(x. H. F. Nuttali 
77 
lice under ordinary conditions prevailing after an infective meal. 
The spirochaetes vanish in lice in which they subsequently reappear. 
Just before their reappearance, usually on the sixth day, but at times 
on the third to the fifteenth day, the lice have been found to be most 
infective. The infectivity of the louse does not depend upon the 
presence in its body of visible spirochaetes, in fact when spirochaetes 
reappear and attain their full size the lice are non-infective (Nicolle). 
This, coupled with the observation that human blood is infective during 
the apyrexial stage when spirochaetes cannot be found in the blood 
(Sergent and Foley), certainly bears out the view which I have upheld 
with others, that spirochaetes are Protozoa, for they obviously undergo 
a cyclical development in both the vertebrate and arthropod hosts. 
Nicolle advances the view that the typical spirochaete is incapable 
of multiplication or that it rarely divides, and that multiplication and 
consequent infectivity are entirely or largely confined to the minute 
forms which may coincide with the “coccoid bodiesof some authors, 
assuming that they are not ultramicroscopic. 
It has been proved that lice do not convey relapsing fever by their 
bites. As Nicolle and his colleagues have shown, persons may be bitten 
many thousands of times by infective lice with impunity. Infection 
takes place through the lice being crushed upon the skin which is 
commonly excoriated by the self-inflicted scratches of the individual 
harbouring the lice. Infective material may, moreover, be carried on 
the fingers to the nose or eye, and it has been demonstrated experi¬ 
mentally that the spirochaete is capable of invading the system through 
intact mucous membranes. 
The period of incubation, under'experimental conditions, as observed 
in man and monkeys following infection through the excoriated skin 
or intact mucous membrane, is 6-8 days, when crushed lice in the 
infective stage are used. This corresponds with clinical experience. 
A single infective louse, crushed upon the excoriated skin, has produced 
relapsing fever. Persons have been experimentally infected by placing 
infected lice upon their persons without their knowledge. Happily 
for thege subjects and for the experimenters who intentionally infected 
themselves, the course of the disease can be cut short by the arsenical 
treatment discovered by Ehrlich. 
