516 Dr. C. Morley Wenyon [March 8, 



Fleas which have become infected on one rat will wander from rat 

 to rat, and in a short time the infection will spread. Very careful 

 experiments have been made which prove that the actual bite of the 

 flea does not convey the infection. Fleas can be studied very con- 

 veniently by fixing them on fine wire passed round the thorax, as is 

 done by showmen. A flea so tethered can be easily handled and its 

 movements controlled, while the droplets of blood which it ejects 

 while feeding can be received on to slides and examined with the 

 microscope. Such a flea I have kept alive for about four months. 

 A tethered flea can be fed on a rat in the blood of which trypano- 

 somes are present. Subsequently it can be fed on one's own wrist. 

 and it will be found that after the expiry of about six days the drop- 

 lets of ejected blood will contain small trypanosomes which have 

 become established in the rectum of the flea. The insect is now in 

 the infective condition. The flea can be placed on an uninfected rat, 

 and while it is feeding care can be taken to protect the skin so that 

 no droplets of blood are allowed to fall upon it. The droplets can 

 be received on to a cover-glass held behind the flea. With a fine 

 pipette the droplets can be sucked up from the cover-glass and intro- 

 duced into the mouth of another uninfected rat This experiment 

 was repeated many times with different species of flea— the rat flea, 

 the dog flea, the human flea, and the Indian plague flea — always with 

 the same result. The rat on which the fleas actually fed never became 

 infected, whereas those which received the droplets in the mouth 

 did so. It is thus clear that the infection is spread, not by the bite 

 of the flea, but by the droplets of infective blood which the flea 

 ejects from its rectum. The trypanosome of the rat has availed 

 itself of the gluttonous habits of the flea in order to effect its trans- 

 mission from rat to rat. The flea continues to void trypanosomes 

 for the remainder of its life. 



A trypanosome which produces a disease of human beings in 

 certain parts of South America resembles the harmless trypanosome 

 of the rat in the mechanism of its conveyance. This form is peculiar 

 in that within the vertebrate host it multiplies chiefly within the 

 muscle-fibres, but also in other organs like the thyroid gland, as 

 small rounded bodies which finally grow into trypanosomes which 

 invade the blood. Other trypanosomes, like those of sleeping sick- 

 ness and the harmless one of the rat, multiply in the trypanosome 

 form while they are swimming about in the blood stream. In the 

 case of the South American disease, or Chagas' disease, as it is called 

 after its discoverer, the trypanosomes which have developed in the 

 muscle-fibres make their way into the circulating blood, whence they 

 are sucked up by a large reduviid bug which lurks in the crevices of 

 wood and comes out at night to bite its victims. The trypanosomes 

 multiply in the intestine of the bug, and, as in the case of the rat 

 trypanosome, they finally establish themselves in the posterior portion 

 of the intestine or rectum. From this situation they are voided in 



