Fungi are eukaryotic organisms that comprise yeasts, moulds (filamentous fungi) and higher fungi (mushrooms and toadstools). They are widely distributed in the environment and can survive in extreme conditions where nutrients limited. Most fungi are saprophytes (living off dead organic matter) in soil and water and are vital to the carbon cycle. Certain fungi are also of great commercial value in the production of bread, alcohol and antibiotics.
Yeasts are the simplest of the fungi. They are unicellular, spherical in shape and reproduce by budding. In some yeasts, including the medically important genus Candida, the buds elongate to form filaments (pseudohyphae).Moulds are composed of numerous microscopic branching, filamentous hyphae, known collectively as mycelia, that are involved in gaining nutrients and reproduction. The reproductive mycelia produce spores, termed conidia, either by asexual or sexual reproduction from opposite mating strains. Spores are disseminated in the atmosphere, enabling fungi to colonise new environments.Certain pathogenic fungi are dimorphic, being a yeast form when invading tissues but a mould when living in the environment. The exception is Candida, which forms pseudohyphae when in the body.
Diseases caused by fungi
The study of fungi is called mycology, and the diseases they cause, mycoses. Mycoses are classified depending on the degree of tissue involvement and mode of entry into the host.
Superficial – localised to the epidermis, hair and nails but can extend deeper into keratinised tissue.
Subcutaneous – confined to the dermis, subcutaneous tissue or adjacent structures
Systemic – deep infections of the internal organs caused by:
– primary pathogenic fungi that infect previously healthy persons
-opportunistic fungi of marginal pathogenicity that infect the immunocompromised host.
Superficial mycoses These are the most common mycoses of humans and are acquired from the environment, infected humans or natural animal hosts.
Pityriasis versicolor is an infection of the superficial skin and hair which is prevalent in the tropics. Fungi that invade deeper into keratinised cells are termed dermatophytes. These diseases are often called tinea or ringworm because of the characteristic red inflammation with central clearing that forms at the site of infection. Dermatophyte infection affects various sites:
e.g. scalp (tinea capitis), foot (tinea pedis: ‘athletes foot’) and groin (tinea cruris). Although becoming less common, tinea pedis is still the most common fungal infection in the United Kingdom.
Yeast infections are most commonly caused by Candida albicans and are confined to the vagina, mouth and soft skin (candidiasis or ‘thrush’). A commensal of the vagina and gastrointestinal tract, the organism can flourish if ill health, impaired immunity or antibiotic treatment alter the normal bacterial flora.
Subcutaneous mycoses These are infections caused by a number of different fungi that arise from injury to the skin. They usually involve the dermis, subcutaneous tissues and muscle. The fungi commonly live saprophytically on thorn bushes, roses and tree bark, from which wounds can occur. Certain occupational groups (e.g. florists and agricultural workers) are more at risk from infection. Such mycoses are difficult to treat and may require surgical intervention.
Systemic (deep) mycoses These are life-threatening invasive infections caused by a variety of fungi.
The primary pathogenic fungi infect previously healthy persons and are caused by dimorphic fungi normally found in soil. Infection usually arises from inhaling spores, and the lungs are the main site of infection. However, dissemination to other organs and the central nervous system can occur. The incidence is largely confined to endemic areas in North and South America.
The opportunistic fungi infect persons who, usually, have some serious immune or metabolic defect, are on broad-spectrum antibiotics or immunosuppressive drugs, or have undergone major surgery.
Other fungi-related diseases
Certain fungi may indirectly cause human infections. Constant exposure to fungal spores in the atmosphere can induce respiratory allergies, particularly among certain occupational groups (e.g. Farmer’s lung). Some mushrooms and toadstools cause poisoning if ingested. Certain moulds produce toxic secondary metabolites (mycotoxins) that can contaminate food.
Diagnosis and treatment
The diagnosis and treatment of common mycoses and related diseases is by microscopy and culture of lesions or serology. Antifungal agents are discussed elsewhere.