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Trypanosoma lewisi 
It seems therefore characteristic of the flea transmission of T. lewisi 
that it occurs after the lapse of a period during which the fleas are 
incapable of communicating the parasite. This result is in accord with 
that obtained by Minchin and Thomson. 
We are however inclined to think that infection occasionally follows 
the immediate transference of the fleas to a ‘ clean ’ animal, in other 
words that the so-called mechanical transmission occurs and that it 
may be followed by the latent or non-infective period which precedes 
the infective period when distant transmission takes place. In support 
of this view we would cite the following experiment. 
Experiment 10. Fleas were used which had been raised in the laboratory, in 
the manner advised by the Indian Plague Commission, in large glass boxes in 
which a flea-infested rat had been placed. Under such conditions the fleas readily 
multiply in the filth which soon collects in the box. 11 of such fleas were fed for 
one hour on a rat whose blood had shewed T. leiuisi for 12 weeks. After 24 and 
72 hours the fleas were again fed on a ‘ clean ’ Rat D. Rat D became infected in 
seven days. After this the fleas were fed at intervals on a series of ‘ clean ’ animals, 
but they did not infect the rats again until the .34th day. 
Ill all experiments where we excluded the possibility of ‘ mechanical 
transmission’ five out of nine rats became infected, but where the 
possibility was not excluded five out of six experiments had a positive 
result. The difference in the results observed may have been due to 
mechanical transmission having been the means of infecting in some of 
the experiments. The average number of fieas for each experiment 
was the same in the two series. 
If mechanical transmission of T. leiuisi by fleas takes place we 
thought that T. hrucei should also be transmitted mechanically, but 
the results of six experiments were negative. We allowed fleas to feed 
on a rat heavily infected with T. hrucei and then transferred them to 
clean rats, but in no case did these rats become infected. 
We obtained no evidence of hereditary transmission of trypanosomes 
in fleas although we experimented with a large number (about 60) 
raised from the larva removed from a flea-breeding box in which an 
infected rat had been kept for many weeks. The flea imagines produced 
no infection when placed on ‘ clean ’ rats. 
Certain conditions may influence the transmission of T. leiuisi by 
fleas. For instairce Petrie and Avari say that the hot season is the more 
favourable to transmission because they found fewer fleas on the rats 
and more cases of infection. We think that a favouring factor is 
connected with the ' wildness ’ of the flea, for we found that with fleas 
