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Tsetse flies and the development of Africa

The Seventeenth Stephen Paget Memorial Lecture was delivered at the Annual General Meeting of the Society at Manson House, Portland Place, on Tuesday, June 8th, 1048.

 

 

Here, in the problem of the tsetse fly and trypanosome diseases, we have a story of research which is now beginning to produce results of the greatest intellectual interest and practical value.

 

Introductory

Tsetse flies {Glossina) arc blood-sucking insects virtually confined to tropical and sub-tropical parts ofAfrica. When they take up blood from a man or an animal infected with one of the trypanosome diseases they take up also the trypanosome and may then transmit this to other human beings or other animals. The trypanosomes which infect man are, generally speaking, different from those found in animals, so that the human and the animal diseases are to be thought of separately. One should remember also that the human disease, sleeping sickness, is practically always trans­mitted by the tsetse fly and not by any other biting insect or through any other channel. The corresponding infections of animals, known collec­tively as nagana, are transmitted in the main by tsetse flies, but to a con­siderable extent also by other biting flies, at least in certain parts ofAfrica.

As to the importance of these matters, the human disease, or sleeping sickness, has been one of the major medical problems in all parts ofWest Africa. One notes, for instance, that in Nigeria during the last 15 years the Health Department has treated just about 500,000 cases of human sleeping sickness, caused by the trypanosomes transmitted by tsetse flies: that is in a population of rather over 20 millions, so that the disease is clearly a major medical problem. In East Africa, on the other hand, though epidemics may arise—and in the past the most shattering and devastating epidemics have occurred—yet, in general, the total number of cases or of deaths to be attributed to sleeping sickness is rather a matter of hundreds than of thousands. At present, then, inEast Africathe human disease is not usually a major cause of sickness or death of the populations, though there is, of course, the ever-present danger of an epidemic.

The cattle diseases, or nagana, are, on the whole, much more serious. Their importance is negative rather than positive, for it lies not so much in the thousands of cattle who sicken and die, but rather in the fact that domestic animals are kept away by their owners from enormous areas occupied by tsetse: apart from the seriousness of this from the standpoint of human nutrition—the absence of beef and of milk—it also involves the absence of draught and plough animals. The tsetse has thus had an important bearing on the historical shaping of African civilisation.

 

The total area ofAfricaheld by the tsetse fly has been mapped and is roughly estimated as 4,500,000 square miles.   It is twice the area of theUnited Statesor 75 times the area ofEnglandandWales.   Such units are perhaps more convincing than a figure of so many million square miles.

 

Methods of Study

Perhaps we shall understand the subject better if I tell you how it is studied in the field. Take first the studies made on the tsetse fly itself. In the last century, when Europeans like Livingstone and the great African travellers started going into tropicalAfrica, the tsetse was known to be dangerous. Livingstone, writing in 1857, drew attention to this small fly, 'with its tiny proboscis, which could yet cause great oxen to die. He asked how this could come about, and he wrote: "The poison-germ, contained in a bulb at the root of the proboscis, seems capable, though very minute in quantity, of reproducing itself." Surely a most remarkable use of reasoning and imagination. The people of the period knew as a matter of observation that these dangerous biting insects were confined to certain types of woodland, and that some of them were strictly limited to waterside strips while others spread more widely over the countryside, though not universally. They knew also that there were certain types of vegetation —or bush—which were dangerous and were often referred to as fly belts.

In the period between the two wars that elementary field observation was developed by a man called Swynnerton, a man untrained in science, but with a very acute analytical brain. He and his scientific colleagues made intensive studies of this problem inTanganyika, as others did, inWest Africa. In this period the relation of fly to vegetation became more capable of being understood; the entomologists produced a mass of evidence defining the relation of the insect to particular types of vegetation.

 

There are in the main two types of problem. Along the side of rivers there may be a ribbon of thick and generally evergreen trees and bushes inhabited chiefly by the Glossina palpalis. This is in contrast with G. morsitans, and flies of other species which live, not in ribbon development along the water, but widely overAfricain enormous areas, but selecting in each area just this type of vegetation or that.

A great deal of information on this relation between the insect and plant life has come about from the development of a technique called the "fly round." These fly rounds are carried out by an entomologist or a trained African with assistants. They walk over a certain route at regular intervals, perhaps once a week. The route is planned to traverse many different types of vegetation, and in each section of it these men catch all the tsetse flies they can, recording the number caught, the species and so forth. If that is done over a long period figures are obtained which are of great comparative value: it is possible to say that the flies occur in a certain sort of vege­tation and not in another. By using the fly round and other subsidiary techniques, a great deal has been learned, though always rather qualitatively. The basal fault of the fly-round method is this, that the number of flies that can be caught depends partly on the total number of flies in the place but also on their activity, and their activity depends very much on the kind of weather. For example, on a gloomy day one may get a small number, whereas on the following morning, when it is bright, the number may be found to have increased tenfold. One cannot distinguish population and activity: we can never assess the total fly population by this method. The method cannot become quantitative.

