ROCKY MOUNTAIN FEVER. 73 



season to the next. It is not yet known whether 

 sleeping-sickness is inherited in the tsetse fly. Ma- 

 laria is not so transmitted in the mosquito. 



There may be varying grades of chronicity in soatii African 

 both the animal and insect hosts. Thus South Af- 

 rican tick fever of man (a spirillosis) is semi- 

 chronic, consisting of several recurrences followed 

 by recover}^ and it is probable that the tick may 

 acquire the disease from the patient during any 

 one of the recurrences. In the tick, however, the 

 disease is chronic and hereditary. 



In these cases the method of maintenance is 

 clear, and in the presence of a sufficient number 

 of insects the conditions are favorable for a 

 thorough infection of the inhabitants. Thus it is 

 that in many tropical districts there are none who 

 do not fall victims to malaria sooner or later. 



In Eocky Mountain spotted fever we have an Rocky Monn- 



i p ,.,. 1'ixi T tain Spotted 



example of a condition in which the disease is Fever. 

 acute in the various animal hosts, including man, 

 but chronic in the carrier, the tick. In order that 

 fresh ticks may acquire the disease it is necessary 

 for them to feed on a susceptible animal in com- 

 pany with infected ticks, or shortly following a 

 feed by the latter. Since several hundred larvae 

 or nymphs may be found on one of these animals 

 at the same time, it is readily seen how this may 

 be accomplished. While certain of the small ani- 

 mals (ground-squirrel, ground-hog, rock-squirrel, 

 and perhaps others) are in hibernation the 

 virus still lives in the eggs, larvae and nymphs; 

 and when the animals "come out" in the spring 

 certain of them become infected through the bites 

 of the larvae or nymphs and then are in condition 

 to infect fresh ticks. Hence the disease is kept 



