Impact of permethrin impregnated bednets on child mortality in Kassena-Nankana district, Ghana: a randomized controlled trial.

A community-based randomized, controlled trial of permethrin impregnated bednets was carried out in a rural area of northern Ghana, between July 1993 and June 1995, to assess the impact on the mortality of young children in an area of intense transmission of malaria and no tradition of bednet use. The district around Navrongo was divided into 96 geographical areas and in 48 randomly selected areas households were provided with permethrin impregnated bednets which were re-impregnated every 6 months. A longitudinal demographic surveillance system was used to record births, deaths and migrations, to evaluate compliance and to measure child mortality. The use of permethrin impregnated bednets was associated with 17% reduction in all-cause mortality in children aged 6 months to 4 years (RR = 0.83; 95% CI 0.69-1.00; P = 0.05). The reduction in mortality was confined to children aged 2 years of younger, and was greater in July-December, during the wet season and immediately after (RR = 0.79; 95% CI 0.63-1.00), a period when malaria mortality is likely to be increased, than in the dry season (RR = 0.92, 95% CI 0.73-1.14). The ready acceptance of bednets, the high level of compliance in their use and the subsequent impact on all-cause mortality in this study has important implications for programmes to control malaria in sub-Saharan Africa.

Entomological evaluation of the Gambia's National Impregnated Bednet Programme.

Entomological studies were conducted in paired study villages in three of the five study areas used for the epidemiological assessment of the Gambia's National Impregnated Bednet Programme. Baseline data collected in 1991 were compared with post-intervention data from 1992, when one of each village pair (from areas II, III and V) was included in the treatment programme in which villagers' nets were dipped in permethrin. In a longitudinal study, indoor-resting densities of Anopheles gambiae s.l. in the treated villages were significantly reduced, when compared with their paired untreated village, in areas II (t = 3.32, 13 degrees of freedom, P = 0.006) and III (t = 3.71, 13 degrees of freedom, P = 0.003). However, this was not associated with higher outdoor-biting rates in the evenings in the treated villages than in the controls. The reduction in vector population was most evident in area II, where the treated village was relatively isolated and 74% of the population slept under a treated net. Anopheles gambiae s.l. were also collected during two fortnightly periods in area V, where the sporozoite rates in 1991 had been relatively high (7.65% and 6.07%). There was no significant decrease in the sporozoite rate in the treated village in area V in 1992, despite the fact that this village was over 5 km from its nearest neighbouring village. However, the proportion of villagers sleeping under a treated net in this village was < 50%. No clear evidence for an epidemiologically significant 'mass killing effect', resulting from the village-wide use of treated nets, emerges in this or any other study undertaken in The Gambia. However, such an effect is indicated by results from Salikene, where the village mosquito population was relatively isolated and where the majority of the population slept under a treated net. This situation is unlikely to be repeated in most Gambian villages in the primary-health-care scheme, since treated and untreated villages are generally interspersed and the mosquito populations move freely between them. We must therefore conclude that, on a national scale, the nets provide an exceptionally high level of personal rather than community protection. The mechanisms underlying the personal protection are, however, still unclear.

Socio-economic risk factors for malaria in a peri-urban area of The Gambia.

Successful control of malaria depends upon a detailed knowledge of its epidemiology, including knowledge of the social and economic factors that influence its prevalence. Little is known about the socio-economic factors that influence the prevalence of malaria in tropical Africa. Therefore, we undertook such a study in over 350 Gambian children with malaria resident in a peri-urban area with seasonal transmission, using the case-control approach. Malaria was associated with poor quality housing and crowding and with travel to rural areas, where the level of malaria transmission is higher than in urban centres. No association was found between the risk of malaria and the overall education level of parents or guardians of study children. However, the knowledge of malaria possessed by mothers of cases of malaria was less than that of controls, suggesting that further education of the study community on the causation of malaria and on ways of preventing it could be of value.

Socio-economic determinants are not major risk factors for severe malaria in Gambian children.

Only a small proportion of subjects infected with Plasmodium falciparum develop severe disease. Why this should be is not fully understood. To investigate the possible importance of socio-economic variables on the severity of malaria in Gambian children we undertook a case-control study of 384 children with severe or mild malaria. Few differences were found between the 2 groups. Children with severe malaria had a longer duration of symptoms when recruited than mild cases but this difference was largely accounted for by the fact the most children with severe disease were recruited at a referral hospital, whilst mild cases were recruited at a primary health care facility nearer their home. There was no difference between groups in the time before mothers sought some form of health care. Mothers of children with severe disease were less ready to take their child to hospital than mothers of mild cases, suggesting that education on the importance of taking a child with features of malaria to a health centre as soon as possible might have some effect on the development of severe disease. However, overall, the results of this study suggested that socio-economic and behavioural factors are not the major determinants for severe malaria in African children.

