ANALYSIS OF THE EFFICACY, PRACTICALITY AND COST
OF VARIOUS TSETSE AND TRYPANOSOMIASIS CONTROL TECHNIQUES
AS USED IN ZIMBABWE

V. Chadenga¹

INTRODUCTION

The Branch of Tsetse and Trypanosomiasis Control (TTCB) in Zimbabwe's Department of Veterinary Services is tasked with organizing and implementing the permanent control of trypanosomiasis, a disease transmitted by tsetse flies. Zimbabwe has an area of 390, 757 square kilometres of which almost half the country, some 180,000 square kilometres, can be occupied by tsetse flies if control measures are not undertaken and tsetse flies spread to their ecological limits (Fig I). Such a situation can put at risk a third of the national cattle herd, denying draught power and other benefits to a significant proportion of the country's population.

According to the agroecological survey of Zimbabwe by Vincent and Thomas (1962) about 240,000 square kilometres is not suited for growing crops due to erratic rainfall and poor retentive properties of the soils. The recommended land use in such areas is extensive livestock production, sometimes with production of drought-tolerant small grains. This being the case, the presence of tsetse becomes a major constraint to optimal exploitation of these areas.

The Government of Zimbabwe spends large sums of money on resettlement programmes to address the imbalance in land distribution, which was in existence for a long time. However, the presence of tsetse flies and the disease trypanosomiasis seriously affect Government efforts to make resettled farmers engage in productive and profitable farming. In view of this fact, the Branch's major objective of progressive tsetse eradication will have the effect of eliminating a major constraint on land use and enable farmers to optimise the use of a natural resource base within a policy framework in which the disease trypanosomiasis is not a major determinant.

Tsetse and trypanosomiasis control in Zimbabwe is characterised by remarkable achievements interrupted in the 70's by war which rendered control operations impossible.

¹ Assistant Director
Tsetse and trypanosomiasis Control Branch
Department of Veterinary Services
P.O. Box 8283
Causeway, Harare
Zimbabwe

Fig 1 : CURRENT TSETSE INFESTED AREAS IN ZIMBABWE, 1991

The History of Tsetse and Trypanosomiasis Control in Zimbabwe

In Zimbabwe, the emphasis is on vector control. Early attempts to control tsetse flies involved bush clearing and elimination of wild life (Lovemore, 1960).

At the end of the last century, the rinderpest pandemic which entered Zimbabwe in 1896 devastated domestic livestock and cloven-hoofed game populations. The destruction of so much game and livestock had the effect of eradicating tsetse flies from much of the country save for a few isolated pockets mainly along the Zambezi Valley (Map I).

By 1900, game populations showed signs of recovery and so did tsetse flies. Tsetse infestation increased and hunting operations were undertaken to deny tsetse flies their blood meal. It had been demonstrated by Jack (1933) that intensive hunting could eliminate Glossina morsitans, and Chorley applied this method along a wide front of the Zambezi Valley. Once the efficacy of game destruction as a method of tsetse and trypanosomiasis control was clearly shown, the method was formally adopted by Government in 1933.

This method was used with some success, but it was clear that game destruction could not achieve permanent results. Furthermore public opinion became vehemently opposed to this method of tsetse control to a point where Government was forced to appoint a Commission of Enquiry with a brief to identify a new organisation which would develop techniques of tsetse control less embarrassing than the destruction of game (Thomas et al, 1955). The Commission decided as an interim measure, setting up of game free, cattle free defence corridors, i.e. a modified form of hunting, controlled and confined to specific corridors. However, the defence corridor was not 100% effective, as G. morsitans were being recorded in areas supposedly tsetse free. In addition, it became clear that in spite of intensive hunting some animals were not eliminated (Lovemore, 1963; Child & Wilson 1964).

Supplementary tsetse control measures had to be introduced and immediately applied. During the period 1955 to 1960 bush clearing, aimed at denying refuge sites to tsetse, was also adopted as a strategy for dealing with the tsetse menace. The other methods were application of residual insecticides from the ground, and aerial spraying (which had been tried and found ineffective between 1953 and 1954). While accepting that game destruction, bush clearing and ground spraying have obvious disruptive effects on the environment, the authorities of the time decided to continue using these strategies in view of the deteriorating trypanosomiasis situation, and in the absence of any other equally effective technique.

