80 TEXTBOOK OF ZOOLOGY 



of necessary relations for production of the disease is broken. Mos- 

 quitoes are destroyed by draining swamps which serve as breeding 

 places, by placing mosquito fish (top minnows) in the pools to eat 

 the larvae, or by covering the water with a film of oil which keeps 

 out air and smothers the larvae as well as discourages females from 

 laying eggs in such water. Another means by which the chain may 

 be broken is to cure the carriers by killing all of the Plasmodia in 

 their blood by use of quinine, properly administered under a physi- 

 cian 's direction. Quinine is a specific drug for this disease. 



Texas Fever. — The small sporozoan, Babesia higemina, causes this 

 disease in cattle by destroying red blood corpuscles. The red cor- 

 puscle count of the host may be reduced from an average of 7,000,000 

 per cu. ml. to less than 1,000,000 per eu. ml. The disease is trans- 

 mitted from cow to cow by the cattle tick and its young. 



Nagana, similar to African sleeping sickness in man, dourine, a 

 sexual disease of the horse, and surra, are all diseases of domesticated 

 animals and are caused by trypanosomes. In some parts of the world 

 they have considerable economic importance. 



There are many other diseases that are rather similar to the above 

 which may be caused by Protozoa, although the organisms have not 

 been specifically isolated. Such diseases as Rocky Mountain spotted 

 fever, transmitted by the Rocky Mountain spotted fever tick and 

 fatal to man ; dengue or breakbone fever, a very unpleasant and un- 

 comfortable disease, transmitted by the yellow fever mosquito Aedes 

 (Stegomyia) ; as well as perhaps rabies, scarlet fever, typhus fever, 

 smallpox, and trachoma should be considered with this possibility. 



The cost of the above-mentioned and other Protozoa to man 

 throughout the world in money, loss of time, and suffering is almost 

 inestimable. A good protozoologist is one of our most valuable eco- 

 nomic assets. 



