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CTA. 1988. Tackling trypanosomiasis. Spore 17. CTA, Wageningen, The Netherlands.
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Despite many years of baffle, the number one' problem facing livestock in Africa is still the indomitable tsetse fly. The limitations on raising livestock imposed by the typanosomiasis that it spreads pose increasing problems for African countries...
Despite many years of baffle, the number one' problem facing livestock in Africa is still the indomitable tsetse fly. The limitations on raising livestock imposed by the typanosomiasis that it spreads pose increasing problems for African countries with growing meat demands. Pluridisciplinary research is now concentrating on eliminating or at least reducing the number of flies and on limiting their effect on animal health through the use of more resistant breeds. For livestock in Africa, life is no picnic. In fact, it is quite the opposite: although the green pastures are tempting, they are also home to the flies that kill. Unfortunately, those areas that are most favourable to livestock -those with lush pastures and abundant rainfall -- are the most infected with tsetse flies. Of the ten million square kilometres that they inhabit, there are seven million km2 of pasture zones that are practically out of bounds for zebra herds. This is a desperate situation for the many countries that must deal with large increases in their population and thus the production of meat. Those countries in the Sahel where livestock raising is traditional can no longer meet the needs of neighbouring countries. The deficit has also been increased by recent droughts and the disappearance of pastures in arid regions. Herders are being continually forced to move their animals south in the search for better grazing areas. But the large Sahelian zebus are threatened by this incursion into tsetse fly territory. Only the small beef bulls from Guinea and a few breeds of sheep and goats are resistant to trypanosomiasis. Losses of meat production attributed to this problem are estimated at FF 57 million per year. How is it that this disease, which has been known for so long, remains more than ever a problem? To understand, this problem, one must be familiar with the different strains of trypanosomiasis that are involved in what is more commonly known as 'sleeping sickness', a disease that also affects humans beings. The largest role is played by the smallest actor, the trypansome parasite that is responsible for the infection. It is a protozoan flagellate that invades the blood, Iymphs and various tissues. There are many species, but three of them are particularly dangerous for livestock in Africa The disease is transmitted to animals through a vector, which is commonly the tsetse fly or flies as there are more than 30 species or sub-species. These humming insects are brown flies that prefer shady and humid areas where the temperature is never too high: savanna thickets, forests or forested paths, and banks of rivers or lakes. It is by biting the animals that the fly injects the trypanosomes that will gradually contaminate them entirely. In its advanced form, 'Nagana' (the name given by some herders for this disease) is mortal to cattle. But even chronic, low-level infections are serious because they can result in high rates of abortion, mortality of young animals, weight loss, reduction of milk production, and a lowering of stamina which makes such animals useless for draught work. But infected animals are not completely innocent victims. Once infected, they constitute trypanosome reservoirs which, in turn, infect the flies that feed on them. It is thus a vicious circle that links the animal to the vector and the vector to the animal. To break it, all fronts must be tackled at once or one of the vectors must be completely eliminated. To get rid of all of the infected animals is impossible because even though wild animals are resistant to trypanosomiasis they still act as hosts. Priority has thus been given for a long time to controlling tsetse fly populations. The emphasis is now on integrated pest management that, according to the variety of situations encountered, includes different techniques that have proven their effectiveness. Pluridisciplinary research involving entomologists, geneticists, wildlife specialists and veterinarians is proving indispensable for developing appropriate methods that take into consideration all factors. Reducing fly populations Tsetse fly traps are being used increasingly as one way to reduce numbers. Different models and screens have been developed by teams at ORSTOM (the French development research agency) and IEMVT/GTZ (the French tropical veterinary medicine institute /and the German development research agency) for use in both West and Central Africa. The oldest is the bi-conical model in blue tissue with black screens, colours that attract tsetse flies. It allows their captors to use the insects for scientific studies or to destroy them if it is used with insecticide, generally a pyrethrum of low toxicity to mammals. Black and