578 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



confirm this conclusion. Much more recently Wolbach (1919) has 

 found an organism which he has described as Dermacentroxenus ric- 

 kettsi, which he considers to be the cause of the disease. He finds 

 that it is intracellular in mammals and in ticks and intranuclear in 

 ticks. It is reasonably certain that some of the native small animals 

 furnish the reservoir of the disease and the destruction of ground 

 squirrels and the dipping of domestic animals to destroy ticks have 

 been tried with good results in the Bitter Root Valle3 r . The tick 

 most closely associated with this disease is Dermacentor andersoni 

 Stiles (D. vem/stus Banks), but it has been experimentally shown by 

 Maver that the disease may be transmitted experimentally by several 

 species of ticks. 



The Rocky Mountain spotted fever investigation has been marked 

 by the martyrdom of several of the investigators. Dr. T. B. Mc- 

 Clintic, of the United States Public Health Service, died of the fever 

 while engaged in its investigation, and Dr. A. H. McCray died in the 

 same way later while working for the Montana State Board of 

 Health. Doctor Ricketts himself, to whom the confirmation of the 

 tick relation in this disease is due, while he passed successfully 

 through his work with spotted fever, died later of typhus fever in 

 Mexico while studying the relation of lice and typhus, which will be 

 treated in a later paragraph. 



There has been no systematic arrangement of the accounts so far 

 given of the discovery in the carriage of disease by insects, but I have 

 considered them rather chronologically, following one account of 

 a discovery by the story of the next one. But, arriving now at ap- 

 proximately the middle of the first decade of the present century, 

 the work in this direction all over the world has been so intense and 

 discoveries have been so various, or have followed one another so 

 rapidly, that any strict chronological sequence must be abandoned 

 and, in fact, it will be possible to touch only briefly upon a few of 

 the principal discoveries. 



Antedating the investigation described in the last few paragraphs, 

 were the experiments in the transmission of bubonic plague by fleas 

 carried out successfully by Simond in 1898. This terrible disease, 

 which from time to time has ravaged the Old World in epidemic form, 

 is supposed to have caused the death of 25,000.000 people on the 

 Continent of Europe in the fourteenth century. And in modern times 

 it has raged in oriental countries, causing, for example, for many 

 years in India, an average of nearly 1,000.000 deaths annually. The 

 causative organism of the plague is Bacillus pestis, described by 

 Kitasato, the famous Japanese investigator. The establishment of 

 the identity of the disease with the one which occurs in rats had been 

 made by Yersin in 1894 and Simond's transmission of the disease bv 



