Antoon de Vos
ANTOON DE VOS is an FAO wildlife biologist with wide experience in Africa.
In Africa, where the ecological equilibrium is known to be extremely delicate, tsetse control agencies should be particularly on the alert for any undesirable results or side-effects of eradication schemes. The continent's growing need for food, due to its fast-rising population, calls for an expansion of meat production but animal trypanosomiasis is, at this time, preventing cattle raising in most of tropical Africa.
Animal trypanosomiasis and human sleeping sickness are spread by tsetse flies (Glossina spp.), which inhabit a broken semi-humid and humid belt stretching across the continent from the Atlantic to the Indian Ocean.
The Food and Agriculture Organization (FAO) of the United Nations, in cooperation with various other assistance agencies, is launching a campaign for the eradication of these diseases and of the tsetse fly. If successful, cattle herds alone could be boosted from 100 to 120 million head, the supply of milk would increase considerably and protein scarcity could be greatly reduced.
Unfortunately, as yet there exists no simple, foolproof and cheap method for the eradication of carrier flies without causing some harm to the environment.
Control of these diseases can be achieved either by drug treatment of the host or by eliminating the vector, or a combination of both. The selective breeding of resistant cattle is another possible approach.
Two methods of tsetse control were being used until recently - bush clearing and wildlife destruction. They have been almost entirely dropped because they were found to be ineffective.
The thinking behind bush clearing was that the modification of the fly habitat would inevitably result in the disappearance of the flies. Although this was a correct assumption, many projects involving bush clearing failed to bring the expected result because, once the trees and shrubs were removed, follow-up efforts to maintain the areas in a cleared condition were omitted or inadequate. AS a consequence, bush encroachment took place and re-invasion by tsetse flies was only a matter of time. For example, millions of dollars in foreign aid have been spent in various countries for bush clearing operations using expensive heavy machinery. Because the local people were neither ready nor prepared to use the cleared areas for intensive agriculture or grazing, resprouting of the trees and shrubs took place, resulting in denser cover for the tsetse flies than before clearing operations were initiated.
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Eradication of trypanosomiasis aims at expanding meat and dairy production for an increasing African population. But similar efforts in the past have resulted in wasted resources, severe environmental damage and needless slaughter of wildlife, after which the tsetse flies returned worse than before. The present campaign should safeguard the natural environment. But most important, it should be a part of integrated land-use and development operations so that cleared land will actually be utilized for agriculture and settlements and not abandoned again to ever worse tsetse infestations. SEARCHING FOR TSETSE-FLY SPECIMENS IN BOTSWANA keeping land cleared is not easy |
THE AFRICAN TSETSE ZONE Can the land that is cleared actually be settled?
Many species of wildlife are reservoirs of pathogenic trypanosomes and because of this, early parasitologists advocated their extermination. Wildlife destruction was based on the idea that if the preferred hosts, like warthog, could be eliminated, tsetse flies and trypanosomiasis would also vanish. Hundreds of thousands, and more likely millions of head of game have been destroyed with this purpose in mind in Rhodesia, Botswana, Zambia, Uganda and other countries during the past half century. Thus, game elimination was initiated in Rhodesia in the early 1920s, and this was continued until the late 1960s. The officially recorded toll of the systematic hunting operations up to 1963 was approximately 800000 head of game animals of all kinds (Roth, 1973).
Although these efforts did result in the almost complete eradication of some game species, they were not really effective in the elimination of tsetse flies because the populations of game which managed to survive usually increased, and suitable habitat conditions for the flies continued to exist. It has also been demonstrated that many small mammals play a role of alternate hosts for trypanosomiasis, which makes it much more difficult to control the disease.
It is almost unbelievable that it should have taken three decades of game slaughter after World War II to convince the authorities concerned that this approach was basically ineffective. If the game that was destroyed in the slaughtering operations had been properly used, presumably these actions could have been excused. But, in fact, most of the game was left where it was killed without any use being made of it.
The environmental impact of insecticide applications has also been considerable and environmental contamination has not been sufficiently considered. Side-effects have been reviewed by Koeman and Takken (1976), based on studies made mainly in Botswana, Kenya, Nigeria, Uganda and Zambia.
At present insecticides are applied on a large scale in most tsetse control programmes of the 19 African countries for which information is available. In the majority of operations, DDT, dieldrin or endosulfan have been used either in ground sprays or by aerial spraying. But during the past decade aerial applications have become predominant because fly belts are very extensive and, in general, this method is cheaper and faster than ground spraying.
Many efforts have been made to transfer the insecticides as efficiently as possible to the flies. In aerial spraying this may be achieved by a proper choice of the type of aircraft, atomisers, spray formulation and selection of vegetation to be sprayed. Unfortunately, quite often insecticides have not been applied carefully or were deposited beyond the confines of the target area because of wind drift. The need for skilled pilots should be stressed to ensure effective application with minimal adverse impact. They should be specially trained for this work.
