Fungi and disease

Fungi and disease

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, andare 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 rep-resents 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 or-ganism which, although usually harmless, can act as a pathogen in individuals whose resistance to infection has been lowered. Other opportunistic mycoses include candidi-asis (‘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 con-sidered 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.

Many fungi produce natural mycotoxins; these are secondary metabolites, which, if consumed by humans,can cause food poisoning that can sometimes be fatal. Certain species of mushrooms (‘toadstools’) including the genus Amanita contain substances that are highly poisonous to humans. Other examples of mycotoxin ill-nesses include ergotism and aflatoxin poi-soning. Aflatoxins are carcinogenic toxins produced by Aspergillus flavus that grows on stored peanuts. In theearly 1960s, the turkey industry in the UK was almost crippled by ‘Turkey X disease’, caused by the consump-tion of feed contaminated by A. flavus.

It is thought likely that all animals are parasitised by one fungus or another. Extraordinary though it may seem, there are even fungi that act as predators on small soil animals such as nematode worms, producing constrictive hyphal loops that tighten, immobilising the prey.

Fungi also cause disease in plants, and can have a devastating effect on crops of economic importance, either on the living plant or in storage subsequent to harvest-ing. Rusts, smuts and mildews are all examples of common plant diseases caused by fungi. The effects of fungi on materials such as wood and textiles.