Fungi and disease

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A limited number of fungi are pathogenic to humans (Table 8.1). Mycoses (sing: mycosis) in humans may be cutaneous, or systemic; in the latter, spores generally enter the body by inhalation, but subsequently spread to other organ systems via the blood, causing serious, even fatal disease.

Cutaneous mycoses are the most common fungal infections found in humans, and are caused by fungi known as dermatophytes, which are able to utilise the keratin of skin, hair or nails by secreting the enzyme keratinase. Popular names for such infections include ringworm and athletes' foot. They are highly contagious, but not usually serious conditions.

Systemic mycoses can be much more serious, and include conditions such as histo-plasmosis and blastomycosis. The former is caused by Histoplasma capsulatum, and is associated with areas where there is contamination by bat or bird excrement. It is thought that the number of people displaying clinical symptoms of histoplasmosis represents only a small proportion of the total number infected. If confined to the lungs, the condition is generally self-limiting, but if disseminated to other parts of the body such as the heart or central nervous system, it can be fatal. The causative agents of both diseases exhibit dimorphism; they exist in the environment as mycelia but convert to yeast at the higher temperature of their human host.

Aspergillus fumigatus is an example of an opportunistic pathogen, that is, an organism which, although usually harmless, can act as a pathogen in individuals whose resistance to infection has been lowered. Other opportunistic mycoses include candidiasis ('thrush') and Pneumocystis pneumonia. The latter is found in a high percentage of acquired immune deficiency syndrome (AIDS) patients, whose immune defences have been compromised. The causative organism, Pneumocystis carinii, was previously considered to be a protozoan, and has only been classed as a fungus in the last decade, as a result of DNA/RNA sequence evidence. It lives as a commensal in a variety of mammals, and is probably transmitted to humans through contact with dogs.

The incidence of opportunistic mycoses has increased greatly since the introduction of antibiotics, immunosuppressants and cytotoxic drugs. Each of these either suppresses the individual's natural defences, or eliminates harmless microbial competitors, allowing the fungal species to flourish.

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The term vaginitis is one that is applied to any inflammation or infection of the vagina, and there are many different conditions that are categorized together under this ‘broad’ heading, including bacterial vaginosis, trichomoniasis and non-infectious vaginitis.

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