PROTOZOAN PARASITES 



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have been derived from parasites originally intestinal in the verte- 

 brate, which found their way not merely into the mucosa' of the 

 bowel, but later into the blood-stream, in which they would be 

 fairly well protected, and from which they would at first escape 

 by becoming encysted in the intestinal wall, and then, bursting into 

 the bowel, make their way to the exterior with the f^cal matter 

 and so infect a new host. When blood-sucking animals became 

 evolved, a new cycle would be open for them^ — -viz., from the verte- 

 brate through the blood-sucker back to the vertebrate— and thus 

 the old method by way of the alimentary canal would be lost. 



The origin of the protozoan blood-parasites of man may therefore 

 be twofold: (a) from man's intestine into the blood; (&) from an 

 invertebrate's alimentary canal into the blood of man. 



The result of one or other of these methods is well illustrated by 

 the malarial parasite, which passes through its sexual cycle in an 

 Anopheline mosquito, which is its definitive and, according to the 

 view adopted, its primary or secondary host. When the infected 

 Anopheline bites a man, the parasite enters into the blood-stream, 

 in the red cells of which it develops and undergoes its asexual 

 cycle. Man is therefore the intermediary, and either the primary 

 or secondary, host of the malarial parasite, as mentioned above. 



The blood-stream of man contains two different elements — 

 (i) liquor sanguinis; (2) cells- — {a) red, {p) white. Therefore the 

 parasite has two possibilities before it — -either to live in the liquor 

 or in a cell, or partially in one and partially in the other; and this 

 last is what generally happens — -i.e., the parasite lives so much of 

 its cycle in a cell and so much in the blood-stream. It would, how- 

 ever, appear that there is a great phylogenetic tendency for pro- 

 tozoan parasites to leave the liquor sanguinis and to reside in red 

 cells (the malarial parasite) or in white cells or in endothelial cells 

 (the Leishman-Donovan parasite). 



Infection of the Embryo. — -Protozoan blood parasites apparently 

 can be arranged in two categories as regards the infection of the 

 foetus, for some, like the malarial parasite (which is generally con- 

 sidered to be incapable), do not traverse the placenta, while others, 

 like the Spirochcetes and Treponemata, can do so. With regard to 

 the infection of invertebrate eggs the matter is quite different, for 

 many of these parasites infect the eggs, thus carrying the germs of 

 disease into a new generation of blood-suckers. 



It would appear as though the intracellular stage enabled the 

 parasite to grow (e.g., consider Schaudinn's history of the develop- 

 ment of HcBMOproteus noctucE in the little owl) and to multiply 

 [e.g., note the development and multiplication of the malarial 

 parasite) . 



In the evolution of such hsematozoan types some authorities 

 (Woodcock) hold that the flagellate forms living in the blood- 

 stream are to be considered the most primitive, and that the more 

 truly cellular the parasite becomes the more it has evolved. Hence 

 the HcBMoflagellata, or parasites freely moving in the liquor san- 



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