A comparison of use of a pyrethroid either for house spraying or for bednet treatment against malaria vectors. A trial of bed nets (mosquito nets) as a malaria control strategy in a rural area of The Gambia, West Africa. Insecticide-impregnated bed nets for malaria control: varying experiences from Ecuador, Colombia, and Peru concerning acceptability and effectiveness.
In an intensely malarious area in north-east Tanzania, microencapsulated lambdacyhalothrin was used in four villages for treatment of bednets (provided free of charge) and in another four villages the same insecticide was used for house spraying. Another four villages received neither intervention until the end of the trial but were monitored as controls. Bioassays showed prolonged persistence of the insecticidal residues. Light traps and ELISA testing showed reduction of the malaria vector populations and the sporozoite rates, leading to a reduction of about 90% in the entomological inoculation rate as a result of each treatment. Collections of blood fed mosquitoes showed no diversion from biting humans to biting animals. Incidence of re-infection was measured by weekly monitoring of cohorts of 60 children per village, after clearing preexisting infection with chlorproguanil-dapsone. The vector control was associated with a reduction in probability of re-infection per child per week by 54-62%, with no significant difference between the two vector control methods. Cross-sectional surveys for fever, parasitaemia, haemoglobin and weight showed association of high parasitaemia with fever and anaemia and beneficial effects of each intervention in reducing anaemia. However, passive surveillance by resident health assistants showed no evidence for reduced prevalence of fever or parasitaemia. Net treatment consumed only about one sixth as much insecticide as house spraying and it was concluded that the former intervention would work out cheaper and nets were actively demanded by the villagers, whereas spraying was only passively assented to. An intervention trial was undertaken in a rural area of The Gambia to assess the impact on malaria morbidity of the use of bed nets. Bed nets were allocated at random among a group of 16 Fula hamlets, where they were previously rarely used. The incidence of febrile episodes with associated malaria parasitaemias throughout the rainy season and the prevalence of splenomegaly and parasitaemia at the end of the rainy season were determined in 233 children aged 1-9 years who slept under bed nets and in 163 children who did not. Bed nets were used correctly by the children in the study cohort, but direct observations showed that a significant number of children left their nets for a period during the night. There was no significant difference in the incidence of clinical attacks of malaria or in any other malariometric measurement between the 2 groups. Thus, bed nets were not effective in reducing malaria morbidity in this group of children. The apparent protection from bed nets demonstrated in previous retrospective surveys may have been due to an increased number of infective bites being received by exposed individuals sleeping close to users of bed nets. Between 1991 and 1994, an intervention program with permethrin- and lambdacyhalothrin-impregnated bed nets was carried out over a period of nine months in each of five endemic, malarious areas of Ecuador, Peru, and Colombia. This program was evaluated through household surveys, blood sampling, in-depth longitudinal studies, and entomologic analysis. Eighty-four communities (including approximately 35,000 individuals) were paired according to malaria incidence, size, and coverage with bed nets and then randomly allocated to intervention and control groups. The results showed that peoples' acceptance of the measure was related to their perception of an immediate protective effect against insects. The effectiveness of the bed nets, measured as a reduction of malaria incidence in intervention communities as against control communities, showed large variations between and within the study areas. The protective efficacy varied between 0% and 70% when looking only at the postintervention differences between intervention and control groups. The average protection was 40.8% when considering a four-month incidence of clinical malaria attacks and 28.3% when considering a two-week malaria incidence. Important factors for the success of the bed net program were insect susceptibility to pyrethroids, high coverage with impregnated bed nets, high malaria incidence, good community participation, high mosquito densities when people go to bed, and a high proportion of Plasmodium falciparum. In one area, where DDT spraying in the control communities was executed, the effectiveness of bed net impregnation was slightly better than that of spraying.
Field entomological studies confirm that EMRs have no effect on preventing mosquito bites. Therefore there is no justification for marketing them to prevent malaria infection.