However, by 1963, Government had banned game destruction and bush clearing. Instead, ground spraying was to be the major weapon for tsetse control and remained so for almost twenty years.

Tsetse and Trypanosomiasis Control in Zimbabwe is currently achieved through the following strategies:

1. Ground spraying
2. Aerial spraying
3. Odour-baited and insecticide-treated targets
4. Cattle dipping
5. Trypanocidal drugs.

The merits of each of the above techniques are discussed in turn and Fig II shows the extent to which the different techniques were used in Zimbabwe during the period 1983 – 90.

1) Ground Spraying

The use of persistent insecticide applied from the ground has a long history in Zimbabwe's tsetse control operations. The technique is based on the knowledge that tsetse flies spend a greater part of their life resting in cool, shady places provided by trees, holes, etc. and directing a persistent insecticide at these sites should achieve a good control measure. In Zimbabwe, the insecticide of choice for this purpose has been the chlorinated hydrocarbon DDT, used as a 75% wettable powder which is applied discriminately to essential habitat of tsetse flies. Reports by Cockbill (1967) and Robertson and Kluge (1968) indicate good progress in arresting the advance of G. morsitans orientalis in the south-east of the country as well as in joint operations with South Africa and Mozambique.

The amounts of DDT used for control operations during the period 1968 to 1990 are given in Table 1.

Sufficient information was gathered over the years to conclude that the insecticide DDT is persistent, relatively cheap, effective and the workforce find it easy to use. In a typical ground spraying operation in Zimbabwe, teams of operators using knapsack spraying machines dispense a four percent aqueous solution of DDT to the point of run off, to the resting and refuge sites known to be favoured by the fly. The amount of active ingredient over the entire treatment area ranges between 20 – 25 kg per square kilometre.

Large areas of Zimbabwe have been cleared of tsetse using DDT, with the most spectacular success being operations conducted in the south-eastern region of the country.

While Zimbabwe has achieved significant successes using DDT for tsetse control, it is clear that the technique has its limitations. When using ground spraying alone, it is difficult to consolidate areas that are cleared of tsetse flies, and this means that repeat spray applications are inevitable. This becomes expensive, as well as being a cause for environmental concern. In addition, ground spraying places heavy demands on transport labour and strict operational supervision.

In view of these facts, the Branch is de-emphasising, but not banning use of DDT for tsetse control operations. This decision is based on the knowledge that DDT is a relatively cheap insecticide with low mammalian toxicity and the persistence appropriate for tsetse control work. In addition to this, work undertaken in Zimbabwe by scientists from the Natural Resources Institute (NRI) in the UK concerning the environmental effects of DDT used to control tsetse flies, gave the Branch confidence to retain ground spraying as one of the many strategies for controlling tsetse. Generally, the report concluded that there was no evidence of permanent and substantial ecological damage arising from DDT use for ground spraying. The effect of DDT on non-target organisms were found to be transient and moreover, no adverse effects of DDT on vertebrate populations in the tropics had been demonstrated. Besides, DDT and its residues are known to disappear more quickly in the tropics than in temperate regions (Douthwaite, 1990).

Fig. 2 GRAPH OF VARIOUS CONTROL TECHNIQUES 1983 – 1990

There have been efforts to introduce other insecticides for ground spraying, such as the synthetic pyrethroid deltamethrin. In the most recent trial, 20% Deltamethrin suspension concentrate was diluted at the rate of 1 litre plus 399 litres of water giving a spray liquid of 0.05% deltamethrin. The results were good, but at between Z$1,150 – $1,300 per square kilometre, use of deltamethrin for ground spraying is more expensive than either ground spraying with DDT or the use of targets.

Table 1: Quantities of DDT used in ground spraying operations in Zimbabwe, 1968 to 1990

Source: Annual reports of the Tsetse and Trypanosomiasis Control Branch.

Aerial Spraying

Basically, aerial spraying is the sequential ultra low volume (ULV) aerial application of an insecticide. In Zimbabwe, endosulfan at 20 – 24 g/ha was used to treat large tsetse infestations. Deltamethrin (Glossinex: Cooper Zimbabwe) at 0.25 g/ha was also used with limited success. Such an application of low dosages of endosulphan and deltamethrin make aerial spraying highly specific for tsetse, with little effect on non target organisms.

