Factors affecting a full-scale implementation programme

The different achievements in the large scale trials in different pans of the world suggest that a full-scale implementation of impregnated nets can be very complicated and requires more extensive interdisciplinary research. Many factors responsible for the effectiveness of the implementation should be studied at the beginning and the results should be used for guiding the implementation.

4.1.4.1 Local epidemiological conditions

The nature of malaria transmission in any single area is always different from others and should not be generalised (Molineaux et al. 1988). There are many factors related to malaria transmission. However, the major relation of the vectorial capacity and the prevalence of malaria have long been established (Ross, 1911) and several models of malaria transmission have been satisfactorily used to explain the different epidemiological conditions in different parts of the world (Molineaux, 1985). According to these transmission models, a large reduction of vectorial capacity in a holoendemic area needs to be achieved before the parasite prevalence starts to decrease. In a hypoendemic area, only a small reduction of the vectorial capacity will result in a considerable reduction of the parasite prevalence.

These may partly explain the un-satisfactorily results in high endemic areas in Africa and Papua New Guinea, and impressive results in low endemic areas in China.

However, Snow et al (1988c) suggested that, considering the long-term malaria control goals in highly endemic areas, the 63 - 72% efficacy achieved in The Gambia might be the most suitable level for that endemic area in which the natural immunity was still allowed to be built up by a lower number of infective bites.

4.1.4.2 Pyrethroid resistance

The principal resistance mechanisms of pyrethroids may include (Miller, 1988):

a) penetration resistance, where the composition of the insect’s exoskeleton becomes modified in ways that inhibit pyrethroid penetration;

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b) site-insensitivity, where the chemical site of action for a pyrethroid becomes modified to reduce sensitivity to the active pyrethroid form.

c) metabolic resistance, where the metabolic pathways of the insect becomes modified in ways that detoxify the pyrethroid.

d) behavioral resistance, where insect behaviour becomes modified so the insect no longer come into contact with the pyrethroid deposit;

Physiological resistance of anopheline mosquitoes to DDT and other classes of insecticides have been found in many areas. These are mainly caused by insect’s knockdown-resistance (kdr) gene and multi-function oxidase enzymes (WHO, 1986). Pyrethroids posses similar mechanisms of action and resistance as does DDT. Cross-resistance o f kdr type resistance between DDT and pyrethroids have been found in many species. This causes wide concern with the cross-resistance problem that may be anticipated when using pyrethroids in areas of DDT-resistant mosquitoes (Elliott et al. 1978; WHO, 1986). However, Malcolm (1988) recently reviewed the global situation of pyrethroid-resistance in anophelines and concluded that there was rather weak evidence o f kdr-type resistance in anophelines reported around the world.

An.stenhensi was the only Anopheles so far in which there was strong evidence of this type o f resistance. Chakravorthy and Kalyanasundaram (1992) found that An.stephensi selected by LDJ0 of permethrin for 5 generation resisted to permethrin, cypermethrin, alphamethrin and deltamethrin; and adding of synergistic chemicals (piperonyl butoxide) to permethrin, did not overcome the development of resistance.

Organophosphate might be substituted for pyrethroids if resistance to the latter arises.

Pyrethroids are detoxified by several esterases which are susceptible to inhibition by organophosphate and carbamate insecticides. Therefore, the use of pyrethroids mixed with these insecticide are reserved on toxicological grounds (WHO, 1989).

Another important mechanism of resistance is behavioral resistance. Several year after the implementation, shifting of biting times of the vectors from post-midnight to pre-midnight and biting habits from indoor to outdoor, could be expected (Charlwood & Grave, 1987). These are the same behavioral changes which were found after DDT house spraying (Ismail et al. 1975) and difficult to prevent. More infection may be contracted outdoor and outside the village. People may bring the impregnated nets with them and sleep under the nets when they stay

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overnight outside the house to protect themselves from mosquito biting. This has been practised and found effective in rice-field huts in China with exophilic An.dirus (Li Zuzi & Lu Baolin: quoted in Curtis et al. 1990). However, in high transmission areas, other personal protection, such as repellents (Lindsay and Janneh, 1989), and vector control measures should be integrated (WHO, 1983).

4.1.4.3 Human factors

These are suspected to be the major factors responsible for the failures in large-scale intervention trials of untreated nets in The Gambia (Snow et al.

1988a) and impregnated nets in Malaysia (Hii et al. 1987). The villagers and children are not always in the nets when the vectors come to their houses. The mosquitoes usually have a chance to feed on these people before they retire or during the night when they get out of the nets for either indoor or outdoor activities. Moreover, they may even sleep outside the nets due to the uncomfortable hot climate. These problems may be partly related to the knowledge and perception of people about the risk of malaria and the proper use of bed nets; and partly related to intrahousehold cultural preferences. Other behaviours are related to the knowledge and perception of the additive effects of insecticides on the nets. In some areas, nets may be already widely used and it is common to have them washed frequently (MacCormack &

Snow, 1986; Rozendaal et al. 1989). This has been proved to drastically reduce the efficacy of the impregnated nets. Health education therefore needs to be launched before or at the beginning of the implementation of impregnated bed nets. Before that, surveys of the sociocultural preferences of the local people of bednets and their use, are essential for understanding the community and achieving effective cooperation (MacCormack & Snow, 1986). These may also serve as an important guideline for improving the technical aspects of designing and treating nets to synchronise with the sociocultural conditions (Rozendaal et al. 1989).

4.1.4.4 Costs and affordability

permethrin-impregnated bednet was $2.05. The cost per year including the cost of preferable considering the long-term self-reliance goal.

In China, bednets are domestic products. After the initial successful interventions, impregnated bednets have been widely used in a full-scale control programme covering millions of people in endemic areas (Lu Baolin, 1991).

The costs of this new implementation was estimated and compared with those of DDT house spraying. The cost of treatment with deltamethrin varied from about $0.05 in Sichuan Province to $0.10 per capita per year in Jiangsu Province. It was estimated to be about half to one-quater of the cost of DDT residual sprays and is about half the cost of permethrin treatment in Jiangsu Province in 1986.