294 MICROBES AND TOXINS 



firms the great progress it has realized. It is to be hoped that 

 the future will confirm these promises and make of this drug 

 one of the great victories of therapeutical science. 



Observations on immunity in Protozoal Diseases. 

 Strains resistant to Sera. The future of Chemio- 

 therapy. — To chemiotherapy, also, we owe much new 

 knowledge on the immunity to the protozoal diseases, and 

 on immunity in general. 



The question of the dose is here of capital importance ; the 

 object is to begin and to finish the treatment at one blow, but 

 it must be a sledge-hammer stroke. Small doses cure for a 

 time without producing immunity, and relapses therefore occur. 

 In the treatment of a relapse the trypanosome is found to be 

 different. Sometimes it becomes more sensitive to the action 

 of the drug. Sometimes it gives the impression " that as the 

 result of the absorption of those parasites killed by the atoxyl, 

 the body acquires a degree of immunity which then interferes 

 with their normal development " (Ehrlich). As a rule, however, 

 such immunity is ephemeral. 



But what is most commonly observed in following the course 

 of a first attack on to a relapse, and again from one relapse to 

 another, is a diminution in the efficacy of the remedy. The 

 question is, is it the body or the parasite which has changed ? 

 We find it is the parasite, for when inoculated now into a fresh 

 animal it still resists the same drug. The injection of a drug 

 in small doses is in fact the best way to render a trypanosome 

 resistant to it, and further to create a strain of trypanosome which 

 resists the remedy and preserves this resistance even after 

 hundreds of passages, transmitting it as an acquired character. 



Trypanosome strains have been created in the laboratories 

 resistant to trypanred, to benzidine dyes, atoxyl, and tartar- 

 emetic. The resistance is a group resistance, i.e., it applies to 

 all drugs of the same series or chemical family : a trypanosome 

 resistant to trypanred is still sensitive to the arsenic bodies ; it 

 resists, however, also the benzidine dyes, which are a good deal 

 different from trypanred. It can be seen by testing various 

 drug-groups against the same resistant strain that biology agrees 