Work conducted in Zimbabwe, and described by Hursey and Allsopp (1983, 1984) demonstrated that aerial spraying is a feasible and rapid technique for dealing with emergency situation, as well as treating large areas with less manpower than required in ground spraying operations over comparable areas.

However the 1988 aerial spraying operation proved that the technique is poorly effective in hilly terrain and cannot guarantee eradication. While aerial spraying can achieve good control, the operation requires a high level of technical sophistication, strict timing of repeat sprays and areas sprayed are open to reinvasion and therefore require protection. In addition to this, there is the added public concern about blanket spraying of insecticides into the environment.

Notwithstanding that aerial spraying has been used successfully in Zimbabwe, the technique is expensive in comparison with alternative techniques and is not recommended for routine use. More appropriate circumstances for aerial spraying might include dealing with a widespread disease situation where an immediate break in the transmission of the disease is being sought.

Odour Baited and Insecticide Treated Targets

Various trials undertaken in Zimbabwe have shown that tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen can be controlled by attracting them to visual targets that are baited with odour attractants and coated with insecticide. The first trial was on an island of 5 square kilometres on Lake Kariba (Vale et al, 1986) followed by a large trial on 600 square kilometres in the Zambezi Valley near Chirundu (Vale et al, 1988). The results from these trials suggested that targets could be used for large scale campaigns and they offer several advantages over conventional methods of control.

Since the trial in Chirundu, targets have been used at deployment densities of 4 per square kilometre in large scale campaigns throughout Zimbabwe for the control of G. morsitans. The environmental impact of the technique is low as the effect on non target organisms is minimal (Vale et al 1988). There is the added advantage that problems of reinvasion are minimised since tsetse can only invade a few kilometres into the target areas. While producing a slow steady decline in tsetse densities, targets have the major advantage of working all year round. They achieve eradication within 12 – 18 months at costs slightly more than ground spraying in the case of G. morsitans operations. It appears that targets can eradicate G. pallidipes at lower deployment densities than those required for G. morsitans, and in this situation tsetse control with targets is cheaper than any other insecticidal method of control.

Large-scale target operations using targets at 4 per square kilometre eradicated mixed G. morsitans and G. pallidipes populations from Umfurudzi in the north-east of Zimbabwe and have proved technically effective in various areas south of Lake Kariba.

Research on minimum number of targets required to achieve eradication is being undertaken in an EEC-funded target trail taking place in Mashonaland Central Province in the north of Zimbabwe.

The critical density of targets required for eradication G. pallidipes in a short time is not less than 4 per square kilometre. For G. morsitans the findings suggest that even 4 targets per square kilometre produce an inadequate rate of population decline and there is no detectable effect on G. morsitans under 4 targets per square kilometre.

Currently 54,000 targets are deployed throughout tsetse infested areas in Zimbabwe and tsetse populations are showing expected declines.

Table 2: Effects of target densities on G. pallidipes and G. morsitans (Turner, D.A. 1990)

Cattle Dipping

As a result of experiments at Rekomitjie Research Station and a large field trial at Nyanga in the estern region of Zimbabwe, it was demonstrated that cattle dipped regularly in a suspension concentrate of deltamethrin act as highly effective and lethal baits for tsetse flies, achieving a high level of trypanosomiasis control (Table 3).

Table 4: Comparative data on application rates and costs of applying “Decatix” and “Spot On”

The application of insecticide to cattle is not a new idea having been used in the 1940's (Vanderplank 1947, Whiteside 1949). The technique has improved since then with trails in Zimbabwe using persistent synthetic pyrethroids. The insecticide is administered to cattle as either a dip wash of deltamethrin (Decatix, Wellcome Zimbabwe) or pour-on formulation (Spot On, Wellcome Zimbabwe). The dip wash, deltamethrin 5 % a.i. suspension concentrate, has a target strength of 0,00375 % while Spot On is a 1% deltamethrin pour on formulation applied at the rate of 1ml\10 kg body mass.

