Permethrin-treated Bed Nets Research Articles

Seasonal abundance and biting behaviour of Anopheles punctulatus and An. koliensis in Malaita Province, Solomon Islands, and a trial of permethrin impregnated bednets against malaria transmission.

Seasonal abundance of the malaria vectors Anopheles punctulatus Dönitz and An.koliensis Owen in Bilimanu, an isolated inland village with forty-two houses in Malaita Province of the Solomon Islands, was monitored over 28 months by means of all-night landing/biting catches at one site during June 1985 to September 1987. Totals of 1250 An.punctulatus and 141 An.koliensis were collected, the latter being the largest number of this species ever caught at any locality in the Solomons. Bednets impregnated with permethrin 0.5 g/m2 were introduced in December 1986 to be used at night by all 190 villagers for protection against malaria vectors. Bioassay tests with An.punctulatus blood-fed females exposed under nets for 10 min resulted in 100% mortality up to 50 weeks post-impregnation. For An.punctulatus, the main vector species, the mean catch (indoors + outdoors) per man hour was 2.9 (range 0.7-13.2) before a cyclone on 19 May 1986, and, 0.66 (0.2-2.7) after the cyclone. The vector survival rates were usually high before the cyclone, but erratically lower thereafter for An.punctulatus. An.koliensis disappeared after the cyclone. Both An.punctulatus and An.koliensis consistently showed higher rates of biting man indoors than outdoors and their diel biting cycle showed a peak around midnight. Outdoors, the parous proportion of An.punctulatus was twice the nulliparous, and nearly so indoors. Following intervention with permethrin-treated bednets, the mean catch of An.punctulatus fell to 0.35 per man-hour (monthly range 0-1.5). The Plasmodium falciparum malaria infection rate reduced from 10% pre-intervention to zero in September 1987, 9 months after intervention, and then rose again.(ABSTRACT TRUNCATED AT 250 WORDS)

The Effectiveness of Permethrin-Impregnated Bed Nets against Malaria for Migrant Workers in Eastern Thailand

A randomized, double-blind, field trial was carried out to compare the effectiveness of permethrin-treated bed nets with that of untreated nets as a method of malaria control for migrant workers in eastern Thailand. The study was conducted using 261 subjects in eastern rural areas that are known to be highly endemic for multidrug-resistant Plasmodium falciparum infection. One hundred twenty-six subjects used treated nets, while 135 used untreated nets. During the 35 weeks of observation, 23 subjects using treated nets and 33 workers using untreated nets developed 28 and 51 episodes of malaria, respectively (P = 0.029). The reduction in risk per subject due to treated nets was 0.06. The residual effects of permethrin were tested using a World Health Organization standard bioassay. Anti-mosquito activity was found to be present in the nets for more than 16 months. We conclude that because of the failure of the development of safe, effective, long-lasting prophylactic agents, integrating the use of impregnated nets with large-scale primary health care programs may be a partially effective method for controlling malaria in eastern Thailand.

Experimental hut trials of bednets impregnated with synthetic pyrethroid or organophosphate insecticide for mosquito control in The Gambia

