A.M JORDAN
Introduction
Man and tsetse
Prospects for control of animal trypanosomiasis
Discussion
References
Although mechanical transmission of African animal trypanosomiasis may be important in some localities, there is no evidence that the disease can persist in the absence of tsetse flies. Wells (1972) considered that various factors, especially the difficulty of detecting very low density tsetse populations, have contributed to conclusions that trypanosomiasis exists in some locations where tsetse are apparently absent. Other factors which can contribute to such a conclusion include the movement of trypanosome-infected livestock from tsetse-infested to tsetse-free areas and the recrudescence of parasitaemia in chronically infected animals. The conclusion that normally tsetse-transmitted trypanosomiasis cannot be maintained in the absence of Glossina spp. is supported by data collected from extensive areas of northern Nigeria. In these areas tsetse have been eradicated by the use of persistent insecticide spraying, and by consolidating reclaimed land through intensive human use (Jordan, 1986). In such areas animal trypanosomiasis is no longer a significant problem (Putt et al., 1980).
This paper therefore takes as its baseline the all-important role of tsetse in the maintenance of African animal trypanosomiasis.
Without the fly there is no sustainable disease problem. However, between the extremes of abundant tsetse, presenting what may subjectively be called a "heavy" trypanosomiasis challenge to domestic livestock, and tsetse-free and hence disease-free localities, there are a variety of different patterns of fly and disease distribution. These patterns are not static, either in space or time. Therefore, one objective of this paper is to attempt to demonstrate that the dynamic nature of animal trypanosomiasis is primarily determined by movements and changing densities of populations of the vectors of the disease, particularly of the fly belts of tsetse of the morsitans group. The causes of advances and recessions of the fly belts are not fully understood although man, through effects on tsetse habitats and hosts, has had a major impact in the past and, as human numbers increase, is having an increasingly profound impact today.
The fusca group
Some of the most significant effects of man on tsetse occur in the fusca group. But as, in general, the twelve species comprising this group feed very little on domestic livestock, these effects are usually not reflected in a corresponding effect on animal trypanosomiasis. The reason is that these species are predominantly inhabitants of either tropical forest or forest outliers in the savanna and, as such, rarely come into contact with domestic animals. Most areas of humid primary and secondary forest in Africa are rapidly being felled by man either for timber or for cultivation. This is not a new process-few of Africa's forests are primary in nature-but what is different today is that, whereas in the past there was a long period of fallow permitting the regrowth of forest vegetation between successive short periods of cultivation, now, because of increasing demand for land, the period of fallow is either too short for the reestablishment of forest vegetation or nonexistent as plantation crops, such as coffee and cocoa, replace the natural vegetation.
In densely populated West Africa the once continuous distribution of a number of species of the fusca group has been broken up into isolated pockets. Elsewhere the distribution of other species is also now discontinuous. As this process of disappearance of the fusca group is only of limited academic interest from the animal trypanosomiasis point of view, it will not be further discussed. In the longer term, however, there may be some effect on the epizootiology of the disease, as species of the fusca group can be heavily infected with trypanosomes, and they certainly contribute to the maintenance of the. reservoir of infection in wild animals.
The morsitans group
The effects of man on populations of the morsitans group are dramatic and are profoundly affecting the distribution and abundance of these major vectors of animal trypanosomiasis. Because these species of tsetse occupy the savanna woodlands of Africa (often suitable locations for domestic livestock) and because they will feed readily on cattle, sheep and goats, they form the basis of the African animal trypanosomiasis problem. In some areas they are present in such numbers as to completely exclude domestic livestock; it is in such areas that most cases of the disease occur.
Whereas these often remote areas suitable for the morsitans group (the Grenzwildnisse of Ford, 1971) may be thought of as relatively static, they have in fact experienced a number of profound disturbances even during recorded history. One occurred at the end of the nineteenth century when a major panzootic of rinderpest swept through Africa, killing enormous numbers of the wild and domestic hosts of tsetse. As a result the fly disappeared from extensive areas and many of the "advances" of the fly which have been described since then have not been advances at all but have simply been a recolonizing of lands lost at the time of the rinderpest panzootic. The second major disturbance of the status quo in the fly belts is now in progress. It is more intense in some areas than in others and is related to the numbers of man.
There are many examples from various parts of Africa (Jordan, 1986) which illustrate the relationship between human populations and populations of the morsitans group. The occupation by the fly of a new area can often be related to a decrease in the human population and the evacuation of an area to an increase in the human population. A useful rule of thumb was drawn up by Nash (1948) for G. morsitans submorsitans in West Africa which also applies to other subspecies of G. morsitans elsewhere and probably to other species of the morsitans group as well. Thus these species occur where there are up to 40 people per square mile (015 km-2), are less abundant at population densities of 40-100 per square mile (15-39 km-2) and are absent where the population exceeds 100 per square mile (39 km-2).
The continent-wide trend at the moment and for the foresee-able future is for human populations to increase, especially in the urban areas, but often in rural areas as well. Hence, the overall trend is for the area infested by the morsitans group to decrease, but there are many exceptions to this general conclusion. The main difference is that although human populations are expanding at much the same rate throughout tropical Africa (usually in excess of 3% per annum) this expansion was initiated from very different baselines. In densely populated West Africa, especially Nigeria, and in Rwanda and Burundi, many former fly belts have disappeared or are in the process of disappearing. Elsewhere, in sparsely populated countries there has been negligible impact of man on the fly belts, and large areas in and around them remain unavailable to domestic livestock or can only be used at risk. In Zambia, for instance the overall human population density is only about 8 km-2, and many people are concentrated in the urban areas, compared to some 110 km in Nigeria (Jordan, 1986).