Table 3 THE INCIDENCE OF TRYPANOSOMIASIS IN THE TRAIL AREA FROM MAY, 1983 TO JUNE, 1987

Fig 3 : COMMUNAL AREAS UNDER DECATIX AND SPOTON IN ZIMBABWE 1991

The pour on which is applied monthly is less subject to error than dipping. It is less wasteful of chemical than dipping where small numbers of animals are to be treated at any one location.

Ongoing tsetse surveys and trypanosomiasis surveillance data confirm cattle dipping as a viable strategy where cattle exist in reasonable numbers.

the technique requires minimum infrastructure and in areas where dips are not available, pour on formulations have been substituted, with good results. Cattle dipping and pour ons are now actively and widely used to enhance other tsetse control strategies throughout Zimbabwe. Currently 192,000 cattle are under the programme spread over an area 28,300 square kilometres of tsetse infested territory (Fig. 3). Cattle dipping and pour on as used in Zimbabwe, has proved flexible and effective.

Given the relatively low cost of applying the technique, cattle dipping including pour on, is by far the cheapest technique available where cattle are already routinely and regularly assembled for veterinary inspection and other services.

Table 5: Incidence of Bovine Trypanosomiasis based on Parasitological Examination of Blood Smears during the period May, 1990 to March, 1991 in Zimbabwe.

Trypanocidal Drugs

While Zimbabwe emphasizes vector control as the only permanent strategy for eradicating trypanosomiasis, infected cattle presented at dip centres are routinely given the curative drug diminazene aceturate. In the period May 1990 to April 1991 only 1,657 inoculations were administered compared to 250,000 cattle considered at risk in tsetse infested areas.

This is further evidence that cattle residing in tsetse infested areas in Zimbabwe, are under a low trypanosomiasis challenge. This is also further testimony of the success of vector control programmes.

Chemoprophylaxis through isometamidium chloride offers protection for as long as five months (Connor, 1990) and in Zimbabwe the drug was used on a large scale in the North-Eastern Region in the early 1980's following a period when tsetse control operations had collapsed. The high challenge prevailing at the time, demanded protection of cattle until such time that a control campaign could be instituted. The drug was administered at 0,5 mg/kg body weight but currently no animals are under prophylactic protection in Zimbabwe.

The success and emphasis on vector control programmes is such that chemoprophylaxis and chemotherapy are marginalised as strategies for combating trypanosomiasis. The annual voted expenditure for the Branch is Z$13,000,000 of which Z$10,000 is reserved for purchase of drugs. On the other hand, the amount reserved for tsetse eradication programmes is Z$5,000,000.

The provision for drugs is unlikely to be exceeded, given the fact that cattle in tsetse infested areas have been placed under deltamethrin dip and pour-on treatment, further reducing tsetse challenge. However, drugs will remain part of tsetse control armoury to be used as and when required. The use of so many different techniques has been successful to a point where recorded cases of trypanosomiasis were less than 500 during the period May 1990 to May 1991 compared to 25,000 cases in 1981/82.

Cost Analysis of Various Tsetse Control Techniques

Given the range of tactical options available for tsetse control, the Government of Zimbabwe sought under British bilateral aid funding an agricultural economist, whose brief is to undertake a detailed study of financial, economic and social aspects of tsetse control in Zimbabwe. The objective was to examine the justification for the present level of expenditure on tsetse control, and to evaluate and compare the costs of alternative techniques which might be used. Such an analysis would serve the important purpose of providing an improved basis for decision making in the allocation of resources. Mr John Barrett has been seconded to the Branch from the Natural Resources Institute in the UK since 1987, and the comparative costs given in Table 6 and 7 are based on his work (Allsopp and Barrett, 1988; Barrett, 1989a; Barrett, in preparation).

Direct costs relate to insecticide, targets and baits, flying time, staff and transport. Indirect costs cover construction of access roads and airstrips as well as approximate administrative over heads.

Ground spraying and target deployment operations are more expensive in broken terrain where elaborate access is required and productivity per team is reduced.

As shown in the Table 6, costs of various tsetse control techniques vary according to strategies used and the type of terrain as well as productivity of spray or target deployment teams.

Table 6: Summary of Costs of Different Approaches to Tsetse Control

Case A: Flat terrain with relatively easy access

Case B: Rugged terrain with difficult access

Ground spraying with deltamethrin and aerial spraying are the most expensive techniques available and their use in tsetse control campaigns cannot be justified in economic terms where the cheaper techniques are feasible. On the other hand, targets compare favourably with ground spraying with DDT and are cheaper than aerial spraying.

