Human and agricultural resources
The impact of the tsetse
Conclusions
Bibliography
J.Slingenbergh
The author is Animal Health Officer (Tsetse Control),
Animal Production and Health Division, FAO, Rome
In Ethiopia the rural agricultural sector makes up 85 percent of the total population of 49 million (FAO, 1991a) and accounts for 95 percent of all crop and livestock production. With an agricultural GDP growth averaging 1.1 percent over the last decade, the rural sector has not kept up with the country's population growth of 2.9 percent. This human population factor hinders the growth of agricultural output while the increase in land pressure leads more and more to the use of exploitative farming practices and, consequently, results in land degradation.
Ninety-two percent of the human population and 78 percent of the livestock population are concentrated in the highlands, which constitute 55 percent of the total land area. It is here that ecosystems are most fragile. Furthermore, part of the northern highlands is subject to recurrent drought conditions which cause acute food shortages. The combined effect of demographic changes and depletion of the resource base results in a growing imbalance between the amount of arable land available and that required. These factors are believed to affect the food security of the entire nation.
Farming systems
The predominant farming system in the highlands is characterized by mixed livestock and crop production, with livestock playing a vital role in agricultural activities. Animal draught power is widely applied in crop production and the availability of oxen is essential for the ploughing of heavy vertisols. However, high stock levels are also associated with the deterioration of soil and vegetation. While major efforts have already been initiated to enhance conservation based on sustainable agriculture and to simultaneously improve productivity rates, the pressures on land continue to increase.
Land suitability studies carried out in areas of low population density in southeastern, southern, southwestern and western Ethiopia revealed that the tsetse-infested lowlands in the western part of the country have the best potential for expanded agriculture, provided that the tsetse/trypanosomiasis constraint can be overcome (FAO, 1987).
In addition to preventing the development of animal agriculture in these lowland areas, trypanosomiasis is a direct problem in the uplands where an increasing number of livestock are being forced into contact with tsetse flies. With the gradual saturation of the non-infested highlands, pressures on the tsetse-infested resources have increased steadily, as has the intensity of contact between the fly and animals. While livestock is pushed into contact with the tsetse, distribution of the fly itself is also dynamic, and farming areas at higher altitudes are being progressively invaded by tsetses.
This phenomenon can be understood better when taking into account the specific topography of the western and southwestern parts of the country where long, often steep-sided tsetse-infested valleys penetrate into the highland plateau. Fly progress has been recorded in the Rift Valley and in the Omo and Abai River systems (Balls and Bergeon, 1966; 1970; Hutchinson, 1971; McConnel and Baker, 1974; Langridge, 1974; 1976; Fuller, 1978; Chater, 1982; FAO, 1984).
Although fly advances still occur at higher altitudes, they are curtailed by the lower temperatures. Pupal development becomes prolonged, emerging flies have less fat reserves and fly mobility is restricted. Highlands above 2 000 m are generally free of tsetse flies. Below the 2 000 m contour, some six million head of cattle are exposed to infective fly bites (FAO, 1991b). Efforts to contain the disease are still largely based on the use of trypanocidal drugs but vector control is gaining importance. Odour-baited attractant devices have recently been tested in the Didessa Valley, a tributary of the Blue Nile, and have proven to be an effective tool for the control of two different tsetse species: Glossina morsitans submorsitans and G. tachinoides.
The Didessa Valley
Aerial photos of the Upper Didessa Valley, taken during the early 1970s before the tsetse invasion, show that habitation previously extended down to the valley floor at 1 400 m. The land-use pattern was related to topography and relief, and cropping areas in the valley were mostly confined to the slightly undulating terrain toward the valley walls. The intensity of land use increased with altitude. Coffee plantations were found in the uplands in patches of dense evergreen forest. The vegetation on the valley floor consisted mainly of scrub-woodland and open grassland used for extensive grazing and fuelwood collection. Intact gallery forest occurred along the main river and principal tributaries.
Prior to the tsetse invasion, the farming system comprised rain-fed cropping of maize, teff and sorghum as well as limited amounts of wheat, barley and pulses. Around the homes, enact, coffee, vegetables and spices were cultivated. Livestock provided draught power, milk, skins and manure for the kitchen garden.
