Malarial Parasites 497 



and, after a most painstaking investigation, the details of which are 

 given in a paper which can be found in the International Medical 

 Annual,* 1895, made the second great discovery in the parasitology 

 of malarial fever. He found that, as Manson thought, the mosquito 

 is the definitive host of the parasite, but that the matter is much less 

 simple than was imagined, for the organisms taken up by the mos- 

 quito undergo a complicated life cycle requiring about a fortnight 

 for completion, after which, not the water into which the mosquito 

 might fall and into which its contained organisms might escape, but 

 the mosquito itself becomes the agent of infection. In other words, 

 the parasites taken up by the mosquito, after the completion of the 

 necessary developmental cycle, are returned by the mosquito to 

 new human beings, who thus become infected. Thus it was shown 

 that malaria is not a miasmatic disease at all, but that it is an in- 

 fectious disease whose parasites divide their life cycle between man 

 and the mosquito, each becoming infected by the other. The only 

 r61e of the swamp is to furnish the mosquitoes, and since these are 

 only more numerous where swamps are numerous, but may occur 

 without swamps, , the not infrequent occurrence of malarial fevers 

 .apart from swamps is also explained. Ross further discovered that 

 aU mosquitoes are not equally susceptible of infection, and, therefore, 

 not all able to spread the infection. Grassi, in Italy, quickly 

 followed with the demonstration that human paludism was 

 transmitted in the same manner, and only by mosquitoes of the 

 genus Anopheles. 



There remained, however, one more important fact to be eluci- 

 dated, and one more mysterious body to be accounted for, viz., the 

 "flagellated" body that had misled Manson. This was found by 

 MacCallum f to be but the spermatozoit of the male parasite. While 

 observing one of the malarial parasites of birds — Plasmodium dan- 

 liewskyi — he saw one of these "flagella" swimming away from its 

 parent parasite, and followed it carefully, moving the slide upon 

 the stage of the microscope. It, and others of its kind, approached 

 a large globular parasite, to which one effected an attachment and 

 into which it entered. MacCallum realized that he had observed- 

 the sexual fertilization of the organism. In 1900 two demonstra- 

 tions of momentous importance were made. First, Sambon and 

 Low went to Italy, to one of the most pestilential parts of the 

 Campagna Romana, and lived there during three months of the 

 most malarious time of the year in a mosquito-proof house, taking 

 ■every precaution to avoid mosquitoes, and escaped infection; 

 second, anopheles mosquitoes infected in Italy, by biting malarial 

 patients, were taken to England, where they were permitted to 

 bite Dr. P. J. Manson and Mr. George Warren, both of whom, after 

 a period of incubation suffered from malarial paroxysms and showed 



* E. B. Treat & Co., New York, 

 t "Journal of Exper. Med.," 1898, ni, 117. 

 32 