The cheapest technique is the application of insecticides to cattle, either as dip wash or pour on. However, the strategy relies on availability of cattle in reasonable numbers and production of cattle at inspection centres for treatment. Once operations move out of cattle holding areas the technique cannot be applied.

While the expense of target technology compares favourably with other strategies, it is still costly with major items of expense being transport, manpower and the need to revisit targets for purposes of re-treatment, etc. However, unlike ground or aerial spraying, target technology has the potential of being cheaper and current research work in Zimbabwe is focussing on applying a higher initial dose of pesticide say 0.4% a.i. of glossinex, 20% a.i. (Cooper, Zimbabwe; see Fig 4). This means the current target treatments per year will be reduced to one or two, saving Z$11,46 per year in manpower and vehicle costs, and this translates to a net saving of Z$8,46 per year per target. Given that 54,546 targets are currently deployed in Zimbabwe, the total annual saving would be Z$461,459 (Shereni 1990) FAO Report. Table 7 contrasts the costs of different insecticide treatment regimes with number of visits. The other way of making targets cheaper is by identifying more attractive odours which would make them effective at a density of 1 per square kilometre instead of 4 per square kilometre as at present.

The economic analysis pertaining to different approaches to tsetse and trypanosomiasis control, has offered decision makers in the Branch, an improved basis for choosing techniques appropriate to a whole range of operational circumstances.

Technical Feasibility and Practicality of Various Tsetse Control Techniques

The availability of such an armoury of tsetse control techniques has afforded the Tsetse and Trypanosomiasis Control Branch an opportunity to assess not only the technical feasibility of each technique but also other important issues such as cost-effectiveness and environmental impact.

While ground spraying has been used with greater success against G. morsitans and in flat terrain, the technique has serious limitations which militate against its use given the current distribution of tsetse flies in Zimbabwe. The areas that are still tsetse infested in Zimbabwe are characterized by rugged terrain, occupied by dense populations of G. pallidipes and some of the areas are National Parks. Given the increasing environmental concerns over use of DDT, the imperfection of ground spraying against G. pallidipes, heavy demands for labour and the inability of the technique to provide year round protection against reinvasion, it is becoming increasingly difficult to justify DDT use for tsetse control in Zimbabwe today, notwithstanding the fact that large areas of country were cleared of tsetse flies, using DDT. Ground spraying like aerial spraying is a technique which has limited flexibility. Once a mistake is made, e.g. overdosing, spraying wrong places, etc. there is nothing that can be done to rectify the error apart from spraying the affected area again. Our experience is that up to 30% of an area that is ground sprayed is lost to reinvasion, requiring repeat spray applications which are costly both financially and in terms of environmental impact.

TABLE 7

Cost comparison of targets with different insecticide treatment regimes according to number of services required.

Note: All costs based on prices as at March 1991.

Source: J. Barrett, Tsetse and Trypanosomiasis Control Branch, Harare.

Fig: 4 Target cost in relation to insecticide use and frequency of servicing

The aerial spraying campaigns in the western and north-eastern regions of Zimbabwe produced good results against G. morsitans and in flat terrain. However, the technique proved poorly effective against G. pallidipes and in rugged terrain. The high technical demands, strict adherence to timing, costs and inability to check reinvasion, make aerial spraying inappropriate for routine operations.

The limitations associated with traditional methods of tsetse control, led to a 20 year research effort which culminated in 1988, in a field trial of targets in the Zambezi Valley.

The targets appear to offer satisfactory solutions to the shortcomings associated with other techniques. The costs of targets compare with those of ground spraying since they have the added advantage of a high carry-over value for hardware, are effective all year round with negligible environmental pollution.

Target operations in Umfurudzi (North-East Region, 1986) and on-going EEC-funded target trial in Doma (Northern Region) demonstrated that in such broken terrain, the target method can cause satisfactory rates of collapse of both species of tsetse present in Zimbabwe. However, eradication of the population under attack depends on maintaining a consistent rate of target servicing schedules thereby eliminating the possibility of reinvasion.

