Chapter 6: Tsetse control and eradication

The aim of control is to raise the death rate of the flies above the birth rate for a sustained period. If the area is completely isolated by natural or artificial barriers, this will eventually eradicate the tsetse population; if the area is not isolated, the level of reinvasion will determine the level of control.

If traps are being used, there is the option of whether to use an insecticide or not. The advantages of using an insecticide are:

- traps are not 100% efficient. Some flies will land on the trap, but will not enter. These will only be killed if the trap is treated with an insecticide. The rate of reduction will, therefore, be faster using treated traps.

- an insecticide treated trap will work even if damaged, whereas a small hole in the cage of a non-treated trap can render it useless.

- traps can be much simpler and cheaper in design, the ultimate being a simple disposable cloth target, and require much less maintenance.

The disadvantages of using an insecticide are:

- although the rate of reduction may be faster, the control operation may be more expensive and dependent on the availability of insecticide.

- if an insecticide is used, arthropod tsetse predators such as ants, spiders, robber flies (Asilids) and Bembex wasps may also be killed. Such predators, as well as birds and lizards, tend to colonise the traps and may help to raise trap efficiency.

- insecticides have to be carefully handled to ensure the safety of handlers and the environment, although in this case its use is very limited.

The decision on whether or not to use insecticide will depend on the trap type, the species of tsetse, and whether the objective is control or eradication.

Biconical traps in West Africa have usually been treated with insecticide, especially with G. palpalis as the trap is not very efficient for this species.

Pyramidal traps have been shown to be very effective against G.f. quanzensis in Congo when not treated with insecticide, and maintained in part by the local community. In some situations against G.f. fuscipes, pyramidal traps have been treated only when first deployed, to get a rapid initial reduction.
Targets, of course, have to be treated with insecticide, or possibly a chemosterilant.

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Choice of insecticide
Insecticides used on targets must possess two qualities.  Firstly they must be effective and readily transferred to the fly, which may only contact the treated surface for a fraction of a second.  Secondly they must be persistent under varying climatic conditions so that the targets do not have to be treated too often.

The main chemicals utilised nowadays are the synthetic pyrethroids.  The most widely used being deltamethrin supplied as a 20% suspension concentrate (s.c. ).  It used to be diluted to a 0.1% or 0.05% w/v spray liquid.  The present recommendation, however, is to use a more concentrated spray liquid (0.3%) in order to improve persistence at active levels, about 7 months, and thus reduce servicing costs.

The persistence of insecticides can be economically enhanced by the addition of UV absorbers, such as 2,4 - Dihydroxy -Benzophenone.  This would further increase spraying intervals and reduce costs.

For application to cattle as live attractive targets, the usual formulation is an 18.75% suspension concentrate, diluted to give a spray liquid concentration of 0.00375-0.0065% active ingredient (a.i.).

Refer to vol. 3, section 3.5.2. for the calculations to make up the diluted spray liquid from the suspension concentrate.  Briefly, to determine the amount of water to be added to one litre of concentrate, the concentration of the s.c. is divided by the required concentration of the spray liquid and subtracting 1.

e.g. for target spraying:

No. litres of water =   20     -1   =    199 l.
                              0.1

Thus 199 litres of water have to be added to one litre of 20% s.c. to make up 200 l of spray liquid at a concentration of 0.1%.

e.g. for cattle dipping:

No. litres of water =    18.75    -1   =   4999 l.
                             0.00375

Thus 4999 l of water have to be added to one litre of s.c. to make up 5000 l of dip liquid at a concentration of 0.00375%.

For the pour-on treatment of live animals a 1% suspension concentrate of deltamethrin or flumethrin is applied at the rate of 1 ml for each 10 kg of body weight. Thus for a 200 kg animal, 20 ml of the 1% s.c. would be applied. Other pyrethroids have been tested, including alpha-cypermethrin.

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Insecticide Application
Specific detail on the concentrations of insecticides to be used and their frequency of application will vary according to the product selected and the local climatic factors influencing the length of persistence. The figures given below are guidelines only and the optimum parameters for use should be determined through local field trials.

(a) Hand spraying of targets
The initial application of insecticide to targets is usually at double the strength used for subsequent applications at 2-4 month intervals applied by a knapsack pressure sprayer (see Vol.3. Section 4.3.1. on how to use a knapsack sprayer). The target is sprayed on each side, treating the cloth and netting similarly, if this design of target is used, to the point of run off.