Village trial of bednets impregnated with wash-resistant permethrin compared with other pyrethroid formulations.

A village-scale field trial of pyrethroid-impregnated mosquito nets was undertaken in The Gambia, West Africa, in the Mandinka village of Saruja (13 degrees 13'N, 14 degrees 55'W) during July-November 1989. Nearly all the villagers possessed and used their own bednets. Anopheles gambiae is the main vector of human malaria in the area. An experimental wash-resistant formulation of permethrin was compared with standard emulsifiable concentrate (EC) formulations of permethrin and lambda-cyhalothrin, versus placebo-treated bednets. Target concentrations of pyrethroids on bednets were permethrin 500 mg/m2 and lambda-cyhalothrin 25 mg/m2. The experimental design involved random allocation of a treatment to one net per family. Whereas 68% of people questioned said they washed their nets fortnightly, observations during the 16-week trial period showed that only 4/130 (3%) of nets involved in the trial had been washed as frequently as once per month. Early morning searches for mosquitoes under bednets (1 day/week for 16 weeks) found significantly more mosquitoes (60% An.gambiae) in placebo-treated nets than in pyrethroid-treated nets. The numbers found with each of the three pyrethroid treatments did not differ significantly from each other. Insecticidal efficacy of the treatments was tested by bioassays using wild-caught unfed mosquitoes exposed to netting for 3 min. Linear regression analysis of bioassay mortality against number of times that a net had been washed by villagers showed that nets impregnated with the wash-resistant permethrin retained their insecticidal properties better than nets impregnated with lambda-cyhalothrin or with the standard permethrin formulation.

Entomological risk factors for severe malaria in a peri-urban area of The Gambia.

A study was undertaken of possible entomological risk factors for severe malaria in a peri-urban area of The Gambia. Households of children who had experienced a severe or a mild attack of malaria and of matched controls were visited and their characteristics recorded. Mosquitoes were then collected in the bedrooms of study subjects using both insecticide spray catches and light traps. Mud-walled buildings and bedrooms without ceilings were found more frequently in the households of children who had experienced malaria than in those of controls. Only one difference, which may have arisen by chance, was found between the households of severe and of mild malaria cases; animals were recorded significantly more frequently in the compounds of the severe cases. Mosquito catches reported on two occasions 2 weeks apart showed good reproducibility, indicating that measurements made at the time that a child presented with malaria were likely to reflect those that would have been present at the time that the child was infected. Anopheles gambiae s.l. was found on average more frequently in the bedrooms of children who had experienced malaria than in those of the controls but no difference was found in mosquito numbers between bedrooms of severe or mild cases of malaria. The human blood index and sporozoite rate were similar in mosquitoes caught in households of malaria cases and in those of controls. No evidence was found to support the hypothesis that the level of exposure to malaria-infected mosquitoes is a risk factor for the development of severe malaria.

A malaria control trial using insecticide-treated bed nets and targeted chemoprophylaxis in a rural area of The Gambia, west Africa. 7. Impact of permethrin-impregnated bed nets on malaria vectors.

The impact of permethrin-impregnated bed nets on malaria vectors was studied in 6 pairs of villages during the rainy season in 1989. In each pair, the residents of one village had their nets treated whilst those of the other remained untreated. Routine collections of mosquitoes were made outdoors in the early evening using human-biting collections, and indoors with insecticide sprays, light traps and by searches under bed nets. Mosquitoes of the Anopheles gambiae complex, An. gambiae sensu stricto, An. arabiensis and An. melas, were present in large numbers for 5 months of the study period. These mosquitoes were susceptible to permethrin as judged by bioassay results. Outdoor human-biting rates in the early evening in communities with treated bed nets were similar to those in communities with untreated nets. In villages with treated bed nets most biting occurred outdoors in the early evening with little taking place under impregnated nets. The insecticidal activity of permethrin-impregnated bed nets, dipped by the local population, provided good individual protection against mosquitoes throughout the rainy season and bed nets remained effective even when washed up to 3 times. There was little to suggest that the use of insecticide-treated nets reduced the survival of mosquito populations in villages with impregnated nets. The absence of the expected village-wide effects of net impregnation may have resulted from the circulation of mosquitoes between villages with treated and untreated nets. The proportion of mosquitoes which fed on humans did not differ significantly between villages with treated and untreated nets.(ABSTRACT TRUNCATED AT 250 WORDS)

Variation in attractiveness of human subjects to malaria mosquitoes (Diptera: Culicidae) in The Gambia.

