29th July 2023

Background

Fungi are eukaryotic organisms found in abundance worldwide. They encompass a vast range of species with widely varying biology. Fungi can be differentiated from other organisms by their chitin-containing cell wall and cell membrane, which contains ergosterol (as opposed to the cholesterol found in animal cells).

Fungi can be morphologically characterised as yeasts or moulds. Yeasts are unicellular organisms which undergo cell division via budding. In contrast, moulds form multicellular structures, which through branching hyphae, form a network termed mycelium. Mould releases spores (conidia), allowing the organism to dispersion and spread. Dimorphic fungi can switch between each growth form, typically growing as moulds at 25ᵒC but yeasts at 37ᵒC (i.e. body temperature).

Fungal infections are an overlooked group of diseases, accounting for more deaths worldwide annually than malaria and tuberculosis. Host immunity to fungi is generally very high, so infection in most hosts will be limited to superficial sites (e.g. mucosal candidiasis). Invasive infection often requires either immunocompromise (e.g. advanced HIV, solid organ transplantation, haematological malignancy) or indwelling prosthetic material (e.g. central venous access devices).

This section focuses on some of the most important fungal infections to be aware of for the MRCP examination. Other sections which feature content on fungal infections include the Antimicrobials, HIV and Infections in Immunocompromised Hosts topics, which should also be referred to.

Antifungal drugs

Numerous antimicrobial drugs active against yeast and moulds exist, which can be used to treat fungal infections. The 3 main classifications of systemic antifungals, their mechanism of action, and their indications are summarised in the table below.

Further information can also be found in the Antimicrobials topic.

Table 1: Summary of antifungal drugs