128 RICKETTSIA 



The unnamed rickettsia from the bird mite is another in- 

 tracellular representative discovered by Noller and men- 

 tioned by Sikora, 1920. 



The rickettsia in the harvest mite is highly questionable. 

 Sikora (1920) mentions it as a recently discovered cause of 

 Tsutsugamushi disease, information which came by word of 

 mouth. 



Dermacentroxenus rickettsi (Plate I, fig. 6), the cause of 

 Rocky Mountain spotted fever, has been included among 

 rickettsia by a number of authors. It is therefore included in 

 the above table. Nevertheless, comparison with Rickettsia 

 prowazeki, while showing a number of common features, 

 brings to light many differences between the organisms in 

 morphology and in behavior in the insect vector. 



Dermacentroxenus is less bacterium-like than any of the 

 rickettsias and in multiplicative form always shows red and 

 blue staining materials. It does not appear in thread-like or 

 filamentous forms as does Rickettsia prowazeki. 



In the louse, Rickettsia prowazeki continues to multiply 

 indefinitely in the stomach epithelium; it eventually causes 

 the death of the louse through suspension of digestion. Der- 

 macentroxenus, however, after a stage of active multiplication, 

 largely intranuclear, floods all the tissues of the tick and then 

 diminishes in numbers, leaving behind in certain tissues in- 

 cluding the salivary gland forms different from the multipli- 

 cative forms, and which Wolbach regards as a resistant stage. 



For the present we prefer to keep separate dermacentroxe- 

 nus a parasite in Arachnida from the rickettsia which are 

 parasitic in Insecta. 



Cross immunity experiments, now in progress, show that 

 guinea-pigs which have recovered from typhus do not react 

 as do normal guinea-pigs to inoculation with Rocky Mountain 

 spotted fever. While typhus immune guinea-pigs develop 

 spotted fever, they all show a lengthened incubation period 

 and lower temperatures. The mortality was also strikingly 

 reduced by about 50 per cent. 



