A.M. Jordan
INTRODUCTION
In the early decades of the 20th Century, severe epidemics of human trypanosomiasis affected many African countries. The then colonial authorities attempted to combat these epidemics primarily by the development of trypanocidal drugs and their use in surveillance and treatment campaigns, assisted, in some countries, by research on the ecology of vector species and subsequent attempts to control the vector, mainly by removing the vegetation on which it depended for its habitat.
At this time, little attention was given to animal trypanosomiasis but as the epidemics of the human disease receded and as the numbers of man increased, with corresponding demand for increased outputs from domestic livestock, so the animal disease assumed greater importance. Efforts to control animal trypanosomiasis also depended primarily on the use of trypanocidal drugs and on vector control and, particularly in the latter approach, benefitted greatly from the experience gained in the combatting of the epidemics of sleeping sickness. Because of the specialised nature of the techniques employed, the control of the disease became, in many countries, the responsibility of specialised units, usually within the Departments of Veterinary Services, and became a ‘special case’. Was - and, more importantly, is - this distinction justified and desirable? Or is it a quirk of history that should no longer be prepetuated?
THE ROLE OF TRYPANOSOMIASIS
It is generally accepted that animal trypanosomiasis has had a major influence on the history of tropical Africa. Certainly the fly belts of species of the Glossina morsitans group had a profound influence on the routes followed by the tribes who introduced cattle into Africa south of the Sahara from about 5000 BC onwards. During successive migrations, densely infested fly belts had to be avoided as the only way of preventing the fully susceptible, never previously exposed, animals dying from trypanosomiasis. The power of the disease to exclude the keeping of susceptible breeds of cattle and other domestic livestock is still apparent today and many maps have been published showing the distributions of cattle and tsetse flies as being almost exactly the converse of one another. Today, the distinction is not as clear as it used to be, as trypanocidal drugs can permit cattle and tsetse to co-exist. Even today, however, heavily infested fly belts are virtually devoid of cattle.
Although other animal diseases also cause heavy losses of domestic livestock, there is no evidence that any one disease has had the same profound influence on the distribution of livestock in Africa as has trypanosomiasis. East Coast fever, also arthropod-transmitted, has undoubtedly played an important role in African ecology, but this aspect of the disease, and especially its interrelationships with trypanosomiasis, has been relatively little studied. Like other important diseases, such as rinderpest, perhaps its main difference from trypanosomiasis is that it comes and goes and although huge losses occur in epidemics, in other years losses can be minimal. The rinderpest pandemic at the end of the 19th century killed enormous numbers of cattle (and wildlife) but in a few years it had burnt itself out and numbers began to recover. Other diseases, such as helminthiasis, can cause heavy production losses, and are responsible for many sickly populations of domestic animals, but rarely wipe out or exclude them from extensive areas. Many traditional livestock owners in Africa today still see trypanosomiasis as the most significant disease threat to their cattle.
On balance, the evidence would seem to support the view that only trypanosomiasis has had a major effect on the distribution of cattle in Africa, although other diseases may sometimes be directly responsible for more deaths. Perhaps to this extent trypanosomiasis should be considered as different from other diseases, but does this justify a different approach to the control of trypanosomiasis compared with other diseases?
THE IMPLEMENTATION OF CONTROL OF TRYPANOSOMIASIS
1. Trypanotolerant breeds of livestock
Taking advantage of the trypanotolerant trait of certain breeds of domestic livestock is essentially a 'passive' approach to the control of trypanosomiasis, requiring no direct intervention. As such they will not be considered further in the context of this paper. It is, however, noteworthy that although these breeds are primarily considered of importance because of their tolerance of trypanosomiasis, some, such as N'dama cattle, also show increased tolerance towards other diseases, such as helminthiasis, compared to breeds of cattle more susceptible to trypanosomiasis in those ecosystems where they predominate.
2. Chemotherapy
With the exception of Bostswana and Zimbabwe, the use of trypanocidal drugs is the main method of controlling trypanosomiasis in all those countries in which the disease occurs. It can be argued that cattle only occur in many parts of Africa because of the availability of these drugs. Ideally their application should be under the control of qualified veterinary authorities - either through government departments or, increasingly in favour, through private veterinary practices. In fact, this is rarely the case and most trypanocidal drugs are applied by the livestock owners themselves, usually in the absence of definitive diagnosis of the disease. In this respect trypanocidal drugs are no different from other veterinary products which are also administered, when available, by traditional owners of livestock. Sometimes trypanosomiasis is the most important disease present, sometimes it is not.
