354 



NA TURE 



[January 20, 19 10 



(2) Malarial infection is not known to take place by any 

 other method. 



Experiments further showed, as I have mentioned 

 already, the very remarkable fact that avian malaria can 

 only be transmitted by culicine mosquitoe".. and human 

 malana only by anopheline. If human blood containing 

 the parasite be taken up by a Culex, the parasite canno't 

 develop, but is digested up, along with the blood. The 

 same thing happens to the parasite of avian malaria when 

 taken up by an Anopheles. 



Following on these experimental discoveries, the develop- 

 ment of the parasite was studied microscopically in all 

 countries by a great number of observers, amongst whom 

 we may mention especially Grassi in Italy and Schaudinn 

 in (lermany. By their combined labours the complete lifc- 

 history of the parasite has been worked out in the greatest 

 detail, revealing one of the most fascinating chapters in 

 natural history. 



(An account was then given of the development of the 

 malarial parasite, illustrated by a diagram.) 



My second example, sleeping sickness, is also a disease 

 that has been long known, though without attracting, 

 until recently, so much attention as malaria. It was first 

 observed in the West Indies in negro slaves imported from 

 the west coast of Africa, the region in which it appears 

 to be endemic. It was observed that the negroes suffering 

 from It were not infectious, and th.-it the disease did not 

 spread to others— a fact easily explained bv what is now 

 known about the transmission of sleeping sickness, namely 

 that It IS effected by flies of the genus Glossina, commonly 

 known as tsetse-flies, which are confined at the present 

 time to the African continent. 



Of recent- years this previously obscure disease has 

 forced_ Itself on the public attention bv its having spread 

 from Its native haunts on the west of Africa and invaded 

 regions previously free from its presence. In our pro- 

 tectorate of Uganda, in particular, it has caused terrible 

 mortality, completely extirpating the natives in some p.nrts 

 and numbering also many Europeans amongst its victims' 

 I do not propose here to enter into the distressing symptoms 

 of this deadly disease, but only to deal with what may be 

 termed its natural history. 



Before it is possible to understand clearly the nature of 

 sleeping sickness it is necessary to sav a few words about 

 .similar diseases in animals. It was' well known to all 

 African travellers from the time of Livingstone that 

 domestic animals, especially cattle, horses, and dogs, were 

 liable in Africa to a peculiar fata! disease known as 

 nagana. caused bv the bite of blood-sucking flies of the 

 .gemis Glossina, the tsetse-flies, of which there are several 

 species abundant in various ports of Africa. It was sup- 

 posed that the fiy produced and injected a virus which 

 caused the disease. 



The nature of nagana was first made clear bv .Sir 

 David Bruce, who found that the cause of the disease was 

 the pre.sence in the blood of a minute flagellate or trypano- 

 some, since named Try\,anosoma brucii. and that the 

 tsetse-fly did not generate the parasite, but was merelv 

 the unwitting agent in transmitting it from infected to 

 he.nlthy animals. 



When the enldemic of sleeping sickness broke out in 

 Uganda, the Royal Societi', at the request of the Govern- 

 ment, aopointed a commission to investigate it, and Sir 

 Havid Bruce was sent out as a member of the commission. 

 .Atrvpanosome was found bv Castellani in the cerebro- 

 spinal fluid of sleeping-sickness patients, and it was shown 

 liv Bruce and his assistants that this trvpanosome was the 

 cause of the dise.ase, and that it was tmnsmitted from 

 sick to healthy persons bv the bite of the local species of 

 tsetse-flv, Glossina palftalis. It was proved bv subsequent 

 researches that the trvpanosome causing sleeping sickness 

 was identical with one that had been discovered 'oreviouslv 

 in the blood of negroes in Gambia, and n.imed 

 /. ffamltonse bv Dutton. In short, it was proved that 

 sleeping sickness of man is a trvpanosome disease similar 

 tn nagana of animals, but produced bv a different species 

 of trvpanosome, transmitted bv a different species of tsetse- 

 flv. and running a somewhat different course. Whereas 

 Trvbanosoma hrucU remains in the blood of its victims 

 until their death, T. pamhirnse is found in the blood in 

 the early stages of the disease, but spreads, probably 

 NO. 20gQ, VOL. 82] 



