Opportunistic and Ubiquitous Mycoses

Opportunistic and ubiquitous mycoses are infections caused by fungi that are common in the environment or in the normal human flora and that usually cause serious disease only when host defences are weakened. Candidiasis, invasive aspergillosis, and cryptococcosis are the leading examples, and as the population of immunocompromised people has grown, these infections have become a major cause of deep and life-threatening fungal disease.

Definition

Fungal infections caused by ubiquitous environmental fungi or members of the normal flora that typically produce significant disease only when host defences are impaired; their distribution is worldwide and their occurrence is governed chiefly by host susceptibility rather than geographic exposure.

Scope

The topic covers fungi that are widely distributed — as commensals like Candida or as ubiquitous environmental moulds like Aspergillus — and that act as opportunists in the setting of immunosuppression, neutropenia, critical illness, or indwelling devices. It is a reference overview of the category, not individualised clinical guidance.

Key concepts

• Opportunism dependent on host immune status
• Commensal fungi (Candida) as endogenous source
• Ubiquitous environmental moulds (Aspergillus)
• Neutropenia, immunosuppression, and critical illness as risk states
• Indwelling devices and disrupted barriers
• Invasive candidiasis and candidemia
• Cryptococcosis and infection in advanced HIV

Mechanisms

Unlike the endemic fungi, opportunistic pathogens generally do not need an unusual exposure: Candida species already colonise mucosal surfaces, and Aspergillus conidia are inhaled by everyone from the ambient air. Disease arises when host defences fail — for example through neutropenia, defective cell-mediated immunity, broad-spectrum antibiotic use, mucosal or skin barrier breakdown, or indwelling vascular devices — allowing these fungi to invade and, in the case of Candida, to enter the bloodstream (Pappas, 2016; Patterson, 2016; Rippon, 1988). Because the determining factor is the host rather than geography, these infections are ubiquitous in distribution and frequently deep and disseminated in tissue depth.

Clinical relevance

Opportunistic mycoses are central to the care of immunocompromised and critically ill patients, in whom they are an important cause of morbidity and mortality, so awareness of host risk factors is part of recognising them. This entry summarises the category and its microbiology for reference and is not a guide to diagnosing or treating any individual patient.

Epidemiology

Invasive candidiasis is among the most common healthcare-associated fungal infections and Aspergillus is a leading cause of invasive mould disease, while cryptococcal disease remains a major problem in people with advanced HIV; global estimates place these opportunistic infections among the principal contributors to serious fungal disease worldwide, with their burden tracking the size of at-risk populations (Bongomin, 2017).

History

Candida, Aspergillus, and Cryptococcus were long known as environmental and commensal fungi, but their prominence as invasive pathogens grew sharply in the later twentieth century with the expansion of cancer chemotherapy, transplantation, intensive care, and the HIV pandemic, a transition reflected in the evolution of medical mycology and codified in texts such as Rippon's Medical Mycology (Rippon, 1988).

Seminal works

• pappas-2016
• patterson-2016
• bongomin-2017

Frequently asked questions

What makes a fungus an 'opportunist'?

An opportunistic fungus is one that is common in the environment or normal flora and usually harmless, but that can cause serious infection when the host's immune defences or barriers are impaired.

Why are these mycoses called ubiquitous?

Their causative fungi are found everywhere — as part of the normal human flora or as common environmental moulds — so exposure is universal and disease depends on host susceptibility rather than living in a particular region.