1. Nylon bednets impregnated with different insecticides were evaluated in 1988 against wild adult mosquito populations, mostly Mansonia africana (Theobald) and Anopheles gambiae Giles sensu lato, entering experimental verandah-trap huts in The Gambia. Each bednet had six 10 x 10 cm holes made in the walls to simulate torn conditions and permit female mosquitoes to enter and feed on sleepers. 2. Individual net treatments, determined by gas chromatography of net samples from before and after 12 weeks use of the bednets, were: permethrin 670 +/- 159 and 405 +/- 190 mg/m2 (40% loss), cypermethrin 37 +/- 8 and 16 +/- 9 mg/m2 (57% loss), deltamethrin 10 +/- 7 and 10 +/- 8 mg/m2 (no loss), lambda-cyhalothrin 2.6 +/- 0.9 and 1.6 +/- 0.5 mg/m2 (38% loss), pirimiphos-methyl 4017 +/- 117 and 1160 +/- 319 mg/m2 (71% loss). 3. Washing three times in the traditional manner with local cow-fat soap reduced the initial dosages by about 85% of cypermethrin and lambda-cyhalothrin, 99.8% of pirimiphos-methyl and left no detectable residues of deltamethrin or permethrin. 4. The unwashed permethrin-treated bednet reduced the number of mosquitoes entering a hut by 60% of An.gambiae s.l. and 68% of Mansonia spp. This deterrency was less pronounced with the other insecticides and was lost by washing the bednets. 5. Each insecticide, especially lambda-cyhalothrin and pirimiphosmethyl, caused significant mortality rates of mosquitoes that entered huts with impregnated bednets, and prevented the majority of An. gambiae s.l. and Mansonia females from bloodfeeding. Washing completely removed the efficacy of deltamethrin and permethrin treated bednets, whereas nets treated with cypermethrin, lambda-cyhalothrin or pirimiphos-methyl remained significantly insecticidal after washing. 6. Aerial toxicity from the pirimiphos-methyl treated bednet killed 80% of An.gambiae s.l. confined overnight in the hut at the end of the trial, whereas the pyrethroid-treated bednets gave negligible mortality rates of mosquitoes. 7. Sleepers using the bednets had no medical symptoms significantly associated with any of the treatments. On the contrary, from 216 interviews, 4/10 complaints were associated with the use of untreated nets (P approximately 0.05), perhaps because sleepers were kept awake by mosquitoes and became more aware of any ailments. 8. It is concluded that permethrin tends mainly to deter mosquitoes from house-entry, enhancing personal protection, whereas the other insecticides kill higher proportions of the endophilic mosquitoes, which would give better community protection against malaria transmission.

Permethrin‐impregnated bednets reduce nuisance arthropods in Gambian houses

The prevalence of bedbugs (Cimex hemipterus L.), chicken ticks (Argas persicus Oken) and headlice (Pediculus capitis De Geer) was surveyed in a rural area of The Gambia. At the beginning of the study 37.5% of children's beds were infested with bedbugs and 3.9% with chicken ticks, whilst the prevalence rate of pediculosis in children under 10 years old was 28.8%. Both bedbugs and headlice were clustered within compounds. Headlice prevalence increased with hair length and they were more common on girls than boys. Following this cross-sectional survey all bednets in the sixteen hamlets were either dipped in permethrin or a placebo. About 4 months later it was found that bedbugs and chicken ticks had disappeared from homes in which the bednets had been impregnated with permethrin. There was no reduction in hamlets with placebo-treated bednets. The rate of acquiring headlice between the two surveys was reduced by 91.1% in children who slept under insecticide-treated bednets compared with children with placebo-treated bednets. There were also significantly fewer day-flying and crawling insects, except earwigs, in homes of children who slept under insecticide-treated bednets compared with those with placebo-treated nets. These additional benefits of permethrin-treated bednets should contribute to their widespread acceptance and utilization by the community for personal protection.

Impact of permethrin‐treated bednets on malaria transmission by the Anopheles gambiae complex in The Gambia

Malaria vector mosquitoes belonging to the Anopheles gambiae complex were studied in four hamlets in The Gambia. All inhabitants were given bednets treated either with a placebo (milk) in two hamlets or with the pyrethroid insecticide permethrin (500 mg/m2) in two other hamlets. Malaria transmission occurred mainly during a few weeks of the rainy season, in September and October 1987. The indoor resting densities of mosquitoes in permethrin-treated hamlets were reduced, and we estimated over 90% reduction in biting on man by An. gambiae Giles sensu stricto in these hamlets. No mosquitoes were found under permethrin-treated bednets compared with eighty-one recovered from placebo-treated bednets. Mosquitoes exited more readily from rooms where permethrin-treated bednets were used than from rooms with placebo-treated nets. The annual mean probability that a child would receive an infective bite was estimated to be 0.09 in hamlets with insecticide-treated bednets, compared with 1.9 where placebo-treated bednets were used. Permethrin-treated bednets are therefore recommended as a means of effectively reducing the risk of exposure to malaria transmission, particularly in areas of low seasonal transmission.

