CHAPTER IX 



TRANSMISSION OF DISEASES BY INSECTS 



It is commonly known that some kinds of insects are of vital impor-' 

 tance to man as agents in the transmission of certain diseases. In 

 recent years immense progress has been made in our knowledge of insect- 

 borne diseases. 



MALARIA 



So far as is known, malaria is transmissible only through the agency 

 of mosquitoes. 



The malaria "germ," discovered in 1880 by the French army surgeon 

 Laveran, may be found as a pale, amoeboid organism (Laverania, 

 Fig. 273) in the red blood corpuscles of persons afflicted with the disease. 

 This organism (schizont, 2) grows at the expense of the haemoglobin of 

 the corpuscle (3-5) and its growth is accompanied by an increasing 

 deposit of black granules (melanin), which are doubtless excretory in 

 their nature. At length, the amcebula divides into many spores (mero- 

 zoites, 6) which by the disintegration of the corpuscle are set free in the 

 plasma of the blood. Here many if not most of the spores, and the 

 pigment granules as well, are attacked and absorbed by leucocytes, or 

 white blood corpuscles, while some of the spores may invade healthy red 

 corpuscles and develop as before. The period of sporulation, as Golgi 

 found, is coincident with that of the " chill " experienced by the patient; 

 and quinine is most effective when administered just before the sporula- 

 tion period. The destruction of red blood corpuscles explains the pallid, 

 or ancemic, condition which is characteristic of malarial patients. In 

 three or four days the number of red corpuscles may be reduced from 

 5,000,000 per cubic millimeter the normal number to 3,000,000; and 

 in three or four weeks of intermittent fever, even to 1,000,000. 



Authorities recognize at least three species of malaria parasites 

 affecting man: (i) the tertian (Plasmodium vivax), with an asexual cycle 

 of forty-eight hours, causing the fever to recur every two days; (2) 

 the quartan (P. malaria), with a cycle of seventy-two hours, causing 

 fever every third day; and (3) the subtertian or malignant form (Laver- 

 ania falciparum) of which there are three varieties (perhaps species) , 

 with cycles of twenty-four or forty-eight hours, according to the variety. 



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