By Tristan Wang ’16
History of Dermatophytes
One of the most common forms of human disease, estimated to affect more than a fifth of the world’s population, come from diseases with fungal origins—also known as mycoses (1). While superficial in infection, dermatophytosis or ringworm is a parasitic fungal diseases of the skin or external material on the skin and it constitutes the vast majority of these mycotic diseases (1).
History credits three biologists for the creation of medical mycology: Robert Remak, Johann L. Schönlein, and David Gruby, each of whom studied dermatophytes at one point or another (2). In 1835, Remak observed microscopic structures from crusts of favic lesions (favus is ringworm of the scalp) after which Schönlein identified the specimen having fungal origin (2). Remak ultimately describes the fungi as Achorion schöenleinii in honor of his mentor Schönlein; however it was Gruby who is considered the founder of dermatomycology (2). Independently of Remak and Schönlein, Gruby published papers from 1841 to 1844 about the about the cause and nature of Favus expanding significantly what biologists then knew about dermatophytes (2).
Around 1890, Raimond Sabouraud advanced knowledge of dermatomycology by studying extensively into the taxonomy, morphology, and treatment of dermatophytes, even classifying these fungal agents into four genera (three of which are still current to mycologists) (2). Even more importantly, Sabouraud developed a medium for culturing dermatophytes that, after a few modifications, is even still used today and is named in his honor: Sabouraud glucose agar (2).
Taxonomy and Evolution
Because dermatophytes exhibit both asexual (anamorphic) and sexual (teleomorphic) reproductive forms, some species are able to straddle two different classifications based on what form they are in, although it is important to note that not all dermatophytic species have been found to reproduce sexually (2). Any species capable of reproducing sexually fall into the teleomorphic genus Arthroderma while all anamorphs fall into one of three genera: Microsporum, Epidernophyton, and Trichophyton (2). All four genera are categorized into the family Arthrodermataceae of the order Onygenales, an order known to specialize in degrading tough proteins (Ascomycota) (2).
Functionally, dermatophytes can be categorized into different ecological roles. Geophiles normally live in the soil as saprophytes (digesting dead organic matter) and exist mainly in locations of keratinous (a skin-related protein) material (3). Occasionally, geophiles become dermatophytes and infect animals, but if they do, it usually occurs though contact with the soil (3). Zoophiles on the other hand, include species that rarely grow as saprophytes and can be considered as animal pathogens, often with a limited host range (3). Interestingly, it can be argued that because of their specialty in animal infections, zoophiles are able to survive in long dormant states on contaminated animal materials (3). Lastly, anthropophiles chiefly infect humans and occasionally animals (3). Dermatophytic species significantly vary in their ability to invade hair, nail, and skin on their unique enzyme production and nutritional demands (3).
Phylogenetic studies have shown that pathogenic dermatophytic species share a close ancestry with soil-borne, non-pathogenic ancestors and that it is possible that these species specialized in their food source (4). For example, Microsporum canis is known to infect of cats with the occasional human or dog host (4). Human infections by Microsporum canis however loses virulence and viability with each successive human-to-human transmission implying a specialization in cat hosts and material (4).
Epidemiology and Immunology
Dermatophyte-induced infections (dermatophytosis) predominantly feed on keratin, the fibrous structural protein that makes up the majority of outer skin in addition to hair and nails (3). As a result, a variety of enzymes like keratinases are produced for protein degradation (2). This communicable disease affects only mammals (with the exception of Microsporum gallinae in fowl) and can grow on substrates ranging from hair, feathers, horns, and hooves (2).
In medical terms, dermatophytic diseases are named based on the anatomic locations of the infection. Tinea barbae refers to ringworm on the beard and mustache, tinea capitis on the head, tinea corporis on glabrous skin (skin without natural hair), tinea cruris on the groin (“jock itch”), tinea favosa on the favus (scalp), tinea manuum on hands, tinea pedis on feet (“athlete’s foot”), and tinea unguium on nails (onychomycosis) (2). These terms are more fluid as a single dermatophytic species can infect several atomic sites and several atomic sites can be caused by a single species (2).
Dermatophytic infections is usually limited to dead keratinized tissue of the skin also called the stratum corneum and usually do not invade further on a immunocompetent host (2). Reactions to infection vary based on response to metabolic compounds produced by the dermatophyte, location of infection, and strain of fungus but the disease itself is generally harmless or irritating (2). Humoral and cell-mediated defensive reactions through lymphocytes, neutrophils, and macrophages are able to remove infection either through oxidative and nonoxidative mechanisms (2, 5). Removal by antibody means can help limit the growth of some fungi but some studies have shown that patients with chronic infections often have the highest level of antibodies implying that antibodies do not help with infection removal (2, 5). Fortunately, studies have shown that immunity to dermatophytosis can be achieved after infections (2).
Infections with humans are often spread through abrasions exposed to communal aquatic facilities and sharing towels and clothes (2). Females are generally infected more by tinea unguium and corporis whereas males more often serve as hosts for tinea cruris and pedis (1). Worldwide, tinea unguium, corporis, and pedis make up more than 80% of all infections (1).
Current Trends and Cures
A cocktail of fungicides including griseofulvin, tolnaftate, haloprogin, and allylamines have been used to combat dermatophytosis in the last 100 years (2). And although some species have virtually disappeared from humankind, like the eradication of T. schoenleinii, M. audouinii, and M. ferrugineum, other species have begun to show resistance to drug therapy in the last 30 years (2). More studies are needed to fine-tune these cures.
What dermatophytic diseases lack in severity, they make up for in longevity. Skin diseases have been around for centuries and given these trends, it would not be surprising for people to continue to live with them a while longer. Nevertheless, what we have learned from dermatophytes in terms of mycology and immunology is so staggering that maybe living with these fungal critters for a couple more centuries may not be necessarily bad.
- Vena, Gino A., Paolo Chieco, Filomena Posa, Annarita Garofalo, Anna Bosco, and Nicoletta Cassano. “Epidemiology of Dermatophytoses: Retrospective Analysis from 2005 to 2010 and Comparison with Previous Data from 1975.” New Microbiologica 35 (2012): 207-13.
- Weitzman, Irene, and Richard C. Summerbell. “The Dermatophytes.” Clinical Microbiology Reviews 8.2 (1995): 240-59.
- Simpanya, Mukoma F. “Dermatophytes: Their Taxonomy, Ecology and Pathogenicity.” Biology of Dermatophytes and Other Keratinophilic Fungi. Bilbao, Spain: Asociación Espanñola De Micrología, 2000. 1-12.
- Summerbell, Richard C. “Form and Function in the Evolution of Dermatophytes.” Biology of Dermatophytes and Other Keratinophilic Fungi. Bilbao, Spain: Asociación Espanñola De Micrología, 2000. 30-43.
- “Fungal Infection in HIV-infected Persons. American Thoracic Society.” Am J Respir Crit Care Med American Journal of Respiratory and Critical Care Medicine 152.2 (1995): 816-22.