EFFECTS OF PROTOZOAN PARASITES UPON THEIR HOSTS 201 



the belief of Liihe, Mesnil, and others, that the original forms of some 

 at least of these organisms were flagellated protozoa which have lost 

 their motile organs and assumed an intracorpuscular or cytozoic mode 

 of life with the accession of parasitism in man. Also, it appears from 

 such cases that the original hosts were insects and not man, so that 

 here at least man w T ould appear to play the part of intermediate or 

 secondary host. 



The further deductions which some recent observers have made 

 (notably Hartmann and Kisskalt, and others), that all hemosporidia 

 are to be traced to flagellated ancestral forms, and that the group as a 

 division of the sporozoa should, therefore, be abandoned, does not 

 follow from the evidence and cannot be sustained at the present time 

 (see p. 269). 



III. EFFECTS OF PROTOZOAN PARASITES UPON THEIR 



HOSTS. 



The malevolent effects of various kinds of protozoan parasites on 

 their hosts may be either chemical or physical in nature, and due to 

 products of their own metabolism, or to mechanical destruction of 

 cells and tissues. The majority of the former type give rise to anti- 

 bodies which may persist for varying periods, thus setting up an active 

 or a passive immunity. 



Beyond the fact that they differ in different cases, little is known 

 about the chemical effects produced by protozoan parasites. Nor is 

 the definite action known in many instances. In the case of malaria 

 the pyrexial attacks are supposed, by the majority of authorities, to 

 be due to the liberation of a toxin contained in the pigment melanin 

 which is elaborated by the parasites. The sudden precipitation of this 

 pigment in the blood upon dissociation of the merozoites causes intoxi- 

 cation and convulsions. Celli, Gualdo, Montesano, and others have 

 produced similar convulsions by inoculation with the serum of malarial 

 blood without the organisms, while, as Thayer points out, the coinci- 

 dence of the convulsions with the schizogony of the parasites and the 

 liberation of these pigmented substances, when taken together with 

 the degenerative changes often found in the brain, nerves, liver, and 

 kidney, all point to the conclusion that some toxic substance or sub- 

 stances are present. 



A widespread effect of protozoa is the lysis set up by their presence 

 in cells and tissues. This was clearly worked out by Councilman and 

 Lafleur ('91) in the case of amebic dysentery, where the parasites 

 penetrate the submucosa, where they cause the cells to jellify and 

 degenerate. Similar destructive changes are brought about by the 

 organisms of trachoma, of rabies, and of smallpox. Neurorydes- 