(b) Immersion of targets in insecticide
There are some advantages in immersing targets in insecticide prior to deployment. This avoids the need for spraying equipment, and allows better control of dosage. To get the correct dose, mg of active ingredient per trap or target, first determine how much liquid a given number of traps or targets will absorb. If for example, one target absorbs 100 ml liquid, and the dose rate is 100 mg per target, each 100 ml of insecticide liquid should contain 100 mg active ingredient.

After immersion for 15-30 minutes, remove the targets to dry. It is better to place them horizontal on the ground for this purpose, rather than hanging them up, as this will reduce run off. To prevent pollution, this must not be done close to a river or other water source.

Care should be taken when using this method as laboratory tests indicate that, unless all the targets are remove from insecticide at the same time, the first removed will absorb most of the active ingredient so that by time the third or fourth is taken out hardly any insecticide will be left.

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(c) Application to cattle
Dipping is the most economical way to apply the insecticide to the cattle, providing dipping facilities are already available. Animals should be dipped every two weeks, although dipping every week may be necessary in wet weather. Hand spray pumps can also be used to spray individual animals using the same formulation used for dipping, but this is usually more costly and wasteful.

If no dipping facilities exist a Pour-on can be used. In this method the correct volume of the insecticide formulation is applied by hand, using a special applicator, starting from in front of the shoulder running back to behind the hip. For small numbers of animals, special insecticide dispenser bottles (Fig. 28A) can be used to apply a line along the back of the animal. For large herds, T-bar applicators (Fig. 28B) permit more rapid application but must be well maintained. Drench guns can also be used, but care must be taken as the insecticide solvent may attack plastic parts in the drench gun. Pour-on should be applied at 2-4 weekly intervals.

One other method which has been tested is the use of animals fitted with ear-tags which release insecticide gradually over a period of time. These, however, have not proved very effective, giving a mortality rate of less than 16%, and a knockdown rate of less than 41%.

Fig. 28 Applicators for pour-on to cattle A. squeeze-and-pour applicator; B. T-bar applicator.

The safety and environmental protection measures for insecticide application and disposal must be followed as detailed in vol.3 (sections 3.8 & 4.2.3.), in particular:

- great care must be taken during application not to get insecticide on the body, and especially the eyes;

- all staff after handling insecticides should thoroughly wash themselves, their clothes, all spraying equipment and any vehicles used for transporting insecticide;

- all contaminated water should be drained into a washing pit situated well away from any rivers, wells or other potential sources of drinking water. In particular animals must not be sprayed by a river, water must be transported to the spraying site;

- stored insecticides must be secure against theft, and empty containers should be destroyed by knocking a hole in the top and bottom of tins and burning cartons so that they are not subsequently used for domestic purposes.

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General principles
It has been suggested that attracted flies should be sterilized and released, rather than killed. The females can then no longer reproduce and the sterile males will mate with other fertile females, which will then produce non-viable progeny.

Potentially this approach is more efficient, especially in areas where access is difficult and an even distribution of traps or targets impossible. The sterilized males may also have an effect on populations well outside the zone.

Choice of chemosterilant
Until recently the only chemosterilants potentially suitable for this purpose were metepa or bisazir, however they are both very dangerous chemicals to handle and should not be used in the field.

More recently a much safer alternative has been developed. This is the juvenile hormone analogue, known as pyriproxyfen. A dose of 0.02 ug in 1 ul acetone applied to an adult female will stop development of offspring at the pupal stage for the lifetime of the female.

A commercial formulation of pyriproxyfen in oil (Cereclor S45(R) is now available. It has been tested in the field by using a mutton cloth cone in the cage. Each cone was dipped in a 25% solution of Cereclor in acetone which after evaporation, retained about 4 ml of Cereclor giving a sterilant concentration of about 2 mg/cm2 of cloth. Flies passing through the cage received a sufficient dose to sterilise them. Formulations tested on targets are promising, but so far not enough pyriproxyfen has been picked up by males, during the very brief contact with a target, to cause sterility of females mating with them.

Prospects for chemosterilization
Trials using F3 traps over a small area of about 12 km², have shown that emergence rates of pupae of G. pallidipes and G.m. morsitans fell to about 3% and 30% respectively of control levels after 3 months. If the method was applied over a sufficiently large area, it should, therefore, be as effective as insecticide-treated targets.