A great advance has been made inTanganyikaduring the last dozen years by Jackson, one of Swynnerton's disciples, who has invented a new approach to the subject. Putting the matter very simply, whatJacksonhas done is this: he goes out with his fly boys and catches, say 500 tsetse flies, marking each one with a little spot of paint, so that it is recognisable.

Then he liberates them, waits a day or two until the marked flies have mixed up with the others, and then captures a sample of flies: the percentage of marked flies in the sample is determined, and from that he can deduce the total population. Allowance has to be made in such an estimate for the births and deaths of flies during the interval, and also for immigration and migration; reputable ways of making these corrections have been worked out. By developing this methodJacksonhas been able to get actual figures of tsetse fly in the square mile. This is a very great advance, whether we are trying to assess the problem in a quantitative way or to measure the results of campaigns against the insect.

In many other directions work of interest has been going on. It is quite easy to discover trypanosomes by dissection of the tsetse, and so far as those which affect man are concerned it is very rare to find them: one nearly always finds that less than 1 in 1,000 of the flies caught in the field is infected with those trypanosomes. Even if you work in the laboratory, and feed your flies on an infected man or animal, you still do not get anything like 100 per cent, of the victims developing infection and handing it on. What is it, then, that causes 1 in 10 or 1 in 100 flies to develop the infec­tion and become dangerous, all the other flies remaining uninfected although given the opportunity of becoming infected? We do not know the complete answer to that question, but we have identified some of the factors which influence the matter. This gives me the opportunity of emphasising the very great importance nowadays of studying not merely the tsetse flies as such (or anopheles in the case of malaria, or fleas in the case of plague); we should give much more thought to the infected individual insect which is responsible for the transmission of the disease. It is this insect which is the key in the epidemiology of sleeping sickness, or whatever disease we are considering.

 

Methods of Attack

So much for the study that is proceeding. Now let me tell you about the methods of attacking tsetse flies. There is, first of all, the attack by way of vegetation. In the simplest type of problem, Glossina palpalis inhabits a waterside fly belt 10 yards wide and many yards long. This problem can be attacked in various ways, e.g., by clearing with axes ; or one can go on catching tsetse flies with the help of fly boys until they are exterminated. In another type of the fly belt, the widespread type, entomologists have learnt so much about what the fly must have in the way of vegetation that skilled men can carry out discriminative felling of bushes and trees, re­moving only what is essential to the insect. There are places in Africa where by felling 5 to 10 per cent., or even less, of the bush, tsetse have been exterminated. This work is still in the experimental stage: it cannot yet be carried out on a big scale because it is so skilled and demands such ' special knowledge. But it is going forward, in East andWest Africa, with success. It may be greatly assisted by aerial photography, for it is very easy to map vegetation and identify particular associations of plants from air photographs.

A second method is being successfully applied to the problem of G. morsitans, a species which is not very much interested in man. The African will settle in a fly belt leaving behind him patches of bush and so making the maximum contact between himself and the insect, which is evidently dangerous ; but with planning that danger can be averted. As man comes in, game goes out, and with it this species of tsetse. In certain types of African country it is now known how many people ought to settle to the square mile so as to hold all the area. The Tanganyika Medical Department has been using human settlement for 20 years, choosing areas of fairly good soil, and putting on them the right number of people to maintain and hold the land. This is difficult because it demands an understanding of so many sciences ; one must consider not only the fly and the trypanosome it carries, but social anthropology and agriculture. The African has a great sense, not of individual land ownership, but of tribal and family land ownership, and* you can only settle the people safely in the land which is their own, paying due regard to their social system.

Moreover, most of the land now held by G. morsitans has been left unoccupied because it is not the best land ; the soil is bad or there is a shortage of water, so that agricultural development is a matter of con­siderable difficulty. The medical people put villagers on the land at a density sufficient to hold it against tsetse, but the agricultural experts tend to say that this density is too high and that soil exhaustion will follow. I talked to one agricultural officer and asked him, " We have these settle­ments with people in good health; the crops are good: the settlement is 20 years old. How long a period of settlement will satisfy you cautious agricultural gentlemen? "—a question to which I have not obtained any answer.

There is also the method, of game destruction, which is effective, and proved to be so if you are dealing with G. morsitans. The justification for game destruction is twofold: first of all, it is a matter of experience that if you.destroy game and do not put in cattle at first, G. morsitans dis­appears, i.e.. morsitans must have game to feed on, in the absence of cattle. Secondly, the game animals are a reservoir of trypanosomes which cause nagana in cattle, horses and other domestic animals.

