O ECONOMIC ZOOLOGY 



species. This is the organism that causes one of the oldest and most 

 widely distributed diseases of man, malaria, which was the first disease 

 that was proven to be directly caused by a protozoan parasite. Until 

 near the beginning of the present century both the cause and the mode 

 of transmission of malaria were unknown, though Lancisi, in 1718, 

 ventured the statement that mosquitoes or gnats might introduce 

 certain poisonous substances, found in swampy regions, into the human 

 blood and thus cause malaria. It has been recognized for ages that 

 malaria was more prevalent in damp and swampy regions and the 

 disease owes its name Mai-aria to the commonly held idea that it 

 was due to bad air or miasmas found hanging over swamps and such 

 places; it was also generally thought that the disease could be contracted 

 by drinking bad water. 



In i88iDr. La veran discovered a curious new parasite in the blood 

 of malarial patients, which he claimed was the cause of the disease. 

 This discovery did not attract much attention until some years later 

 when Laveran and Manson independently suggested that the germ might 

 be transmitted to man by the bite of some blood-sucking insect. 

 After years of investigation Major Ronald Ross, of the English army, 

 finally worked out the life history and proved that the insect re- 

 sponsible for the transmission of human malaria is a mosquito belonging 

 to the genus Anopheles, while bird malaria is transmitted by the genus 

 Culex. 



The life-history of the sporozoan is essentially the same in the three 

 types of the fever, the main difference being the different lengths of the 

 developmental periods, the tertian fever producing the attack or 

 chill every 48 hours, the quartan fever every 72 hours and the pernicious 

 fever every day. 



The whole life-history in both hosts, man and anopheles, is briefly as 

 follows: the organisms are introduced into the blood when the 

 mosquito bites. These minute, elongated sporozoa move about in 

 the blood plasma and finally enter the red blood cells. According 

 to some authors they do not actually enter the cells, but it is difficult 

 to see how the following processes could take place unless they do. 

 In the blood cells they grow and give off poisons that are contained 

 within the cells. At the end of their developmental period, 48 hours, 

 for example, in the tertian fever, they break down into numerous spores, 

 and it is the escape from the corpuscles of these countless spores, together 



