377 
Morphology. —Length, on an average, 2.0/4, by o ‘4/4 
broad ; undulations six to eight in number, about 2/4 
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 fevers 
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 2 4/4 by 0*45/* broad; the undulations, 
eight to ten in number, 2*2/4 in width. ( Vide 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 
