December 6, 1900] 



NA TURE 



m 



in Indian birds of the parasite with which that species of insect 

 is liable to be infected, afforded pretty conclusive evidence that 

 the mosquito never derives the germs of malaria from the 

 larva and can acquire them only by biting some infected 

 animal. 



But although the mosquito theory was thus demonstrated, 

 there remained a link wanting in the chain of biological 

 sequence. The flagella which Manson regarded as spores were 

 destitute of malarial pigment, whereas the smallest corpuscles 

 seen by Ross in the stomach wall invariably possessed it. How 

 was this inconsistency to be explained ? What was the relation 

 of the unpigmented flagellum to the pigmented corpuscle ? The 

 answer had been already independently supplied. 



1 was present at a sitting of the Zoological Section of the British 

 Association at the Toronto Meeting in 1897, when Dr. 

 MacCallum, a young pathologist of the Johns Hopkins 

 University at Baltimore, read a paper describing the results of 

 an investigation in which he had long been engaged into 

 another form of malaria parasite, Halteridium, especially 

 common in crows. He told us, and he illustrated his statements 

 with preparations under the microscope, that he had distinguished 

 differences; which he regarded as fundamental, between the 

 spherical bodies seen in the shed blood of a bird affected with 

 that parasite. Though alike in size, some had a more granular 

 protoplasm than the others, which had a more hyaline 

 aspect ; and he had observed that the more hyaline 

 ones alone emitted flagella! These, after wriggling themselves 

 free from the parent cell, swam away till they approached some 

 ■corpuscle of the other, more granular, sort ; when the first that 

 reached it plunged into its substance and disappeared, while all 

 others were, by some amazing provision, absolutely refused 

 entrance. Here, ther, was witnessed, in an exceedingly low form 

 of animal life, a process of fertilisation identical with that which 

 occurs in an echinus or a fucus. The flagella were neither more 

 nor' less than spermatozoa, and the more granular cells were 

 ova. As the result of the fertilisation, the female cell was seen 

 by MacCallum to alter its shape in the shed blood and assume 

 an elongated form to which the term vermiculus was applied. 

 This new creature was possessed of wonderful powers of locomo- 

 tion, sometimes in its powerful career piercing through the sub- 

 stance of a red corpuscle.^ Nothing could well be imagined 

 better adapted for penetrating the layer of cells that line the 

 stomach of the mosquito ; and as the vermiculus retained its 

 pigment, Ross's pigmented bodies were naturally accounted for. 



These observations of MacCallum's might seem at first almost 

 too wonderful for credence ; but they have been fully confirmed 

 by others. 



It appears to be doubtful whether Halteridium ever produces 

 the "rosace" form, with its attendant sporulation ; but there 

 is no doubt that the process of fertilisation seen in that parasite 

 occurs in human m;alaria. MacCallum himself observed the act 

 of conjugation in the crescentic human form ; though he did not 

 see the subsequent development of the vermiculus. Koch made 

 'a further step by observing the vermiculus of Proteosoma 

 in blood from the mosquito's stomach.- And, finally, our 

 medallist Grassi, who in other ways has made most im- 

 portant contributions to this subject, has, in a recent work 

 {vide Grassi, " Studi di uno Zoologo sulla Malaria," Roma, 1900), 

 accompanied by very beautiful illustrations, not only described 

 the presence of vermiculi in abundance in the blood in the 

 stomach of mosquitoes during the first two days after biting 

 patients affected with malaria, but he has traced and figured the 

 pigmented bodies of the smallest size in the tissues of the 

 stomach in the immediately succeeding period, these bodies 

 retaining in some instances the elongated form of the vermiculus 

 after passing through the layer of epithelium that lines the cavity 

 of the organ. 



It has thus been abundantly established that the parasites of 

 malaria are present in the patient's blood in two distinct forms, 

 one sporulating asexually in the human system and causing the 

 attacks of fever, the other undergoing sexual development in 

 the body of the mosquito. That both forms are developed from 

 the spores introduced by the mosquito is certain. At what stage 

 they begin to develop their respective peculiarities is not yet 

 quite made out. The crescent form is peculiarly favourable for 



■ 1 FzV/^ On the Haematozoan Infection of Birds, by W. G. MacCallum, 

 "iliX)., Journal of Experimental Medicine, vol. iii. No. t, 1898. 



2 Vide Ueber die "Entwickelung der Malaria Parasiten, K. Koch, Zeit- 

 s'chrift'.fiir Hygiene und Infectious ' Krankheiten, Band xxxii., 189^. 

