September 22, 1923] 



NA TURE 



445 



been worked out and the mode of its transmission from 

 insect to man ascertained. The late Sir Patrick 

 Manson, working in Amoy, had shown (1878) that the 

 larvae of Filaria bancrofti undergo growth and meta- 

 morphosis in mosquitoes^ but the mode of transference 

 of the metamorphosed larvae was not determined until 

 -1900. Nearly two years after the last meeting in 

 Liverpool the part played by the mosquito as host and 

 transmitter of the parasite of malaria was made known 

 by Ross. In addition to these two cases, at least eight 

 important examples can now be cited of arthropods 

 proved to act as carriers of pathogenic organisms to 

 man — e.g. Stegomyia— yellow fever; Phlebotomus — 

 sandfly fever ; tsetse-flies — sleeping sickness ; Cono- 

 rhinus — South American trypanosomiasis (Chagas' 

 disease) ; Chrysops — Filaria (Loa) loa ; the flea Xeno- 

 ■psylla cheopis — plague ; the body-louse — trench fever, 

 relapsing fever, and typhus ; and the tick Omithodorus 

 — African relapsing fever. 



In selecting examples for brief consideration I propose 

 to deal very shortly with malaria, although it is the 

 most important of the insect-carried diseases, because 

 the essential relations between the Anopheles mosquito 

 and the parasite are well known. There still remain 

 lacunae in our knowledge of the malarial organisms. 

 Ross and Thomson (1910) showed that asexual forms 

 of the parasite tend to persist in small numbers between 

 relapses, and suggested that infection is maintained 

 by these asexual stages. Such explanation elucidates 

 those cases in which relapses occur after short intervals, 

 but the recurrence of the attacks of fever after long 

 intervals can only be explained by assuming that the 

 parasites lie dormant in the body — and we know neither 

 in what part of the body nor in what stage or condition 

 they persist. Nevertheless, the cardinal points about 

 the organism are established, and preventive measures 

 and methods of attack based on a knowledge of the 

 habits and bionomics of Anopheles have been fruitful 

 in beneficial results in many parts of the world. 



If we desire an illustration of the vast difference to 

 human well-being between knowing and not knowing 

 how a disease-germ is transmitted to man, we may 

 turn to the case of yellow fever. When this pestilence 

 came from the unknown, and no one knew how to 

 check it, its appearance in a community gave rise to 

 extreme despair, and in many cases was the signal for 

 wholesale migration of those inhabitants who could 

 leave the place. But with the discovery that Stego- 

 myia was the transmitting agent all this was changed. 

 The municipality or district took steps to organise its 

 preventive defences against a now tangible enemy, 

 and the successful issue of these efforts, with the 

 consequent great saving of life and reduction of human 

 sufTering in the Southern United States, in Panama, 

 in Havana, and in other places, is common knowledge. 

 It is a striking fact that during 1922 Central America, 

 the West Indies, and all hut one country of South 

 America were free from yellow fever, which had ravaged 

 these regions for nearly two centuries. The campaign 

 against Stegomyia is resulting, as a recent Rockefeller 

 report points out, in yellow fever being restricted to 

 rapidly diminishing, isolated areas, and this disease 

 seems to he one which by persistent effort can be 

 brought completely under control. 



In 1895 Bruce went to Zululand to investigate the 



NO. 2812, VOL. I 12] 



tsetse-fly disease which had made large tracts of Africa 

 uninhabitable for stock, and near the end of the same 

 year he issued his preliminary report in which he 

 showed that the disease was not caused by some poison 

 elaborated by the fly — as had been formerly believed — 

 but was due to a minute flagellate organism, a trypano- 

 some, conveyed from affected to healthy animals by a 

 tsetse-fly (Glossina morsitans). In 1901 Forde noticed 

 an active organism in the blood of an Englishman in 

 Gambia suffering from irregularly intermittent fever, 

 and Button (1902) recognised it as a tr}'panosome, 

 which he named Trypanosoma gambiense. In 1902 

 Castellani found trypanosomes in the blood and cerebro- 

 spinal fluid of natives with sleeping sickness in Uganda, 

 and suggested that the trypanosome was the causal 

 organism of the disease. The Sleeping Sickness 

 Commission (Bruce and his colleagues) confirmed this 

 view, and showed that a tsetse-fly, Glossina palpalis, 

 was the transmitter. Since then much has been learnt 

 regarding the multiplication of the trypanosome in 

 the fly and its transference to man. For some years 

 this was believed to take place by the direct method, 

 but in 1908 Kleine demonstrated " cycHcal " trans- 

 mission, and this was shown later to be the principal 

 means of transference of T. gambiense. In 1910 

 Stephens and Fantham described from an Englishman, 

 who had become infected in Rhodesia, a trypanosome 

 which, from its morphological characters and greater 

 virulence, they regarded as a new species, T. rhodesiense, 

 and its " cyclical " transmission by Glossina morsitans 

 was proved by Kinghorn and Yorke. Recent reports 

 by Duke and Swynnerton (1923) of investigations in 

 Tanganyika Territory suggest that direct rather than 

 cyclical transmission by a new species of Glossina is 

 there mainlj^ responsible for the spread of a trypano- 

 some of the T. rhodesiense type. 



The impossibility of distinguishing by their morpho- 

 logy what are considered to be different species of 

 trv'panosomes, and the difficulty of attacking the fly, 

 are handicaps to progress in the campaign against 

 .sleeping sickness, which presents some of the most 

 subtle problems in present-day entomology and proto- 

 zoology. Here also we come upon perplexing con- 

 ditions due apparently to the different virulence of 

 separate strains of the same species of trypanosome 

 and the varying tolerance of individual hosts — on 

 which subjects much further work is required. 



The relation of fleas to plague provides one of the 

 best and most recent illustrations of the necessity for 

 careful work on the systematics and on the structure 

 and bionomics of insects concerned in carrj'ing patho- 

 genic organisms. Plague was introduced into Bombay 

 in the autumn of 1896, and during the ntxt two years 

 extended over the greater part of Bombay Presidency 

 and was carried to distant provinces. The ^Indian 

 Government requested that a commission should be 

 sent out to investigate the conditions. The commis- 

 sion, which visited India in 1898-99, came to the 

 conclusion (1901) that rats spread plague and that 

 infection of man took place through the skin, but — 

 and this is amazing to us at the present day — " that 

 suctorial insects do not come under consideration in 

 connection with fhe spread of plague." Further 

 observations, however, soon showed this conclusion to 

 be erroneous. Liston found in Bombay in 1903 that 



