Comercio exterior

Lindsay and Gibson (1988) traced a history of mosquito nets back to the sixth century B.C.in Egypt. They later seemed to be developed independently in many isolated communities around the world. The original idea was probably for uninterrupted sleep. Most of the early records of use of bed nets were among the aristocrats and they were luxuriously decorated. Later, the idea spread and they were

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used by women, children and fishermen. Nowadays, mosquito nets are still commonly used among ordinary people in the tropics.

4.1.1.1) Proper use of untreated nets

In order to derive maximum protection, Coveil, in 1943 (quoted in Farid 1988), suggested as follow:

1. The net material should be white to allow easy detection of mosquitoes; all sides should consist of netting (with only a lower strip of calico) to allow free access of air.

2. A large bed and net should be used, in order to minimise the possibility of pushing bare hands, knees or elbows against the net during sleep, so allowing mosquitoes to bite through the net from the outside.

3. The net should be tucked in all round, under the mattress.

4. If the net was lifted from the bed by day, the bottom should be closed and it should be let down before dusk, stretched and evenly tucked under the mattress.

5. Upon retiring the net could be loosened along one side of the mattress just sufficient to allow the person to crawl into bed, and then be tucked back in place again from the inside.

6. Any rents or tears should be mended or repaired. Before retiring, the interior should be searched for any mosquitoes or tears. It is a wise precaution to spray the inside with a pyrethrum space insecticide before getting under the net.

Today, all these suggestions are still valid. However, entomologists have long recognised that keeping a net in place is a problem. In fact, they even used bednets for sampling mosquito populations in the field (Lindsay & Gibson 1988).

4.1.1.2) Impact of untreated nets on malaria transmission i) Effects on anophelines

In The Gambia, Port and Boreham (1982) used an experimental hut trial to study the effects of bed nets on feeding by An.eambiae in the field. They also made holes in the nets to simulate the usual condition. The results showed that

all the nets tested reduced mosquito feeding and this reduction inversely correlated with the number and size of holes in the nets. They also found that more unfed female anophelines left the huts with the nets than the hut with no net to seek for blood meals elsewhere. This suggested that bed nets were an effective means of reducing attacks by anophelines. Similar findings were also found by Charlwood (1986) in Papua New Guinea and Lindsay et al (1989) in The Gambia. In Tanzania, however. Lines et al (1987) simulated a badly tom net by cutting 8 holes of 10 x 20 cm. into it and found that it gave no more protection than without nets. Although the reduction of man-vector contact was shown. Port and Boreham (1982) found little impact of bed nets on malaria transmission especially in highly endemic areas. In these areas a high percentage of anophelines were infected and there was usually significant man-mosquito contact in the early part of the night prior to retiring.

Lindsay et al (1989) reported that children under bednets still received more than one infective bite per year, suggesting that they were not effectively protected from malaria infection by sleeping under untreated bednets in The Gambia.

ii) Effects on disease

There have been a few records in the past on the effectiveness of bed nets in preventing malaria attacks mainly from military experience (Lindsay and Gibson, 1988). Recently, retrospective case-control studies in Columbia (Banguero, 1984) and Thailand (Butrapom et al. 1986; Fungladda et al. 1987) have shown that mosquito net users are significantly exposed to a lower risk of malaria attacks than non-users. An observational cohort study in The Gambia (Bradley et al.

1986) and an intervention study in Kenya (Nevill et al. 1988) showed similar results.

However, longitudinal surveys of school children of a village of 1,000 in an area with perennial transmission in Congo, showed no significant differences between the malaria incidence rates in a bednet user group and those in a non-user group (Trape et al. 1987). Moreover, a randomised control intervention trial of bed nets by Snow et al (1988a) in The Gambia, which used a village as an intervention unit, showed that the risks to children age 1-9 years getting malaria attacks were not correlated with the use o f bed nets. They concluded that bed nets were not effective in reducing malaria morbidity in this group of children because a significant number of them were found leaving their nets for a period during the night. The apparent protection

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from bed nets demonstrated in previous retrospective studies might have been due to an increased number of infective bites being received by exposed individuals sleeping close to users of bed nets. These confirmed the previous entomological findings (Port and Boreham, 1982; Lindsay et al. 1989a). However, these findings cannot be generalised to the other areas which have lower levels of transmission.

In conclusion, untreated bednets may be useful for individual protection against malaria if properly used in low endemic areas, but at the same time, the

3. Mosquitoes may enter nets from underneath through slits in mattress supports which are made of woven string or maze stalks.

4. Users may be bitten before they go to bed.

5. Mosquitoes may be waiting around for their chance to obtain a blood meal when the users get out of the nets during the night.