C. Strickland and N. H. Swellengrebel 
443 
taken from infected wild rats transmission was obtained much more 
readily than with those bred and fed in the laboratory. Sir David 
Bruce and his collaborators (1910) have noted a similar phenomenon 
in the transmission of T. gambiense by Glossinae. 
Fleas seem to be able to transmit infection whether they have fed 
on a rat recentl}^ infected or on one which has been long infected. We 
have obtained transmission of T. lewisi from a rat which had only been 
infected one to four days, and from a rat on the other hand which had 
been infected for three and a half months. 
(c) The mechanism of infection. It has been assumed that fleas 
infect rats with T. lewisi during the act of blood sucking, the parasites 
being introduced into the host through the fleas’ mouth parts. In view 
of this assumption it seemed somewhat strange that we should only 
find parasites in the fleas’ hindguts : as has been described in a previous 
paper by us. If we assume that after all the ‘ small trypanosomes ’ are the 
infective agents we could hardly expect them to be carried forward from 
the hindgut through the pyloric valve and the proventriculus to the 
mouth parts, for both valves are designed to prevent regurgitation {see 
Jordan and Rothschild, 1908). Moreover we have never found small 
trypanosomes in the midgut of fleas whose hindgut has been swarming 
with this form of the parasite. As we have seen we have succeeded 
repeatedly in infecting rats upon which infective fleas were placed and 
allowed to feed naturally, that is by being allowed to crawl about 
and remain upon the host. With the object of learning more about 
the mechanism of infection attempts were repeatedly made by us to 
infect rats by allowing infective fleas to bite them through fine gauze, 
but the results were always negative : thus 
Experiments 11—16. A large number of infected fleas (20—30) in six separate 
experiments were first starved for several days and were then fed respectively on six 
clean rats through fine gauge. Most of them were observed to feed and could be 
seen distended with blood afterwards, but none of the ‘ clean ’ rats became infected ; 
but when the same fleas were afterwards placed upon another lot of six clean rats 
and allowed to feed naturally, the latter all became infected. 
Experiment 17. Eighteen fleas which had fed on an infected rat E for one 
hour were fed through fine gauze on a series of clean rats for about a month without 
any result, but when the only two which survived this time were put on and left on 
a clean rat and allowed to feed naturally, this latter became infected. 
Experiment 18. A batch of about 200 fleas were first starved for a few days and 
were then fed on eight clean rats through fine gauze. After this they were let loose 
on six other clean rats. Whereas none of the rats on which the fleas fed through 
gauze became infected, tlie rats on which they were let loose all became infected. 
Parasitology in 
29 
