377 



Morphology. Length, on an average, 2O/^, by 0*4^ 

 broad ; undulations six to eight in number, about 2/* 

 between the summits of each. They stain uniformly, 

 with the exception that frequently a minute clear spot 

 can be seen about the middle of the length. 



Mode of Transmission. Is unknown, but in spite 

 of negative experimental evidence, bugs or lice are the 

 probable carriers. 



Pathogenicity. In monkeys the incubation period 

 varies from one to four days, depending on the dose 

 inoculated. The attack lasts two to six days, and there 

 is usually one relapse. Rats and mice cannot, as a 

 rule, be infected directly by human blood. A passage 

 through a monkey is first necessary. In rats, the 

 incubation period varies from a few hours to a few 

 days. The attack lasts one to three days. Spiro- 

 chaetes are scanty. Relapses occur, but last only a 

 day. Rats always recover. 



2. Sp. duttoni. The cause of African tick fever. 



Symptoms. The attack lasts one to four days. 

 The apyretic interval lasts about three days. The 

 usual number of relapses is uncertain, they may be as 

 many as six. 



Blood Examination. Spirochaetes, even at the 

 height of the fever, are nearly always scanty. 



Morphology. The spirochaeta is, when fully 

 grown, about 24^ by 0*45^ broad ; the undulations, 

 eight to ten in number, 2 '2^ in width. (Fide Plate V.) 



Pathogenicity. It differs from S. recurrentis in 

 that nearly all the ordinary laboratory animals, except 

 cats, can be infected. Rats are most easily infected. 

 They frequently shew immense numbers of parasites, 

 and large tangles can be found. Several relapses may 

 occur, unless the animal succumbs. An animal that 

 has recovered from an attack of S. recurrentis is still 