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General principles
The required density of traps or targets depends on a wide range of factors including the objective, eradication or control, the tsetse species and technique used, the level of natural density-independent mortality and the vegetation types within the control zone.

Trap/target density for savanna spp.
In Kenya, NGU traps baited with acetone and cow urine, and subsequently also octenol, have been used to control G. pallidipes and G. longipennis, without insecticide, placed at an average density of about 2/km², but concentrated in dry season habitats at up to 8/km². The same trap has also been used for control of G. pallidipes in Rwanda (4/km²) and Somalia (4/km²).

Biconical traps, without odours, were used to control G.m. submorsitans in Burkina Faso at about 33/km². The addition of odours, acetone and octenol, enabled this density to be reduced to about 5-6/km².

Insecticide-treated targets have been used in Zimbabwe, Kenya, Ethiopia, Rwanda and Burkina Faso for control of both G. pallidipes and G. morsitans. In Zimbabwe, where the objective was eradication, both the R- and S-type targets have been used, baited with acetone, octenol and more recently phenols. Densities of 1-4/km² have been tested in the past, with about 2/km² now recommended for G. pallidipes and 4/km² for G.m. morsitans.

In Kenya higher densities of about 5-6/km² of this target design have been used for control of G. pallidipes. The areas covered were much smaller than in Zimbabwe which might explain the need for higher target densities. In Zambia, with either S-type targets and all-black targets, the target density was 4/km² for G.m. centralis.

Unbaited insecticide treated blue targets were tested against G.m. submorsitans in Burkina Faso, before odours had been developed. The target density used was much higher at 16-25/km². Now that odours are available, the recommended placement density has been reduced to 5/km².

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Trap/target density for palpalis species

For riverine species, the traps or targets are often set at regular intervals along a river. Hence the density is expressed in terms of numbers per linear kilometre rather than per square kilometre. Placement densities have generally varied between about 3-10 per linear kilometre.

A total of 600 insecticide treated biconical traps were used to control G. tachinoides and G.p. gambiensis over 62 km of the Leraba River, about 10/km, in CÌte d'Ivoire. Later, larger scale control programmes used biconical traps at a density of 3.3/km with very good results. Except in very dense gallery forest, the density can be lowered to 1.6/km once tsetse numbers have been reduced. In Burkina Faso, biconical traps were used against the same species at a density of 5/km.

Insecticide-treated blue targets have also been used to control the same species, but higher densities are needed. In 1980, 876 targets were set along 79 km of the Leraba River, about 11/km. Other operations included the deployment of 1630 targets along 163 km of gallery forest in the Sirasso Region of CÌte d'Ivoire, 10/km, and in Burkina Faso where 7200 blue targets were used along 600 km of river, 12/km.

Sometimes the habitats of the palpalis group are not strictly riverine, and we again have to express trap/target densities in square kilometres. In the Vavoua focus of human trypanosomiasis, blue targets were set at about 180 per km² to control G. palpalis, 15,592 targets were set over 86 km² mainly consisting of coffee and cocoa plantations. Subsequently 41,000 of the more effective blue and black targets were set over 1300 km², 32/km².

In the Daloa region of CÌte d'Ivoire, a comparison was made of the percentage reduction in numbers of G. palpalis using varying densities of insecticide treated monoconical traps. Effective control was maintained with 400-800 traps/km², but not with 100-200 traps/km².

In the Congo, peridomestic populations of G. fuscipes and G. palpalis have been controlled using non-insecticidal pyramidal traps set around villages at a density of 26 traps per village.

In Uganda, insecticidal pyramidal traps were initially deployed at a density of 8-10/km² to control G.f. fuscipes. After two years the numbers were reduced to less than 5/km² to ensure continued suppression of the fly population. Where distribution is linear along a river, traps are placed at 5 per kilometre.

It can be seen that generally trap/target densities used for palpalis species have been much higher than those used to control morsitans species. This is partly because odours have not been available until very recently for the latter species, and also because some species, e.g. G. fuscipes, are less mobile and hence higher trap densities are needed to avoid leaving pockets of flies.

Now that odours are available for G. tachinoides, it may be possible to reduce trap/target densities for this species, but no effective odours have yet been discovered for G. palpalis.

