78 INFECTION AND IMMUNITY. 



Rocky Mountain spotted fever not only is inher- 

 ited in the tick, but both larvas and nymphs may 

 acquire the disease by feeding on infected blood 

 and transmit it by biting when they reach the sub- 

 sequent stage. Hence both hereditary infection 

 and stage to stage infection occur in this case. 



It is questionable whether any disease can be 

 maintained in an insect indefinitely through inher- 

 itance alone. Hollers 9 found that South African 

 tick fever could be carried into the third genera- 

 tion of ticks (Ornithodoros moubala), neither the 

 second nor third generation of ticks having had 

 opportunity to suck diseased blood in the mean- 

 time. The experiment was not carried beyond this 

 point. In relation to Bocky Mountain spotted 

 fever not more than 50 per cent, of the infected 

 females transmit the infection to their offspring, 

 hence maintenance through inheritance alone is 

 considered as impossible. 



Certain considerations relative to the course of 

 infection in insects, the incubation period which 

 may intervene between the moment of their inocu- 

 lation and the appearance of infectivity, and, in 

 addition, the duration of their infectivity, are of 

 interest and importance. These points may be 

 discussed briefly and only in a general way. 

 innocuous- It is striking that diseases which are trans- 

 ss Ge?ms mitted by insects appear to have little pathogenic 

 influence on the insects themselves. The malarial 

 plasmodia penetrate the wall of the stomach, reach 

 the body cavity and eventually the salivary glands, 

 and this, it would seem, without compromising 

 seriously the health of the mosquito. The flea 



9. Ztschr. f. Hygiene, 1907, Iviii, 277. 



