Endemic Systemic Mycoses

Endemic systemic mycoses are deep fungal infections caused by thermally dimorphic fungi that live in particular soils and environments and are usually acquired by inhaling their spores. Unlike the opportunistic moulds, several of these fungi can cause disease even in people with intact immune systems, and because each fungus is tied to a specific geographic niche, the infections cluster in defined endemic regions. Histoplasmosis is the central example.

Definition

Deep fungal infections caused by geographically restricted, thermally dimorphic fungi that are typically acquired by inhalation of environmental spores and can establish systemic disease; they are termed primary or endemic systemic mycoses because they can affect immunocompetent hosts within their endemic zones.

Scope

The topic covers the classic endemic dimorphic mycoses — histoplasmosis, coccidioidomycosis, blastomycosis, paracoccidioidomycosis, and related infections such as talaromycosis — emphasising their shared biology of thermal dimorphism, inhalational acquisition, and geographic restriction. It is a reference overview and does not provide individualised diagnosis or treatment.

Key concepts

• Thermal dimorphism (mould in environment, yeast or spherule in tissue)
• Inhalation of conidia as the portal of entry
• Geographic endemicity tied to environmental reservoir
• Primary pulmonary infection with potential dissemination
• Disease in immunocompetent as well as immunocompromised hosts
• Histoplasma, Coccidioides, Blastomyces, Paracoccidioides, Talaromyces
• Reactivation and latency

Mechanisms

The defining feature of these organisms is thermal dimorphism: they grow as a filamentous mould in the soil at ambient temperature and convert to a yeast form (or, for Coccidioides, a spherule) at human body temperature. Inhaled conidia reach the lung, undergo this conversion, and establish a primary pulmonary infection that is often self-limited but can disseminate, especially when cellular immunity is impaired (Wheat, 2007; Rippon, 1988). Because each species occupies a distinct environmental niche, exposure and therefore disease are concentrated in defined regions, which is what makes these mycoses both systemic in depth and endemic in distribution (Benedict, 2017).

Clinical relevance

Endemic systemic mycoses are clinically important because they can mimic tuberculosis, malignancy, and other lung diseases, and because travel or residence history in an endemic area is a key clue to recognising them. This entry describes the category and its microbiology for reference and is not a basis for diagnosing or treating any individual patient.

Epidemiology

Each endemic mycosis is geographically anchored to the range of its causative fungus, and within those ranges large numbers of people may be exposed, often with subclinical infection; histoplasmosis and coccidioidomycosis in particular account for substantial morbidity in their endemic zones, and shifting environmental and travel patterns have been changing where these infections are recognised (Bongomin, 2017; Benedict, 2017).

History

Histoplasmosis was first described at the start of the twentieth century, and over subsequent decades the dimorphic nature and geographic endemicity of Histoplasma, Coccidioides, Blastomyces, and Paracoccidioides were established and brought together as the endemic systemic mycoses in classic texts such as Rippon's Medical Mycology (Rippon, 1988). Recognition that their ranges and recognised distribution are not fixed has been a more recent theme (Benedict, 2017).

Seminal works

• wheat-2007
• bongomin-2017
• rippon-1988

Frequently asked questions

What makes a mycosis 'endemic'?

Endemic mycoses are caused by fungi that live in specific soils and environments, so the infections occur mainly in the defined geographic regions where those fungi are found.

What is thermal dimorphism?

It is the ability of these fungi to grow as a mould in the cooler environment and switch to a yeast or spherule form at human body temperature, which is central to how they cause systemic infection after being inhaled.