122 FUNGUS DISEASES OF MAN AND ANIMALS 



no longer susceptible to parasitism by this fungus. Infection of the 

 adult by M. Audouini is very rare. 



M. Canis, on the contrary, is a pathogen of cats and dogs and in- 

 vades the human host by accident from an animal source. It does 

 not cause epidemics in man although transmission may occur in 

 instances where there is intimate exposure, as within the family. 

 Both children and adults may be infected. Unlike M. Audouini, M. 

 Canis typically excites considerable reaction on the part of the host, 

 the lesions frequently being edematous and exhibiting pustules. Al- 

 though this difference in the clinical type of lesion may be considered 

 typical, exceptions are too frequent to make it a dependable method 

 of determining which of the two species is involved in a given case. 

 As a result of the host reaction in most cases there is a tendency 

 toward spontaneous shedding of hairs infected by M. Canis and, as 

 Lewis and Hopper ^^ have shown, thorough scrubbing with soap and 

 water may suffice to cure the infection without the use of fungicidal 

 ointments. 



Similar relationships can be pointed out in the case of several other 

 dermatophytes. Trichophyton violaceum and T. tonsurans are path- 

 ogens of man; they cause chronic dermatophytosis in which the in- 

 fected hair stubs are so firmly anchored that their removal requires 

 special attention, and subsequent fungicidal treatment may be re- 

 quired for a considerable time. By contrast, T. faviforme, which is 

 a pathogen of cattle, and the granulosum and asteroides varieties of 

 T. mentagrophytes, which are pathogens of the horse and other ani- 

 mals as well as man, when they attack man, cause boggy lesions of 

 the type designated kerion. From this type of lesion the infected 

 hairs are spontaneously shed. In rare cases true granulomata are 

 produced. There is occasionally such a severe pustular folliculitis 

 that when the patient is seen in the clinic infected hairs may be 

 difficult to find and several attempts may have to be made before 

 the fungus is isolated in culture. In general, the granular varieties 

 of T. mentagrophytes are more apt to be of animal origin and to 

 evoke a more marked tissue reaction in the human host than are 

 the cottony varieties of that species. There are exceptions, however, 

 io the usual correlations between the type of fungus and the type of 

 lesion, as Dowding and Orr ^^ have pointed out. 



The species causing dermatophytosis of the foot in man show a 

 high degree of adaptation to this host, in many cases. They may 

 grow for months or years in the epidermis or nails without causing 

 subjective symptoms, becoming clinically important only at rare 

 intervals. A clinical flare-up may be due to altered host resistance; 



