SYSTEMIC MYCOSES 123 



a changed environment in the interdigital spaces of the foot due to 

 increased activity on the part of the host, change of season or cli- 

 mate, or to some other unrecognized factor. In any case the fungus 

 is a frequent inhabitant of the epidermis and nails of the feet, and 

 the reactivation of a quiescent lesion is probably a more important 

 factor in the frequent occurrence of dermatophytosis of the foot than 

 is reinfection from an exogenous source. 



Systemic Mycoses. In spite of the contrasts just cited between 

 the lesions caused by dermatophytes of human origin on the one 

 hand, and of lower animal origin on the other, they become insig- 

 nificant when one contrasts the dermatophytoses with the generalized 

 mycoses. The dermatophytes, despite their adaptation to specific 

 hosts, form a rather closely interrelated group of fungi. They have 

 certain morphological characteristics and physiological adaptations 

 in common, and they appear to be primarily parasites of man and 

 animals. Dermatophytes have been isolated from stable litter and 

 manure, and they undoubtedly grow saprophytically on desquamated 

 epithelium, hairs and similar debris, but their presence in such en- 

 vironments and such substrata is secondary to their parasitism of 

 man and animals. 



The systemic mycoses, on the other hand, are with few exceptions 

 caused by fungi which are rarely found in man and animals, but 

 apparently have a natural habitat elsewhere and attack man and 

 animals only under exceptional circumstances. In American blasto- 

 mycosis, sporotrichosis, chromoblastomycosis, the type of actinomy- 

 cosis caused by the acidfast Nocardia asteroides, mycetomas of vari- 

 ous types, aspergillosis, and other rarer mycoses, the evidence indi- 

 cates that the fungus has a natural habitat in soil, vegetation, or 

 possibly in some cases in another animal host. The fungi causing 

 these mycoses apparently need to be introduced into the human host 

 by inhalation or subcutaneously by accident on splinters or thorns 

 and under special circumstances in which sensitization may be an 

 important element. These mycoses are not contagious, transmission 

 directly from person to person occurring very rarely if at all. 



The factors influencing the incidence of these deeper mycoses vary 

 to some extent with the individual mycosis and can best be discussed 

 later in connection with the particular infection. It may be said in 

 general, however, that age may be a factor in some mycoses, occupa- 

 tional exposure appears to be important in others, residence is im- 

 portant in the case of those with an endemic distribution, and re- 

 peated exposure with ensuing sensitization may be important in 

 several. 