The tsetse invasion and subsequent retreat of the population from the Upper Didessa Valley began during the 1970s (USAID, 1976). The dramatic impact on the farming and other land-use systems can be summarized as follows:
· almost complete loss of all livestock types;· contraction of cropped area owing to the loss of work oxen;
· shift in the crop mix from high-value teff to maize, with production of the latter often based on hand cultivation;
· increased reliance of the homestead garden for food supplies;
· dramatic lowering of nutritional status and farm income;
· abandonment of homesteads in many localities;
· re-invasion of scrub-woodland into the old cultivation areas, thus providing increased land cover suitable for the tsetse.
A detailed assessment of the effect of tsetse advances on livestock and crop production was carried out in Chelo, near the upstream limit of tsetse distribution in the Didessa River system (FAO, 1988). Before the fly invasion, average cereal production per household was 1.6 ha with the use of 2.2 oxen. Following the invasion by G. m. submorsitans, the farmers lost 80 percent of their livestock. A first survey, carried out in 1986, revealed that 60 percent of the cattle were positive for trypanosomiasis. The disease had affected each and every household. Equines appeared most susceptible and had died first, followed by cattle. Small ruminants were also disposed of as farmers tried to replace their draught oxen by selling whatever stock remained. By 1986, cereal production had declined to 0.7 ha per family, using 0.8 oxen. The area was no longer self-sufficient in food production and an increasing number of people migrated to the highland shoulder. Because of the risk of the tsetse spreading further up the Didessa River, and given the evidence that flies were spreading across the watershed into the adjacent drainage system, the situation called for operations to prevent further tsetse advances.
Tsetse ecology and trypanosomiasis epidemiology
Two species of Glossina occur in the Upper Didessa Valley: G. m. submorsitans and G. tachinoides. G. m. submorsitans occupies the woodland on the floor and sides of the valley and only during the short dry season, after the bushfires, does this species seek the shelter of vegetation on the drainage lines. G. tachinoides, on the contrary, is found in the dense vegetation of the main river and rarely disperses far into the tributaries or drainage lines where the evergreen vegetation is less dense.
In epidemiological terms, the more widespread G. m. submorsitans is of greater importance than G. tachinoides. The advance of G. m. submorsitans resulted in an exponential increase in the annual trypanosomiasis incidence in livestock even at the 1 900 to 2 000 m range (see Fig.). The period between disease transmission and diagnosis is approximately one month, and monthly disease incidence was found to be strongly correlated with the mean daytime temperatures of the preceding month. This confirms that abundance of the vector is regulated by temperature. From the valley floor at 1 400 m as far as the top of the escarpment at 2 000 m, the fly density decreases over two orders of magnitude, with no flies detectable on the highland shoulder. Analysis of the daily activity pattern revealed that, with increasing altitude, fly activity becomes more and more restricted to the afternoon hours. At 1 700 m and above, flies were seen to be active only during the late afternoon.