During experimental hut trials to assess the efficacy of insecticide-treated bednets against malaria mosquitoes, we observed that human subjects varied consistently in their attractiveness to mosquitoes. Attractiveness was assessed by estimating the numbers of wild Anopheles gambiae Giles mosquitoes entering a hut in which a man was sleeping, and the numbers of human-bloodfed An. gambiae sensu lato collected from each hut each morning. Five trials were carried out at Wali Kunda in rural Gambia during 2.5 yr. During each 6-wk trial a man slept under a bednet in each of the six huts. Morning collections of mosquitoes from the room, enclosed verandas, and window traps of each hut provided estimates of the number of mosquitoes that had entered during the night. Blood meals were analyzed using an ELISA technique to identify those mosquitoes feeding on humans. Specimens were collected by field workers, not the subjects; therefore, sampling was independent of the subjects' ability to catch mosquitoes. Moreover, the trials were designed to measure the relative attractiveness of individual sleepers to mosquitoes, allowing for other sources of variation (i.e., among huts, bednets, nights, and day of the week). Attractiveness of men to mosquitoes differed significantly among individuals as indicated by the consistent differences between the numbers of mosquitoes entering each man's hut and the numbers feeding on each man. However, the two measures of attractiveness were apparently independent of each other: subjects who attracted consistently high numbers of vectors into their hut did not necessarily have high numbers of mosquitoes feeding on them. These findings support the view that some individuals within a community are at greater risk from mosquito-borne pathogens than others.

Pyrethroid-treated bednet effects on mosquitoes of the Anopheles gambiae complex in The Gambia.

The response of Anopheles gambiae complex mosquitoes to men sleeping under insecticide-impregnated or untreated bednets in six verandah trap huts was studied during the dry season in The Gambia. With this type of hut it was possible to collect live and dead indoor-resting mosquitoes and estimate the number of wild mosquitoes which entered, bloodfed on man, and exited each night. Bednets were treated with emulsions targetted to leave deposits of 25 mg/m2 lambda-cyhalothrin, or 5, 50 or 500 mg/m2 permethrin, diluted from emulsifiable concentrates (EC), or a blank formulation similar to the EC except that the permethrin was omitted; the sixth net was left untreated. Nets and sleepers were rotated between huts on different nights, the design being based on a series of Latin squares and conducted double-blind. Permethrin-impregnated bednets deterred mosquitoes from entering the huts. The degree of deterrency was proportional to the dosage of permethrin. This effect was also caused by the blank formulation and therefore attributed to other components of the formulation, rather than to the permethrin itself. The net impregnated with 500 mg permethrin per square metre gave the best individual protection, reducing mosquito bloodfeeding by 91% compared with untreated nets. However, lambda-cyhalothrin was proportionately more insecticidal than permethrin at doses of equivalent deterrency. At this stage of research, it remains conjectural whether chemical deterrency or killing of malaria vectors is better for community protection.

Malaria in a peri-urban area of The Gambia.

A clinical and entomological survey of malaria was carried out in Bakau, a peri-urban coastal settlement in The Gambia, from June 1988-May 1989. Only 41 of a cohort of 560 children, aged from three months to nine-years-old, experienced a clinical episode of malaria during the observation period. The majority of cases were identified at clinics and not by regular community surveillance. In Bakau Old Town episodes of malaria were more common on the periphery of the settlement, adjacent to typical anopheline breeding sites, than in the centre. Overall malaria cases were not significantly clustered in space and time, although three pairs of cases among children sleeping in the same room at the same time were identified. A cross-sectional survey in November, at the end of the rainy season, revealed a point prevalence parasitaemia of 2.0% and a spleen rate of 0.3%. All malariometric parameters measured were much lower than any found in comparable studies undertaken in rural areas of the country, reflecting the low number of malaria vectors, Anopheles gambiae complex mosquitoes, found in Bakau. Chloroquine consumption, sleeping under bednets, houses with ceilings, the use of insecticide aerosols and burning traditional mosquito repellents may all have contributed to the low prevalence of malaria in the study area.

Following movements of resistant populations of Simulium soubrense/sanctipauli (Diptera: Simuliidae) by means of chromosome inversions.

Following larval insecticide treatment, resistance to the organophosphates temephos and chlorphorxim appeared in a population of the Simulium soubrense/sanctipauli group of the S. damnosum complex on the Bandama Rivers, Ivory Coast. Resistant populations were subsequently identified from other river basins in the north of Ivory Coast. A small chromosomal rearrangement, IIL-A, was found to be associated with resistant populations. Monitoring the presence of this inversion was used to detect the spread or resistance eastwards into untreated areas.