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Distribution of traps/targets
Once the overall trap/target density has been decided upon, their distribution will depend greatly on the species present, the vegetation types and whether one is trying to control or eradicate. For control, traps should be concentrated in more favourable habitat, where they catch most flies, and at lower densities in other areas, especially if such areas are mainly affected by temporary dispersal. For eradication, all areas where flies occur must be covered.

If the vegetation is relatively uniform, then traps must be relatively evenly distributed. If the vegetation is not uniform then traps can be concentrated within suitable vegetation.

It is still not clear how regular the distribution of traps and targets needs to be within suitable vegetation. This is especially important for savanna species, because if the targets have to be distributed on a grid pattern, this will involve considerable effort and cost in putting in a network of access tracks.

A density of 4 devices/km2 can be achieved in several ways. A regular grid of tracks half a kilometre apart with traps set at the intersections would give a regular distribution. Less regular distributions would be obtained from cutting only parallel tracks through the area at 1km apart, traps set every 250 m, or 2 km apart, traps set every 125 m.

The further apart the tracks, the lower the costs for track making and trap/target servicing. The effect on the fly population will depend largely on the mobility of the flies. If they move very little, putting lines 2 km apart could leave viable tsetse populations between lines. If they move a lot, most flies will encounter the devices, even if the lines are 2km apart. Fly mobility is likely to be more restricted in dry conditions, so tracks need to be arranged with care to avoid leaving fly concentrations.

For G. morsitans, the recommendation is to space lines at 1 km apart and set baited S-type targets at 250 m intervals. For G. pallidipes and G. longipennis, baited NGU traps or S-type targets could be used for control on tracks through dry season habitat 1-2 km apart and spaced at about 250 m intervals.

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Choice of trap/target sites
For thicket species, such as G. pallidipes, site selection depends on climate and vegetation. Where it is very hot, optimal sites depend very much on the time of year. Forested areas will give very high catches at the end of the dry season, but poor catches during the rains; the situation is the opposite in more open areas, although very few flies move more than 1 km out into open grassland. The best all-year sites are in the shade of large Acacia trees bordering riverine thickets and forest. The best sites on the Kenya coast, for the same species, are about 5 m from the edge of thicket and forest patches.

For G. morsitans, target densities should be higher in riverine vegetation, but targets should also be distributed fairly evenly through deciduous woodland.

For G. palpalis and G. tachinoides in gallery forest riverine habitats, the trap or target should be put as close as possible to the edge of the river. Traps hung over or floating on the river, on air filled plastic containers, have proved very effective. In the Vavoua focus in the forest zone, screens are put along the edges of forest patches, and additionally in camps, working and resting areas and at watering points, bridges and fords.

For G.f. fuscipes, optimal sites are along the border between two habitats of which at least one is wooded. The best sites are at the boundary between relict forest and plantation, forest and path, and forest and Lantana bush. Traps should also be set along rivers, and by swamps and water holes. They should contrast well with the background vegetation, and light should fall on the top of the trap.

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Mode of action of barriers
A barrier to prevent reinvasion can be made by concentrating a higher than normal density of traps or targets along the edge of the control/eradication zone. If the objective is local eradication, the barrier must intercept all the flies, or at least such a high proportion that populations cannot re-establish themselves in the eradication zone. For control, the barrier need not be completely effective, but the level of reinvasion will set the degree to which the population can be reduced.

A barrier acts in two ways to prevent flies entering a control/eradication zone.

(a) Interception of invading flies
Because there is a very high density of traps or targets in the barrier zone, flies invariably encounter one or more of the attractant devices, and are killed.

(b) Reduction of fly density near the barrier
The density of tsetse flies is greatly reduced on both sides of the barrier zone, the distance over which the flies are reduced outside the treated area will depend on the mobility of the flies. If the flies move about a lot, the numbers will be reduced for several kilometres outside the zone. If the flies do not move very much, numbers will only be reduced close to the barrier. Because there is a lower density of flies near the barrier, there is less chance of any getting through into the control zone.

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Barriers for savanna species
Trap barriers for savanna species were first tested in West Africa against G.m. submorsitans where they were present with a riverine species. In order to exclude the savanna species, 12 transects 6 km long and 250-500 m apart were cut at right angles to the river, with insecticide treated biconical traps and screens placed alternately at 100 m intervals. This pattern was reinforced by two lines of traps and screens at 50 m intervals sited along the river for a distance of 30 km. This density could possibly be considerably reduced if odours were used with the traps.

