EFFECTS OF PROTOZOAN PARASITES UPOX THEIR HOSTS 201 



the belief of Liihe, ]\Iesnil, and others, that the orio-inal forms of some 

 at least of these organisms were flagellated protozoa which have lost 

 their motile organs and assumed an intracorpuseular 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 would 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. 



Bevond the fact that thev 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 lii)eration 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 

 blof)d without the organisms, while, as Thayer points out, the coinci- 

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

 lil)eration 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 suii- 

 stances are present. 



A widcvspread efl'ect of ])r()tozoa is the lysis set up by their privscncc 

 in ceJIs and tissues. This was clearly worked out by ( 'onncilnian and 

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

 penetrate the subnuicosa, where tlicy cause the cells to jclliry and 

 degenerate. Similar destructive changes are biouglit al)out by the 

 organisms of (radionia, of rabies, and of smallpox. Xriiron/rfrK 



