362 BIOLOGY OF THE PROTOZOA 



each infected individual became a source of contamination for 

 other insects of the same type. The cycle thus established by 

 adaptation to the different kinds of host would continue indefinitely. 

 However this may be with trypanosomes, and it seems to be the 

 most probable hypothesis, there is little doubt about it in the case 

 of malaria parasites. Here the original host was the mosquito in 

 which fertilization and development take place in the gut and gut 

 wall while the sporozoites are liberated in the body cavity. In all 

 of these cases the protective cysts which all strictly gut parasites 

 form and which safeguard the germs against an unfavorable external 

 environment are quite unnecessary. The second host replaces the 

 cyst. 



Effects of Protozoan Parasites on the Host. — Pernicious effects of 

 parasites depend largely upon the site of parasitism, cytozoic forms, 

 for example, being far more destructive than celozoic, coccidia 

 more often fatal than gregarines or Cnidosporidia or intestinal 

 flagellates. In general the more recent the association of host and 

 parasite the more serious are the effects upon the host, but with 

 physiological adaptive responses on the part of both host and 

 parasite, a balance is ultimately established which leads to com- 

 mensalism or even to symbiosis (as in the association of termites 

 and hypermastigida). South African cattle are little if at all 

 affected by Trypanosoma brucei, but European cattle succumb. 

 Domestic cattle and the wild animals of Africa thus become carriers 

 of the disease. 



Functional derangement of the host may be brought about in 

 different ways some of which may be due to occlusion or massing 

 of parasites in bloodvessels, ducts or lymphatics, thus shutting off 

 the blood supply and food of vital organs. Thrombus formation 

 in capillaries of the brain or of other vital organs, due to massing 

 of parasites, makes tropical or pernicious malaria the most dreaded 

 of malarial diseases. The characteristic lethargy and accompanying 

 symptoms of African sleeping sickness are due to lack of nourish- 

 ment and atrophy of nerve cells in the base of the brain, caused by 

 the occlusion of smaller bloodvessels by accumulations of parasites 

 and lymphocytes in the perivascular spaces. Or impairment of 

 function may be due to the destruction of large numbers of secreting 

 cells— the coccidian Cyclospora karyolytica, for example, destroys so 

 many secreting cells of the intestine that the disease in ground 

 moles is fatal in 100 per cent of cases (Schaudinn). Secondary 

 organic complications may be due to the overactivity of vital 

 organs— thus in malaria so much hemoglobin is liberated that the 

 liver cannot take care of it all and the excess is passed on to the 

 kidneys, resulting in hemoglobinuria and functional impairment of 

 the excretory organs. 



Secretions by parasites in many cases cause cytolysis of tissue 



