Microsporum

• TRANSMISSION ROUTE: Inhalation

Definition

Microsporum is a genus of fungus that causes infections of the skin, hair, and nails.

Natural Habitat and Features

Microsporum species are widely distributed throughout the world, although some have restricted geographic distribution. Their natural habitat may be soil (geophilic species), animals (zoophilic species), or humans (anthropophilic species). Among soil species are cookei (also isolated from cat, dog, and rodent hair) and gypseum (also isolated from rodent fur). Nanum is found both in soil and on animals (swine). Animal species include canis (cats and dogs), gallinae (fowl), and persicolor (field rodents). Species with humans as the natural host include audouinii and ferrugineum.

Microsporum is the asexual (mitosporic) phase of the fungus. The sexual (teleomorphic) phase is assigned to the genus Arthroderma. Microsporum are molds with septate hyphae, that is, filaments with partitioned cavities. Conidia, asexual sporelike reproductive bodies, may be borne directly on the hyphae or on conidiospores, branching structures designed to bear conidia. Microconidia are one-celled, solitary, and oval- to club-shaped. Macroconidia are multicellular, spindle-shaped (fusiform), and have spiny (echinulate) or rough transparent (hyaline) walls. Microsporum grow on Sabouraud’s dextrose agar at 77° Fahrenheit (25° Celsius). After seven days of incubation, the colony varies between 1 and 6 centimeters. The color of the colony varies depending on the species.

Pathogenicity and Clinical Significance

Microsporum is a dermatophyte, a fungus that causes dermatophytosis, a superficial infection of the skin, hair, or nails. Epidermophyton and Trichophyton are two other genera of fungi with species that cause dermatophytosis. Most pathogenic Microsporum species infect the skin and hair. In the United States, canis, transmitted to humans from pet dogs and cats, is the most common cause of skin and hair fungal infections. It is also often the cause of mild tinea capitis (scalp and hair infections) and tinea corporis (trunk, leg, and arm infections). Less frequently, canis and gypseum are implicated in tinea manuum (ringworm of the hands). The anthropophilic species audouinii and ferrugineum can cause contagious tinea capitis, especially in children. Persicolor only infects the skin. Nail infections (onychomycosis) caused by Microsporum species are rare. Most nail infections are caused by Tricophytoses, Candida, or other fungal species.

Overall, the risk of infection with Microsporum species is low. However, pathogenic fungi can be introduced by contact with contaminated soil or infected animals, such as household pets, or with infected persons or with objects (such as a shared comb or towel) that have been used by infected persons. Barber shops, hair and nail salons, and gyms are public locations where a person may come into contact with contaminated items.

Pathogens colonize the surface of the body by destroying keratin, the major protein found in skin, hair, and nails. Infections spread laterally, with sharp, advancing margins. They do not become invasive. As the infection was thought to look like the burrows of worms, the common name for Microsporum infections, in general, became “ringworm.” The root name in Latin for many forms of Microsporum infections is tinea, which means “worm.”

Many cases of infection with Microsporum are asymptomatic. Other, mild cases may be self-limiting or they can be managed with nonprescription topical agents. Persistent, recurring, or severe cases require directed medical treatment.

Drug Susceptibility

Griseofulvin was once the drug of choice for treatment of infections caused by Microsporum species and other dermatophytes. One of this drug’s main drawbacks was that it often took several months to effect a cure, particularly in cases of tinea capitis. Monthly drug monitoring was required too.

More convenient, safer, and more effective agents are now available and preferred. Topical formulations of azoles (midazole, clotrimazole, and miconazole, and sulconazole) may be sufficient treatment for mild, localized cases. Using them avoids the risk of side effects that can accompany the use of oral medications. For persistent, recurring, or more virulent infections, oral therapy with itraconazole or terbinafine is now the drug of first choice.

Standardized in vitro susceptibility tests for antifungal agents used to treat infections caused by Microsporum have not been established. Nonstandardized in vitro comparison studies of the three major oral agents have been carried out with selected Trichophyton species. Griseofulvin yielded higher minimum inhibitory concentrations (MICs) than did itraconazole, which had higher MICs than did terbinafine. However, the meaning and clinical significance of these data require further investigation and might not hold if tested against Microsporum species. In limited clinical studies, itraconazole and terbinafine appeared to show greater efficacy against fungal skin infections than did griseofulvin. Depending on the study, itracanozole and terbinafine were comparable in efficacy, or terbinafine was superior.

Bibliography

"Everything You Want to Know About Ringworm." Healthline, 27 Aug. 2024, www.healthline.com/health/ringworm#risk-factors. Accessed 4 Feb. 2025.

Richardson, Malcolm D., and David W. Warnock. Fungal Infection: Diagnosis and Management. New ed. Malden, Mass.: Wiley-Blackwell, 2010.

Ryan, Kenneth J., and C. George Ray, eds. Sherris Medical Microbiology: An Introduction to Infectious Diseases. 5th ed. New York: McGraw-Hill, 2010.

White, Gary M., and Neil H. Cox. Diseases of the Skin: A Color Atlas and Text. 2d ed. Philadelphia: Mosby/Elsevier, 2006.