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C.O.885
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is obvious that animals which can become naturally infected by Glossina palpalis, unless the fact is very exceptional, are an added danger to the community.
Meantime we must provisionally suppose that only the human being and the fly need be seriously considered as agents in the spread of infection, that the vast majority of flies, where human beings are scanty or absent, are uninfected and harm- less in themselves, and that, if fly could be eliminated from places of human concourse such as mentioned above, those existing elsewhere would run little chance of becoming infected.
It is probable that only a small percentage of flies in an infected area (taking it as a whole) carry infection, though there may be certain very limited localities where the percentage may be, at times, comparatively much higher. The average percentage of human infection also is very hard to determine with any exactness, because, of the difficulty, except by such comparatively tedious, and, to many natives, terrifying measures as gland-puncture, of obtaining as a standard an even approximately true percentage in any given district or Ay-area. The palpation of glands, in such places as Uganda and Usoga, where such diseases as itch or craw-craw and syphilis are very common, is apt to be very misleading (Note 5).* But, unless the average per- centage of infected files were low, it is difficult to understand how, in certain highly-infected localities where for months and months every native has been daily exposed, along with sick people, to swarms of flies, any single soul has escaped alive. Many Europeans, too, both before and since Glossina palpalis became known as the carrier of the infection of sleeping sickness, have been bitten by it and yet have so far escaped the disease, and in this there is some analogy with the relation of the bites of Anopheles, to infection with malaria. In the case of spirillum fever, on the other hand, where infection is hereditary in the invertebrate, and a large proportion of the ticks on certain roads and in certain districts is infected, practically all Europeans who expose themselves to their bites contract the fever. It seems fairly certain, also, that those trypanosomes which have been found, in certain tested localities, to exist in from 0.5 to 8 per cent. of the flies examined, are not the Trypanosoma gambiense and do not cause disease when inoculated into susceptible animals, so that even these small percentages may be too large in the case of T. gambiense. The flies are forest or jungle insects, not in any sense "domestic," and the great bulk of them will always exist in places where human beings are scanty or absent and infection is rare. Thus the risks are practically confined to places of human concourse, to which, therefore, the preventive measures of clearing can be chiefly limited. It is important, however, that these clearings should be carried out not only in infected but also in non-infected fly-areas.
In spite of all the time and labour that has been expended in anatomical and bacteriological study in connection with Glossina palpalis, T. gambiense and sleeping sickness, and in spite of all the expert skill which has been engaged in this task, we do not yet know, so far as I am aware (for I have not at present seen the account of the last stage of the work of the Royal Society's Commission in Uganda nor any very recent publication dealing with the subject, †) whether the trypanosome undergoes a cycle of development in the fly, nor, if so, at what stage or stages of its evolution the bite of the latter is infectious, nor, in any case, for how long a fly carrying trypanosome may remain infectious, nor do we know even the average duration of the fly's life in natural conditions, supposing that it is capable of life-long infection.
This being so, we cannot, in applying preventive measures, however sure we may feel of an ultimate proportion of success, obtain any clear idea either of how soon we may expect diminution in the infectivity of depopulated infective areas, nor within what time we may relax our precautions, nor can we gauge even the approximate risks of infection in such areas. It is evident that the shorter the period of infection in the fly, the less chance there is of it infecting by its bite after once acquiring the infection, and the less will be the number of persons whom it is likely to infect, since, as a rule, it naturally feeds only once in 48 hours; and the converse of course is also true. Analogy with other protozoal organic infections points to the probability of a cycle of development in the fly, of a hon-infective period of evolution preceding the infective stage, and of a long, if not life-long,
* NOTE 5.—It is but a rough test in the hands of experts. As a guide for persons unacquainted with the characteristic glands of trypanomiasis it is quite untrustworthy: "moreover, glandular swelling is not invariably perceptible in every case.
† See note p. 21.