1 Odour attractants
Appâts odorants
Sustancias olorosas de atracción
|
Fly/trap |
Odours |
Index1 |
|
Glossina morsitans submorsitans |
||
|
Biconical |
Urine 1.3 - 2.0 |
|
|
Octenol |
1.6 - 3.0 |
|
|
Acetone |
1.8 - 2.6 |
|
|
Urine + octenol |
4.1 - 4.4 |
|
|
Urine + acetone |
2.4 |
|
|
Acetone + octenol |
2.9 |
|
|
Acetone + urine + octenol |
4.4 - 10.0 |
|
|
Monoconical |
Acetone + urine + octenol |
8.8 |
|
Glossina tachinoides |
||
|
Biconical
|
Urine |
1.2-1.6 |
|
Octenol |
1.3 |
|
|
Acetone |
1.0 - 1.1 |
|
|
Urine + octenol |
2.0 - 2.1 |
|
|
Acetone + urine + octenol |
1.6 - 2.0 |
|
|
Monoconical |
Acetone + urine + octenol |
1.4 |
1 Index = the increase in trap catch by a particular treatment compared with the catch of untreated control traps.
2 Glossina morsitans submorsitans catches on an electric net surrounding various odour-baited objects
Captures de Glossina morsitans submorsitans sur un filet électrique entourant divers objets servant d'appât odorant
Captura de Glossina morsitans submorsitans en una red eléctrica que rodea cebos de olor
|
Object |
Mean catch |
|
Monoconical target |
488 |
|
Tree-trunk (20 cm diameter) |
108 |
|
Tree-trunk + blue band |
347 |
3 Glossina morsitans submorsitans catches in a biconical trap and electrified targets - Captures de Glossina morsitans submorsitans dans un piège biconique et sur des cibles électrifiées - Captura de Glossina morsitans submorsitans en una trampa bicónica o en blancos electrificados
|
Design |
Size |
Colour |
Mean catch |
|
Biconical trap |
Standard |
Standard |
57 |
|
Target |
1 m2 |
Blue |
201 |
|
Target |
1 m2 |
Blue/black |
160 |
|
Target |
1 m2 |
Black |
86 |
Testing bait systems
In order to identify appropriate and cost-effective bait systems for the control of G. m. submorsitans and G. tachinoides under prevailing conditions in the Didessa Valley, a series of trials were carried out to test different visual and olfactory attractants. The experiments were done using a Latin square design with a minimum of two replicates during both wet and dry seasons from mid- 1986 to mid-1989.
Table 1 indicates that, for G. m. submorsitans, a combination of octenol (20 to 80 mg/day), acetone (20 to 40 g/day) and cow urine (40 to 80 g/day) obtained the highest catches. G. tachinoides was much less responsive to odours in general and acetone appeared to have no effect on attraction of this species. All three odours were therefore used in control operations against G. m. submorsitans but only octenol and cow urine in the control of G. tachinoides. Monoconical traps were found to be superior to biconical traps and, as the former are also cheaper and easier to deploy, they are routinely used in surveys and control activities for both species. Other experiments in the area indicated, unlike findings from other countries, that increased doses of octenol result in an increased catch.
Comparison of different types of targets, all baited with acetone (500 mg/hour), octenol (50 mg/hour) and urine (300 mg/hour) indicated no difference in the mean catch of G. m. submorsitans on:
· a monoconical target (613 flies caught on a surrounding electric net);
· a standard black Zimbabwe target with flanking mosquito netting (601 flies);
· a target of similar size but with the mosquito netting replaced by blue cloth (623 flies).
In further experiments with the same doses of odour attractants, a comparison of monoconical targets was made with catches on tree-trunks with and without a 1 m band of blue cloth wrapped around the tree-trunk at 0.3 to 1.3 m above ground level (see Table 2).
Many flies landed on the tree-trunk but even more landed when the band of blue cloth was present, although the catch was less than that of the monoconical target.
In a different experiment without odour attractants, the standard biconical trap was shown to catch fewer flies than an electric net surrounding a blue, blue/black or black 1 m2 target. The targets caught more flies as the amount of blue increased (see Table 3).
From these results, it was concluded that the most cost-effective and simple device for the control of G. m. submorsitans in the Didessa Valley was the odour-baited, insecticide-impregnated, predominantly blue cloth suspended from trees or wrapped around trunks. For the control of G. tachinoides it was decided to use odour-baited, insecticide-impregnated monopyramidal targets.
Community-based tsetse control schemes
Tsetse control operations were carried out in four different ecological situations, varying from densely infested woodland on the valley floor to agriculturally productive higher-altitude areas where flies could hardly be detected.
Common to all control operations was the active participation of the farming communities. They assisted in the survey and control activities and were chiefly responsible for the servicing of the bait devices or targets. This included replacement of odours as necessary, monthly respraying of the targets with 100 mg active ingredient (a.i.) deltamethrin and clearing of vegetation around the targets. When targets were first positioned, each team of about ten farmers was guided and supervised by two tsetse control technicians. Progress was monitored with monoconical traps and by the technical staff's examination of local cattle herds for trypanosomiasis.
The type of bait systems used for G. m. submorsitans were modified in view of the results of the bait trials described above but they always included acetone, octenol and cow urine as odour attractants. The placement pattern and density of the bait systems or targets varies according to the type of vegetation. Most targets are positioned in the woodland, along the drainage lines and at the foot of the hills at a density of three to ten per km2. The monopyramidal targets used against the riverine G. tachinoides are placed on both sides of the river at intervals of about 100 m. These targets have to be removed during the wet season when flooding occurs.
Chelo
Operations in Chelo began at the end of 1986 when 100 odour-baited targets were located in an area of 30 km2, comprising four villages on the floor of the Didessa Valley at the upstream limit of G. m. submorsitans distribution. Prior to the control activities, more than 25 flies/trap/day were caught but the targets caused a rapid decline of the population to below the detectable level within four months after the operation had begun.
