THE ECONOMIC IMPORTANCE OF ANIMALS 221 



yellow fever. At the time of the American occupation after the 

 war with Spain, the experiments referred to above were under- 

 taken. The city was cleaned up, proper sanitation introduced, 

 screens placed in most buildings, and the breeding places of the 

 mosquitoes were so nearly destroyed that the city was practically 

 free from mosquitoes. The result, so far as yellow fever was con- 

 cerned, was startling, as you can see by reference to the chart. 

 Notice also the rise in the death rate when the young Cuban 

 Republic took control. How do 

 you account for that ? We all know 

 what American scientific medicine 

 and sanitation is doing in Panama 

 and in the Philippines. 



Other Protozoan Diseases. — 

 Many other diseases of man are 

 probably caused by parasitic pro- 

 tozoans. Dysentery of one kind 

 appears to be caused by the pres- 

 ence of an amoeba-like animal in the 

 digestive tract which comes usually 



through an impure water supply. s^^^T^;. *(lftrHowid.r"°" 

 Smallpox, rabies, and possibly other 



diseases are caused by protozoans. Smallpox, which was once the 

 most dreaded disease known to man, because of its spread in 

 epidemics, has been conquered by vaccination, of which we shall 

 learn more later. The death rate from rabies or hydrophobia has 

 in a like manner been greatly reduced by a treatment founded on 

 the same principles as vaccination and invented by Louis Pasteur. 

 Another group of protozoan parasites are called trypanosomes. 

 These are parasitic in insects, fish, reptiles, birds, and mammals 

 in various parts of the world. They cause various diseases of 

 cattle and other domestic animals, being carried to the animal in 

 most cases by flies. One of this family is believed to live in the 

 blood of native African zebras and antelopes ; seemingly it does 

 them no harm. But if one of these parasites is transferred by the 

 dreaded tsetse fly to one of the domesticated horses or cattle of 

 the colonist of that region, death of the animal results. 