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infectiveness of the invertebrate host. But all this, in the present imperfect state of our knowledge of trypanosomes and of trypanosomiasis, remains nothing more than suspicion.
The facts which I have adduced, for they can scarcely be called arguments, in favour of a low percentage of infection among the flies of a given area are, so far as they go, also in favour of the limited duration of infection in the fly. In addition, also, I will relate the case of a small settlement of seven huts which I came upon in Usoga some four years ago. These huts were situated on the point of a small and narrow peninsula of a few acres in extent, the whole of the point for 60 or 70 yards being covered with jungle, which practically surrounded the little settlement, while, beyond this, the base or neck of the peninsula was formed of 100 yards or so of low swampy ground extending landwards between wide areas of papyrus. The point itself swarmed with fly as far as the jungle extended, but, on the swampy ground beyond, there were few or none. The inhabitants were 19, of whom all but five children, two women and one man had sleeping sickness, and these people (for no one else lived anywhere near), with four or five goats and a few fowls and perhaps an occasional hippopotamus and some wild birds, must have formed the whole available source of food for the flies on this small area. I was not able to follow the history of these people, for the peninsula was soon afterwards deserted, but of my 14 canoemen I know the history of eight for the next six months, and only one of them happened to develop sleeping sickness, though probably they had undergone many other exposures to infection. Of three servants who were with me none has any sign of sleeping sickness to this day, nor have I myself, though not one of us could avoid being bitten by the swarming and persistent flies at that stricken spot. One would suppose that there would be hardly a fly there that had not many times bitten one or more of the eleven inhabitants who were certainly infected, most of whom were lying about freely exposed to their attacks at the time of my visit, and it appears to me that, if the infection in the fly were lifelong, or even of considerable duration, most or all of us who were bitten then should have become infected, and I cannot help but think, in consequence, that there is some ground for hope that the period of infection in the fly is either of very short duration or is made up of very brief intermittent stages. It is even possible, still, that the conveyance of the infection by Glossina palpalis, rather than by other Glossing, or by any blood-sucking insect, may be merely owing to a prolonged viability of the trypanosome in the interior of this fly, that a migration rather than an evolution takes place, and that it is not a true host.*
I understood from Dr. A. C. H. Gray that the trypanosomes which one finds swarming in the intestine of a varying small percentage of Glossina palpalis were found by Tulloch and himeslf to be non-pathogenic to animals susceptible of infection by Trypanosoma gambiense. There is doubtless more than one type of these trypanosomes, besides individuals resembling T. gambiense, but, so far as I can ascertain, no one has yet certainly demonstrated the presence of this trypanosome in G. palpalis after all the blood of a meal has disappeared by complete digestion.
In my opinion the question of the duration of infection in the fly and of the capacity of other species of Glossina than Palpalis of carrying the infection are the most important and most urgent for investigation at the present moment.
That wholesale clearing of the foreshore and for some distance inland will effectually eliminate Glossina palpalis, so long as the clearing is maintained on the foreshore, has been conclusively proved at Entebbe, where, for the last five months, from March 31st-August 31st, 1906, it has been impossible to obtain a specimen from places in which it lately swarmed, whilst, immediately beyond the limit of clearing, it remains as numerous on the foreshore as ever.
The following experiment, which is now in progress, will, I hope, lead to definite results of scientific and economic value on this point. I propose that the extended clearing of the foreshore of Entebbe Bay, which had already been recommended as highly necessary, shall be carried out thus:-
A strip of 200 yards in length having been cleared along the water's edge to a width of 30 yards, the results as affecting the fly will be carefully watched, and, if necessary, the width will be increased by strips of from five to ten yards until the fly disappears. If the width of 30 yards proves sufficient, the next strip of 200 yards
• I have since read the "Preliminary Report on G. palpulis in its Relation to T. ganbiense, &c.,' by Minchin, Gray and Tulloch. It appears that their conclusions from direct experiment point, like my deductions from general observation, to a short period of infectivity in the fly.