652 MALARIA. 



The hypothesis that inuliiriul infection results from the bites of mos- 

 quitoes was advanced and ably supported by Dr. A. F. A. King, of 

 Washington, D. C, in a paper read before the Philosophical Society on 

 February 10, 1883, and published in the Popular Science Monthly in 

 September of the same year. In 1894 Manson supported the same 

 hypothesis in a paper published in the British Medical Journal (Decem- 

 ber 8), and the foUowing year (18!>5) Ross made the important dis- 

 covery that when blood containing tiie crescentic bodies was ingested by 

 the mosquito these crescents rapidly underwent changes similar to 

 those heretofore descri])ed, resulting in the formation of motile fila- 

 ments, which ])ecome detached from the parent body and continue to 

 exhil)it active movements. In 1897 Ross ascertained further that 

 when blood containing crescents was fed to a particular species of mos- 

 quito, living pigmented parasites could be found in the stomach walls of 

 the insect. Continuing his researches with a parasite of the same class 

 which is found in })ii'ds, and in which the mosquito also serves as an 

 intermediate host. Ross found that this parasite enters the st(miach wall 

 of the insect, and. as a result of its devel()])inent in that locality, forms 

 reproductive bodies (sporozoites), which sul>se<iuently find their way to 

 the vcneno-salivary glands of the insect which is now capable of infect- 

 ing other l)irds of the same species as that from which the blood was 

 o])tained in the first instance. Ross further showed that the mos(|uito 

 which served as an intermediate host for this parasite could not trans- 

 mit the malarial parasite of man or another similar parasite of birds 

 (halteridium). These discoveries of Ross have l)een confirmed by 

 Grassi, Koch, and others, and it has been shown that the mosquitoes 

 which serve as intermediate hosts for the malarial parasites of man 

 belong to the genus Anopheles^ and especially to the species known as 

 A n ojjh tie^^ da v i (/<-/'. 



The question whether mosquitoes infected with the malarial parasite 

 invaria])ly become infected as a result of the ingestion of human blood 

 containing this parasite has not been settled in a definite manner, but 

 certain facts indicate that this is not the case. Thus there are localities 

 noted for being extremely dangerous on account of the malarial fevers 

 contracted by those who visit them, which on this very account are 

 rarely visited by man. Yet there must be a great abundance of infected 

 mosquitoes in these localities, and especiall}' in low, swampy regions in 

 the Tropics. If man and the mosquitoes are alone concerned in the 

 propagation of this parasite, how shall we account for the abundance of 

 infected mosquitoes in uninhabited marshes { It appears probable that 

 some other vertebrate animal serves in place of man to maintain the life 

 cycle of the parasite, or that it may be propagated through successive 

 generations of mosquitoes. 



It is well known that persons engaged in digging canals, railroad cuts, 

 etc., in malarious regions are especially liable to be attacked with one or 



