Fungal infections are very common in humans across the globe. According to some estimates, it is believed that about 70% of the human population experiences fungal infection at one time or the other. Fungal infections in humans are mostly restricted to the outer surface of the body, such as the skin, nails and hair. These infections tend to be recurrent or persistent in nature.
However, they rarely lead to serious complications, and are usually treatable. At the most, fungal infections are a cause of mental distress and social awkwardness. Itching and skin rash, which are common features of fungal infections, can be quite bothersome to the affected person and others in social settings.
How fungal infections develop?
Fungi have a very wide distribution in the environment. They are present on the surfaces of our bodies, as well as inside our bodies. We can contract them through the soil, or through contact with domesticated animals. Everybody is exposed to fungi in the environment. However, not everybody gets fungal infections of the skin, hair and nails.
A fungal infection occurs only when certain conditions are present that facilitate the growth of fungi on the surface of our body. Examples of conditions that increase the risk of developing a fungal infection include dryness or dampness of the skin (as in sweating), breaks in the skin barrier, use of unhygienic public amenities, and physical contact with other infected persons, animals or soil.
It is important to stress that not every person who is exposed to the risk factors will go on to develop a fungal infection. The status of the immune system of a person and the presence of other competing microbes on the skin also have a role in keeping fungal infections in check. Also, some people may have a genetic predisposition to getting fungal infections.
Fungal infections in humans usually occur on the skin, hair and nails. There are specific types of fungi that are suited to grow in these conditions, and have, therefore, a predilection for humans.
Types of Fungi
Fungal infections in humans are mostly caused by two types of fungi: (1) yeasts, and (2) dermatophytes.
Yeasts are single-celled fungi that are capable of infecting both the skin surface as well as the cavities inside the human body. One of the best known examples of yeasts that causes infection in humans is Candida albicans. This species of fungi can infect the body surface (especially skin areas where chaffing has occurred), the mouth (causing thrush or oral candidiasis), and the vagina (thrush or vaginal candidiasis). Candida prefers the skin surface and the mucus membranes that line the body cavities.
Dermatophytes are fungi that require keratin for their growth. For this reason, they are the most common type of fungi to infect the keratin-rich human skin, nails, and hair. There are multiple species of dermatophytes that can cause human infections. All dermatophytes have special keratin-digesting enzymes that help them feed on the keratin protein found in the outer layers of the skin, nails, and hair.
Dermatophytes are also commonly referred to as ringworms, because they cause a characteristic skin rash that is circular in shape. Microsporum and Trichophyton are the two main species of dermatophytes that cause most human skin infections. These dermatophytes can survive in the environment for years by forming spores, which germinate only when the conditions for infection are ideal. Infections caused by dermatophytes can be treated easily. However, they also tend to recur a lot.
Candida albicans (abbreviated as C. albicans) is the most well known species of yeast that causes human fungal infections. Other species within the Candida genus that can cause human infections include C. tropicalis, C. glabrata, C. parapsilosis, and C. guilliermondii. Candida albicans occurs naturally in the human digestive tract and on the skin. However, it does not cause infections until some favorable conditions appear.
Loss of immune function (such as in immunocompromised patients) and changes in the natural skin environment may allow unchecked growth of Candida albicans to happen, resulting in candidiasis. Skin infections caused by Candida albicans are not as common as skin infections caused by dermatophytes. Candidiasis of skin usually occurs in skin areas where deeper tissues are exposed (such as on skin folds and inflamed or chafed skin).
Immunocompromised patients (such as AIDS patients) and diabetics (especially those who are obese) are more commonly affected by candidiasis. Diaper rash may also be caused by Candida in some cases.
Previously known as Pityrosporum, Malassezia is a yeast that is found on the surface of human and other animal skin. Under normal circumstances, the growth of Malassezia is kept in check by other microbes on the skin (collectively termed the skin microflora). Therefore, the mere presence of Malassezia on the human skin is not sufficient to cause skin infection.
Under conditions where Malassezia becomes pathogenic, various skin and scalp problems (such as seborrheic dermatitis, tinea versicolor, and dandruff) may occur. Abnormal immune response to Malassezia on the skin is thought to contribute to the pathogenesis of dandruff and seborrheic dermatitis. Infections caused by Malassezia are usually restricted to the outermost layers of the skin.
Infections by Malassezia are one of the most common fungal skin infections in AIDS patients.
Trichophyton fungi (especially Trichophyton rubrum) are the most common causes of dermatophyte skin infections in humans. Jock itch and athlete’s foot are common fungal infections caused by fungi in the Trichophyton genus. Other species of Trichophyton that cause human skin infections include T. tonsurans, T. interdigitale, and T. verrucosum. Most fungi of the Trichophyton genus are specific to humans (technically known as anthropophilic).
Infections caused by these fungi are highly contagious, and spread easily among humans. However, humans are not the only source of Trichophyton infections. Some species of Trichophyton (such as T. verrucosum) can spread from cattle to humans. Some species of Trichophyton can also be acquired through contact with soil.
Fungi of the Microsporum genus are also common causes of fungal infections of skin, hair and nails. These fungi can be anthropophilic, zoophilic, or geophilic. However, they all can cause human infections. Examples of some of the common Microsporum species that can cause human infections include M. audouinii, M. canis, M. ferrugineum, M. distortum, M. nanum, and M. gypseum.
Two species comprise the Epidermophyton genus. However, only one of these two species (E. floccosum) is responsible for human infections. Examples of human infections caused by E. floccosum include jock itch (tinea cruris), athlete’s foot (tinea pedis), ringworm on body (tinea corporis), and fungal nail infections (onychomycosis).
However, epidermophyton does not seem to cause infection of the human scalp hair (tinea capitis). Like other dermatophytes, E. floccosum causes superficial skin infections, and only invades deeper tissues in immunocompromised patients.