722 INFECTION AND IMMUNITY. 



numbers of ticks." Similar parasites were found 

 in no other species of animals. 



Stiles, in later investigations, could not confirm 

 the results of Wilson and Cbowning, being unable 

 either to find the parasites which they described in 

 man, or to accept the tick-gopher hypothesis. 



McCalla and Brereton infected two individual? 

 "oth'er successively by the bite of a tick which they had 

 Animal^. removec j f rom one O f their patients. 



Ricketts and his collaborators have shown that 

 spotted fever can be reproduced with great con- 

 stancy in the guinea-pig by the injection of in- 

 fected blood or the organs or eggs of infected 

 ticks. The symptoms of spotted fever in the 

 guinea-pig appear after an incubation period of 

 from two to five days. There is a sudden rise in 

 temperature to 105 or 106 F., with a general- 

 ized roseolar eruption. Swelling and hemorrhage 

 of the scrotum or vulva occurs. The symptoms are 

 diagnostic when they occur typical!} 7 . The mon- 

 key, rabbit, horse and at least five species of small 

 wild animals have a greater or less degree of sus- 

 ceptibility. 



Ricketts and King, working independent!} 7 , were 

 able to transmit spotted fever from diseased to 

 normal guinea-pigs by allowing ticks which had 

 fed on diseased pigs to bite normal pigs. Ricketts 

 and Wilder were able to show that up to 50 per 

 cent, of infected ticks transmitted the infection to 

 their young. It was found that nymphs develop- 

 ing from infected larvae were infectious for guinea- 

 pigs, and that in a similar way adult ticks devel- 

 oping from nymphs were able to transmit spotted 

 fever. Naturally infected ticks were discoveerd in 



