The impact of permethrin-impregnated bednets on malaria vectors of the Kenyan coast

Abstract. The effects of introducing permethrin-impregnated bednets on local populations of the malaria vector mosquitoes Anopheles funestus and the An.gambiae complex was monitored during a randomized controlled trial at Kilifi on the Kenyan coast. Pyrethrum spray collections inside 762 households were conducted between May 1994 and April 1995 after the introduction of bednets in half of the study area. All-night human bait collections were performed in two zones (one control and one intervention) for two nights each month during the same period. PCR identifications of An.gambiae sensu lato showed that proportions of sibling species were An.gambiae sensu stricto > An.merus > An.arabiensis.
Indoor-resting densities of An.gambiae s.l. and the proportion of engorged females decreased significantly in intervention zones as compared to control zones. However, the human blood index and Plasmodium falciparum sporozoite rate remained unaffected. Also vector parous rates were unaltered by the intervention, implying that survival rates of malaria vectors were not affected. The human-biting density of An.gambiae s.l. , the predominant vector, was consistently higher in the intervention zone compared to the control zone, but showed 8% reduction compared to pre-intervention biting rates - versus 94% increase in the control zone.
Bioassay, susceptibility and high-performance liquid chromatography results all indicated that the permethrin content applied to the nets was sufficient to maintain high mortality of susceptible vectors throughout the trial. Increased rates of early outdoor-biting, as opposed to indoor-biting later during the night, were behavioural or vector composition changes associated with this intervention, which would require further monitoring during control programmes employing insecticide-treated bednets.     

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.     

Permethrin-impregnated bednets are more effective than DDT house-spraying to control malaria in Solomon Islands

Abstract. A field trial compared DDT house-spraying with permethrin-impregnated bednets for malaria control in Solomon Islands from 1987 to 1991. Mortality-rates of malaria vector Anopheles farauti in exit window traps were 11.6% from an untreated hut, 10.1% from a hut sprayed with DDT 2 g/m², and 98% of those from a hut in which the occupants used bednets treated with permethrin 0.5 g/m². Since bioassays of the DDT-sprayed walls (15 min exposure in W.H.O. standard test cones) gave 77% mortality of Anfarauti , it was concluded that the insignificant impact of DDT could be explained by the exophilic behaviour of endophagic vectors, whereas the greater impact of permethrin was attributed to the more effective exposure of Anfarauti females to the impregnated bednets - attracted by the occupants. The parous rate was higher indoors, except in the area with permethrin-impregnated bednets. It was therefore concluded that permethrin-impregnated bednets reduced the mean longevity of Anfarauti and hence its vectorial capacity. The circumsporozoite (CS) antigen positivity rate of Anfarauti in the DDT area was 0.18% outdoors, significantly less than 1.42% indoors. In the comparison area CS rates were 0.65% outdoors and 0.75% indoors. CS antigen was not detected in Anfarauti from the bednet area, indicating the apparent prevention of malaria transmission. As DDT spraying was so much less effective, it was discontinued in 1993 and permethrin-impregnated bednets are now the principal malaria control method in Solomon Islands.     

Effects of community-wide use of lambdacyhalothrin-impregnated bednets on malaria vectors in rural Sierra Leone

Abstract. The effect of community-wide use of bednets treated with lambdacyhalothrin 10mg/m² on the malaria vector Anopheles gambiae (forest form) was evaluated in Sierra Leone. Sixteen similar villages near the town of Bo were randomly allocated either to remain without nets or to receive treated bednets for all inhabitants, with effect from June 1992. Mosquitoes were sampled using human biting catches on verandas, light-trap catch (beside an occupied untreated bednet), window exit-trap catch and pyrethrum spray collections. During the first year of intervention (June 1992 to July 1993) the treated bednets provided personal protection for people sleeping under them, but had very little impact on densities of An. gambiae collected on human bait. The human blood index (HBI) of An.gambiae was not affected (HBI = 99% in villages with and without nets). An.gambiae parous rates were significantly reduced in all intervention villages, but malaria sporozoite rates fell in only some of the villages. These results are intermediate between those obtained from other projects in Tanzania and Burkina Faso, where treated bednets reduced man-biting, parity and sporozoite rates, versus The Gambia where treated bednets had no significant impact on any of these factors. Possible reasons for these contrasted findings are discussed.     

