a wide range of tests that suggest and confirm histoplasmosis is desirable. Identifying Histoplasma capsulatum in culture from biopsy specimens, sputum, or bronchoalveolar lavage supports a precise positive diagnosis.

3. New perspectives for the treatment of histoplasmosis

The decision to initiate treatment for histoplasmosis depends on the form and clinical presentation of the disease: Self-limited forms usually do not require treatment, in contrast to moderate and severe forms with persistent symptoms, extensive lesions, or chronic conditions. In these cases, administering an effective treatment with minimal side effects, such as lipid amphotericin B followed by itraconazole for a sufficient period of time, is reasonable [10].

The use of nanocarriers for drug delivery has demonstrated their potential as an alternative and versatile technological platform for the treatment of intracellular infections caused by fungi of the species H. capsulatum [11]. However, future research is needed to improve the outcomes of this disease.

The chapters included in this book address all of these challenges related to the epidemiology, diagnosis, and treatment of histoplasmosis.