ORAL THRUSH (ORAL CANDIDIASIS)

Oral thrush (oral candidiasis) is a localized fungal infection caused by Candida albicans in the oral cavity of humans such as the tongue, lips and gums. Oral thrush is a fungal infection that affects the mucous membrane of the mouth. Thrush or oral thrush is an infection of the mouth caused by the Candida fungus which is a yeast cell. Oral candidiasis is characterized by the formation of whitish mucoid plaques on the mucous membranes of the mouth. The infection is known as endogenous mycoses because the causative agent (C. albicans) is part of the normal flora of the human body, found especially in the gut, mouth, vagina and respiratory tract. Mycoses due to Candida species in humans occur following the suppression of the body’s immunity (for example, in HIV/AIDS individuals). Other predisposing factors such as prolonged antibiotic usage and the use of immunosuppressive drugs are other risk factors to the development of oral candidiasis in humans. Oral candidiasis is strictly opportunistic mycoses that develop in people with poor immunity.

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The disease can also occur in newborns delivered through Candida infected birth canal (vagina) of already infected women. It is noteworthy that candidiasis in newborns is not an endogenous fungal infection but a contracted disease that occurs in infants from mother to child especially when the birth canal or vagina is heavily contaminated with C. albicans. This type of infection only occurs in newborns as a result of mother-to-child transmission of the infection. Vulvovaginitis is a common Candida infection in pregnant women, and it is usually characterized by itching and irritation and discharge from the vagina of affected women. Oral thrush (which can also be known as mouth thrush) is one type of candidiasis in humans. Other clinical forms of candidiasis include vaginal candidiasis, cutaneous candidiasis, mucosal and mucocutaneous candidiasis and systemic candidiasis. Other yeasts of medical importance aside Candida species include Saccharomyces species, Torulopsis speciesand Cryptococcus species.

Candida albicans is responsible for most of the candidiasis episodes in humans. Generally, Saccharomyces species are generally known as GRAS (generally recognized as safe) organismsbecause they are used in food production and they are also important production organism in most medical, pharmaceutical, and industrial applications. GRAS organisms are not pathogenic in nature; they are safe to humans; and they possess little or no pathologic effect in humans or animals. However, S. cerevisiae is a colonizer of mucosal surfaces of the body; and it is also a normal flora of the respiratory tract and the gastrointestinal tract of humans. Infections due to S. cerevisiae are mostly notable in immunocompromised individuals; and the organism is mostly implicated in causing fungaemia (presence of yeast or fungi in the blood). Other Candida species such as C. kruzei, C. tropicalis, C. parapsilosis and C. guillermondii can also cause mild candida infections in humans.

C. glabrata (previously known as Torulopsis glabrata) is another species of Candida that is highly resistant in nature to some commonly used antifungal agents; and it causes nosocomial infections and mucosal candidiasis because of its ability to attach to epithelial cells. C. glabrata thrives in acidic environment; and it causes some ‘mild’ pathological conditions in women, especially around the vaginal region. Candida infection is not limited to the mouth; and thus, it can occur in other parts of the body including the vagina of women. Small amounts of Candida albicans are also present in the mouth, digestive tract, and skin of most healthy people; and this beneficial fungus are normally kept in check by other bacteria (which are normal microflora) in the body. However, stress, diseases and the irrational use of medications such as antibiotics, birth control pills and corticosteroids can disturb the natural balance of the fungus in the body. This will cause C. albicans to grow out of control and cause thrush in the affected host. The use of such medications weakens the immune system and cause Candida to thrive. Nystatin, fluconazole, amphotericin B and itraconazole can be used to treat thrush.

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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