CHAPTER XVIII 



PROTOZOA AND DISEASE 



Protozoa which when Uving in the bodies of other animals are capable 

 of producing disease in those animals are termed pathogenic. Many such 

 Protozoa are known. 



113. Pathogenic Protozoa. — Among the Mastigophora are the try- 

 panosomes (Fig. 27 C). One of these, found in certain parts of tropical 

 Africa, produces a disease known as trypanosomiasis, or, because it is 

 characterized by a loss of consciousness, sleeping sickness. These 

 trypanosomes are transmitted from one person to another by the so-called 

 tsetse flies. The sleeping sickness of Africa should not be confused with a 

 disease in this country which exhibits the same symptom and which 

 sometimes goes by the same name; in the latter case the loss of con- 

 sciousness is not caused by an animal parasite but is due to congestion in 

 the blood vessels of the brain. 



Among the Infusoria are forms belonging to the genus Balantidium 

 (Fig. 30 E), which cause a type of dysentery known as balantidial dysen- 

 tery. This is most common in the tropics. 



Among the Sarcodina are ameba-like parasites, usually acquired 

 through drinking contaminated water, which are the cause of a serious 

 and often fatal form of dysentery known as amebic dysentery. Here 

 also belong the parasitic organisms found in the mouth which produce 

 the condition known as pyorrhea. 



Sporozoa, however, includes a larger number of the protozoan parasites 

 of man than do all the other classes together. Among these one of the 

 most common is the malarial-fever parasite. The life history of this 

 organism will be given in detail to illustrate the life cycle of a pathogenic 

 protozoan, though it is more complex than that of many other types 

 (Fig. 33). The spirochaetes, which cause syphilis and other diseases, are 

 by some authorities considered as belonging to the Protozoa, while others 

 consider them intermediate between the Protozoa and'the Bacteria and 

 more closely related to the latter. 



114. Malarial Parasite. — The malarial parasite may exist in the blood 

 of man, where it undergoes a series of asexual generations which may 

 continue for many years and even through the lifetime of the person. 

 The individual parasite lives in a red blood corpuscle, into which it enters 

 while in the spore stage. Then it changes to a form resembling a minute 

 ameba. It feeds upon the contents of the corpuscle and when full grown 



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