958 DISEASES CAUSED BY FILTRABLE VIRUS 



might represent a group of parasitic, and perhaps pathogenic, or- 

 ganisms not hitherto observed. Von Prowazek did not believe 

 these small bodies to be bacteria, and from the beginning took the 

 position that they were more likely to belong to, or be closely 

 related to the protozoa. Da Rocha-Lima, who has studied them 

 particularly in their relationship to typhus fever, gave the appear- 

 ances which he saw in typhus lice the name of Rickettsia prowazeki. 

 In the course of numerous investigations upon the etiological signifi- 

 cance of these peculiar appearances, especially in connection with 

 Typhus fever, Rocky Mountain Spotted Fever, and Trench Fever, many 

 workers have confirmed the observations of the earlier observers, and 

 while it is quite impossible at the present time to classify them with 

 any degree of certainty either with the bacteria or the protozoa, the 

 various forms described possess sufficient similarity to each other to 

 warrant the establishment of a tentative group. In describing them 

 in a separate section we do not mean to imply that, at the present 

 time, it is absolutely certain that they are parasites. But this seems 

 so likely, and their etiological relationship to the diseases mentioned 

 has been suggested by so many careful investigations, that clearness 

 of treatment at the present time fully justifies such segregation into 

 a separate group. 



The appearances which are classified together as Rickettsia are 

 minute, ovoid or bacterium-like bodies. They are, as a rule, ex- 

 tremely small, the smallest forms being more minute than the 

 smallest known bacteria, measuring about 0.3 to 0.5 of a micron. 

 Larger forms more bacillary in appearance, may be observed, and 

 it is suggested that the Rickettsia bodies go through a develop- 

 mental cycle. The small forms often appear in the "diplo" form 

 and some German observers have described capsule-like halos around 

 groups of two. 



They are all very difficult to stain. The ordinary aniline dyes 

 stain them either very faintly or not at all. Prolonged staining 

 with Giemsa gives them a faint reddish blue tinge. They do not 

 retain the Gram stain. 



They are non-motile. 



Up to the present time none of the Rickettsia have been cul- 

 tivated, with the exception of one form observed in the sheep louse 

 which grows aerobically on glucose-blood-agar. 



All of them have an insect host which acts, in the case of the 

 pathogenic Rickettsia, as the transmitting agent, 



