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39.
VI.--The Decline of the Epidemic as indicated by the Death Returns.
(See Appendix.)
The ratio of infected to healthy persons is a factor in determining the freedom with which the disease spreads. If conditions are otherwise the same, chances favouring transmission increase with this ratio. This was always recog- nized, and segregation of the sick advocated as a measure for the suppression of the epidemic.
40. If other factors remain the same, therefore, every epidemic must reach explosive virulence, and every resident of a region where infection, if introduced, can spread at all would eventually contract it. This actually happened in some communities located on small islands or on the mainland shore.
41. In 1906 it was believed that the epidemic had reached such a state of explosive virulence on all the islands. It was reported and believed that more than ninety per cent. of the islanders were already infected at that time.
It was assumed that the reduced number of deaths which had been returned during the year or two preceding was due to reduced number of "possible victims," and that the entire population of the islands, and, therefore, of the mainland shore wherever it was exposed to tsetse to a comparable extent, would be completely wiped out.
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42. The death returns prove most conclusively that these fears were greatly exaggerated. Instead of more than ninety per cent. of the island population being infected there was less than ten per cent., and the decrease in number of deaths from the islands certainly, and from the mainland probably, was accompanied by a decline in the death-rate (per thousand), which was almost as great.
43. It is also conclusively shown that, although the number of sick people increased rapidly during the earlier stages of the epidemic, and conditions thus became much more favourable, in one way, for its further continuation, other factors, more than proportionately unfavourable, came into play, and more than compensated. As a result a turning point was reached long before all the people had contracted infection. This stage in the progress of the epidemic was reached in different years in different groups of islands, and in different districts on the mainland shore, and cannot be shown to have had a definite relation to anything else than the course of the epidemic itself. Many factors which might possibly be held accountable had it taken place everywhere at the same time may thus be eliminated, and there can be little doubt that a reduction in the extent to which the flies fed upon man was the compensating factor actually responsible.
44. At the same time when the depopulation measures were carried out, the number of infected persons, and with their deaths the source of infection for the remainder, was steadily diminishing. If the micro-parasite could not hold its own it is inconceivable that it would be able to increase from small beginnings to the point of becoming epidemic under such conditions, and recolonization of these regions in such manner as to preclude their recurrence would be to preclude the possibility of infection spreading if it chanced to be reintroduced.
45. From all that can be learned of the manner in which the surviving inhabitants of the islands were living at the time when they were induced to evacuate their country, conditions were considerably more favourable for the transmission of infection than would be possible were recolonization to be permitted in the manner here proposed.
VII. Conditions along the Buddu Coast in 1906.
46. It is possible, of course, that the decrease in number of infected persons upon the islands was due to something else than mere decimation of the population, such as the elimination of the more susceptible persons and the survival of the relatively immune. This theory would not agree with the undoubted fact that on some small and badly fly-infested islands the entire population was affected and the epidemic increased to explosive proportions. Neither does it agree with the results of Dr. van Someren's study of conditions along the Buddu coast in 1906, which are to the effect that it makes no difference whether conditions unfavourable to transmission come about as the result of epidemic or precede it.
47. A large part of this coast was found to be thinly, or not at all, infested by tsetses, and only at a few points were conditions with respect to tsetse and population at all similar to those which prevailed on the islands and lake shore generally. At these points (Sebweru and Kaziru) the villages near the shore had
48.
been practically depopulated as the result of the epidemic, which had assumed large proportions in 1904.
At other points where the fly was scattering or absent infection was found to be present, but appeared not to be spreading locally, even though tsetse was also present. The most conspicuous and significant example was the village
at Dumo.
49. This consisted of about fifty huts, all of which were located within a thousand yards of infested shore. A few of the huts and plantations were actually within the ordinary "fly range." A part of the huts drew their water from the lake at points where fly was quite common. About a mile distant was a reach of shore which was then densely infested. The fishing grounds were opposite the village, and off infested shore.
50. Dr. van Someren was of the opinion that these conditions were favour- able to epidemic, but, although he specifically mentions, he fails to take into full con- sideration, the following points of difference between this village and most others in the infected zone: (1) The regular canoe landing was free of fly. (2) The most of the huts drew water from a stream where there was no fly. (3) The infes- tation along the shore opposite the village and fishing grounds and where the other watering places were located, although fairly dense, was not excessive. (4) The population had no occasion to go habitually into the region near at hand where the infestation did exceed the average.
51. Non-occurrence of epidemic at this particular village was not due to absence of infection. It had been introduced, not once, but many times; and not only recently, but at least two years before. In 1904 three persons had died from sleeping sickness, in 1905 one person, in 1906 one person, and four more were found to be suffering from the disease. Every one of the five recent cases had come from some other locality where epidemic occurred, and there was nothing to indi- cate that they had contracted the disease locally. The presumption is strongly against it, for approximately the same number of sick were found in other villages located well back from the lake, and in localities where there were not nearly so many flies or none at all.
52 Every infected person that came to live in the village might have brought about epidemic, but none of them did, and it is difficult to conclude otherwise than that the disease could not increase and disperse under conditions no more favour- able than these. In addition to the infected persons which actually lived and died there, it is certain there were many others temporarily resident, for it happens that the locality was a halting place for many of the canoes engaged in traffic along the important trade route which followed the western shore of the lake, and practically all of which, if south bound, must have come from or touched at a badly infected locality.
the
53. Conditions such as these are considerably worse than could recur under Bukakata standard" for recolonization, for this does not permit regular watering places to remain uncleared nor villages to be located so near the lake shore. Neither could infection be so continually reintroduced from extraneous sources, except in times of epidemic.
54. The village was not depopulated until 1907 or 1908. A former chief or headman was seen and interviewed. He stated that eleven of its inhabitants had died, a figure which corresponds with Dr. van Someren's data and indicates that [that] epidemic did not follow in the period subsequent to his visit and before depopulation.
55. With relation to the ban subsequently placed on all canoe traffic on the open lake, and the apparent necessity for the destruction of all the native canoes, it is significant that along a thirty-mile reach of shore to the north- and southward of Dumo, and which was for the most part fly free or only lightly infested, were many villages the inhabitants of which owned many canoes. There were no restrictions on fishing at that time nor on traffic with the badly infested islands and mainland to the east and northward.
56. Nevertheless, the people in these villages appeared not to have suffered to any greater extent than the people in inland districts who traded with the coast dwellers and who went about their business by overland routes. It made no differ- ence that was measurable in the several villages in which Dr. van Someren made an examination of the inhabitants whether traffic was conducted by canoes or other- wise. As far as the evidence goes, it is entirely to the effect that infection was
"infective areas 29 only contracted in the populated
(to be mentioned again presently) which were equally accessible from either the water or from the land.