The first target barriers against G. pallidipes and G.m. morsitans consisted of odour-baited S-type targets placed at a density of 30/km2, and concentrated in a 5 km band around the eradication zone. Since then, targets have been concentrated in a narrower band (500-2000 m wide) at similar densities, especially when protecting areas that have been ground or aerial sprayed. Four or five trap lines are used, spaced at 400-500 m, with targets at 100-200 m intervals (100 m interval on the 'front line').

If the aim is to control over a relatively small area, less than 500 km2, rather than eradicate, a high density barrier is not likely to be cost-effective, since the numbers of attractive devices needed is so high. In this situation, a barrier zone about 4 km wide with traps at 10-20/km 2 can be used to reduce though not eliminate reinvasion.

Depending on the degree of reinvasion it may not be necessary to maintain a specific barrier when the objective is control, as the operation itself will ensure the reduction of invading fly populations. However, such a situation will produce an edge effect which would have to be assessed as to whether the effects were tolerable or not ,taking into consideration the economics of barrier construction and maintenance.

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Barriers for riverine species
The most detailed work on barriers for riverine species has involved non-insecticidal blue biconical traps to prevent reinvasion of G. palpalis and G. tachinoides. A 10 km long barrier of non-insecticidal traps placed at 100 m intervals has been shown to be highly effective. Despite the experimental release of very large numbers of marked G. palpalis, tsetse were not able to penetrate more than about half way up the barrier.

A 7 km barrier of insecticide-treated biconical traps or a 10 km barrier of untreated traps is now recommended. A combination of blue targets and biconical traps, to reduce the cost, may also be effective.

Effectiveness of barriers
Providing a barrier has been in place for some time to reduce local fly populations, it can be very effective if properly maintained. This is a main factor affecting barrier efficiency.

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Logistics of trap/target deployment
Trap/target deployment in savanna areas can be very labour intensive because a large cleared area is needed to protect them from fire. In Zimbabwe, a target team consists of a field officer, a field assistant, a labour supervisor, a driver and 20 labourers; transport is provided by a lorry and a landrover. Between 20 and 80 targets per day can be deployed or serviced.

For community-based control operations, there is usually more manual labour but less transport available, and setting may have to be done using bicycles, motor bikes or only a small vehicle. Site selection, clearing and setting at a rate of 10-15 traps a day can be done using a small 4-wheel drive vehicle and 3 people.

Deployment of traps along riverine gallery forest for control of G. palpalis and G. tachinoides can involve the use of a boat. Two men in an engine-powered canoe can cover about 15 km a day. Targets were deployed in the Vavoua area by the farmers themselves, 38,500 targets were deployed in 3 weeks. In Uganda, pyramidal trap deployment is carried out by a four man locally recruited technical team who work closely with the community.

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Frequency of servicing
Trap/target maintenance or servicing is one of the most important parts of any control operations. It is also unfortunately one of the most expensive components, and anything which can be done to reduce the servicing interval is beneficial.

The frequency of servicing depends on the level of damage experienced. If it is undertaken every two months, and 6% of the traps become non-functional during this period, the trap density should be 3% greater than that recommended in order to compensate, assuming that on average that the damaged traps are operating for half the time.

In Zimbabwe, servicing of targets has been carried out about every four months, with an extra visit during the wet season. The higher rate of insecticide application (300 mg a.i.) has reduced maintenance to two or three times a year. In Kenya traps are serviced about every two months. Barriers around local eradication zones may have to be serviced very frequently to remain effective, in Sideradougou, Burkina Faso, trap barriers have been serviced every two days by men on bicycles.

Obviously, it is more critical to ensure the correct functioning of non-insecticidal traps than it is of targets as any holes in the capture devices of the former will negate their effect.

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Damage prevention
Some traps and targets will always be damaged in the field, but damage levels should be minimised as much as possible.

Servicing protocols
To ensure that traps and targets are effective and efficient regular servicing is essential. It is advisable to draw up a protocol (list of activities) to be followed for the servicing of the traps, and to make a list of all the items needed, much time can be wasted otherwise. See Table 6.1 for a typical list of the items that may be needed.

