Control and prevention

vector control. Knowledge of the habits and behaviour of the local vector is necessary before embarking on methods of vector control. The principal method is to modify the environment so that it is unsuitable for the fly, but not to cause so much damage that the water table is affected or soil erosion results. With the riverine type of habitat, areas of the forest tunnel are cleared removing all the dense undergrowth, but leaving the big trees with their extensive root systems to prevent erosion of the river bank (Fig. 15.17). Clearance should be continued for 0.5 km on either side of a river crossing, water collection place or inhabited area.

In East Africa, where extensive forest provides a habitat for the fly, the forest margin is pushed back from any place of habitation. A band of at least 1 km, preferably 2 km, should be left between the area of habitation and the forest. This must also include any cultivated area and regulations are required to prevent people from moving into the cleared part to start new cultivation. Ring barking is a more economical method of forest clearance than cutting down every tree.

Where forest clearance is impractical, insecticides can be used. This is easiest along the course of substantial rivers using a boat, spraying the forest on either side. In the savannah-type habitat, isolated thickets can be treated. Extensive insecticidal application to miombo forest is inappropriate. Insecticides have to be repeatedly used, whereas forest clearance is permanent and

Table 15.5. A simplified key to Glossina of medical importance and their favoured habitats.

Hind tarsi 1.

All segments dark above

Only two distal segments dark above Abdomen obviously banded dorsally Abdomen dark, unbanded dorsally

Distal two segments of front and middle tarsi without dark tip

Last two segments of front and middle tarsi with dark tip Bands of abdomen very distinct and sharply rectangular Bands on abdomen rounded medially and less distinct

In summary, the vectors of T. b. gambiense are G. palpalis G. tachinoides G. morsitans and of T. b. rhodesiense G. morsitans G. pallidipes G. fuscipes G. swynnertoni G. tachinoides in SW Ethiopia

Favoured habitats are:

a. Lake and riverside, fringing forest b. Fringing forest without permanent water c. Miombo woodland, thickets and 'game' savannah woodland d. Restricted to northern Tanzania 'game' savannah woodland

G. palpalis group (1)

G. morsitans group (2) G. tachinoides G. palpalis (W. Africa) G. fuscipes (E. Africa) G. pallidipes

G. swynnertoni G. morsitans

G. palpalis G. tachinoides G. fuscipes near Lake

Victoria G. pallidipes G. morsitans G. swynnertoni the relative costs of these two techniques needs to be considered.

Trapping can also control the vector. A well-designed trap will collect enough flies to considerably reduce the biting risk. An effective trap has a fine metal mesh treated with insecticides, which is shaded to attract tsetse flies. These are rapidly killed when they touch the screen, but this must be cleaned regularly to work efficiently.

The fly can bite through thin clothing, so taking preventive action from being bitten in a tropical climate is difficult.

alteration of the human habitat Sleeping sickness has been responsible for large movements of people from their traditional homelands, either by choice or by government action to avoid an epidemic. Moving people away from the sleeping sickness areas is the ultimate method of control, but one to be taken only when all else fails.

The preferable alternative to moving populations is to modify the habitat so that it is unsuitable for transmission. Methods of forest clearance have already been described, while providing water supplies will remove the reliance on obtaining water from rivers.

The density of population largely determines the endemicity, as mentioned above. Two different approaches can be taken:

• keep the population close together and clear an area of forest around them;

• encourage the people to spread out very widely so that they partially clear a large area of forest.

In the first method, the people are safe as long as they remain within the village, but once they pass through the forest, they are subjected to a considerable number of bites. In the second alternative, people will become infected in the initial stages of forest clearance, but once this has been done, then protection will be much greater and more use can be made of the land. In the initial period of forest clearance, a surveillance service will be required to find these pioneer cases. The most unsatisfactory solution is a moderately large population spread evenly over the area; this is the potential situation for an epidemic.

parasite reduction A surveillance service should be set-up and all cases treated (see below). Finding cases in the early stages of the disease, not only increases the chance of successful treatment, but also removes a potential source of infection to tsetse flies.

Another approach to reducing the parasite reservoir in T. b. rhodesiense is to destroy the animal population. This used to be practised on a wide scale, but animal conservation has now questioned the wanton slaughter of animals. In most cases, it will be found that the human reservoir is more important than the animal, but where there is evidence that flies are becoming infected from this alternative source, then game can be killed or driven off.

Treatment of cases requires hospitalization as the drugs are highly toxic. Suramin is effective in early and intermediate cases of both T. b. rhodesiense and T. b. gambiense. When the CNS is involved, Melarsoprol is the drug of choice. Eflornithine is very effective in all stages of T. b. gambiense infection (including cerebral), but is not effective in T. b. rhodesiense. Pentamidine has been used as a prophylactic against T. b. gambiense to people at special risk. There is no prophylactic against T. b. rhodesiense infection.

Surveillance In a sleeping sickness area, a surveillance service should be set up. Sleeping sickness workers are recruited more on their knowledge of the local community than their medical skills, as the simple techniques of gland puncture or making a blood slide can easily be taught. The workers cover a set area and take slides from people with symptoms of persistent fever and headache, or those who pursue a particular occupation, such as hunters, honey collectors or wood-cutters. In T. b. gambiense infection, palpation for neck glands can provide a useful estimate of prevalence. In an epidemic of T. b. rhode-siense, a mass blood slide examination can be performed in the worst-affected areas to detect asymptomatic cases.

The illness in animals is more extensive than in the human population and veterinary services often set-up extensive surveillance and control programmes, so combining efforts with them can be of value.

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