Fungal Respiratory Infections

Fungal respiratory infections are diseases of the airways and lung parenchyma caused by fungi, including endemic dimorphic fungi such as Histoplasma and Coccidioides and opportunistic molds and yeasts such as Aspergillus. Their behavior depends heavily on the host: many cause limited or asymptomatic illness in healthy people but produce severe, sometimes invasive disease in those with impaired immunity.

Definition

Fungal respiratory infections are infections of the respiratory tract and lung parenchyma caused by fungi, ranging from self-limited pneumonitis in immunocompetent hosts to chronic cavitary or invasive disease, with severity strongly modulated by the host's immune state.

Scope

This entry covers the main groups of pulmonary fungi (endemic dimorphic fungi and opportunistic molds and yeasts), routes of acquisition, the decisive role of host immune status, the spectrum from self-limited to invasive and chronic disease, and epidemiology including geographic distribution. It is a reference overview of how fungal respiratory infections are understood and categorized, not a diagnostic or treatment protocol.

Key concepts

• Endemic dimorphic fungi (Histoplasma, Coccidioides, Blastomyces)
• Opportunistic molds and yeasts (Aspergillus, Cryptococcus, Pneumocystis)
• Inhalation of spores as the route of acquisition
• Host immune status and opportunistic infection
• Invasive versus chronic versus allergic forms of aspergillosis
• Geographic and environmental distribution

Mechanisms

Most pulmonary fungal infections begin with inhalation of fungal spores or conidia from the environment — soil, decaying matter, or bird and bat droppings for certain endemic fungi. In an immunocompetent host, innate and cell-mediated immunity usually contain the organisms, often leaving only a self-limited pneumonitis or asymptomatic infection. When host defenses are impaired — by neutropenia, corticosteroids, transplantation, advanced HIV, or other immunosuppression — fungi can proliferate and produce progressive disease, including angioinvasive infection in which molds such as Aspergillus invade blood vessels and disseminate. Pre-existing lung cavities or structural damage can also be colonized, giving rise to chronic forms such as aspergilloma and chronic pulmonary aspergillosis.

Clinical relevance

Fungal respiratory infections are an important consideration in immunocompromised patients and in people exposed in endemic regions, and recognizing the influence of host status and geography is central to understanding this group of diseases. This description supports educational understanding of the entities and is not a basis for diagnosing or treating an individual patient.

Epidemiology

The endemic dimorphic fungi have characteristic geographic distributions, so exposure history and region are central to their epidemiology; histoplasmosis and coccidioidomycosis, for example, are concentrated in particular areas where the organisms reside in the environment (Wheat, 2007; Galgiani, 2016). Opportunistic molds and yeasts cause disease disproportionately among immunocompromised hosts, and invasive aspergillosis is an important cause of morbidity in profoundly immunosuppressed patients, while chronic pulmonary aspergillosis complicates pre-existing structural lung disease (Patterson, 2016; Denning, 2016).

History

The endemic mycoses were characterized through twentieth-century studies linking regional outbreaks to environmental exposure, and recognition of opportunistic fungal pneumonia expanded markedly with the growth of cancer chemotherapy, organ transplantation, and the HIV epidemic, which produced large populations of immunocompromised hosts. Improved fungal diagnostics and antifungal therapy subsequently reshaped the recognition and classification of invasive and chronic pulmonary fungal disease.

Seminal works

• patterson-2016
• wheat-2007
• galgiani-2016

Frequently asked questions

Why are fungal respiratory infections more common in immunocompromised people?

Intact cell-mediated and innate immunity usually contain inhaled fungi, so many infections are mild or asymptomatic in healthy hosts; when immunity is impaired, the same organisms can proliferate and cause severe or invasive disease.

What is the difference between endemic and opportunistic fungal lung infections?

Endemic dimorphic fungi, such as Histoplasma and Coccidioides, occur in specific geographic regions and can infect healthy people exposed there, whereas opportunistic fungi, such as Aspergillus, mainly cause serious disease in people with impaired immunity or damaged lungs.