Table 6.1 : Items required for setting/servicing traps

- pangas and slashers large and small hammer
- side poles and odour pegs brush
- centre poles and 3" nails acetone and funnel
- cage sticks and strings acetone bottles & tops
- thin wire and wire cutters acetone rain covers
- ruler and tray cow urine (50% and 100%)
- traps and cages urine containers & tops
- staplers and staples felt tip pen
- needle and cotton geneterrent
- drinking water and cup trap book and pen

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When servicing, a record should be kept of the condition of traps and of the number of flies caught in each. Traps in consistently poor sites can then be moved to better ones, and areas of invasion detected. If a preservative such as diesel oil is used the flies can be counted whereas if polythene bag cages are used, the depth of flies in each cage gives a rough measure of how good each site is, provided there is no loss by ant damage. Although some traps in prominent sites may be left with the flies uncleared so that people can see what they are for, it is generally better to clear dead flies out when servicing, since rotting flies may reduce the catch or rot the trap. A typical protocol for servicing traps is given below:

Table 6.2 : Protocol for servicing traps

1. Shake down dead flies in the cage until they are level, and then measure the depth of dead flies with a ruler. Record if there is any ant activity. Empty the cage of dead flies and reseal; dispose of the dead flies at least 5 m from the trap.

2. Check trap for holes and tears, especially in the cone and at the top of the cage. Repair minor damage on the spot; replace with another trap if the damage is major. Check trap supports and tighten wire ties if necessary. Brush the trap, especially the target and shelf.

3. Refill odour dispensers, replace with fresh urine every six months. Check dispensers; make sure the hole of the acetone dispenser is clear and remove spiders and cobwebs, especially under the rain cover.

4. lear all regrowth of vegetation to ground level for at least 2 m around the trap.

5. Check cloth for colour fading and replace or restore with dye as necessary.

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Activities 2. to 5. above have also to be carried out for insecticide treated traps and targets, but in addition one other activity is necessary, this is:

(a) Respraying with insecticide
Targets are only effective if there is sufficient insecticide deposit to kill flies that land on the target for a few seconds. They must therefore be retreated at regular intervals. This should be done at 2-4 month intervals depending on the insecticide used and its concentration. The interval also depends on many other external factors, especially the degree of exposure to sunlight and rain, and the materials used for the targets.

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The first modern large scale trial of targets against G. pallidipes and G.m. morsitans was carried out in a 600km² area, the 'Rifa Triangle', in Zimbabwe using S-type targets. Targets were treated with deltamethrin, and baited with acetone (or butanone) and octenol. After 9 months, tsetse numbers in the centre of the block had been reduced by 99.99% and numbers had also been reduced along the barrier perimeter for 5-10 km outside, which greatly reduced the formerly strong invasion pressure.

Subsequently the method has been widely used in Zimbabwe. In the Umfurduzi Wildlife area, eradication of G.m. morsitans was achieved after 9 months using targets at 4/km².

In Western Province of Zambia, G.m. centralis has been controlled using S-type targets baited with acetone and octenol, later replaced by all-black targets. In the trial block (500km²) targets were set at 3.8/km², whilst in the main block (1500km²) the overall density was 2.3/ km², concentrated in habitats suitable for tsetse at 3.7/km². Local eradication was achieved in the trial block, but small residual populations remained in parts of the main block. A community education programme minimised theft or damage of the targets.

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In Kenya, S-type targets, baited with acetone and octenol, have also been used against G. pallidipes in Galana Ranch and Lambwe Valley, baited in addition with cow urine. Eradication was successful with a target density of 5/km² over a small part of Galana Ranch (25km², and was later extended to control tsetse over a larger part of the ranch by the ranch owners. In Lambwe Valley a similar target density was used, and fly populations were greatly reduced (99.9%), although residual populations remained, at the end of the first year, in some of the thickets. This was probably because targets were mainly put around the perimeter of thickets and, especially in the dry season, there was insufficient fly movement out of the thickets for adequate contact with the targets. The main constraints were theft of targets in Galana, and destruction by fire in Lambwe.

In Nguruman, Kenya, NGU traps made locally, were deployed over an area of about 110km² at an overall density of about 2/km². These were concentrated in areas most suitable for tsetse, G. pallidipes and G. longipennis and in surrounding trap barriers. During the dry season a reduction of about 99% was achieved, but during the rains the reduction was about 90% because of seasonal reinvasion. After the pilot trial, local people set up a community development project to manage and finance tsetse control over about 1000 km², and have achieved similar reductions in tsetse numbers. The same traps have also been used successfully in Somalia on a community participation basis.

