WILLIAM H. TALIAFERRO 693 



account of the reproduction-inhibiting property are unable to reproduce. Moreover, 

 they are relatively so scarce that microscopic examination fails to reveal them. Hence, 

 they simply exist there for a time and are unable to set up an infection. The lysin 

 eventually sweeps them away. In the recent work of Regendanz and Kikuth the 

 hypothesis is advanced that there is no lysin formed, but that after reproduction is 

 inhibited, various non-specilic agencies, such as phagocytosis by the reticulo-endo- 

 thelium, continuously remove the adult trypanosomes. In view of the unpublished 

 work of Coventry previously cited, I cannot agree with them in this belief. 



COMPARISON OF THE LETHAL AND NON-LETHAL TRYPANOSOMES 



Among the trypanosome species, as has been already indicated, there are some 

 that are pathogenic and others that are non-pathogenic. Furthermore, according to 

 a growing mass of evidence, some that are fatal to man and domesticated animals 

 may live harmlessly in their natural vertebrate hosts. For example, T. gambiense 

 causes sleeping sickness in man, but seems to produce no observable symptoms in the 

 infected antelope and other wild game of Africa. 



In accounting for these facts, a comparison of the resistance acquired by a host 

 against the pathogenic forms and by a rat against the non-pathogenic T. lewisi is 

 very illuminating. In the first case the host either acquires no resistance (mouse) or 

 periodically forms a trypanolysin (guinea pig, dog, etc.) which is never permanently 

 effective because it does not kill all the parasites and those which remain eventually 

 make good the periodical depopulations. In the second case the host first produces 

 an antibody which completely inhibits reproduction (cell division) in the parasites, 

 and thereafter each number crisis is so much gained by the host since those that re- 

 main are incapable of repopulating the blood. It seems possible, then, that whether 

 a given species is pathogenic depends upon the reaction of the host and that the 

 formation of a reproduction-inhibiting antibody, in conjunction with some trypano- 

 cidal mechanism, is the immunological basis of non-pathogenicity. Certain experi- 

 mental findings bear out this hypothesis. Thus, there is evidence that when a rat 

 does not form the reproduction-inhibiting reaction product in infections with T. 

 lewisi the parasite produces a lethal infection. Brown (1914a and b) studied a strain 

 of T. lewisi which was pathogenic to rats, and in considering his second paper it is 

 probable that what he considers the chief anomalies of this strain are due to the fact 

 that the parasites reproduced longer than normally. Similarly, the author has ob- 

 served apparent lethal infections of T. lewisi in which reproduction of the parasites 

 was never completely inhibited. In the splenectomized rats of Regendanz and Kikuth 

 (1927), in which no reproduction-inhibiting reaction product was formed, the infec- 

 tion progressed steadily until the death of the host — in other words, the infection 

 resembled an infection with a pathogenic species in the mouse. 



In trying to explain the fact that those species of trypanosomes which are lethal 

 to man and domesticated animals can live in the blood of certain wild animals without 

 producing symptoms, van Saceghem (1923) postulates a reproduction-inhibiting im- 

 mune property which causes the pathogenic trypanosomes to become harmless com- 

 mensals. As yet, however, van Saceghem has not demonstrated such an immune 

 property by suitable immunological experiments as the author and others have done 

 in the case of T. lewisi infections. 



