RESISTANCE TO THE ARSENICALS 



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Trypanosomes resistant to the usual arsenicals (e. g., atoxyl, tryparsa- 

 mide, oxophenarsine, etc.) are generally resistant to many other arsenicals 

 but to different degrees. Two examples of such cross-resistance will be 

 given for illustration. The accompanying tabulation shows the concentra- 



tions necessary to kill trypanosomes in 6 hr (Yorke and Murgatroyd, 

 1930). An acriflavine-resistant strain showed the same pattern of resistances 

 to the arsenicals as the atoxyl-resistant strain. The tabulation below 



shows the minimal effective doses subcutaneously to clear the blood of 

 mice of trypanosomes (Schumacher and Schnitzer, 1956). A strain made 

 resistant to butarsen showed essentially the same pattern, but was much 

 more resistant to butarsen itself (resistance factor > 18). It has generally 

 been found that strains resistant to atoxyl are not resistant to phenyl- 

 arsenoxide or the inert-substituted derivatives, whereas some increase is 