through the lymphatic channels, into the cerebro-spinal 

 fluid, and then causes the peculiar nervous symptoms which 

 give the disease its name. The rapid spread of sleeping 

 sickness into regions where it was previously unknown 

 is an indirect consequence of the occupation of the African 

 continent by European Powers. Formerly the native 

 tribes were constantly at war with one another, and a 

 negro never travelled any great distance from his own 

 village. Now caravans move in every direction, and doubt- 

 less in this way the disease has been spread by porters and 

 other natives already infected with the trvpanosome 

 coming into regions where tsetse-flies abound, and there 

 infecting the flies, which in their turn have disseminated 

 the infection amongst the previously healthy population. 



.'\Ithough it was proved experimentally that the disease 

 is propagated by tsetse-flies, the exact method by which 

 this is effected has remained hitherto somewhat mysterious. 

 It was proved that the infection could be conveyed by 

 what may be termed the direct mechanical method ; that 

 is to say, if a fly has sucked recently the blood of an 

 infected person, its proboscis may contain living trypano- 

 somes, and if it inserts its proboscis, immediately or 

 a short time afterwards, into the skin of a healthy person, 

 it may convey the infection simply by means of its con- 

 taminated proboscis. Experiments showed that infection 

 in this direct manner only took place up to forty-eight 

 hours after the fly had fed on the infected subject, and 

 all attempts to obtain infection with flies at a longer 

 interval than forty-eight hours gave negative results. 

 Experimental evidence was therefore lacking for the exist- 

 ence of a developmental cycle of the parasite in the fly, 

 although it was argued by many writers that for various 

 reasons such a cycle must exist. Quite recently, however, 

 a positive result has been obtained by Prof. Kleine, director 

 of the German Sleeping Sickness Commission in German 

 East .Africa. Experimenting with nagana by feeding a 

 batch of flies first on an infected animal and then on a 

 long succession of healthy animals, he has made the most 

 interesting and important discovery that the flies are not 

 infectious at all until some three weeks after their first 

 feed, and that then they infect every animal upon which 

 they are fed. This result indicates that the incubation 

 period — that is to say, the time occupied by the parasite 

 in its cycle of development in the fiy — is far longer than 

 anyone had suspected, and that the negative results of 

 former investigators are to be explained by their experi- 

 ments not having been extended over a sufficiently long 

 period. It must be borne in mind that to those working 

 in tropical .'\frica it is often difficult, or even impossible, to 

 obtain a suflicient number of experimental animals for 

 such a protracted series of experiments. 



From Prof. Kleine's experimental results it is evident 

 that the trvpanosome of nagana, and doubtless of sleeping 

 sickness also, does undergo a cycle of development in the 

 tsetse-fly, and the way is now open for the microscopist 

 to rush in and to observe what becomes of the parasite in 

 this long period that el.apses between its being taken up 

 by the fly and being given out again. We may expect 

 that a fascinating and wonderful history will be made 

 known of the transformations and migrations, the amours 

 and the increase of the trvpanosome in the bowels of the 

 unconscious tsetse-fly : and we seem now to be in sight of 

 a solution to the baffling problem of the transmission of 

 diseases caused by trvpanosomes. 



The third disease I have chosen for mv discourse, namely, 

 vellow fever, is one sufficiently well known to everyone, 

 by repute at least. There is no need for me to describe 

 at length the dreaded " Yellow Jack," a malady often 

 fatal, and always excruciatingly painful. The connection 

 of this disease with mosquitoes has long been suspected, 

 and has recently been proved conclusively by both the 

 American and French commissions sent out to study the 

 disease. The mosquito in this case is neither a Culex 

 nor an Anopheles, but one belonging to a distinct genus, 

 namely, Sies^omyia fasciata, sometimes called the tiger- 

 mosquito. It has been proved conclusively that the mos- 

 quito does transmit yellow fever, and it has also been 

 nroved that the disease is not communicated by direct 

 infection or contagion through contaminated clothes or 

 dwellings ; and here let me direct attention to one great 

 obstacle to conducting experiments on yellow fever — !'■"*. 



