376 GENERAL PRINCIPLES 



cattle, horses, and in fact, all the domestic mammals, as well 

 as wild mammals, are affected by a disease known as Tsetse 

 fever. It is fatal to such a degree that " large areas are closed 

 to colonization" where the disease is endemic. Trypanosoma 

 Brucei is the cause of the fever, but Tsetse is the name of a fly. 

 The natives have long known that the fever only occurs in 

 districts in which the Tsetse fly is found, and there is now no 

 doubt that this fly, in stinging affected cattle becomes itself 

 infected and then carries the germs to uninfected cattle. There 

 are several species of Tsetse fly (Glossinia), and of these, prob- 

 ably more than one is responsible for the spread of Tsetse fever. 

 The sleeping sickness is also carried by Tsetse flies. 



767. The domestic animals of South America, southern 

 Europe and northern Africa, and the countries bordering on 

 the Indian Ocean, are also affected by different types of Trypano- 

 some diseases. In these cases other flies and mosquitos are 

 the principal agencies of infection, but lice and fleas may per- 

 form the same office. The Trypanosomes of fishes are carried 

 by leeches. 



768. Of more direct interest to us is the parasite of malarial 

 fever. There are at least three varieties of this, producing the 

 " tropical, " the " tertian, " and the "quartan" fevers, respec- 

 tively. At a certain stage there are found numerous minute 

 bodies floating in the blood plasma of the host. These are 

 the "spore" stage of a Sporozoan, Plasmodium. They are 

 vastly smaller than a red blood corpuscle and are capable of 

 amoeboid motion. They attach themselves to a red corpuscle 

 and work their way into it. Here they grow at the expense of 

 the blood corpuscle, and at the end of 48 hours, in the case of 

 the tertian parasite, they divide into a number of "spores." 

 Hereupon the corpuscle goes to pieces and the spores are again 

 floating in the plasma. These spores repeat the cycle just de- 

 scribed and thus a new generation of "spores" is produced on 

 each alternate day. This process may continue indefinitely, 



