76 INFECTION AND IMMUNITY. 



this point clear, let us assume that the disease is 

 not hereditary in the tick. Let some normal larvae 

 become attached to an infected steer. They might 

 acquire the disease from this animal, but could 

 play no part in the infection of others, since they 

 do not abandon this host until they have reached 

 the adult stage and the females are prepared to 

 } a y eggs. The latter then drop and lay their eggs, 

 and, in accordance with our assumption, the larvae 

 which emerge would not have the power of infect- 

 ing further animals. In this case the tick would 

 be a factor in maintenance only in the event that 

 infected individuals should through accident be- 

 come dislodged from one host and subsequently 

 become attached to a susceptible animal. This 

 may occur in some instances. 



East coast The conditions are different in the case of 

 another piroplasmosis of cattle, namely, the 

 Rhodesian or East Coast fever, which occurs in 

 Africa. In this case, as determined by Lounsbury 

 and by Theiler, the brood from an infected female 

 is not infective, but larvae, when fed on diseased 

 blood, are infective after reaching the nymphal 

 stage, and, likewise, when infected as nymphs the 

 adults have the power of transmission (RJiipi- 

 cephalus appendictilatus) . This is known as stage- 

 to-stage infection, and is important in this in- 

 stance, inasmuch as the larvae and nymphs leave 

 the hosts to molt. Following the molt they fre- 



soon become attached to hosts. Then they feed, pass into a 

 quiescent stage, and leaving a white skin, appear as 8-legged 

 nymphs. These also feed, become quiescent for a period, and 

 casting off a white skin are now adults, which are differ- 

 entiated sexually. In some instances, as in the tick trans- 

 mitting Texas fever, both molts occur on the host. More fre- 

 quently, however, other species leave the host to molt. From 

 several weeks to several months are required for the whole 

 cycle. 



