NATURAL IMMUNITY 139 



It is thus evident that in a study of the interrehitions of a host and the 

 parasite both the condition of the host and that of the parasite have to 

 be taken into account. The increase in virulence of Tryjmnosoma lewisi 

 produced by Roudsky was artificial, and it is probable that under natural 

 methods of transmission such a change would rarely, if ever, take place. 

 Nevertheless, the observation is an important one, for it demonstrates 

 that a trypanosome may become modified to such an extent that it will 

 produce infections in animals in which normally it fails to develop. It 

 is a generally accepted fact that the animal trypanosome, Tnj'panosoma 

 hrucei, does not as a rule infect man who is constantly exposed to the 

 bites of infected tsetse flies, yet there occurs in man in Rhodesia a 

 trypanosome which has been given the name Tryjpanosoma rhodesiense, 

 which in all respects appears to be identical with T. hrucei. It is main- 

 tained by some that it is distinct from T. hrucei, and by others that it is 

 identical with it. It has, however, to be recognized that it is quite within 

 the bounds of possibility that the animal trypanosome T. hrucei may 

 occasionally change, for reasons not yet discovered, so that it becomes 

 capable of infecting man, or that man may occasionally be in a condition 

 which will permit infection with the unaltered trypanosome. Duke 

 (1923, 1923a) believes that an outbreak of trypanosomiasis amongst 

 human beings in the Mwanza district of Africa, in which the trypano- 

 some was of the T. rhodesiense type, was due to the inoculation of 

 the animal trypanosome T. hrucei as a result of the lowered resistance 

 of the population after a period of famine and heavy ankylostome 

 infection. 



There are many examples of variation in virulence of parasitic Protozoa. 

 It is well known that if Trypanosoma gamhiense is inoculated from the 

 blood of man into a rat, the type of infection produced is a chronic one, 

 very few trypanosomes being present in the blood of the rat at any one 

 time, the inoculated animal often surviving for many months. In suc- 

 cessive passages in rats the virulence increases, till finally a strain is 

 produced which multiplies very rapidly, so that the blood is soon swarming 

 with parasites, which bring about the death of the host in about ten days. 

 By passage of the strain through a different host such as the guinea-pig 

 this virulence for rats may be largely lost. It is regained, however, by 

 further passage through the rat. Duke maintains that in the spread of 

 sleeping sickness the epidemic outbursts of this disease are due to direct 

 passage of the trypanosome from man to man by mechanical transmission 

 in which some biting insect merely conveys blood from an infected to 

 a healthy person, just as in laboratory experiments the syringe conveys 

 blood from an infected to a healthy animal. It is supposed that in this 

 way the virulence of the trypanosome, which is kept relatively avirulent 



