In approximately three-fourths of 100 strains of dermatophytes tested, exposure for two weeks to even the modest heat of 39 C not only inhibited but was apparently lethal to inocula of the organisms on Sabouraud's dextrose agar. No strain survived similar exposure to 43 C. In most cases exposure to this latter temperature for 72 hours was sufficient to prevent growth of the inocula.
In Sabouraud's liquid shake culture preparations inocula of 24 strains of dermatophytes failed to grow after exposures for one week to temperatures ranging from 39 to 43 C. In many cases exposure to 43 C for only 5½ hours completely inhibited established inocula in such preparations. Such 5½ hour daily exposures for four days proved lethal to all 12 strains of dermatophytes tested in this manner.
The germination of spores of Microsporum gypseum was completely inhibited by exposure to humid heating at 43 C for three days; spores of Trichophyton mentagrophytes required ten days of such exposure for similar complete inhibition of germination.
A technique is described for establishing infections with M. gypseum on the tails of rats. Such infections failed to develop if the tails were kept at 41 C for one week after inoculation. Similar infections established on rat tails for one week cleared after exposure for seven to ten days to 41 C. No spontaneous tendency to heal over a period of one month was observed in the case of control M. gypseum infections on rat tails kept at room temperature.
The results of these studies might suggest clinical trial of prolonged wet heat in the treatment of stubborn dermatophytic infections. There are, however, hypothetical reasons for caution in such clinical trials.