XIII 

 SUMMARY AND CONCLUSIONS 



WE have already stated in the introduction that our results are 

 in general confirmatory of observations made by widely sepa- 

 rated workers and that the work which was independently 

 planned by us serves as a carefully conducted control to a 

 number of researches made under less favorable circumstances 

 in several countries. 



Our experiments with lice were the first to be made with lice 

 from sources absolutely unquestionable, both as to the oc- 

 currence of rickettsia-like micro-organisms and the presence 

 of the virus of any disease. The fact that these lice when 

 nurtured upon typhus patients acquired only Rickettsia 

 prowazeki, in the way of demonstrable micro-organisms, with 

 great regularity (cf . table, p. 44) is in itself very strong evidence 

 for the etiological relationship of Rickettsia prowazeki to 

 typhus. 



The experiments described, with protocols, under the head- 

 ing of "Experiments to Prove the Specificity of Rickettsia 

 prowazeki for Typhus" (p. 51) admit of no other conclusion 

 than that the virus of typhus is not separable in the louse 

 from Rickettsia prowazeki. We could not devise means of 

 determining why lice do not invariably become infective when 

 nurtured under proper conditions upon typhus patients. It 

 would appear from our results that an element of chance enters 

 into the infection of the louse with Rickettsia prowazeki (the 

 virus of typhus). Possibly the louse in order to become in- 

 fected must pierce a capillary at the site of a lesion or locali- 

 zation of rickettsia in the endothelium. This idea implies that 

 the mechanism of typhus transmission is an imperfect one in 

 more than one respect, for it must be remembered that in- 

 fection with Rickettsia prowazeki eventually destroys the louse 



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