Larvicides

• biological control;

• environmental modification.

These are all illustrated in Fig. 3.9.

Adulticides Killing the adult mosquito can either be done while it is flying using a

Surface Hole and defecation bury

DRY SYSTEM

Pour-flush latrine

Pour-flush to soakaway

WET SYSTEM

us ;

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i

Fig. 3.8. Types of excreta disposal systems -

incremental sanitation.

Fig. 3.8. Types of excreta disposal systems -

incremental sanitation.

Simple pit latrine

Improved pit latrine

Composting latrine

Flush toilet to septic tank

Flush toilet connected to sewerage system

Flush toilet to septic tank

Flush toilet connected to sewerage system

Fig. 3.9. Mosquito control methods.

knock-down spray or when it is resting with a residual insecticide. Knock-down insecticides will kill adult mosquitoes at the time of application only, whereas residual insecticides continue to have a lethal effect for a considerable period of time.

knock-down insecticides are used to control epidemics of vector-transmitted disease where an explosive increase in the number of flying adults is responsible. They have been used in malaria epidemics, but have perhaps their greatest value in dengue and the control of arbovirus infections. They are used as space sprays (aerosols) in the house, for mosquito survey counts and for disin fecting aircraft. Knock-down sprays commonly contain pyrethrum, derived from a species of chrysanthemum grown in highland areas of East Africa. They can be dispersed in aerosols, smoke generators (fogging) or ultra-low volume (ULV) aerial sprays.

residual insecticides Residual spraying is the main method for control of mosquito-transmitted disease because the insecticide continues to remain active for 6 months or more. By careful organization, repeated applications made at regular intervals can maintain a continuing killing effect. Ideally they should be sprayed just before the start of the main transmission season, especially in areas where malaria is seasonal.

Residual insecticides act on the resting mosquito. Mosquitoes need to rest after they have taken a blood meal and generally choose the nearest place, which is the wall of the victim's house. If the wall has been sprayed with residual insecticide, then the mosquito will absorb a lethal dose through its legs while it is resting. The insecticide can either be sprayed as an emulsion or wettable powder, as few of the insecticides commonly used go into solution with a cheap and easily obtainable medium such as water. Emulsions are best on non-absorbent surfaces, while wettable powders are suitable for mud, leaf or other poor quality walls. The wettable medium (generally water) soaks into the wall and leaves the powder on the surface. Some of the insecticide is taken into the porous surface, but this gradually comes out, maintaining a steady concentration. Once residual insecticide has been sprayed on a wall, then it must not be washed or painted.

Residual insecticide sprayed on a surface depends upon a number of factors:

• the proportion of active insecticide in the preparation;

• the amount of insecticide mixed with the fluid medium;

• mixing, before and during application;

• the distance from the surface that is sprayed;

• the speed of application.

These are all specified for a particular insecticide and sprayers must be trained to ensure that the right concentration is delivered. A measured area of plaster can be scraped and the insecticide content analysed.

Residual spraying is carried out by a team of sprayers with manually operated spray apparatus covering a village at a time. Houses are emptied and pets and domestic animals restrained at a suitable place some distance away (as they are sensitive to insecticides). Any insects, beetles and lizards that are killed should be swept up and disposed of before the domestic animals are allowed back into the houses. This takes a considerable amount of organization with a strict schedule of notification, followed by spraying. The supervisor answers any questions, ensures that the work is done and arranges logistic support. If residual spraying is not adequately explained to people, then organizational resistance will develop. The target is to spray every dwelling house whether permanently or temporarily occupied.

Deterrents and repellents can be either smokes or applications to the body in the form of creams and solutions. They do not kill the insect, but deter it from biting.

Mosquito coils or heated pads have a combined deterrent and repellent action. They are made with small quantities of pyrethroids in a slow burning base, but other insecticides can be added to enhance the activity. Used in a still atmosphere, they can be most effective. If they do not prevent all the bites, they reduce the number, which is important in filariasis transmission. They reduce the probability of being bitten by an infective mosquito carrying any disease.

The most commonly used repellent is diethyltoluamide (DEET), which can be applied to the person, clothing, tents and mosquito nets. The solutions can either be dissolved in methylated spirit or emulsified with water and applied to the surface. It is not absorbed by synthetic fabrics and a cotton or wool base is essential if it is to remain for some time. Four weeks of activity is given if continuously exposed, but if the garments (such as a shawl or leg bands) are kept in a polythene bag, then repellent action can continue for 3— 6 months. Precaution should be taken while applying DEET to the skin as some individuals are sensitive, while neurological toxicity can be produced in children. Natural repellents made from eucalyptus oil are preferable for application to the person.

Mosquito nets and personal protection Personal protection is a valuable precaution in reducing the number of mosquito bites.

Clothing that covers the arms and legs especially if combined with a repellent can protect an individual most effectively. With the appearance of widespread insecticide resistance, greater reliance must now be placed on personal protection.

The use of mosquito nets is a well-tried method of personal protection. Mosquito nets are fitted to the bed and the edges tucked under the mattress. A knock-down spray applied prior to retiring will prevent any mosquitoes entering the net when the occupant goes to bed. Young children should be placed under nets before it gets dark. If the custom is to sleep on a mat on the floor rather than a bed, then mosquito nets can still be used. The sale of subsidized mosquito nets can be an effective method of malaria control, if they are subsequently treated with an insecticide.