blue mono-conic traps have recently been successfully tested; they are more effective in the dry season and are less expensive. Pyramid traps commonly used in East Africa have also been tested with favourable results. Screens are easier to make than traps and can produce the same results. Thousands are now being used in many regions of Africa. They are made from blue cotton or black and blue polyester and cotton, and are fixed or pivoting according to the region and the species of tsetse to catch Not all tsetse flies behave alike, of course, so detailed studies are needed to determine the best equipment to be used in each situation. To increase the effectiveness of these techniques, olfactory products are currently being tested. Octenol, acetone and some phenols made from cattle secretions and excretions seem to be good attractors for savanna species in both Zimbabwe and West Africa. Research is also being done on sex pheromones but field tests have only just begun. The results obtained from these environmentally safe techniques are undeniable. But the installation and periodic recharge of insecticide in the traps pose costly, logistic problems. Theft and damage are also contra-indications to their use in certain regions and countries. Their effectiveness is also limited during the rainy season because the water washes away the insecticide. Such traps are thus often used in conjunction with other techniques, notably to reinforce and prolong the action of ground or aerial application of insecticides, a more expensive technique that is difficult to repeat often. The association of two techniques that has given the most spectacular results is that achieved by CRTA (the Centre for Research on Animal Trypanosomiasis) in Bobo-Dioulasso, Burkina Faso, which is working in collaboration with IEMVT and GTZ. Combined with traps, the release of sterile males is a good 'finishing' touch that enables the near elimination of those females that escape from traps. The objectives of this research centre are directed both towards the vector (the tsetse flies) and the parasites responsible for the disease (trypanosome) through the use of ecologically acceptable methods (sterile males, traps, trypanotolerant species). Two characteristics of the tsetse fly reproduction system facilitate their biological control. The females mate only once or a small number of times at the beginning of their adult lives and conserve in their seminal receptacles the sperm that will fertilize their eggs throughout their entire reproductive life. The male reservoir of sperm is also constituted only once during the pupa stage. It is thus possible to sterilize these males by irradiating them with Gamma rays that disrupt their chromosome cells. By releasing large numbers of sterile males over a long enough period, the reproduction of tsetse flies can be reduced. To implement programmes covering vast areas, CRTA has developed techniques to reproduce tsetse flies in laboratories. Female flies are fed with blood from local abattoirs. The males, separated from the females and anaesthetized, are irradiated before being released in zones where traps installed during the dry season have already reduced the number of flies. In two years, 3,500 km2 of pasture zone have been totally freed of tsetse flies in Burkina Faso. Vigilance must of course be maintained, and isolation barriers established through the use of traps and screens to prevent re-invasion of these areas. After the success of these programmes, similar campaigns are being planned for Ethiopia and northwest Ghana. Nigeria has also been trying for several years to launch a massive tsetse fly control programme. It is likely that CRTA will soon see its activities develop beyond the boundaries of Burkina Faso by making its equipment and expertise available to neighbouring countries joining the battle against trypanosomiasis. Treating trypano In areas where the fight against tsetse flies is less well organized, the use of medicines to treat livestock or to prevent the appearance of the disease is the only solution available. Whether curative or preventative, these trypanocides or trypanopreventers are widely used when tsetse fly populations are not too high and when a limited number of treatments is enough to maintain livestock in good health. But when dealing with high levels of fly populations they become too expensive for herders. Their systematic or improper use can also build resistance as there have not been new trypanocides on the market for more than 25 years. These are also used in some special cases such as the transportation of animals through infested areas, the use of draught animals or for areas in which integrated pest management programmes have not yet been implemented. For these reasons, the use of trypanocides became indispensable in Ethiopia for the settlement of refugees in the eastern part of the country. Other techniques to protect cattle are also being tested, and include a dressing applied to the back of the animal as well as ear-rings impregnated with pesticide that protect their entire body. These methods have already been used against