Advances in the Diagnosis of Histoplasmosis: An Update into the Laboratory Diagnosis of Histoplasma capsulatum

Abstract

Histoplasmosis is a life-threatening systemic infection caused by the thermally dimorphic fungal pathogen Histoplasma capsulatum, whose natural reservoirs are thought to be soil enriched with bird excreta and bat guano. Infection is most frequently diagnosed in the Americas, but it has been reported worldwide. Among immunocompetent individuals, histoplasmosis is mostly asymptomatic and spontaneously self-limited. However, in patients with an acquired or congenital T cellular immunity deficiency, histoplasmosis is responsible for an important morbidity and mortality. In HIV infected patients, its extrapulmonary or disseminated form, known as progressive disseminated histoplasmosis (PDH) became an AIDS-defining illness, especially in Latin America, where an estimated 15,000 new infections and 9,000 deaths occur among people living with HIV (PLHIV). Histoplasmosis diagnosis is still challenging and limited in many countries. Symptoms of PDH are often nonspecific, and among PLHIV, they may be similar to those of other diseases, especially tuberculosis, complicating diagnosis and treatment. As PDH is associated with high mortality rates due to late diagnosis rapid detection of histoplasmosis and specific treatment are needed. In this regard, some progress has been recently achieved, such as the publication of World Health Organization (WHO)/Pan American Health Organization guidelines for diagnosis and management of PDH among PLHIV and the inclusion of Histoplasma antigen testing in the WHO list of Essential Diagnostics. Conventional laboratory methods used for diagnosis of histoplasmosis, such as culture and histopathology, pose many challenges, including the need for complex laboratory infrastructure and staff with mycology training, a turnaround time of up to several weeks, and limited sensitivity. Diagnosis using serologic methods is further complicated by a low antibody test sensitivity when performed in immunocompromised individuals. Several molecular “in-house” tests were also developed and showed promising results but none of these tests are commercially available, and their standardization and validation are still pending. Detection of urine Histoplasma antigen has been described as the best option to diagnose PDH due to its high sensitivity. However, crossreactivity with other related fungi is common. Moreover, this assay is expensive and only performed in a few laboratories. Novel protein antigen candidates have been recently identified and produced by DNArecombinant techniques in order to obtain standardized and specific reagents for the diagnosis of histoplasmosis, in contraposition to the unspecific antigens or crude extracts used. This chapter describes the currently available diagnostic assays, highlighting their strengths and limitations, and reports the latest approaches to achieve reliable and rapid diagnostic tests for histoplasmosis.