VIII 

 RICKETTSIA 



1. HISTORICAL REVIEW 



"RICKETTSIA" is the group name given by da Rocha-Lima 

 (1916 2 ) to minute micro-organisms with certain peculiarities 

 found in lice. The name honors the memory of Howard Taylor 

 Ricketts who first described micro-organisms possibly of this 

 type in connection with studies upon typhus (Ricketts and 

 Wilder, 1910). 



Ricketts' report (1910) was meagre and his death from 

 typhus while making his investigations in Mexico City pre- 

 vented the publication of his complete observations and 

 conclusions. Ricketts wrote as follows: 



"(1) In the stained (Giemsa) preparation of the blood of 

 patients, taken on from the seventh to the twelfth day of the 

 disease, we invariably have found a short bacillus which has 

 roughly the morphology of those which belong to the 'hemor- 

 rhagic septicaemia group.' Usually it appears to stain solidly, 

 but on minute examination an unstained or faintly stained bar 

 is seen to extend across the middle. Occasionally two organ- 

 isms are seen end to end. Exact measurements have not been 

 made, but when compared with the size of the erythrocyte, 

 their length is estimated hardly more than two micromilli- 

 meters, and their diameter at about one-third this figure. 

 Certain other bodies, the identity of which is not so clear, may 

 represent degeneration or involution forms of the above. They 

 consist of two stained granules, connected by an 'intermediate 

 substance 7 which is stained faintly blue or not at all. Fre- 

 quently one of these granules or ' rods' is larger than the other 

 and stained a deep purple, whereas the small one takes a faint 

 blue color." 



"(2) In moist preparations of the blood of patients, bacil- 

 lary bodies, with a structure like that mentioned above, have 



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