The application of insecticides to cattle offers the same advantages as targets and are operationally similar in that both are baits that lure tsetse to them and operate all year round to provide a steady decline in tsetse populations. Currently targets are deployed in combination with cattle dipping where cattle and dipping facilities occur.

Land Use and Tsetse Control

Tsetse control in Zimbabwe is mainly concerned with protection of livestock in already settled land facing present or potential trypanosomiasis challenge. Proper utilisation of land reclaimed from tsetse flies is a major concern not only of the TTCB but also other interested groups (Barrett, 1989b). The concern of the Branch in relation to matters of sustainable land use in tsetse-freed areas is reflected in the recent completion of an interdisciplinary study of environmental impact of agropastoral land use in a tsetse-freed part of the mid-Zambezi Valley (Barrett et al, 1991).

This concern is borne out of the realization that tsetse and trypanosomiasis control is inextricably linked with problems of land development usage, human health and wild life conversation. For this reason, the Branch will in 1992 revive the now defunct Tsetse and Trypanosomiasis Control Committee, with members drawn from farmers groups, the Ministry of Health, Departments of Resettlement, National Parks, Agritex and Research and Specialist Services.

The Committee's brief will be to advise the Tsetse Control Branch on the requirements for tsetse control in relation to optimal land usage, human health and conservation of natural resources. The Committee would also foster good working relationships between Government departments operating in tsetse areas. It is felt that such a committee will help to fill a missing gap and raise public awareness on dangers associated with improper land practices.

Discussion

Tsetse and trypanosomiasis control in Zimbabwe is entering a new and decisive phase, thanks to a range of control techniques available to the Branch.

Given the varying conditions of field operations and the dynamic nature of tsetse populations, the Branch retains the capability, and infrastructure to support the techniques described above including ground spraying. The reason for this is that no single technique described is the best for all circumstances. For example, in 1986, the North-Eastern area of Mount Darwin suffered serious stock losses amounting to 300 per month. In such a situation, an aerial spraying campaign covering an area of 3,500 square kilometres had to be launched to achieve immediate break of disease transmission, and to benefit from economies of scale. Ground spraying, target operations or application of insecticide to cattle, would have been inappropriate in such a situation, given the slow acting nature of these techniques.

In Zimbabwe today, most control operations have progressed beyond cattle holding areas and for this reason, the Branch has taken the deliberate decision to make increasing use of targets for almost all of the country's control measures.

However, targets have their imperfections which include cost brought about by the operationally demanding necessity to have them retreated at least three times a year. Given the broken terrain characterising the country's operational areas and in view of the current servicing schedules, getting to targets is now a greater practical problem accentuated by high fuel bills and frequent vehicle breakdowns contributing to a lower work rate and high costs.

Current research in Zimbabwe is looking into ways firstly to circumvent the need to service targets so often, secondly to reduce the number of targets from the current four per square kilometre, and thirdly to design disposable baits.

Given that at present targets are slightly more expensive than ground spraying in broken terrain, the latter technique remains attractive especially in view of the need to create employment.

On the other hand, while application of insecticide to cattle is the cheapest option for controlling tsetse, the technique requires that cattle are present in reasonable numbers.

Integration of the various control techniques discussed was attempted in North-East Zimbabwe. The operation which was conducted in 1986 highlighted problems associated with such a strategy. The problems arise from the fact that the different techniques, i.e. ground spraying, aerial spraying and baits kill tsetse populations at different times and at contrasting rates and offer varying degrees of protection against invasion, so that some flies can diffuse from one area to another and avoid treatment by any of the control techniques (Vale pers. Comm.). Aerial spraying is conducted in about two months (depending on temperature) in the cool dry season killing tsetse rapidly, but flies from outside the treated block, are free to invade between spray cycles, and immediately after the last application. On the other hand, ground spraying must be conducted in the hot dry season to be fully effective in the 4 – 5 months when the flies concentrate in the places where the insecticide is applied, but rapid fly invasion can occur when the insecticide is washed away by the rains.

The Tsetse and Trypanosomiasis Control Branch has over the years received consistent Government funding and this enabled the Branch to build a sound operational capability, infrastructure and capacity necessary for the efficient prosecution of tsetse control measures. The Branch has progressed from the more traditional tsetse control techniques of game destruction, bush clearing through ground and aerial spraying to bait technology, representing the latest tsetse control technique available. All the different techniques discussed have contributed to the sum of the total land that has been cleared of tsetse in Zimbabwe.

