CHAPTER IX 



TRANSMISSION OF DISEASES BY INSECTS 



It is now known that several kinds of insects are of vital importance 

 to man as agents in the transmission of certain diseases. This recently 

 demonstrated role of insects now commands universal attention. 



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 (Plasmodium, 

 Fig. 270) 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. 



Three' types of malaria are recognized: (i) the tertian, in which the 

 paroxysm recurs every two days; (2) the quartan, in which it happens 

 every third day; and (3) the aestivo-autumnal type (Fig. 270). These 

 three kinds are by some investigators thought to be due to different 

 species of parasites; and when, as often happens, the malarial chill occurs 

 every jiay, this is attributed to two sets of tertian amcebulae, sporulating 

 on alternate days. 



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