16.9 Mycoses: the fungus diseases of humans

The fungus diseases of humans are called mycoses and the majority, perhaps all, are not caused by dedicated pathogens, but rather by fungi common in other situations that take advantage of a particularly beneficial set of environmental conditions or of a host with defences weakened in some way (so-called opportunistic pathogens). There are about 135 fungal pathogens that cause diseases of man and domestic animals, and only about 60 species of fungi cause disease in man; of these about 30 cause superficial infections of the skin and about 30 cause subcutaneous, lymphatic or systemic infections (Fig. 10) together with several other species that cause allergic reactions.

The fact that there is a relatively short list of mycoses does not mean that fungus diseases of man are rare. What the human disease fungi lack in diversity, they make up for by being very widespread. Many people (?everybody?) suffers from Athlete’s foot at some time in their life. This is caused by a tropical import called Trichophyton rubrum, but you don’t have to go to the tropics to collect your share because it so likes warm and moist shoes that it is now distributed throughout the temperate climatic zones and is so common that you can be infected very easily. Athlete’s foot is little more than a nuisance though, as it can be successfully treated with over-the-counter remedies.

Generally speaking, we are prone to fungal invasion of the skin, nails and hair because they are exposed to the environment. Athlete’s foot is one example, ringworm is another such infection. This introduces another aspect of fungal biology: ‘ringworm’ is a family of mycoses because the cause can belong to one of two closely related genera, Microsporum or Trichophyton. Each fungus is very specific to a particular part of the body. A range of animals can also suffer ringworm diseases of skin and fur, and that range includes farm animals and pets. The fungi spread readily to humans, which introduces another notion, that of a zoonosis, being a disease that can be transmitted from other vertebrate animals to humans.

Routes of entry and distribution of the fungus diseases of humans
Fig. 10. Routes of entry and distribution of the fungus diseases of humans. Labels at left indicate routes of entry of pathogenic and opportunistic fungi that cause deep and cutaneous mycoses. At right we indicate the principal tissue sites of deep mycoses in comparison with superficial, cutaneous, and subcutaneous mycoses. Based on illustrations produced by Steve Schuenke for the online version of Medical Microbiology, 4th edn, by Samuel Baron (1996), ISBN 0-9631172-1-1 (hardcover), published by the University of Texas Medical Branch at Galveston at http://gsbs.utmb.edu/microbook/.

Another remarkable statistic about human mycoses is that it is now unusual for a woman to go through her reproductive years without at least one significant infection by the yeast Candida albicans. This introduces another source of disease because C. albicans is a normal inhabitant of the human mouth, throat, colon, and reproductive organs. Usually it causes no disease but lives commensally in ecological balance with other microorganisms of the digestive system. However, other factors such as diabetes, old age, pregnancy, but also hormonal changes, can cause C. albicans to grow in a manner that cannot be controlled by the body’s defence systems and candidiasis results, with symptoms ranging from the irritating to the life threatening. For most people candidiasis, like other superficial infections, is irritating; the fungus remains in the outer layers of the skin because the body’s immune defence system prevents the fungus penetrating more deeply. In patients whose immune systems are compromised in some way there is no such defence and the infecting fungus becomes deep-seated, systemic and potentially fatal.

This category of patient includes transplant patients, where the immune system is pharmaceutically modulated to control rejection, and people with AIDS (acquired immune deficiency syndrome), where deterioration of the immune system is caused by the decline in CD4+ T cells, the key infection fighters of the immune system, by the HIV (human immunodeficiency virus) retrovirus. Infections of those with weakened immune systems are called opportunistic infections. Candidiasis is the most common HIV-related fungus infection that can affect the entire body, but there are other opportunistic fungi that typically do not cause disease in healthy people but only those with damaged immune systems; these fungi attack when there is an ‘opportunity’ to infect and can cause life-threatening disease.

Updated December 17, 2016