Given the intensity of research and current control techniques, the eradication of tsetse flies from Zimbabwe is a distinct possibility in the not too distant future.

Acknowledgements

The author is grateful to Mr. J.C. Barrett for assistance with tables on costs and Mr W. Shereni for useful discussions.

REFERENCES:

Allsopp, R. and Barrett, J.C. (1988) Perspectives on the insecticidal control of tsetse flies. Proceedings of a workshop entitled “Regional Development and Implementation of Tsetse Control Strategies in Eastern Africa with emphasis on Targets and Traps” held at the Kenya Trypanosomiasis Research Institute, Muguga, Kenya, 7th–8th March 1988.

Barrett, J.C. (1989a) Cost analysis of odour-baited targets used for tsetse control in Zimbabwe. A paper presented to the 20th meeting of the International Scientific Council for Trypanosomiasis Research and Control, Mombasa, Kenya, 10–14th April, 1989.

Barrett, J.C. (1989b) Tsetse control, land use and livestock in the development of the Zambezi Valley, Zimbabwe. ALPAN Network Paper No. 19, African Livestock Policy Analysis Network, International Livestock Centre for Africa, Addis Ababa.

Barrett, J.C. (in preparation) Cost study on trypanosomiasis control in Zimbabwe. Forthcoming Bulletin of the Natural Resources Institute, Chatham, UK.

Barrett, J.C., Brinn, P. and Timberlake J. (1991) Tsetse control, agropastoralism and land degradation: a case study in Chiswiti Communal Land. Final Report. Unpublished report of the Tsetse and Trypanosomiasis Control Branch, Department of Veterinary Services, Harare.

Burnett, (1954) The effect of poison bait cattle on populations of Glossina morsitans and G. swynnnertoni. Bull. ent. Res. 45, 41.

Cockbill, G.F. (1975) Handbook for use of tsetse field staff. A publication of Tsetse and Trypanosomiasis Control Branch.

Cockbill, G.F., Lovemore, D.F. and Phelps, R.J. (1963) The control of tsetse flies (Glossina: Diptera, Muscidae in a heavily infested area of Southern Rhodesia by means of insecticide discharged from aircraft. Bull. ent. Res., 54, 93 – 106.

Douthwaite, R. (1990) Reports to the Branch.

Ford, J. Control of trypanosomiasis by control of wild life. WHO Transactions.

Hursey, B.S. and Allsopp, R. (1983) Sequential applications of low dosage aerosols from fixed wing aircraft as a means of eradicating tsetse (Glossina spp) from rugged terrain in Zimbabwe. Branch of Tsetse and Trypanosomiasis Control, Harare.

Jack, R.W. (1933) The tsetse fly problem in Southern Rhodesia. Rhodesia agric. J., 30, 365.

Lovemore, D.F. (1987) Tsetse control by chemical means in Malawi, Mozambique, Zambia and Zimbabwe. In: Cavalloro, R. (Ed) Integrated tsetse fly control: methods and strategies. Proceedings of the CEC International Symposium, Ispra, 4–6 March 1986. A.A. Balkema, Rotterdam.

Robertson, A.G. (1968) The Nagupande selective hunting experiment.

Robertson and Kluge, The use of insecticide in arresting an advance of Glossina morsitans Westwood in the South-East Lowveld of Rhodesia. Proc. Trans. Rhod. Sci. Ass., 53, 17–33.

Shereni, W. Strategic and Tactical Developments in Tsetse Control in Zimbabwe (1981 – 1989). Insect Sci. Applic. 11 (3).

Shereni, W. and Pope, A.R.J. An evaluation of Deltamethrin as an alternative to DDT for ground spraying against tsetse flies (Glossina Spp (Diptera : Glossinidae) in Zimbabwe.

Thomas, Sir W.E., Davey, T.H. and Potts, W.H., (1955) Report of the Commission of Inquiry on Human and Animal Trypanosomiasis in Southern Rhodesia. Federation of Rhodesia and Nyasaland, C. Fed. 24, Salisbury.