The palpalis group
Whereas demographic changes in sub-Saharan Africa are having increasingly profound effects on the distribution and abundance of the species of the morsitans group, effects on the species of the palpalis group are much less significant. With one exception in West Africa, there is no evidence that there have been major advances or recessions in response to changes in rural human population densities. Thus the distributions of these species are probably much the same today as they were in the early years of recorded history; they are, of course, absent from areas, draining into the Indian Ocean. The single documented exception is the southward extension of the range of G. tachinoides into man-made habitats in West Africa (Baldry, 1966).
The reason for the resilience of the palpalis group in the presence of man is based on their adaptability to change from feeding on the large species of wild animals to the less obvious components of the wild fauna, such as reptiles, as well as to man and his domestic animals.
The species of the palpalis group occupy habitats ranging from humid forests, through dense riparian vegetation in humid savannas, to sparse riparian vegetation in semi-arid savannas.
The effects of man are different over different parts of this range. Towards the drier limits of the distribution of the palpalis group, the agricultural activities of low-density human populations tend not to make major inroads into the vegetation fringing rivers and streams, thus preserving the tsetse habitat. However, as human populations expand, shifting cultivation becomes more permanent with corresponding progressive degradation of the riparian vegetation rendering it less suitable as a habitat for species of the palpalis group. This process is underway in many areas but has probably resulted in only an insignificant reduction in the overall area infested by these species, although reduction in numbers of flies, and hence the "challenge" they present to domestic livestock, has undoubtedly occurred over wide areas of West Africa.
At the humid limits of the distribution of the palpalis group there has also been much modification of the vegetation, a process of change which is accelerating, but here the effects on the flies have been much less marked. These species are able to adapt to man-made habitats and thus, despite more people and changed habitats and hosts, the flies survive-and often thrive. Dense pert-domestic populations of these species have been described from many localities in West, Central and parts of East Africa. They occupy a wide variety of man-made habitats, often at as high a density as in more natural environments elsewhere. Although these species are not as effective vectors of animal trypanosomiasis as are species of the morsitans group, the implications for the persistence of the disease in such circumstances are obvious.
The key role of tsetse flies in the maintenance of African trypanosomiasis in domestic livestock has been confirmed in the past by campaigns which succeeded in eradicating the fly-and subsequently the disease-from well-delimited areas. There have been two types of successful eradication campaigns-either the clearance of fly infestations isolated by extensive natural barriers from other infestations, or fly infestations which were cleared in a logical manner and followed up by human settlement at a density sufficient to remove fly habitats and hosts and prevent re-invasion. Examples of the former are campaigns which eradicated tsetse from the island of Principe (De Azevado et al., 1962) and from an isolated fly belt in Zululand (Du Toit, 1954). The best example of the latter type of campaign is from northern Nigeria where some 200,000 km2 were cleared of tsetse and where much of the former fly-infested land is now settled and unsuitable for the fly (Jordan, 1986).
Other vector-control campaigns have been less successful and it is now appreciated (although not by all!) that it is easy to kill tsetse flies by well-organized operations but difficult-and expensive-to ensure that cleared areas remain fly-free. Although vector eradication-and hence trypanosomiasis eradication-is a feasible objective in a few locations (the major fly belts of Somalia, for instance, are limited to two river systems and isolated from infestations elsewhere), in most situations disease control, in which vector control can have a role to play, is the only realistic option.
The basic problem is the control of animal trypanosomiasis where domestic livestock and relatively static infestations of Glossina encounter one another. In many areas, realistically, nothing can be accomplished in the short-to-medium term, and the traditional livestock owners will continue to act as they have for centuries, balancing losses from trypanosomiasis if they graze their animals in infested areas against losses from starvation if they do not. Attempts to control the disease will continue to rely primarily on drugs, often applied by the livestock owners themselves with little control of dose rates or frequency of treatments. There is also scope for further exploitation of the phenomenon of trypanotolerance, particularly in those vegetation and climatic zones where such breeds occur. The possibility of increasing their utilization in the drier savannas of Africa, where pastoralists still produce most of the livestock products of the continent, is much less likely. However, pastoralists are having to settle in many parts of Africa, as pressure on land increases, and the traditional livestock owner may become increasingly prepared to keep trypanotolerant breeds if they are available, blurring former sharp distinctions between arable farmers and owners of livestock.
Vector control, as distinct from vector eradication, may also have a role to play by reducing the level of trypanosomiasis challenge to livestock which will in turn encourage the development of land use practices involving livestock. Here the critical factor is cost-the continuing suppression of tsetse populations by insecticidal and most other traditional methods is often easy but always expensive. The recent development of insecticide-impregnated, odour-baited traps and targets which attract and kill tsetse offer the prospect of cheaper alternatives, but they are not a simple panacea as competent supervision and management are still essential.
As far as the long term is concerned, it is clear that a major factor, and in many localities the dominant factor, affecting the epizootiology of animal trypanosomiasis will continue to be the rapidly expanding African human population. In some localities this will have the effect of eliminating or reducing tsetse infestations, whereas elsewhere domestic livestock will be forced into closer contact with fly infestations as competition with arable farmers for fly-free land intensifies. Such a scenario has major implications for computations of levels of "challenge"; differences will not only occur from place to place but also, increasingly, "challenge" will vary from time to time. In densely populated parts of Africa, such as Nigeria, it is not difficult to foresee the disappearance of the fusca and morsitans groups of tsetse except in game reserves where their habitats and hosts are protected, but a low level of challenge from the more adaptable species of the palpalis group will persist for the foreseeable future, certainly in the more humid zones.
An understanding of, and even attempts to forecast, changing levels of the challenge presented by tsetse to domestic livestock will be essential in the future, both for improving our knowledge of the epizootiology of the disease and for the design and implementation of effective strategies for its control.
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