 Exceedingly beautiful microphotographs of different kinds of malaria 

 parasites in various stages of development accompany this article. 



NO. 1623. VOL. d'^ 



this inquiry, as it is the crescents only which discharge the. 

 sexual function ; and they are easily distinguished from the 

 sporulating kind, not only by their shape, but also by their much 

 larger size. 



The development of the crescents has been specially studied 

 by the Italian pathologists, Bastianelli and Bignami,^ who 

 have been able to distinguish the young crescents while 

 still of extremely small dimensions ; and they have made the 

 remarkable observation that, while the crescents are as a rule 

 only found in the blood of the finger when they have arrived at 

 maturity, the young forms are to be seen in internal organs, 

 such as the spleen, but above all in the bone marrow, wh-^re' 

 alone, according to these observers, the youngest recognisable 

 crescents are to be found. 



Seeing that, in whatever part of the body they are, the para- 

 sites always inhabit the blood, it seems difficult to conceive 

 what can be the cause of their preference, at different stages of 

 their growth, for the blood vessels of different regions and 

 organs. But of this we find parallels in several other cases of 

 blood parasites, the most striking, perhaps, being the astonish- 

 ing fact that, of two species of Filaria that infest the human 

 blood, one only shows itself in superficial parts at night, arid is 

 therefore termed Filaria iiocturna, while the other has the name 

 Filaria dmrna, because it only appears by day in the finger 

 blood and retreats into deep parts for the night. 



Ross was not an entomologist, and he was unable to learn in 

 India the names of the species of mosquito with which he had 

 been working, till Daniels, one of the explorers sent out by the 

 Malaria Committee, having gone to Calcutta to confirm or 

 otherwise Ross's work, informed him that his rare kinds, which 

 acted as hosts for the hutnan crescents, belonged to the genus 

 Anopheles, and that the common sort which performed the same 

 office for Proteosoma, belonged to another genus, Culex; It 

 has been a matter of great interest to ascertain whether all 

 mosquitoes which act as conveyers of malaria to man are of the 

 genus Anopheles, and the exceedingly common and numerous 

 species of Culex are guiltless in that respect. Very numerous 

 investigations into this question, and especially those conducted 

 by Grassi and his coadjutors, seem to have proved that such is 

 the case, and that, so far as human malaria is concerned, 

 Anopheles alone have to be considered. ' 



Our other two explorers, Messrs. Christophers and Stephens, 

 have made various important contributions to our knowledge of 

 malaria. Thus, having paid special attention to the very 

 dangerous disease which, on account of one of its symptoms, is 

 termed blackwater fever, they have come distinctly to the con- 

 clusion that it is not a special disorder but a form of tropical 

 malaria. If this is the case, it is of immense practical im- 

 portance ; for it will follow that any means efficacious for 

 ordinary malaria will prove equally so for the deadly black- 

 water fever. 



Another most important fact which they have ascertained, and 

 which was independently observed by Koch, is that in a native 

 population in a malarious region, while the adults may be per- 

 fectly free from the disease, the young children contain the 

 parasites in their blood in an enormously large percentage. 

 Though the disease appears to be much less dangerous to the 

 native children than to new arrivals, implying that they have a 

 degree of congenital immunity, the parasites in the young 

 natives are perfectly efficacious for causing dangerous fever in 

 white people when conveyed to them by mosquitoes. Hence 

 the important practical inference that white people settling in a 

 malarious tropical region should not, as they now commonly do, 

 plant their houses near native settlements, but place them at 

 some considerable distance from them, about a quarter of a mile 

 being apparently sufficient. And Christophers and Stephens 

 in their last communication have gone so far as to express the 

 opinion that the following of this simple rule would go very 

 far indeed towards rendering the malarious tropics healthy to 

 Europeans. 



In a communication to this Society, it is the scientific side 

 rather than the practical that is naturally chiefly dwelt on. Yet I 

 should have been glad, had time permitted, to have referred to 

 the various measures of prevention and treatment of malaria 

 which the light of recent kno\yledge has already suggested, and 

 which have already borne important fruit. I must now content 

 myself with saying that, very various as these measures are, they 

 are all, without exception, based on the mosquito theory. 



1 Vide "Sulla Struttura dei Parassiti Malarici," per G. Bastiani ed A.. 

 Bignami. Societa per gli Studi della Malaria, 1899. 



