G. H. F. Nuttall 
disappear and the mice usually die. When mice harbouring the 
trypanosomes in their blood were bitten by tbe bugs, the flagellates 
resumed the forms (leptomonas, crithidia and trypanosome) which they 
originally possessed prior to entering the body of the mouse. 
The rat trypanosome (T. lewisi) and its mode of transmission. 
Whereas in the fr^ossfna-transmitted trypanosomes the parasites 
enter fresh hosts through the flies’ proboscides, we have another method 
of infection in the case of the rat trypanosome. Trypanosoma leioisi is 
conveyed by several species of flea and by the rat louse ; it is world-wide 
in its distribution and occurs in 2.5-100 °/o of Mus decumanus captured 
in a wild state. Rats may hai’bour the parasite in sufficient numbers 
in their blood to render them demonstrable microscopically for a period 
lasting from a week to seven months: The usual vectors are un¬ 
questionably the common rat-fleas {Ceratophyllus fasciatus and 
Ctenophthalmus agyrtes) although the parasite is transmissible by other 
species of flea {Gtenocephalus canis and Gtenopsylla musculi) as has been 
demonstrated experimentally. Rat-lice {Haematopinus spinidosus) may 
occasionally serve as vectors. 
Trypanosoma lewisi, after being imbibed by the flea, multiplies 
rapidly, chiefly in the hindgut and rectum of the insect. The parasites 
assume a crithidia-like appearance and occur in large bunches attached 
by their flagellar ends to the epithelium, or they occur in cyst-like 
masses within degenerating epithelial cells. Subsequently, the parasites 
resemble the blood forms as seen in the rat. They may then be found 
in vast numbers crowding the hindgut and rectum of the flea. The 
latter becomes infective in four to seven days and it may remain 
infective for 45 days or longer. The flea does not infect the rat through 
its proboscis, and the parasites are not found in the flea’s salivary glands. 
Infection can take place in three ways: the flea harbouring the infective 
forms of the flagellates may be (a) crushed and devoured by the rat, 
(6) the rat may lick its fur upon which an infected flea has just dejected, 
(c) the rat may lick and infect with flea dejecta the wound produced by 
the insect. Fleas in the act of feeding frequently eject excreta which 
may be loaded with the flagellates in the infective stage. We have 
conclusive experimental evidence to prove that infection may occur in 
these different ways. We have then, in the flea, an entirely different 
mode of trypanosome transmission as compared to what we have seen 
in Glossina. 
