RESISTANCE TO THE ARSENICALS 



r59 



Table 6-16 

 Examples of Acquired Resistance to the Arsenicals 



(Yorke et al., 1932). In some cases definite resistance has occurred following 

 a single ex])osure to an arsenical and in others it has required many weeks 

 or months. Spontaneous development of resistance in Trypanosoma equi- 

 perdum without exposure to an arsenical was reported by Eagle and Mag- 

 nuson (1944). Over a period of 2-3 months the resistance of this strain 

 increased progressively until a resistance factor as high as 200 was observed. 

 The duration of the resistance varies to some extent with the degree of 

 resistance; i. e., when onlj^ brief exposure to the arsenical results in a small 

 degree of tolerance, this is apt to be lost rather readily, whereas strains 

 of high resistance maintain their tolerance for very long times. An atoxyl- 

 resistant strain of Trypanosoma rhodesiense was observed for 12.5 years 

 and through 1528 passages in mice without alteration of the degree or 

 specificity of resistance (Fulton and Yorke, 1941), and other instances of 

 maintenance over periods of several months or years have been reported. 

 The resistance in certain instances may possibly be considered as permanent 

 (Voegtlin et al., 1924). Resistance to arsenite in paramecia is lost very 

 slowly, the rate depending, for one thing, on whether the organisms grow 

 vegetatively or conjugation is allowed to occur (Jollos, 1921). In any event, 

 it is quite clear that resistance often implies a change in the organism 

 which is hereditary in the sense that it is maintained through many gener- 

 ations in the absence of the arsenical. 



