CHAPTER XVIII 



PROTOZOA AND DISEASE 



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

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

 protozoa are known. 



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

 panosomes (Fig. 30 C). Certain of these, found in parts of tropical 

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

 characterized by a loss of consciousness, sleeping sickness. These try- 

 panosomes 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 similar symptoms and 

 which sometimes goes by the same name; in the latter case, the loss of 

 consciousness 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. 33 E), which cause a type of dysentery known as halantidial dysen- 

 tery. This is most common in the tropics. 



Among the Sarcodina, one ameba, Endamoeha histolytica, causes a 

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

 Very minute cysts pass from the body and may contaminate food, drink- 

 ing water, or towels. Infection is brought about by ingesting these cysts. 

 This parasite occurs in all parts of the United States but is more com- 

 mon in the tropics or where sanitation is poor. Several other kinds of 

 amebae live in the intestine of man and one in the mouth, but these 

 are all considered to be nonpathogenic. 



There are many species of parasitic Sporozoa. Among these is 

 the malarial-fever parasite, which is pathogenic to man. The life 

 history of this organism will be given in detail to illustrate the hfe cycle 

 of a pathogenic protozoan, though it is more complex than that of many 

 other types (Fig. 36). 



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. 



115. 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. 



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