In general, the range of non-target species affected by helicopter applications, and the resulting damage to the environment, are larger than after more discriminate ground spraying.
Ground spray techniques are particularly effective against those species of Glossina which are, at least seasonally, somewhat restricted in their distribution. Aerial applications of non-residual insecticides, involving five treatments at approximately three weekly intervals, seem to have been successfully used in Zambia, Botswana, and elsewhere. However, the degree of expertise required for carrying out such control programmes is not often available, and also hard to apply. For it involves discovering what is really going on in a "controlled" tsetse population, and among the livestock populations in regard to the actual incidence of new infection, its real impact on production and its sensitivity to drugs. Such problems and difficulties should be carefully weighed before accepting control commitments.
Most insecticide applications studied to date involved extensive deaths among many kinds of mammals, reptiles, amphibians, fish and non-target insects.
Koeman and Takken (op. cit.) arrived at the preliminary conclusion that, in general, mortality in non-target wildlife species continues for a longer period after dieldrin applications (2 to 3 weeks) than after endosulfan applications (approximately one week). There are also differences as far as the spectrum of species affected is concerned: dieldrin tends to affect larger predators while endosulfan does not, but the latter tends to have a more dramatic effect on cold-blooded vertebrates.
As far as long-term effects of aerial spraying are concerned, insectivorous bird populations in particular are severely depressed in numbers and certain species virtually disappear in the treated areas. The latter conclusion refers to results obtained one to two years after application of the pesticide (Ibid.). There are also indications that organisms at the apex of a food chain, like raptors, are most seriously affected.
Long-term effects on non-target invertebrates, fish and other cold blooded vertebrates have not yet been studied in detail. It has been shown in various studies, however, that in particular DDT and dieldrin give rise to measurable residues in fish and other biota.
The problem of environmental pollution may be compounded by concentrations of insecticides in aquatic ecosystems, such as lake basins of central Africa, which lose most of their water by evaporation and not by discharge to the sea.
As for curative and preventive treatment of cattle, the problem is that such treatments must be constantly repeated. At present, immunization of cattle and biological control of tsetse through the introduction of predators are only remote possibilities. And it should be remembered that it is essential to adapt control methods to prevailing local conditions. Usually it is necessary to combine several techniques.
In control situations, the degree of supervision and assessment which has to be exerted in the surveillance and treatment of infection and the evaluation of drug resistance cannot at present be carried out on the scale required by most of the countries involved. These problems of development cannot be readily overcome because they are inherent to the socioeconomic structure of the rural community concerned.
Control campaigns can be successful only if local governments carry them on relentlessly: even a brief interruption might allow the tsetse fly to re-invade pest-free areas, thus ruining previous efforts.
Three major ecological conditions are possible as a result of tsetse-control operations:
· Without an integrated approach to the problem, and especially to the use of land freed from Glossina infestation, the disease could easily spread again.· The eradication of tsetse flies can create an unstable situation, whereby the indigenous ecological and physiological restraints which controlled the disease in pre-technological Africa are disrupted.
· A stable situation is created when an increase in human density and use results in the clearing of bush and the elimination of most wildlife on which tsetse flies normally feed. Under such conditions, trypanosomiasis can no longer be a problem because neither the habitat nor the vector or sufficient alternate hosts exist any longer.
The main objective in reclaiming land from tsetse infestation or in controlling infestations is to increase meat, milk, and other food supplies for the local community. Yet if this objective, at the same time, damages wildlife resources, causing overgrazing and in general downgrading of the land, these negative aspects should be evaluated against it.
Destruction of the wildlife resource in favour of cattle development can only be justified on sound ecological, social and economic grounds. If it can be proven, on these grounds, that wildlife utilization should be continued or improved on the basis of better management, wildlife should be maintained.
Tsetse control operations should be made an integral part of land-development programmes, to avoid under-exploitation - or over-exploitation - of tsetse-cleared land and to minimize undesirable ecological consequences.
Agricultural development of tsetse-infested lands must be reconciled with conservation needs to ensure long-term optimum productivity of these lands. With this in mind all existing natural resources should be considered within the framework of multidisciplinary land-use planning.
AERIAL SPRAYING IN EAST AFRICA more difficult then it appears
But under present conditions sound land utilization is not easy to attain in tsetse-control situations in development areas. The danger of overgrazing if the growth of herds and land-use are not properly planned and controlled will continue to prevail and rapid bush encroachment may ensue, causing tsetse flies to spread again. Therefore, tsetse eradication schemes that are not followed up by properly designed and executed land-use and development programmes will only result in increasing the impact of man's activities on an environment that is ever less capable of sustaining it. In such cases the process of land degradation can only accelerate.
