CHAPTER XXIV 

 THE RELATIONS OF ANIMALS TO DISEASE 



Problems: What is the cause of malaria and how is it carried? 



What causes yellow fever and how is it transmitted f 



What other diseases are carried hy insects ? 



What part does the housefly take in carrying disease germs f 



What are some parasitic worms and how may we combat them? 



Laboratory Suggestions 



Demonstration. Malarial parasites in the blood corpuscles. 

 Exercise. Life history of the mosquito. 

 Demonstration. Trypanosomes — stained slide. 

 Exercise. Life history of the house fly. 



Home project. To determine the breeding places of flies and mosquitoes in 

 my neighborhood. 



Demonstration. Tapeworm and trichina (encysted). 

 Demonstration. Slide showing hookworm. 



Animals as Agents of Disease. — We have already learned the 

 relation of plants to disease ; it is the purpose of this chapter to 

 show how animals play a part in the cause and spread of disease. 

 It is obvious that the relation is twofold. An animal may be a 

 parasite in man, causing certain diseases, or it may, acting as a host, 

 carry a parasite for part of its life history. The malarial parasite 

 and the hookworm are examples of the first type ; the mosquito, 

 which carries the malarial parasite, and the flea, which transmits 

 bubonic plague bacilli, are examples of the second type. 



The Cause of Malaria. — The study of the life history and the 

 habits of the Protozoa has resulted in finding many parasitic 

 forms, and the consequent explanation of some diseases. An 

 amoeba-like parasite, of which at least three species exist, causes 

 different t3TDes of malaria. This disease, not many years ago, was 

 thought to be caused by bad air. (Hence the name, from Italian 

 mala, bad ; aria, air.) But the work of a number of scientists 



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