200 SUMMARY AND CONCLUSIONS 



which is an unusual effect of a parasite upon its intermediate 

 host. 



We were also unable to experiment upon the manner of 

 introduction by the louse of rickettsia into the human. We 

 are certain that Rickettsia prowazeki escapes from the alimen- 

 tary tract with the feces and therefore may be introduced by 

 scratching or by the mouth parts of the louse after becoming 

 soiled with feces. We have not seen rickettsia in the salivary 

 glands or in the esophagus of a louse. 



Incidentally, through the accident which befell Mr. Bacot, 

 we obtained important evidence that the extracellular rick- 

 ettsia, present in a considerable per cent of the lice in 

 Warsaw, are Rickettsia pediculi and the cause of trench 

 fever. 



Our pathological studies, combined with the demonstration 

 of rickettsia in the lesions, add to the clearness of our concep- 

 tion of typhus as a disease, as well as to the proof of the causal 

 relationship of rickettsia. 



The lesions of typhus are located in the blood vessels of 

 the skin, central nervous system, skeletal muscles, and to a 

 lesser degree in a few viscera, heart, kidneys, and testes. 

 Hence, we may say that typhus is a disease of the smaller 

 blood vessels; and we have demonstrated that the parasite of 

 the disease localizes almost exclusively in the vascular endothe- 

 lium. The reaction to the parasite (Rickettsia prowazeki) is 

 shown primarily by degenerative changes, giving rise to 

 thromboses in blood vessels, and by a proliferative reaction 

 on the part of endothelium and neuroglia which give rise to 

 the characteristic microscopic " nodules " of the disease in 

 skin and central nervous system. Death from typhus in man 

 is frequently the direct result of extensive involvement of the 

 brain with the proliferative lesions, in guinea-pigs the rare 

 fatality due to typhus infection is usually caused by the cere- 

 bral lesions. That the virus of typhus is probably present in 

 the cells of the proliferative lesions is shown by the fact that 

 we have transmitted the disease by the injection of an emul- 