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In the Vavoua focus of human trypanosomiasis in CÌte d'Ivoire, blue and black targets were used over an area of 300km² to protect a population of about 25,000 inhabitants. The targets were treated with deltamethrin. The local people deployed and maintained the traps, and took the targets to a specialist team for retreatment with insecticide every 4-6 months. Reductions of 98-99.9% were recorded in the tsetse population, and the level of community participation was very good, especially when tsetse densities were high. After 24 months, medical screening showed that disease transmission had ceased.

Control of G. palpalis and G. tachinoides in northern CÌte d'Ivoire (Korhogo), where animal trypanosomiasis is prevalent, has mainly been carried out using insecticidal biconical traps, which were found to be more efficient than blue targets. The total area treated up to 1987 was 13,400 km 2. Traps were set at 300 or 600m intervals as close to the river as possible; they were kept in place for about 8 months of each year, but removed during each rainy season. Percentage reduction of tsetse generally ranged between 90-99%, with local eradication in some areas. Cattle infection rates in the area decreased by up to 90%.

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In Uganda insecticidal pyramidal traps have been used to control G.f. fuscipes and so reduce human trypanosomiasis in the Busoga focus. Traps were deployed at about 8/km² over some 2850km² . In the five subcounties treated, the percentage reduction in the number of tsetse varied between 96.5% and 97.5% after 6 months, and increased to 99% after 9 months. After 9 months the trap colours had faded, and the traps were replaced. The number of cases of sleeping sickness was reduced by 80-90% after 5 months. The technique was supported by palliative aerial spraying.

In the Niari River sleeping sickness focus in the Congo, G. palpalis has been controlled using untreated pyramidal traps through community participation. Traps were provided free to people in 55 villages, together with kits for mending traps and counting flies and an explanatory booklet and posters. Villagers were responsible for monitoring the numbers of tsetse caught, and for maintaining the traps and moving them to better sites when necessary. Percentage reduction in fly numbers ranged from 87-97%, resulting in a considerably reduced seroprevalence rate, percentage of cases detected by a serological test. Despite the success of the control, the degree of community participation was related to fly density, and many villages did not continue trapping after numbers had been reduced.

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General Principles
Instead of spraying insecticide on to stationary man-made targets, use can be made of cattle, which are to most tsetse species a naturally attractive host, by treating them with insecticide. Although DDT was experimented with in the 1940's and 1950's, it is only since the synthetic pyrethroids have become available that the method has been introduced on a large scale, especially in Zimbabwe.

There are two potential ways in which insecticide-treatment of livestock may reduce the incidence of trypanosomiasis.

- the insecticide may be repellent to the flies, thus reducing the number biting the cattle and hence reducing disease transmission;

- the insecticide may kill sufficient flies to reduce the tsetse population, again reducing disease transmission.

Most experiments have indicated that deltamethrin does not prevent tsetse flies from landing on cattle or targets. However, experiments in Cameroon, also using deltamethrin, showed that treated animals attracted five times fewer flies than untreated animals.

The insecticide may be acting in both ways to reduce the incidence of trypanosomiasis. Although this may seem to be a good thing, if there is any repellency of some insecticides, it will make it more difficult to reduce the number of flies using insecticide-treated livestock or trap/target devices, and this does not seem to be the case.

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Mortality and knock-down
Many of the flies that land on insecticide treated livestock are killed within a short time, the percentage killed is known as the mortality rate. However, some flies are merely immobilised and cannot fly for several hours, but eventually recover. The percentage of flies affected in this way is known as the knock-down rate.

Although it would be better if all the flies were killed outright, it may not matter that some are only knocked down. This is because many such flies are probably eaten by predators, e.g. ants, spiders, birds, before they can recover from the insecticide. The degree of predation may, therefore, vary in different ecological situations according to the availability of predators. This aspect requires further investigation as it could be important, particularly if the objective is eradication.

If cattle are sprayed with deltamethrin, the mortality rate of flies landing on the cow will be about 90% in the first two weeks after spraying. The knock down will remain at about 70% or more for 8 weeks after spraying. Flumethrin gives an initial knockdown of 90%, decreasing to 40% during the first 15 days. This knock down effect also occurs when synthetic pyrethroids are applied on traps and targets.