Sporozoite antibodies and malaria in children in a rural area of The Gambia.

Sporozoite antibody levels were measured in a group of children aged one to nine years resident in a rural area of The Gambia, using an ELISA to the repeat peptide (NANP)40. The prevalence and titre of antibodies varied with age but not with sex or ethnic group. Significant variations in prevalence were recorded within a group of adjacent villages. Children who were seropositive at the beginning of the dry season had higher spleen and parasite rates both at this time and at the end of the subsequent rainy season than did seronegative children, suggesting that they were exposed more frequently to infection. However, seropositive children had fewer episodes of fever accompanied by high levels of parasitaemia than did seronegative children, suggesting that they had a greater degree of clinical immunity. No differences were found in seroprevalence rates or in mean antibody titres between children who slept under conventional or Permethrin treated bed nets and those who did not, even though bed nets significantly reduced the number of bites by vector mosquitoes.

Permethrin-treated bed nets (mosquito nets) prevent malaria in Gambian children

The incidence of clinical attacks of malaria was significantly less in Gambian children aged 1–9 years who slept in villages where all the bed nets (mosquito nets) were treated with permethrin than in children who slept in control villages with placebo-treated nets. Significant differences in changes in spleen size and in packed cell volume were also observed between the 2 groups during the course of a rainy season. No side effect was noted. Treatment of bed nets with insecticide is a form of malaria control that is well suited to community participation and can readily be incorporated into primary health care programmes. Insecticide-treated nets may be more effective in areas of seasonal or low intensity transmission than in areas with heavy perennial challenge.

A trial of permethrin-treated bed nets in the prevention of malaria in Gambian children

A trial was undertaken in a rural area of The Gambia to investigate the impact of permethrin-treated bed nets on malaria. Two groups of children, matched for age, sex, and malaria exposure, were followed through the rainy season of 1985 for illness and febrile episodes. One group of 205 children slept under permethrin-treated bed nets (0.5 g/m 2); 184 children who slept under placebo-treated nets formed the control group. At the end of the rains the children were examined for splenomegaly and blood samples were taken for determination of packed cell volume (PCV) and parasitaemia. Permethrin treatment of bed nets was well accepted and was without side-effects. Children who slept under treated nets had significantly fewer episodes of clinical malaria than control children. However, at the end of the rains there was no significant difference in the prevalence of splenomegaly or parasitaemia or in the mean PCV between the groups. It is suggested that permethrin treatment of nets may have a greater effect on the duration of probing by mosquitoes for a blood meal than on the number of bites received.

Observations on Anopheles gambiae Giles s.l. (Diptera: Culicidae) during a trial of permethrin-treated bed nets in The Gambia

AbstractIn a Gambian village, the peak collections of females of the complex of Anopheles gambiae Giles occurred three months after the onset of the heaviest rains; 32% of these were A. melas Theobald, and 54% of blood-meals were identified as originating from man. Counts of A. gambiae s.l. in rooms containing permethrin-treated bed nets were compared with those in rooms containing placebo-treated nets. The numbers in the permethrin-treated nets were far lower than in placebo-treated nets. A higher rate of exophily was noted in rooms containing permethrin-treated nets. The numbers of unfed A. gambiae s.l. found inside the rooms with placebo-treated nets were significantly higher than those with permethrin-treated nets. However, the proportion fed and the mortality in the exit traps were not significantly affected by permethrin treatment. Three-minute bioassays conducted on four different fabric types impregnated at the same concentration showed that the toxicity varied between the fabrics. Handwashing severely reduced the toxicity and approximately halved the permethrin content. Bed nets are frequently washed in Gambian villages, and this could be a problem in the application of permethrin impregnation of bed nets for vector control. Methods are discussed to overcome this problem at the village level.