Abstract
Dermatophytes are the only group of human-infecting fungi containing species that have evolved into obligate infectious agents. These organisms require infection and transmission from one host to another for the maintenance and dissemination of the species. This evolution from keratolytic soil saprophyte to parasite of keratin-producing tissues also has been accompanied, in some species, by varying degrees of host specificity. Thus, human-infecting agents such as Microsporum audouinii and Trichophyton rubrum are rarely, if ever, found as causes of disease in animals; whereas, other organisms such as M. canis and T. mentagrophytes are exceedingly promiscuous, infecting essentially all mammalian species and even some birds. Furthermore, this evolution toward host specificity also seems to have occurred in particular population groups of the host and in particular geographic areas. Population shifts, changes in health habits, living standards, and travel, and changes in clothing fashions all affect the epidemiology and distribution of the known dermatophytes. These factors induce and select for changes and adaptations in organisms that lead to new species. The purpose of this chapter is to trace the current changes in the epidemiology of human-infecting dermatophytes and to speculate on the changing status (evolution) of some of these species.