Thomson, M.C. (1985) The effect on tsetse flies (Glossina spp) of Deltamethrin applied to cattle either as a spray or incorporated into ear tag. Trop. Pest Management, 33, 329 – 335.

Turner, D.A. (1990) Project reports to the Branch.

Vale, G.A., Lovemore, D.F., Flint and Cockbill, G.F. (1988) Odour baited targets to control tsetse flies in Zimbabwe. Bull ent. Res. 78, 31 –49.

Vanderplank, F.L. (1947) Experiments with DDT on various species of tsetse in the field and laboratory. Trans. R. Soc. Trop. med. Hyg. 40, 603.

Vincent, V. and Thomas, R.G. (1962) The agro-ecological survey. Government Printer, Salisbury, Southern Rhodesia.

Whiteside, E.F. (1949) An experiment to control tsetse with DDT-treated oxen. Bull. Ent. Res., 40, 123.

Whittingham, G.W., Chadenga, V., Ganyani D. and Vale, G.A. (in preparation) The role of odour-baited targets in the control of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) in the North-East Zimbabwe.

ANALYSE DE L'EFFICACITE, DE LA VALEUR PRATIQUE
ET DU COUT DE DIVERSES TECHNIQUES DE LUTTE CONTRE
LES TSE-TSE ET LA TRYPANOSOMIASE UTILISEES
AU ZIMBABWE

RESUME

V. Chadenga

La trypanosomiase est une maladie transmise par les mouches tsé-tsé qui affecte le bétail et constitue un grave facteur limitant de la production agricole en Afrique. Au Zimbabwe, où l'agriculture, et plus particulièrement l'élevage, revêtent une grande importance économique et sociale, la lutte contre les tsé-tsé et la trypanosomiase est une tâche d'importance nationale.

L'historique de la lutte contre les tsé-tsé au Zimbabwe est présenté et les techniques actuellement utilisées sont décrites. Dans ce pays, la prioritéest donné à la lutte contre les vecteurs et à l'utilisation des zones traitées grâce à des implantation rurales planifiées. Des inspections du détail sont conduites chaque mois et le médicament curatif “Berenil” est administré à tous les troupeaux infectés. Dans les zones de forte exposition aux tsé-tsé, le médicament prophylactique “Samorin” est administré.

La Division de la lutte contre les tsé-tsé et la trypanosomiase a obtenu d'excellents résultats avec des pulvérisations de DDT sur les lieux de repos et de refuge des tsé-tsé. Toutefois, des opérations à fort coefficient de main-d'oeuvre telles que les pulvérisations de DDT ne sont plus recommandables; en outre, l'application d'insecticides persistants sur de vastes superficies est écologiquement et politiquement inacceptable. Ces considérations ont conduit à mettre au point trois autres techniques de lutte pour combattre les mouches tsé-tsé:

1. l'épandage aérien, qui est l'application par voie aérienne à volume ultrafaible d'insecticides non rémanents pour détruire toutes les tsé-tsé adultes présentes au moment de la pulvérisation. L'insecticide est épandu en pulvérisations successives à intervalles correspondant à l'émergence des tsé-tsé adultes;

2. l'emploi de cibles porteuses d'appâts et traitées aux insecticides, servant de leurres pour attirer les tsé-tsé. Plusieurs essais effectués au Zimbabwe ont prouvé que les cibles installées à raison de quatre au kilomètre carré peuvent être utilisées efficacement sur différents terrains pour des campagnes de lutte anti-tsé-tsé de grande envergure, et la technique présente plusieurs avantages sur les méthodes de lutte conventionnelles;

3. l'administration de bains au bétail: les essais effectués au Zimbabwe ont montré que l'application régulière de Deltaméthrine au bétail permet de lutter contre les populations de tsé-tsé et de réduire notablement l'incidence de la trypanosomiase.

Les avantages, les limitations et les coût de chacune des techniques sont décrits. Cette multiplicité de stratégies permet à la Division de choisir pour le mieux une méthode ou une combination de méthodes adaptée à une situation donnée. elle a ainsi obtenu des succès notables dans la lutte contre les tsé-tsé, puisque la superficie encore infestée est maintenant réduite à 25 000 kilomètres carrés et que l'on a enregistré moins de 500 cas de trypanosomiase pour l'ensemble de l'année 1990.