ticks and other flies and thus show promise for the control of tsetse. Baths in pyrethrum insecticides repeated every second week have proven successful in Zimbabwe. Cattle protected in,this way also act as traps for the flies but such techniques must remain low-cost for nomadic herders. The ideal solution, of course, would be to find a vaccine but the trypanosomes have more than one way to deal with an animal's natural defence system. When the coat of antigens that covers the parasite comes into contact with the host animal its provokes the development of antibodies to resist the disease. The problem is that these antigens vary during the different stages of infection and the animal never has the time to develop immunity against all of their forms. This explains why it has been so difficult to come up with a standard vaccine In conjunction with CRTA, the Nairobi-based International Laboratory for Research on Animal Diseases (ILRAD) is currently studying these antigens in order to determine which ones do not vary and thus offer some hope of developing immunity. This research is also trying to find simple yet accurate tests for rapid diagnosis, better methods of evaluating the results of various treatments, and ways of determining the level of trypano infection in fly populations in order to estimate the risks in a given area. All of this work has been facilitated by the development of in vitro reproduction techniques for the three main species of African trvpanosomiasis. Breeding trypanotolerance The other major research direction is the development of trypanotolerant animals. A species of humpless bulls that is native to Africa lives and breeds in tsetse-infested zones along with certain small ruminants and other wildlife. But there are relatively few of these animals numbering about eight million for the entire continent. By rapidly increasing the number of such highly resistant animals, one could introduce livestock operations into tsetse-infested regions that could not otherwise be used. For many years now selection programmes have been started, particularly by CRTA. But although Baoule bulls are known for their trypanotolerance this resistance varies considerably from animal to animal. In the absence of criteria that characterize such resistance in animals, the only solution is to expose herds to high tsetse infestations and then remove those animals that are less resistant. This method takes a long time, and it is also necessary to find genetic markers that can easily be found in animal tissues in order to verify their tolerance level. Such markers have been developed by race and CRTA is making progress on their development for individual animals. The identification of the most resistant animals is the first step in building up their numbers. Much work has already been done to detect with ease when such cows are in heat in order to proceed with artificial insemination. Embryo transfer is the final step of this long procedure. By implanting embryos from resistant parents in the uterus of other cows one can accelerate the reproduction and diffusion of selected animals. The first calves bred in this way, have already been born at CRTA and at ILRAD. Despite this progress, resistance mechanisms are far from being completely understood. Environmental effects, infestation levels and nutritional factors have yet to be studied in depth. Can such mechanisms be reinforced or even induced in zebus? Nothing can be said for sure until the basic research has been done to answer this and the other remaining questions. At the same time as such genetic studies, the Trypanotolerance Group at the International Livestock Centre for Africa (ILCA) is working on the economic and biological aspects of the production of such animals. In addition to their resistance, their performance is increased but their weight does not exceed 300 kg. Attention needs to be given to this problem in order to improve their cost-effectiveness There is thus considerable research activity under way to control this increasing problem for livestock operations in Africa. Instead of waiting for a miraculous 'silver bullet' solution, however, attention should be given to a package of techniques that are more effective and better adapted to dealing with site-specific problems. For even if the eradication of tsetse flies would seem to be the best solution, it is not possible everywhere. It demands large material investment, well-trained personnel and strong political commitment because it takes nearly 20 years for an eradication programme to achieve satisfactory results. But no progress can be neglected and other techniques that are carefully applied enable infestation levels and thus the incidence of this disease to be minimized. It is a daily battle that requires constant and continuing attention to overcome this public enemy number one. BlBLIOGRAPHY Reunion de conseil scientifique international de recherche sur les trypanosomiases et leur controle. CSIRTC/OAU/CSTR: ses et leur controle. CSIRTC/OAU/CSTR: 18th meeting, Harare. Zimbabwe, March 1985; 20th meeting, Lome, Togo, November 1987
- CTA Spore (English)