A further 150 km2 were cleared in 1987. Protection from reinvasion was given by the surrounding highlands and a barrier target zone of 3 km in length at the downstream end of the control area. Some additional protection came from the expansion of croplands resulting from the introduction of draught oxen for which farmers received financial assistance through an oxen credit scheme. The barrier was effective and, in the following year (1988), operations were extended and a new barrier zone created downstream.
During the first three years of operations some 450 km2 of land had been cleared to the benefit of 17 villages. Villages assisted by the oxen credit scheme doubled their crop production within three years after the start of tsetse control operations.
Limu Shay
Limu Shay is opposite Chelo on the other side of the Didessa River. Operations in the two locations are considered part of the same programme, aimed at the progressive elimination of tsetse from their furthest limit in the Upper Didessa Valley. The two areas are not comparable, however, as there is much suitable tsetse habitat in the Chelo area, which has high densities of G. m. submorsitans. On the other hand, the most interesting feature of the Limu Shay area is that, before commencement of control operations, there was generally a very low tsetse density, with catches usually of less than one fly/trap/day, while the prevalence of trypanosomiasis in cattle was as high as 60 percent in some villages.
The mosaic of cultivated land, coffee bushes, much open grassland and relatively small areas of woodland is a common feature at these altitudes of the Didessa Valley and clearly provides an adequate environment for sufficient G. m. submorsitans to cause a major trypanosomiasis problem. Control operations began at the end of 1988 when some 600 targets were sited over an area of about 150 km2. A barrier zone of about 25 targets per km: over a distance of 2 km has been constructed between the cold highlands and the Didessa River. The only flies that are still occasionally caught are in the barrier zone. About 12 villages have benefited from the operation.
Photo/Foto: Andreas Depping
Photo/Foto: Andreas Depping
Photo/Foto: Andreas Depping
Photo/Foto: Andreas Depping
Bedelle
Operations in Bedelle were also directed against G. m. submorsitans but the objectives were quite different from those at Chelo and Limu Shay. No attempt to achieve local eradication is being made and the aim is to reduce the prevalence of trypanosomiasis along the top of the escarpment where the tsetse have advanced from about 1 600 to 2 000 m in recent years. The objective is to eliminate any locally established fly populations and reduce the continual invasions by flies from heavily infested regions lower down the escarpment and on the valley floor. A total of 185 monoconical targets have been located along 20 km of the escarpment at an altitude of about 1 800 to 1 900 m. Prior to location of the targets, fly numbers were low, the maximum catch being four flies/trap/day. However, trypanosomiasis was a serious problem and sentinel cattle became infected almost once a year. Suppression of the vector resulted in an 85 percent reduction of the disease's incidence. It is estimated that about 30 villages may have benefited from this operation.
Dembi-Toba
Unlike the other control trials, this operation is aimed exclusively at G. tachinoides. Prior to the control operation, odour-baited biconical traps located close to the river caught an average of 32 flies/trap/day during the dry season. The gallery forest fringing the Didessa River and its tributaries is usually less than 100 m wide but can broaden around confluences, where the highest catches of G. tachinoides were made. Disease transmission probably occurs mainly at watering points.
Prior to control operations, 11 percent of cattle in the area were infected. Control began at the upper limit of the fly territory, at about 1 600 m, and now covers 10 to 15 km of the main river, with further treatment of major tributaries every year. Targets are removed during periods of heavy rain. About ten villages have benefited from the operation.
Photo/Foto: Andreas Depping
Photo/Foto: Andreas Depping
The control operations undertaken by the Ethiopian Government, with assistance from FAO and the United Nations Development Programme (UNDP), have demonstrated the effectiveness of odour-baited and insecticide-treated traps and targets for the control and eradication of tsetse flies under Ethiopian conditions. Implementation can be effectively supported by farmers working on a self-help basis under the direct supervision of tsetse control staff. Current operations in the Upper Didessa Valley are still small-scale and the area will be vulnerable to reinvasion from tsetse populations lower down the river unless action is consolidated by a progressive control campaign aimed at eliminating flies from the whole of the valley. Trypanosomiasis-related problems in populated and cultivated areas have reached levels requiring a wider and longer-term approach. In view of the favourable topography of the western escarpment, it is possible to draw up a programme for other valleys as well. In addition to eradicating flies from the valleys, attempts should also be made to suppress flies near the cold limit in upland areas. Finally, adjustments could also be made to the grazing regime so as to avoid fly contact in those areas where temperature stress restricts fly mobility to the late afternoon hours.
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