The influence of permethrin-impregnated bednets and mass drug administration on the incidence of Plasmodium falciparum malaria in children in Sabah, Malaysia

A small-scale trial was carried out in the Upper Kinabatangan district of Sabah, Malaysia, to determine the effect of using permethrin-impregnated bednets on malaria transmission. A total of 306 nylon bednets with cotton borders, impregnated at a dose estimated to have been 0.062 g permethrin/m2 of nylon netting, were distributed to 139 households in five villages. At the time of distributing bednets, mass drug administration with Fansidar plus primaquine was also administered to the human population to clear all parasitaemias due to Plasmodium falciparum Welch. In another village, for comparison, mass drug administration was the only intervention. After intervention measures in December 1984 and January 1985, the parasite rates in children declined in all villages during the first month, significantly more in the villages with impregnated bednets than in the control, thus proving that the nets had an impact on malaria. However, after about 2 months, parasite rates started to increase again. After 4-6 months, parasite rates in the villages with bednets approached the rate in the control village without nets. The increase in parasite rates was paralleled by a significant deterioration in the quality, physical condition and the degree of non-utilization of bednets. Entomological evaluation proved the efficacy of permethrin-impregnated nets for controlling Anopheles balabacensis Baisas and other anophelines. Bioassays (1 h exposure) of permethrin-impregnated bednets gave 100% mortality initially and 44-61% mortality after 85-106 days. Mosquito collections in treated bednets were significantly reduced for at least 217 days. The project failed to achieve prolonged suppression of malaria transmission for a combination of entomological, sociological and practical reasons which are discussed in relation to the objectives and implementation of future bednet studies.     

Quantitative aspects of the relationship between the sickle-cell gene and malaria

The relationship between resistance to Plasmodium falciparum infection and the frequency and distribution of the sickle-cell gene in populations exposed to endemic malaria transmission is reducible to clear and quantifiable terms. In this review, Trevor Jones examines the prediction of gene frequency changes under selective pressure, the selective advantage to the heterozygote (balanced polymorphism) that the sickle-cell gene provides to individuals in areas with malaria transmission, and the relationship between sickle-cell gene frequency and malaria, as measured by, for example, sporozoite rate and basic reproduction rate. He seeks to clarify what one can infer about malaria transmission from an analysis of the distribution and inheritance patterns of the sickle-cell gene and sickle-cell disease and under what circumstances these inferences should be made.     

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.     

Behaviour of Anopheles albimanus in relation to pyrethroid-treated bednets

Abstract. Responses of the malaria vector Anopheles albimanus to pyrethroid impregnated bednets made of cotton or nylon, compared with untreated nets, were investigated in houses occupied by two people inside and/or outside two bednets, in coastal Chiapas, México. The pyrethroid used was lambdacyhalothrin 30mga.i./m². Bioassay mortality rates of An.albimanus exposed to treated nets for 3 or 15min, rose from 40–55% to 90–100% for nylon nets 3–19 weeks post-treatment, but were consistently lower for treated cotton nets. An.albimanus females (collected unfed on human bait) were released in houses surrounded by curtains for trapping mosquitoes that exited from the house. Floor sheets were used in and around each experimental house for retrieving any mosquitoes knocked-down and/or killed. During post-treatment assessment for 17 weeks, An.albimanus blood-feeding success rates were 23–24% with untreated nets, 14–18% with treated cotton nets and 8–15% with treated nylon nets, significantly reduced when both human baits were inside the treated bednets, but not when one or more baits were outside the treated bednet(s) within the house. Proportions of mosquitoes leaving houses <3 h post-release were 53–59% from houses with untreated bednets versus 65–78% with treated bednets. Except in one case (when both humans were outside treated cotton nets), these increased early exit rates were significant, whether or not the human baits were inside the treated bednets indoors. Mortality rates of An.albimanus females exiting overnight (22.00–06.00 hours) averaged 15–39% from houses with treated cotton and 16–46% with treated nylon nets, very significantly greater than the control mean rates of 6–8% mortality with untreated nets. Observations on wild-caught An. albimanus females marked with fluorescent powder and released indoors revealed that few mosquitoes (3–11 %) actually contacted the bednets unless both human baits remained under them - when contact rates were 22% on treated nylon, 23% on treated cotton and 42% on untreated nets (P= 0.05). The mean resting time was significantly longer on untreated (14.4min) than on treated nylon (5.8min) or cotton (9.5 min) bednets, whereas mean resting times on other surfaces indoors were 16.5–19.8min. Proportions exiting within 2h of release were significantly more from houses with treated houses (33–35%) than with untreated nets (8%). However, mortality rates of mosquitoes that landed on treated nets were very significantly greater (90–100%) than after landing on untreated nets (10%). Thus, despite some excito-repellency, lambdacyhalothrin-impregnated bednets (especially made of nylon) proved to be effective as an alternative to house-spraying against An.albimanus.     

