TKYI'ANOSOMA LKWTSl 617 



We now pass to consider in detail some of the more important 

 trypanosomes. 



Trypanosoma Lewis!. In 1878 Lewis described in rats in India the 

 occurrence of the parasite which now goes by his name, and since that time 

 this trypanosome has been found to be very common in the blood of rats 

 all over the world, though the percentage of animals affected varies in 

 different localities. Though the organism has no importance from the 

 standpoint of human pathology, several significant points arise in con- 

 nection with it. The condition in the rat is of great interest, as, though 

 the infection runs a very definite course, it is very rarely fatal ; in fact, 

 many observers have been unable to produce death by infecting even 

 large series of animals. There is, however, little doubt that a fatal issue 

 does occur sometimes in young individuals, especially when these are 

 infected with strains of the organism imported from other localities. 

 The trypanosome, which is actively motile, is of the usual length but is 

 somewhat narrow, and its protoplasm does not contain any granules. It 

 multiplies by fission, of which Laveran describes two varieties. In one, 

 the organism splits longitudinally and gives rise to smaller individuals 

 than the parent. In the other, the trypanosome loses its ordinary shape 

 and becomes more oval : nuclear division, which is often multiple, then 

 takes place, and on subsequent division of the protoplasm a number of 

 small flagellate organisms result ; these last may attain the full form and 

 size before dividing again, or they may divide when still small. When a 

 rat is infected by injection into the peritoneum, active multiplication goes 

 on in the cavity for a few days and then comes to an end. Very soon 

 after infection the organisms begin to appear in the blood and there 

 rapid multiplication occurs, the extent of which is sometimes so great 

 that the trypanosomes may seem to equal the red blood corpuscles in 

 number. The animal usually shows no symptoms of illness. The 

 infection goes on for about two months, and then the organisms gradually 

 disappear from the blood. In the great majority of cases the rat is now 

 immune against fresh infection. If trypanosomes be introduced into its 

 peritoneum they are, according to Laveran, taken up by mononucleate 

 phagocytes and destroyed. The serum of a rat which has been infected 

 shows agglutinating capacities towards the trypanosomes, causing them to 

 agglomerate in rosettes in which the flagella are directed outwards, and 

 tlie serum of immune rats has a certain degree of protective action if 

 injected along with the organism into a susceptible animal. As has 

 already been noted, this trypanosome has been cultivated on artificial 

 media, on which it multiplies freely, large numbers of small forms being 

 often produced. The-c when injected into rats give rise to the usual 

 infection, but not so rapidly as when blood from an infected animal is 

 used. The organism multiplies at the body temperature, but a lower 

 temperature is ] .referable, and at 20 C. Novy and McNeal succeeded in 

 carrying a growth through many sub-cultures. The trypanosome is very 

 resistant to cooling, and has been exposed for fifteen minutes to the 

 temperature of liquid air (-191C.) without being killed. With 

 regard to this infection Minchin and Thomson have shown that the rat 

 flea, eeratoftkyttwfaueiatut transmits the parasite by the cyclical method 

 (mechanical infection not having been proved). The flea becomes 

 infective about a week after biting, and remains infective for a long 

 period, possibly for the rest of its life. Infection may also take place 

 through another species of flea and through a louse. 



