MEDICAL MYCOLOGY

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The study of pathogenic fungi (i.e., fungal organisms that are of medical importance and causes infection or disease in humans) is known as medical mycology. Pathogenic fungi are fungal organisms that cause disease or infection in humans. Human mycoses may include infections of the outer skin layer, infections of the underlying tissues of the skin, systemic infections, and deep-seated infections or invasive mycoses. Other fungal infections are opportunistic in nature, and only cause disease or infection in humans by chance. Opportunistic fungal infections rarely affect individuals whose immune system is still intact and strong. Opportunistic mycoses usually occur in immunocompromised individuals such as HIV/AIDS patients as well as in people with debilitating diseases like cancer. The elderly, patients on chemotherapy and individuals with other debilitating disease conditions are more prone to opportunistic fungal infections than people with strong immune system. Fungal organisms are major spoilage organisms of foods and other food products, and they also infest household furniture and buildings even though some fungal species are pathogenic in nature. Pathogenically, fungal infections can be exogenous or endogenous in nature.

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Mycoflora are fungi that are normal flora of the human body. Some Candida species are mycoflora found in the human mouth and genital regions such as the female vagina. These mycoflora can become infectious when the body’s immune system becomes compromised or weakened due to irrational antibiotic usage or a debilitating disease. The main portals of entry of fungi that cause endogenous mycoses include the skin, mouth, GIT and the vagina. It is noteworthy that most fungal infections especially those caused by dermatophytes and even some endogenous mycoses are best prevented through an intact host skin which prevent pathogenic fungi from entering the host’s body from the outside.

Fungal infection in human population can cause serious pathological conditions in infected individuals especially in endemic regions where such pathogenic fungi are most prevalent. For example, Paracoccidioides brasiliensis which causes paracoccidioidomycosis, a systemic/invasive fungal infection is most common in South America. Most fungal infections can be treated and managed clinically using antifungal agents. But personal and environmental hygiene also plays critical role in the control and prevention of fungal infection in human population. Good personal hygiene is also essential in preventing some fungal infections (for example, dermatophytosis such as ringworm). However, a broken or injured skin is the common site and portal of entry for environmental fungi to enter the body and become established. Fungal organisms elaborate both beneficial and non-beneficial effects in their natural environment. Beneficial fungi have been exploited by man since time immemorial to produce food, antibiotics and other products that are of economic and health importance to man and his environment. However, some fungal organisms termed as non-beneficial fungi or pathogenic fungi cause a wide range of infections and diseases in human populations, as well as in plants and animals. More so, some fungi cause a variety of food spoilage which amount to great economic loss to food production companies across the globe (Figure 1).

Figure 1. Illustration of fungal spoilage of bread by moulds (for example, Mucor species and Rhizopus species). Food spoilage caused by fungi is a major economic loss and the phenomenon negatively affects food availability. Photo courtesy: https://www.microbiologyclass.com

EXOGENOUS AND ENDOGENOUS FUNGAL INFECTIONS

In exogenous fungal infections, the fungal infection is usually acquired externally from the environment through the inhalation of infectious fungal spores in aerosols or from dust particles. Infectious conidia or spores of fungi are ubiquitous in the environment especially in the soil, and they can easily cause human infection through inhalation of the infectious fungal spores. Inhalation of fungal spores in large doses can predispose humans to a variety of fungal infections. Fungi that cause exogenous infections in man penetrate the body through the respiratory tract system from where they reach the lungs and become disseminated to other parts of the body to cause systemic, invasive or deep mycoses. People living in endemic places of fungal infections or individuals who engage in certain type of job or activity (for example, construction workers, farmers, horticulturists, builders) that cause fungal spores or dust particles carrying fungal spores to become aerosolized can easily inhale infectious fungal spores.

Endogenous fungal infections are caused by fungi that are members of the human normal microflora (for example, some Candida species). Infections caused by endogenous fungi usually occurs when the body’s immune system is compromised or in a bad condition. Most fungal infections are restricted to a particular geographical area while others are distributed worldwide in the environment. The irrational use of antibiotics can displace normal bacterial flora that checkmate the growth of mycoflora in the human body. This allows fungi that are normal flora to overgrow and cause endogenous fungal infection in the affected individual.

Further reading

Anaissie E.J, McGinnis M.R, Pfaller M.A (2009). Clinical Mycology. 2nd ed. Philadelphia, PA: Churchill Livingstone Elsevier. London.

Baumgardner D.J (2012). Soil-related bacterial and fungal infections. J Am Board Fam Med, 25:734-744.

Calderone R.A and Cihlar R.L (eds). Fungal Pathogenesis: Principles and Clinical Applications. New York: Marcel Dekker; 2002.

Champoux J.J, Neidhardt F.C, Drew W.L and Plorde J.J (2004). Sherris Medical Microbiology: An Introduction to Infectious Diseases. 4th edition. McGraw Hill Companies Inc, USA.       

Gladwin M and Trattler B (2006). Clinical Microbiology Made Ridiculously Simple. 3rd edition. MedMaster, Inc., Miami, USA.

Larone D.H (2011). Medically Important Fungi: A Guide to Identification. Fifth edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Stephenson S.L (2010). The Kingdom Fungi: The Biology of Mushrooms, Molds and Lichens. First edition. Timber Press.

Sullivan D.J and Moran G.P (2014). Human Pathogenic Fungi: Molecular Biology and Pathogenic Mechanisms. Second edition. American Society of Microbiology Press, USA.

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