It is generally accepted that this is an unsatisfactory situation, often resulting in inappropriate drug usage, under-dosing and the consequent development of drug resistance. While agreeing that it is unsatisfactory, might it not be realistic to accept the present system, rather than, as is often the case, pretend that it doesn't exist, and try to build on it by encouraging extension workers to instruct owners in 'correct' treatment regimes, at least until veterinary services, effective even in remote rural areas, can be established? Whatever the present and future scenarios might be, there is no justification for considering the day by day control of trypanosomiasis by the use of drugs as being any different from the day by day control of any other animal disease. Whoever administers trypanocides should also administer treatments of other diseases.
3. Vector control
Although tsetse control has great potential and has aroused strong feelings - both for and against - realistically it must be appreciated that, except in a few countries, it has had minimal impact on animal trypanosomiasis compared to trypanocidal drugs. Emotions have been especially aroused by campaigns aimed at the permanent eradication of the vector over large areas; some of these campaigns have been very successful. The techniques employed have been, and continue to be, highly specialised, with no relevance to the control of any other animal disease, and require specially trained personnel to apply them. Under such circumstances, the application of control of trypanosomiasis should, as in the past, be considered as distinct from the control of other animal diseases.
However, present day thinking is moving away from the concept of large-area eradication of the vector and towards a reduction of disease risk by control of the vector, often over relatively circumscribed areas. It is not the purpose of this paper to discuss the merits or otherwise of this approach but it is appropriate to point out that one implication of this change in emphasis is that vector control - like the use of drugs - must continue for the forseeable future and is no longer a once-only operation as is the case with a (successful) eradication campaign. Does this have implications for the application of control measures? The techniques are still highly specialised, with the present emphasis on use of traps or targets. There is, however, increasing emphasis on participation of local livestock owners in control operations. To set this up requires both initial instruction by specialists and, so the evidence so far suggests, a degree of ongoing support if control activities are to be sustained over more than just an initial period of enthusiasm. Owner participation can be inappropriate, especially when animals move over large distances in search of water and/or grazing. With or without owner-participation, tsetse control is still a specialised activity and requires a separate organisation from the application of control measures against any other animal diseases. It is quite unlike sticking a needle in a cow, which requires essentially the same skills irrespective of the contents of the attached syringe.
THE IMPLICATIONS OF CONTROL OF TRYPANOSOMIASIS
There is abundant evidence that there is a plethora of diseases affecting domestic livestock in Africa and that control of any one of these diseases will not necessarily result in significant improvements in animal health and productivity. Animal trypanosomiasis is no different from other important diseases in this respect and there are, therefore, strong arguments in favour of considering the control of this disease within an overall strategy of animal husbandry, including disease control and the optimisation of nutrition. However, one has to start somewhere and international campaigns to control important diseases, such as rinderpest and trypanosomiasis, require specialised approaches and have inevitably to be approached as special issues.
Where trypanosomiasis seems to differ from all other major diseases (section 1) is in its ability to exclude cattle from extensive areas. It is with respect to the implications for resource conservation and changing patterns of land use in such areas, following successful control of trypanosomiasis, that the strongest case can be made for considering trypanosomiasis as different from other diseases. Whereas this issue is at its most dramatic following successful eradication of the vector over extensive areas - immediately opening up a land resource to domestic livestock - it also has to be addressed in the context of continued ongoing control of the disease, whether by drugs or vector control. Trypanocidal drugs have already opened up enormous areas to cattle, raising all the attendant issues of resource conservation, overstocking and land degradation which are generally discussed in the context of vector eradication. The fact that is a less dramatic process than overnight removal of the vector does not make it any less urgent. Similarly, the fact that these processes might be reversed if control is abandoned is, to some extent, irrelevent. These special issues need to be addressed both by trypanosomiasis experts and by those involved with the preservation and optimum utilisation of natural resources. They need to address the issues not only in the context of present and future organised control campaigns against animal trypanosomiasis but, especially, in the context of widespread but poorly understood control of the disease by the traditional livestock owners of Africa and the pressures on them, primarily as a result of increasing human populations, to open up new lands to their animals.