Finally, it must be realized that large-scale elimination of tsetse flies without consolidation of the cleared areas by agriculture creates a situation in which any reduction of expenditure on farming, or relaxation of effort will be followed by catastrophic outbreaks of trypanosomiasis.
Specialists charged with the control of tsetse flies and trypanosomiasis will no doubt argue that total eradication of both flies and the disease should be their ultimate objective. However, several arguments may be invoked to point out the futility of this approach. In the first place, it would be a most costly undertaking; and, secondly, would it be a worthwhile effort from the land-use point of view?
It would seem advantageous in many instances to keep all large and some small tracts of marginal lands for wildlife conservation purposes.
Marginal land on poor soil, having nothing to gain from modification, should be left alone and could be best used for wildlife management and utilization (Lambrecht, 1966). Wildlife has many advantages to domestic stock in terms of husbandry objectives: it is better adapted to the environment, tolerant to trypanosomiasis and does not demand improved pastures, fencing, shelter and other care required by domestic stock. Under proper management, herbivorous wild ungulates could produce protein and by-products on arid land where cattle raising is not feasible.
Considerable information has already been obtained regarding methods of chemical application, insecticides to be applied and the ecology and population dynamics of tsetse flies, but much more research remains to be done before large-scale tsetse control operations can be considered satisfactory from an environmental management point of view.
Research should be pursued and intensified mainly on genetic tsetse control and trypanosomiasis immunology, because these methods have no environmental side effects. Studies of various methods of biological control and of the possible development of hormonal insecticides should be continued. The discovery of a powerful tsetse attractant would be of great value. New drugs, both for the prevention and treatment of the disease in animals subject to tsetse attack also need to be developed.
It is very important that inexpensive biodegradable insecticides be found, with a high degree of toxicity for tsetse flies and a minimal effect on non-target species. Also, improved methods of tsetse control by means of selection of the most adequate pesticides, formulations and applications should continue, in order to reduce as much as possible the environmental impact.
The various aspects of the biology, ecology and population dynamics of tsetse flies should be carefully investigated, as well as the ecology and status of various non-target species of wildlife actually endangered by insecticide applications. There is also an urgent need to investigate the side-effects of tsetse control operations on non-target insects, fish and certain other vertebrates, particularly in relation to pesticide applications in the moister areas.
Environmental degradation has been progressing at an accelerating rate and there does not seem to be any let-up in the foreseeable future. In fact, conditions seem to be deteriorating even more under the impact of a steadily increasing human population pressure (de Vos, 1975).
Unquestionably, trypanosomiasis has been a major factor in the conservation of forest and wildlife resources of Africa and the areas now infested by Glossina constitute a land bank of inestimable potential and importance. Although it cannot be argued that trypanosomiasis infestation should be allowed to continue to guard important forest and wildlife resources, the proposal can be made that, instead, after eradication, all possible efforts should be made to maintain these resources in a good condition. Also, in large-scale operations pre-spray assessments should be made of the ecological value of the various habitats occurring in the areas to be sprayed, and under certain circumstances cleared of vegetation, so as to learn whether damage to fish and wildlife could give rise in certain areas to a serious deterioration of local amenities, such as availability of fish, bee-keeping or loss of species in nature reserves. Special attention should be given to the application of insecticides in vulnerable zones such as riparian forest fringes along streams, which safeguard the survival of many terrestrial savanna species, and also fish species because of the availability of water during the dry season. This implies that attempts should be made to apply pesticides as discriminately as possible by making optimal use of the available techniques and certain combinations of application. For instance, helicopter applications of either dieldrin or endosulfan could be limited to those habitats with a relatively low ecological value, while those with a high value could be sprayed by groundspray techniques, using the most appropriate pesticide available (Koeman and Takken, 1976).
To sum up: although control of trypanosomiasis will be required to allow for an increased availability of human food resources and the elimination of sleeping sickness in tropical Africa, this should not take place while at the same time disregarding or neglecting the ecology of the environment. Proper safeguard should therefore be established to allow for a more satisfactory protection of the natural environment. Under all circumstances, tsetse-control operations should be implemented as part of an integrated land-use and development approach.
KOEMAN, J.H. & TAKKEN, W. 1976 Report on present knowledge of the impact of tsetse control operations. FAO, AGA: TRYP/76/Misc.
BOTH, H.H. 1973 Game conservation and trypanosomiasis. The British Veterinary Journal 129: 407-413.
LAMBRECHT, F.L. 1966 Some principles of tsetse control and land-use with emphasis on wildlife husbandry. E.A. Wildlife Journal, 4(89-98).
CRAWFORD, S.M. AND M.A. CRAWFORD. 1970 An examination of systems of management of wild end domestic animals based on the African ecosystems. In: Animal agriculture, the biology of domestic animals and their use by man, pp. 218-233. H.H. Cole and M. Ronning, eds. Freeman, San Francisco.
