158 THE ANIMAL PARASITES OF MAN 



various observations made from time to time in dealing with malaria. One hypo- 

 thesis only seemed to have a better foundation. Manson (1894), who knew from his 

 own experience the part played by mosquitoes in the development of Filaria from 

 the blood of man, applied this also to the malarial parasites living in the blood, 

 whereby at least the way was indicated by which the Haemosporidia could leave man. 

 The parasites were said finally to get into water through mosquitoes which had 

 sucked the blood of malarial patients, and the germ spread thence to men who drank 

 the water. In some cases the parasites were supposed to reach man by the inhaling of 

 the dust of dried marshes. On the other hand, Bignami believed that the mosquitoes 

 were infected in the open air by malarial parasites which occurred there in an unknown 

 stage and the insects transmitted the germs to man when biting. R. Koch combined 

 both hypotheses, without, however, producing positive proof. R. Ross, then (1897-8) 

 an English military doctor in India, was the first to succeed in this. He had been 

 encouraged by Manson to study the fate of malarial Plasmodia which had entered 

 the intestine of mosquitoes with malaria-infected blood, especially in the case of the 

 Plasmodium (Proteosoma) living in the blood of birds. He showed that the Proteo- 

 soma penetrate the intestinal wall of the mosquitoes, grow and develop into large 

 cysts which produce innumerable rod-like germs, which burst into the body cavity 

 and penetrate the salivary glands. Ross allowed mosquitoes to suck the blood of 

 birds affected by malaria, and some nine days later, let the infected mosquitoes 

 which had been isolated suck healthy birds. After five to nine days Proteosoma 

 were found to occur in the blood of the birds used. The Proteosoma and Halteridium 

 of birds were also further investigated by MacCallum (1897-8), Koch and others, and 

 important results followed. 



In any case Ross (1898) had clearly established the importance of mosquitoes in 

 the spread of malaria among birds. It was now only a question of proving whether, 

 and how far, mosquitoes were concerned with human malaria. Ross himself worked 

 to this end. Here the experiments of Italian investigators (Bignami, Bastianelli, 

 Grassi) 1 were of importance. These investigators studied the fate of malarial 

 parasites in man, produced malaria in men experimentally by the bites of infected 

 mosquitoes, and established that only mosquitoes belonging to the genus Anopheles 

 were concerned, and not species of Cttlex. These latter are only able to transmit 

 Proteosoma to birds. It is true that Culex can ingest the human malarial parasites, 

 but the latter do not develop in them. Development only occurs in species of 

 Anopheles, In Anopheles (and similarly for Proteosoma in Culex] sexual repro- 

 duction takes place ; crescents, spheres and polymitus forms are necessary stages 

 of development in the mosquito. 



With these discoveries the campaign against malaria became more definite. It 

 was directed partly against the transmitters, whose biology and life-cycle were more 

 thoroughly investigated, instead of merely against the infection of the adult Anopheles. 

 The latter do not, as was believed for some time, transmit the malarial germs to their 

 offspring. They always infect themselves from human beings, whereby the relapses 

 appearing in early summer, and the latent infection, especially of children of natives, 

 play a principal part (Stephens and Christophers, Koch). Further, the crusade was 

 directed against the infection of man by the bites of Anopheles. Important results 

 have been obtained in these directions. Low and Sambon in 1900 lived in a 

 mosquito-screened hut in a malarial part of the Roman Campagna for three of the 

 most malarious months and did not contract the disease. In the same year Dr. P. T. 

 Manson was infected with malaria by infected mosquitoes sent from Italy. The role 

 of mosquitoes having been proved, it may be hoped that ultimately the eradication 

 of malaria, or at least a considerable restriction of it, will be achieved. 



1 Grassi, B. (1901), " Die Malaria," 250 pp., 8 plates. G- Fischer, Jena. 



