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Human Disease

Introduction

The principle concept to be covered in this page is that diseases caused by fungi are becoming more common. The reasons are complex, but include use of antibiotics that suppress natural competitors of fungi, use of compounds that suppress the immune system following tissue transplant operations, and infectious organisms, such as HIV, that suppress normal immune responses. In other words, suppression of fungi in the mammalian body is the "norm". When immunological mechanisms fail, fungi become a widespread and unpredictable problem in humans.

The nature of fungal diseases can be categorised in different ways:

  1. Cutaneous infections of the outer layer of skin
  2. Subcutaneous infections where fungi have penetrated through wounds or been injected into the tissue.
  3. Systemic infections, where the fungus becomes established in the host.

Cutaneous Infection

A group of fungi called dermatophytes degrade keratin. They exist on the dead skin of humans provided other environmental conditions are met. They become a problem for the host when the skin remains moist and warm for long periods. Each fungus appears to have specific hosts and is found in specific locations in most cases. However, given sufficient inoculum and appropriate conditions, many dermatophytes can swap hosts and locations.

Many of you will be infected by one of these fungi at some time during your life. Tinea and ringworm are the common names given to the disease caused by dermatophytes. The names are usually associated with the place or shape of the infection reaction. The fungus causes distress because the metabolites it releases cause an allergic or immunological reaction. Removal of the infection removes the response.

The fungi are found in three anamorphic genera of Ascomycota; Trichophyton, Epidermophyton and Microsporum. They transfer from host to host usually as infected skin cells. Thus treatment of the condition and removal of the inoculum are necessary for control of epidemics. For instance, flakes of skin may lodge in carpet of bedrooms or on wet floors of change rooms. The fungi rarely cause significant problems.

Candida may also cause surface infection, but is dealt with below where its properties will be discussed.

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Subcutaneous Infections

Tread on a spike in bare feet and more than rust and soil may penetrate the tissue. Even an insect bite may result in human infection by fungi normally associated with insects. A variety of fungi may become established in specific situations. The fungus usually changes form on invasion and yeast-like cells proliferate in the body. LINK If the fungus spreads through the body, the spread is associated with the lymphatic system.

Spororthrix is a widespread anamorphic Ascomycete that can enter the body through a wound. On entering the host, it converts from a hyphal to a yeast form. The localised infection may resolve itself, but disseminated infection due to spread in the lymph system has been reported. In the worst case, the fungus can spread to joints, bones and internal organs.

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Systemic Mycoses

As the name implies, these fungi establish systemic infection in the body. The most common point of entry for the dimorphic fungi is the lungs. The fungi are of two types, those that can establish infection in immune sufficient individuals, and those that require an immuno-compromised host.

Of the former, the usual source of the pathogen is the soil or plant material. Spores are inhaled and germinate in the lung. The thalli then change form from filamentous to yeast-like and pass into the blood where they are dispersed as yeast cells.

Most individuals who live in locations where the fungi are normally found have antibodies to the soil and plant fungi. Problems usually only develop when adults who have not been exposed come in contact with the fungi. The pathogenic fungi include Histoplasma capsulatum (Histoplasmosis), Coccidioides immitis, Paracoccidioides basiliensis and Blastomyces dermatidis (Blastomycosis)LINK. These fungi are unusual in Australia.

A wide array of saprophytes can initiate disease in immuno-compromised people. Lowered immune competence is associated with involvement of drugs to suppress the immune system such as Cyclosporin A. LINK This drug is now being used widely in transplant operations. The second major cause of immune suppression is disease, especially Acquired Immune Deficiency Syndrome, caused by HIV. The virus, HIV, knocks out the bodies usual response to fungal invasion. Further, prolonged antibiotic treatment, some forms of cancer and prolonged diabetes are associated with reduced immune competence.

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Candida

The fungi Candida albicans, various Zygomycota, Aspergillus and Cryptococcus neoformans are especially associated with compromised immune systems. The list of potential opportunistic pathogens is increasing. Candida is a normal occupant of the mucous membranes and GIT of the body. It can cause major irritation when the pH of the mucosa is disturbed, as happens during pregnancy, use of antibiotics and some illnesses. In women the irritation is most commonly noticed as vaginal candidiasis such as during pregnancy, and in men as balinitis. Young children commonly contract thrush. However, the fungus becomes systemic when the immune response is not sustained.

The other fungi are not found among normal human mycoflora. They tend to be ingested or inhaled. The spores germinate, invade adjacent tissues and immediately spread through the blood stream as yeast cells. They have dramatic effects on the host, made all the more dangerous because of the underlying poor immune response. Lack of experience in identifying the fungi among medical practitioners and the lack of sufficient drugs for effective treatment also impede treatment. LINK

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Conclusion

One view is that all fungi have the potential to cause disease in humans. Only the effective immune system prevents fungi from causing profound disease. The most common problems with fungi relate to use of the surface skin by dermatophytes. Otherwise, the fungi have to be injected into, or lodged in susceptible tissues of individuals with reduced immune competence. For the great majority of people, fungi will not cause more than an irritating reaction. For immunocompromised people, fungal invasion may be fatal.

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References

Kendrick B. (1992) The Fifth Kingdom, Mycologue Publications, Waterloo Ontaria, Canada.

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