Mosquito nets are treated with synthetic pyrethroids, such as permethrin, deltamethrin, lambda-cyhalothrin or alpha-cypermethrin. They deter mosquitoes from entering should the net be torn or kill it if it touches the net. Nylon nets are better than cotton because they absorb less solution and are stronger, but this has to be offset by their greater cost. Additional advantages of treated nets are that they provide some protection to other people sleeping in the same room. They also kill fleas, lice, bed bugs and cockroaches and even if rolled up will still provide some protection. A modification of this method is to treat curtains that are used to cover doors, windows or any opening. These methods are used in community malaria control programmes.

Nets are treated by soaking them in a solution of the insecticide when new or after they have been washed. The amount of insecticide is 200mg/m2 permethrin, 25mg/m2 deltamethrin or 10mg/m2 lambda-cyhalothrin, calculated by measuring the area of the net. Some treated net programmes are using standard sized nets, all of which are made of the same material, to avoid having to measure each one, but a rough approximation can be made by weighing each net. Once nets have been treated, they should not be washed again until just before re-treatment (normally every year) as this decreases the effectiveness of the insecticide.

Some people suffer from nasal congestion when sleeping under a net that has recently been treated with deltamethrin or lambda-cyhalothrin and it is probably better to put it to one side for the first 2 days if either of these insecticides has been used. Otherwise they are perfectly safe and no long-term effects have been recorded.

One of the problems of treating mosquito nets is that they need to be retreated at annual or 6-monthly intervals, so a recent innovation has been long-lasting insecti-cidal nets (LLIN) where the insecticide is impregnated into the fibre of the net before it is woven. Such nets are effective for 4 years or more and, therefore, are being actively promoted for malaria control.

A less satisfactory alternative is to screen the whole house, but this is expensive and a torn area will destroy the whole effect. Air conditioning, by providing a sealed room, generally prevents mosquitoes from entering. Even so, it is preferable to use a knock-down spray in the evening to prevent any mosquitoes that may have entered. The cost of these methods is considerably higher than using treated mosquito nets.

Larvicides Substances that block the breathing apparatus of mosquito larvae and destroy the surface tension (so they sink to the bottom) or poison them are known as larvicides. Kerosene spread on water covers the siphon of the larvae so that it dies from asphyxiation. High-spreading oils have been developed, which inactivate the force of surface tension that larvae use to float on the surface. Insecticides sprayed on collections of water will kill larvae as well as many other organisms (including fish), are expensive and generally objected to by the public and hence are rarely used as larvicides. Such preparations as temephos (Abate), with its very low toxicity, are a notable exception.

Larvicides are not efficient methods of mosquito control, their main use being in urban and periurban areas, especially against culicine vectors. Drains and gutters can be sprayed and temephos added to water containers and septic tanks. Surface sprays must be renewed at regular intervals.

To control Culex quinquefasciatus, the main vector of urban filariasis, which breeds in latrines or soakaways, expanded polystyrene beads can be placed in the pit. The beads float on the surface of the water so larvae are dislodged and prevented from breathing, while the function of the latrine or soakaway is not disrupted. The polystyrene is manufactured as fine granules and when placed in boiling water, it expands into beads.

Biological control The term biological control is used to describe the natural method of reducing vectors. Various natural agents that have been tried include predators such as larvivorous fish, microbial organisms (e.g. Bacillus thuringiensis and B. sphaeri-cus) or modification of the insect itself. Male insects can be sterilized by radiation or with chemosterilants and then released into the environment. If these sterile males compete successfully with the unsterilized males, then the females will not be fertilized. Unfortunately, this technique requires the preparation and release of a sufficient number of males to outnumber those in the natural habitat, which is generally impractical. An alternative technique is to breed mosquitoes that are refractory to the target disease. This can either be through genetic manipulation or by introduction of a closely related natural species. Species replacement, as the method is called, offers some promise because similar, but competitive species can be obtained from different parts of the world.

The problem with any biological method is that nature requires a balance. If a predator destroys all its food supply, then it will die. As a result, an equilibrium is reached where the number of predators and those they prey on remain in sufficient numbers for both to exist. Biological control is, therefore, more an aid rather than a definitive method.

Environmental modification In some situations, it is possible to modify the environ ment to make it unsuitable for the vector. This can include simple methods such as burying tin cans or cutting holes in old tyres to drain water, to clearing vast tracks of forest for tsetse fly control. Any method of environmental modification on a large scale must carefully consider other systems that may be damaged. Clearing large areas of forest can affect the water retention of the soil and deforesting river banks can lead to severe erosion. On the other hand, filling-in or draining a swamp can provide extra land. Eucalyptus trees, which absorb large amounts of water from the soil, can be planted and at a later time, their wood can be used.

Specific methods of environmental modification, such as for trypanosomiasis, will be found under the particular disease, while the emphasis here will be on mosquito control. One of the most successful methods for reducing surface water and preventing breeding places is the construction of subsurface drains. This should be within the ability of most health personnel. The system of drains should follow the contours (Fig. 3.10) and be at least 1.5 m below the surface. The gradient needs to be between 1 in 400 and 1 in 30. Various materials can be used for constructing the drains, such as stones, bamboo or poles laid length-wise in the bottom of the drain. Another method of environmental control is to use a siphon, which flushes out mosquito larvae, or a simple dam as shown in Fig. 3.11.

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