PROTOZOA AND DISEASE 207 



save as restricted localities have become subject to sanitary 

 measures. In 1907 it was estimated that there were some 12,000 

 deaths per year in the United States, principally in the South; and 

 it has been estimated that there may be as many as three milhon 

 cases a year, involving a financial loss of not less than S100,000,000. 

 This condition exists in a country in which there is full knowledge 

 of the causation of the disease and of the preventive measures that 

 have made such regions as the Panama Canal Zone safe habitations. 

 The word " malaria," which means bad air, was originally 

 applied to a group of fevers known to be associated with the air of 

 swampy regions. The idea that such air acts as the causative 

 agent is still prevalent among the ignorant, but if there are no 

 mosquitos to act as intermediate hosts for the parasite there is no 

 malaria. The disease genn in this instance is one of the Sporozoa, 

 a representative of which is the Plasmodium malarioe, which 

 causes the quartan type of malaria. In man the parasite lives 

 in the blood, invading the red corpuscles (Fig. 108), where it 

 fonns merozoites that are Uberated with the destruction of the 

 corpuscles, and in turn invade new corpuscles in which the 

 process is repeated. In this manner a very large number of the 

 red blood cells may be destroyed and the numbers of the parasites 

 greatly increased. Waste products, in the form of melanin gran- 

 ules set free in the blood stream with the Uberation of the mer- 

 ozoites from the disintegrating corpuscles appear to be the specific 

 substances that cause the chills and fever, since the hberation of 

 merozoites and the ague both occur at intervals of about seventy- 

 two hours. After a considerable period of such multiplication, the 

 parasite begins the formation of male and female gametocytes 

 which must be drawn from the blood of man by the bite of a 

 mosquito if they are to undergo the process of conjugation which 

 is necessary for their further development. In the stomach of 

 the new host the final stages of the gametes and the conjugation 

 occur. The resulting cell passes through the epitheUum of the 

 stomach wall and takes up a position as shown in the figure. 

 Then division of the nucleus occurs repeatedl}^ and as the mass 

 grows each nucleus is surrounded by cytoplasm and eventually 

 becomes a spindle-shaped cell or sporozoite. The cyst bursts and 

 the cells thus hberated migrate through the body spaces to the 

 sahvary glands, from which they are ejected with saHva when the 

 mosquito bites a human being. 