Permethrin-impregnated bednets: behavioural and killing effects on mosquitoes

Permethrin-treated pieces of netting and simulated bednets were evaluated against Anopheles gambiae Giles and Aedes aegypti (L.) in the laboratory. When female mosquitoes were allowed to feed on a human arm through pieces of impregnated netting fastened at the end of tubes, doses above 2 g/m2 were required to stop blood-feeding of both An. gambiae and Ae. aegypti. A much lower dose prevented Ae. aegypti from feeding on mice through impregnated netting. When mosquitoes were released in a room and a human subject sat under a permethrin-impregnated (0.2 g/m2) bednet with an arm pressed against the net (mesh 1.5 mm), mosquitoes failed to bite through the net. All the mosquitoes trying to bite through or entering the net through holes cut in it were knocked down within 30 min of release and ultimately died. Permethrin-impregnated wide-mesh (4 or 8 mm) bednets similarly prevented entry and caused high mortality-rates of An. gambiae.     

Comparison of different insecticides and fabrics for anti-mosquito bednets and curtains

Abstract. Various formulations of six insecticides (a carbamate and five pyrethroids), were impregnated into bednets and curtains made from cotton, polyester, polyethylene or polypropylene fabric. For bioassays of insecticidal efficacy, female Anopheles gambiae mosquitoes were made to walk on the fabrics for 3 min and mortality was scored after 24 h. The main concentrations tested were: bendiocarb 400 mg/m², cyfluthrin 30–50 mg/m², deltamethrin 15–25 mg/m², etofenprox 200 mg/m², lambda-cyhalothrin 5–15 mg/m² and permethrin 200–500 mg/m². Field trials in Tanzania used experimental huts (fitted with verandah traps) entered by wild free-flying Anopheles gambiae, An. funestus and Culex quinquefasciatus mosquitoes. Results of testing the impregnated fabrics in experimental huts showed better personal protection provided by bednets than by curtains. Permethrin cis:trans isomer ratios 25:75 and 40:60 were equally effective, and the permethrin rate of 200 mg/m² performed as well as 500 mg/m². Bioassay data emphasized the prolonged insecticidal efficacy of lambda-cyhalothrin deposits, except on polyethylene netting. Most of the impregnated nets (including the ‘Olyset’ net with permethrin incorporated during manufacture of the polyethylene fibre) and an untreated intact net performed well in preventing both Anopheles and Culex mosquitoes from feeding on people using them overnight in the experimental huts. Anopheles showed high mortality rates in response to pyrethroid-treated nets, but only bendiocarb treated curtains killed many Culex. Holed nets treated with either cyfluthrin (5 EW formulation applied at the rate of 50 mg a.i./m²) or lambda-cyhalothrin (2.5 CS formulation at 10 mg a.i./m²) performed well after 15 months of domestic use. Treatment with deltamethrin SC or lambda-cyhalothrin CS at the very low rate of 3 mg/m² gave good results, including after washing and re-treatment.     