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Factors affecting control success

(a) Density of cattle
Research is still continuing on the optimum density of cattle needed in an area to achieve successful control/eradication. Early control efforts directed against G. pallidipes used a density of about 26 animals per km². In Zimbabwe local eradication of G. morsitans was achieved with about 10 cattle/km². In both cases the density of cattle treated was directly related to the numbers in the area and thus available for treatment. Further research is, however, required. Provided the natural live host is at least as attractive to tsetse as current artificial devices then the number of treated animals may be substantially reduced to perhaps 4/km² in the case of savanna tsetse species.

(b) Distribution of cattle
One essential feature of this method is that cattle should be well distributed throughout the control zone. This may be the case naturally for open country savanna species like G. morsitans. It may not be the case for species such as G. pallidipes and G. austeni, which are often concentrated in densely thicketed areas where cattle may not normally graze.

Good control over the grazing patterns of the cattle may, therefore, be essential to control these species successfully. There are some habitats where it may be difficult to take cattle at all, e.g. forests on coral rag in the East African coastal regions.

(c) Feeding patterns of tsetse
The proportion of tsetse that feed off cattle in an area is clearly going to affect the rate at which tsetse numbers decline. If the flies mainly feed on cattle, numbers of tsetse will decline rapidly. If there are many wild hosts available, and tsetse mainly feed on these, then numbers may decline more slowly. Note that this factor may also affect the rate of reduction with traps and targets. Prior to planning the use of cattle it would, therefore, be advisable to determine the percentage of tsetse utilising them as a food source.

(d) Activity period of tsetse
Not all tsetse are active at the time that cattle are grazed in the area. This is particularly true for G. brevipalpis and G. longipennis, since these species tend to fly at dawn and dusk. Traditional herders often avoid infested areas at times of day when cattle are at most risk.

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Case studies
In Zimbabwe, 20,000 head of cattle were dipped fortnightly with deltamethrin in a 2500km² area infested with G. morsitans. Over two years cattle trypanosomiasis was eliminated from areas more than 10km from the Mozambique border, and greatly reduced in areas close to the border, thus indicating that the animals in this area may have not only produced eradication but also served as an effective barrier against reinvasion.

On Mkwaja Ranch, Tanzania, resistance to Samorin resulted in more frequent applications and higher dosages of the drug being required; this was both uneconomical and toxic to the cattle. Introduction of fortnightly dipping with deltamethrin in the dry season, and weekly during the rains resulted in a 90% reduction of G. pallidipes, a 100% reduction of G. morsitans and a 70% reduction of G. brevipalpis in 8 months.

In Zanzibar a deltamethrin pour-on formulation (1% w/v) was applied at about 10 ml per 100 kg body weight to nearly 700 cattle and 200 goats in an area of about 35km² infested with G. austeni. Five treatments were applied at 15-18 day intervals. Apparent densities of flies on white sticky targets had dropped to zero by the end of the trial. Subsequently eradication has been achieved over the northern part of the island, but it has not proved possible to use the technique in parts of the south because of difficulties in getting cattle into the dense forest areas.

On the Kenya coast, a trial group of 70 cattle were treated with flumethrin pour-on, and monitored for trypanosome infection rate and weight gain. They were compared with a control group of 70 cattle on a neighbouring farm. Tsetse, mainly G. pallidipes, were monitored with odour-baited biconical traps on both farms and also in another area 3-11 km away from the trial farm. Comparison of numbers in the same month one year apart showed that tsetse numbers in the trial farm had declined by about 70%, whilst numbers in the control farm and the more distant area had remained about the same. Cattle infection rates dropped from about 35% in both groups of cattle to under 10% in the trial group and to 10-23% in the control farm. Cattle in the trial group had higher weight gains than those in the control group.

The method has also been effective against G. palpalis in Burkina Faso. Two thousand cattle were treated at monthly intervals with flumethrin and tsetse numbers dropped from a pre-treatment peak abundance of 4.7 flies/trap/day to zero after several treatments.

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Effect on tick control
Synthetic pyrethroids are increasingly being used for tick control, and can also be expected to reduce the numbers of other biting flies attracted to cattle. However, it should be borne in mind that if tick control is not established practice, the level of enzootic stability in the area may be affected. In many parts of Africa, tick diseases are held in check by the animals developing a natural immunity to tick diseases when young, which renders them immune to the disease in later life. If regular tick control with acaracides is carried on for more than about a year, it has to be continued indefinitely, as the cattle will start to loose their natural immunity. If the aim is to eradicate the flies, and then cease using acaracides, the calves should not be sprayed and the operation should be completed within 18 months.

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