228 PRACTICAL ZOOLOGY 



the malarial parasite, are said to be pathogenic. Pathogenic 

 Protozoa are all parasitic, living in the alimentary canal, blood, 

 or other parts of the body. Many of them are known to attack 

 man and other animals, but there is still a great deal to be learned 

 about them. Two of the species that cause diseases in domestic 

 animals are Piro plasma bigeminum, which is responsible for 

 Texas fever in cattle (see Chap. XIII, p. 121), and Spiroch&ta 

 gallinarum, which attacks poultry. 



Human diseases that are definitely known to be caused by 

 Protozoan parasites or are connected in some way with these 

 minute germs are malarial fever, yellow fever, syphilis, yaws, 

 recurrent fever, African tick fever, sleeping sickness, amebic 

 dysentery, kala azar, hydrophobia, smallpox, and intestinal 

 catarrh. 



AMEBIC DYSENTERY. Minute amebalike organisms, named 

 Entameba histolytica (Fig. 136, D), are the cause of amebic dysen- 

 tery, and are always found in the alimentary canal of patients 

 suffering from this disease. 



HYDROPHOBIA AND SMALLPOX. Other ameboid organisms 

 accompany hydrophobia and may destroy the nerve cells of the 

 brain. In smallpox similar ameboid organisms attack and de- 

 stroy the epithelial cells of the skin. Whether or not these 

 structures are the direct cause of the disease mentioned or are 

 merely accessories is not known, but they are to be looked upon 

 as dangerous until they are proved to be harmless. 



SLEEPING SICKNESS. In certain parts of tropical Africa 

 flagellated Protozoa of the genus Trypanosoma (Fig. 136, E) 

 cause the disease called sleeping sickness. Trypanosomes are 

 also parasitic in rats and other animals. The species affecting 

 man is carried from one person to another by a certain species 

 of tsetse fly (see p. 99, Fig. 58, A). The parasite, after gain- 

 ing access to the blood of a human being, multiplies with remark- 

 able rapidity. The nervous system of the patient is affected 

 either directly or by a poison secreted by the parasites. The 

 disease may last several months or even years. Irregular fever 