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 Donitz 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 lOmin 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. With re-impregnation of bednets in November 1987, the P.falciparum infection rate was again reduced by March 1988. No effect of treated bednets was observed on the prevalence of P.vivax malaria.     

Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections

Background  

Evolutionary theory suggests that the selection pressure on parasites to maximize their transmission determines their optimal host exploitation strategies and thus their virulence. Establishing the adaptive basis to parasite life history traits has important consequences for predicting parasite responses to public health interventions. In this study we examine the extent to which malaria parasites conform to the predicted adaptive trade-off between transmission and virulence, as defined by mortality. The majority of natural infections, however, result in sub-lethal virulent effects (e.g. anaemia) and are often composed of many strains. Both sub-lethal effects and pathogen population structure have been theoretically shown to have important consequences for virulence evolution. Thus, we additionally examine the relationship between anaemia and transmission in single and mixed clone infections.     

The relationship between female body size and survival rate of the malaria vector Anopheles arabiensis in Ethiopia

Abstract. The relationship between female mosquito body size and survival rate was studied in field populations of Anopheles arabiensis in the Awash valley, central Ethiopia. Body size was quantified by measuring the wing-length. Highly significant correlations were found between size, parousness and insemination. It was concluded that larger An.arabiensis females have a higher probability of survival, being inseminated and producing more egg batches than smaller adults. Implications for vectorial capacity and vector competence of mosquitoes are discussed.     

Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections

Background

Theoretical studies suggest that direct and indirect selection have the potential to cause substantial evolutionary change in female mate choice. Similarly, sexual selection is considered a strong force in the evolution of male attractiveness and the exaggeration of secondary sexual traits. Few studies have, however, directly tested how female mate choice and male attractiveness respond to selection. Here we report the results of a selection experiment in which we selected directly on female mating preference for attractive males and, independently, on male attractiveness in the guppy, Poecilia reticulata. We measured the direct and correlated responses of female mate choice and male attractiveness to selection and the correlated responses of male ornamental traits, female fecundity and adult male and female survival.

Results

Surprisingly, neither female mate choice nor male attractiveness responded significantly to direct or to indirect selection. Fecundity did differ significantly among lines in a way that suggests a possible sexually-antagonistic cost to male attractiveness.

Conclusions

The opportunity for evolutionary change in female mate choice and male attractiveness may be much smaller than predicted by current theory, and may thus have important consequences for how we understand the evolution of female mate choice and male attractiveness. We discuss a number of factors that may have constrained the response of female choice and male attractiveness to selection, including low heritabilities, low levels of genetic (co)variation in the multivariate direction of selection, sexually-antagonistic constraint on sexual selection and the "environmental covariance hypothesis".

     

No relationship between individual genetic diversity and prevalence of avian malaria in a migratory kestrel

Insight into the genetic basis of malaria resistance is crucial for understanding the consequences of this parasite group on animal populations. Here, we analyse the relationship between genotypic variation at 11 highly variable microsatellite loci and prevalence of three different lineages of avian malaria, two Plasmodium (RTSR1, LK6) and one Haemoproteus (LK2), in a wild population of the endangered lesser kestrel (Falco naumanni). Although we used a large sample size (584 typed individuals), we did not find any significant association between the prevalence of the studied parasite lineages and individual genetic diversity. Although our data set is large, the 11 neutral markers typed may have had low power to detect such association, in part because of the low parasite prevalence observed (less than 5% of infected birds). However, the fact that we have detected previous correlations between genetic diversity and other traits (ectoparasitism risk, fecundity) in the study population using the same panel of neutral markers and lower sample sizes suggests that other factors could underlie the absence of such a similar correlation with avian malaria. Differences in the genetics of the studied traits and in their particular basis of inbreeding depression (dominance vs. overdominance) may have led to malaria prevalence, but not other traits, being uncoupled with individual genetic diversity. Also, we cannot discard the possibility that the absence of association was a consequence of a low pathogenic effect of these particular malaria lineages on our lesser kestrel population, and thus we should not expect the evolution of genetic resistance against these parasites.