86 MUSGRAVE. 



8. From this amoebic-bacterial-intestinal compound lesion some process 

 may be evolved changing the metabolism of the parasite and lowering the 

 tissue resistance until a focus of infection in which the bacteria have been 

 eliminated, is established in the liver, brain, spleen or some other organ, 

 and thus a lesion is produced which is due to an interaction between the 

 amoeba and the animal tissues, a condition of true animal tissue-para- 

 sitism. 



9. In considering the occurrence of true parasitism we must never 

 lose sight of the fact that the amceba is altered as well as the tissue, which 

 by change has become capable of being the host of the microorganism. 



Among the higher animal tissue-parasites in which bacteria play no 

 part in the symbiosis, we still have modifications due to environment, just 

 as are shown by pure cultures of bacteria. 



Lingard, 10 showed that the virulence of Trypanosoma evansi is to 

 a certain extent influenced by conditions which affect the pathogenicity 

 of bacteria. This author passed an organism of surra directly from horse 

 to horse through forty animals. During the first part of the experiment 

 the incubation period varied from four and one-half to six and one-fourth 

 days and the duration of illness from twenty-five to forty-two days, but 

 during the latter period the incubation time was reduced to from three 

 to four and one-half days, and the duration of the disease to from three 

 to four days. The author made the further interesting observation that 

 white mice inoculated from a horse in the early stage of his experiments 

 showed an incubation period of from five to five and one-half days and a 

 duration of the disease of five days. The infection was then transferred 

 from mouse to mouse, and between the thirty-seventh to forty-seventh 

 mouse instances occurred in which the incubation was reduced to twelve 

 hours and the course of the disease to from two and one-half to three 

 days. Still more interesting is the fact that a horse inoculated from the 

 forty-seventh mouse lived four and one-half times as long as the control 

 animal. 



Some of the influences which a change of host may exert on parasites 

 are well illustrated in smallpox. As is known, the pustular contents 

 of smallpox when inoculated into monkeys produces a disease which 

 clinically may closely resemble variola in man. When some other of the 

 lower animals, such as the cow, are used for these experiments, the 

 disease reproduced in the animal differs materially in its manifestations 

 and when the virus is again transferred from- the cow to man it manifests 

 itself as vaccinia, which differs from smallpox, and human variola can 

 not be reproduced from this virus. 



Satisfactory explanation of this phenomenon has not been furnished 

 beyond the obvious fact that in some manner the pathogenic character of 

 the organism is changed. Great variations in the pathogenic nature of 



10 Annual Report, Imp. Baet. of India (1900-7). 