The evidence that G. morsitans cannot persist in the absence of game has been known for some 20 years, and one marvels at the ignorance of people and newspapers, which discuss the matter as if it were some curious theory rather than well-established fact. One sees this clearly if a main road goes through an area or if an area is developed for minerals; the game disappears, and with it G. morsitans, no measures to attack the insect itself having been taken. Then again there have been great outbreaks of disease in game animals, such as the rinderpest ; in this case also the disappearance of game has been followed by the disappearance of G. morsitans, and this has happened not once but on several occasions. Then there is evidence of "a different type fromSouthern Rhodesia.Southern Rhodesiawas threatened with G. morsitans which was reoccupying the area from which it had disappeared following an outbreak of Rinderpest and destruction of game. The Government thereupon destroyed the game over a large area, and now some 6,000 or 7,000 square miles of land recovered from the tsetse fly by that measure alone, has become available for cattle. Therefore, whether you think it right or wrong, it is a well-established fact that the destruction of game is an effective way of dealing with G. morsitans.

If we must, as I believe, exterminate game (not only as a measure of tsetse control but for numerous other reasons), it is a duty to set aside adequate reserves and maintain the animals, segregated from cattle and from cultivation.  To this important matter one can only make passing reference.

There is one other way of attacking the problem, and that is by direct attack on the insect by insecticides. People are apt to say, "Can you not deal with these problems by means of D.D.T. or gammexane?" If one has a simple problem of an isolated patch of fly belt, even of 100 square miles, it is probable that one could exterminate the tsetse fly from it by these methods, providing the work is done from the air. But the cost is at present much greater than the value of the land, except where land has a high value for some exceptional reason. It may well be that the use of • insecticides may eventually solve a part of our problem, but I cannot believe that insecticides are going to deal with fly belts that run to tens of thousands of square miles. How can one effectively distribute insecticides throughout such an area.

Again, the disease itself, as distinct from the tsetse, can be attacked and exterminated or nearly so. If one has a sufficient organisation one can find all cases of sleeping sickness early in the disease and treat them. The French, the British and the Belgians have pursued this method of mass diagnosis and mass treatment now for more than 80 years, and because of it sleeping sickness has ceased to be a major threat to human life and health in large parts ofAfrica. Nobody now is seriously afraid of great epidemics of sleeping sickness, for that menace has been countered by the use of drugs, some of which are both curative and prophylactic, giving the individual who has been treated protection against the trypanosome for a period of several months. This should enable the chain of infection to be broken, for if all the people are protected for a period longer than the life of the tsetse no new cases would arise and complete freedom from infection might be obtained.   It looks quite possible.

 

Results

Now I may report to you some of the actual success. I spoke first of the rather simple problem of the fly belt along the waterside—the linear fly belt with G. palpalis in it. In the north ofNigeria," at Anchau, an area was chosen where there was very serious sleeping sickness, due entirely to such waterside fly belts. It was not an elaborate problem though it was • a large one: Nash, of the Nigerian Medical Department, and others, dealt with a very extensive mileage of waterside belt, and at Anchau they now have an area of 700 square miles from which tsetse has been exterminated. On that land there are now 50,000 people as well as a large number of cattle; this is of great significance because cattle are more susceptible to trypanosome diseases than man. The work at Anchau was accompanied by other measures for improving the general lot of the villager—better-planned villages, uncontaminated wells, improved agriculture, and so forth.

The game destruction inSouthern Rhodesiais also a solid and remarkable piece of work. One piece of evidence appealed very much to me. We went out to an area which had been under G. morsitans, and which the entomologist, Mr. Chorley, claimed to have cleared of tsetse by game destruction. Three test herds of cattle, 50 head in each herd, had been put in. The three herds had been kept there for a year, the animals being inspected every three months. All sick animals were examined, and the deaths of any animals were enquired into and only a single case of try­panosomiasis occurred. Unquestionably, that area had been a fly belt and, unquestionably, the test herds showed that the tsetse had been driven out.

I have already referred to the success of discriminative clearing of bush, and I have told you something of the medical settlements inTanganyikawhich are on a very large scale. On these there are now 190,000 people with very large numbers of cattle, where previously there were almost no people and certainly no cattle at all.

You will remark that such success as has come has been brought about by a large variety of methods ; you may ask whether this does not suggest the treatments for the common cold : there are at least a hundred treat­ments of the common cold, and not one of them is of the least good I But the comparison is not a fair one, because this problem of trypanosomiasis in Africa is in itself very complex. There are several species of tsetse, there are different sorts of vegetation and climate, there are different social habits, and agricultural methods, and so forth. But I admit that with more knowledge some of the methods will be shown to be more effective and economical than others, and some of the methods which promised well will be found to be only of historical interest. I trust that I have been able to make it clear that the research of the last 30 years is now delivering very interesting and solid results. I have come back from long journeys inEastern Africain the last three years feeling a considerable degree of optimism about the tsetse situation. The importance of what has actually been achieved is not yet realised, either here or inAfrica.

 

 


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