4 ECONOMIC ZOOLOGY 



wastes away and dies in a terribly emaciated condition, Fig. 3. While 

 it is the natives of Africa that chiefly suffer, the organism attacks 

 whites as well as blacks, and several investigators have sacrificed their 

 lives in studying the disease. 



One of the most serious and fatal diseases of domestic animal in the 

 Orient is Surra. This disease is probably caused by a Trypanosome 

 introduced into the blood by the bite of a fly. 



Another serious disease of cattle in this country is Texas fever. The 

 organism here, Babesia, is considered by some workers to be a flagel- 

 late, by others to belong to the next order, the Sporozoa. 



How the cattle "caught" the disease was a great mystery until 

 Smith and Kilborn discovered that the germ is carried from animal to 

 animal by a tick. The adult female tick is fertilized while hanging 



PIG. 3. Victim of sleeping sickness, shortly before death. (After Calkins, 



Protozoology.) 



to the skin of the ox; she gorges herself with the infected blood of her 

 host and falls to the ground, where she lays an enormous number of 

 eggs. Each larva, when it hatches, is supplied with a little of the in- 

 fected blood ;*it crawls upon a blade of grass where it will die unless 

 another ox brushes against the grass, when the young tick may suc- 

 ceed in attaching itself to the hair of this ox; here it develops into an 

 adult and infects its new host the first time it feeds upon its blood. 

 No wonder the spread of the disease was so mysterious, when one 

 animal could infect another without ever having been within a hundred 

 miles of it. 



Sporozoa. Of these very minute Protozoa the most important, 

 perhaps, is the genus Plasmodium, of which there are probably several 



