II. Blood Parasites 
The blood parasites presented here include malaria, hemoflagel- 
lates, and filaria. Not all of the species of each group actually 
inhabit the blood stream (for example, adult filaria, Leishmania spp.) 
but most are associated with the circulatory system at some stage 
of their development. Possibly the leishmania might be considered 
as tissue rather than blood parasites, but since they are classified 
as hemoflagellates they are presented here with the blood inhabiting 
forms. Another tissue parasite is Dracunculus medinensis which is 
usually put in the same general category as the filariae, and is also 
included with the blood parasites in these charts. 
Malaria 
The life history of malaria is similar to that of the intestinal 
sporozoa, the coccidia, and involves an asexual cycle, schizogony, 
in the human host and a sexual cycle, sporogony, in the vector, a 
species of Anopheles mosquito. The pattern presented here is that 
of Plasmodium vivax but, in general, it is the same for all four 
species. 
Immediately after the introduction of the infective sporozoites 
through the bite of the mosquito, a pre-erythrocytic (exo-erythrocytic) 
development occurs. In man, these exo-erythrocytic stages have been 
demonstrated in the parenchymal cells of the liver. On the basis of 
the knowledge of the prepatent period, this pre-erythrocytic phase 
probably requires a week or more depending on the species involved. 
In P . vivax , Plasmodium ovale , and Plasmodium malariae residual 
exo-erythrocytic stages probably continue in fixed cells during the 
erythrocytic phases, while in Plasmodium falciparum , on the other 
hand, residual exo-erythrocytic stages probably do not occur. The 
presence or absence of residual exo-erythrocytic stages affects both 
the clinical course and the therapy of the disease. 
The asexual stages of malaria include trophozoites and schizonts 
and, except in P. falciparum, all stages of growth may be found in 
peripheral blood. P. falciparum organisms complete their schizogony 
in the capillaries of the internal organs so that usually only the 
rings (young trophozoites) of the asexual forms are seen in circu- 
lating blood. 
In addition to the asexual stages, gametocytes (sexual forms) 
develop in man and in all four species, may be found in peripheral 
blood. Their exact origin is unknown. Some workers believe that 
they develop from certain of the merozoites produced in the erythro- 
cytic schizonts; others, that they come from merozoites formed in 
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