124 RICKETTSIA 



ing, and chemical agents. On the other hand, the virus of 

 trench fever resists 80 C. of dry heat for twenty minutes and 

 drying for many months (Byam and Lloyd, 1919). 



Host Specificity: All rickettsias have insect hosts which in 

 the case of the pathogenic ones are the vectors. All are highly 

 specific for their insect host while the pathogenic ones may 

 infect widely separated mammals. 



Hereditary Transmission: In every instance where careful 

 study has been made it has been found with the exception 

 of the rickettsia of typhus that the organisms pass down 

 through successive generations, in the eggs. Da Rocha-Lima 

 has offered some evidence that this is also true of Rickettsia 

 prowazeki, and Sergent, Foley, and Vialette, 1914 (quoted by 

 Nuttall, Parasitology, Vol. 10), accidentally communicated 

 typhus to a monkey and a man with the offspring of lice which 

 were supposed to be infected only with relapsing fever. 



Classification: Is of course impossible, and it is probable 

 that we have already included under rickettsia a number of 

 very different micro-organisms. The Rickettsia of the sheep 

 louse has little to distinguish it from bacterium; yet we believe 

 the rickettsia of typhus has a number of peculiarities which 

 necessitate its separation at present. The rickettsia-like cause 

 of Rocky Mountain spotted fever, which we prefer for the 

 present to consider under a distinctive name, while resembling 

 in many ways Rickettsia prowazeki, is very unlike the morpho- 

 logically simple rickettsia of trench fever. 



The following table (Table X) indicates the wide distribu- 

 tion of "Rickettsia'' in " insects." One important generali- 

 zation, which we believe to be warranted in view of their 

 great specificity for their insect hosts and hereditary transmis- 

 sion, is that they are forms of micro-organisms primarily 

 adapted to insect tissues with occasional representatives 

 pathogenic for mammals. It must be kept in mind that the 

 grouping of these micro-organisms under Rickettsia can only 

 be tentative with the meagre data so far determined. 



