The effects of different drying methods and sun exposure on the concentrations of deltamethrin in nets treated with K-O Tab tablets.

The method used to dry bednets after they have been treated with an insecticide solution may affect the levels of insecticide and the uniformity of the insecticide deposits on the dry nets. In an attempt to see how the drying method may affect the insecticide deposits on the dry net, and to select the best drying method, laboratory and field studies have recently been carried out (in the U.K. and Iran, respectively). Conventional polyester nets were each treated with a deltamethrin solution (made with one K-O Tab tablet in 500 ml water) and then dried, either while hanging vertically or laid horizontally on the floor, in the sun or shade. The concentrations of deltamethrin in 25-cm2 samples cut from the dry nets (from the inner folds, surface folds, and top and bottom of each net dried vertically, and from the upper and lower surfaces of each net dried horizontally) were then determined using high-performance liquid chromatography (HPLC). Drying the treated nets in the sun or the shade did not make a significant difference to deltamethrin concentrations on the nets. Mean deltamethrin concentrations were, however, higher on the lower parts of the nets that had been hung to dry vertically than on the upper parts of these nets, and greater on the upper surfaces of nets dried horizontally than on the lower surfaces of such nets. In general, the layers and folds of the nets that had been on the outside of the drying nets contained more deltamethrin than the inner folds. These patterns are consistent with the hypothesis that deltamethrin tends to accumulate at the points where the water from the insecticide solution evaporates from the drying nets and also, in the case of nets dried vertically, at the lower points of the drying nets (as the result of gravity). In order to obtain an even and adequate distribution of insecticide, it is therefore not necessary to dry the net in the shade. To achieve a uniform deposit of deltamethrin, the drying net should be folded as little as possible and dried quickly.

Evaluation of the effects of repeated hand washing, sunlight, smoke and dirt on the persistence of deltamethrin on insecticide-treated nets.

Field studies were carried out in Iran to evaluate the effect of various factors (washing, sun, smoke, dust and dirt) on the residual insecticidal activity of PermaNet (a brand of long-lasting insecticidal net), and on nets conventionally treated with deltamethrin (K-O Tab), using bioassay tests. Thirty-two nets were washed five or 15 times, and eight nets were not washed at all. Nets were washed vigorously in cold tap water (17 degrees C, pH 8.9) with a detergent. Hand rubbing continued for 3min. After washing, some nets were exposed to dense smoke from a dung-hay fire for 3min and were also left exposed to the dusty wind between washes. One group of nets was exposed to the sunlight for the full 3-d interval between washes; another was exposed to sunlight for just 3h after each wash; two other groups were kept in the shade. There was a significantly greater loss of activity in nets exposed to the sun throughout the 3-d interval between washes: that is, for a total of 15 to 45 d. However, short sunlight exposure (maximum 3h between washes) during drying did not have any effect. We did not find any significant effect of exposure to dirt, dust and smoke after washing. It is concluded that the effect of sun is much smaller than that of washing, and that drying nets for a few hours in the sun is not harmful.

A randomized and controlled comparison of the wash-resistances and insecticidal efficacies of four types of deltamethrin-treated nets, over a 6-month period of domestic use with washing every 2 weeks, in a rural area of Iran.

In a randomized, prospective, 6-month-long field study in a rural area of Iran, the wash resistances of 200 nets (40 PermaNet, 40 Yorkool and 40 A-Z nets), that their manufacturers claimed be long-lasting insecticidal nets (LLIN), were compared with those of 40 nets conventionally treated with deltamethrin (using K-O Tab tablets). All the nets were kept in routine domestic use and subjected to standardized hand-washing at 2-week intervals. Wild-caught or laboratory-reared Anopheles stephensi were used for the bio-assays of insecticidal activity. The regular washing and domestic use led to reductions in the insecticidal activities of all the treated nets after 6 months. Although the PermaNet nets showed the smallest reduction, they were not significantly better than the conventionally treated nets, which still showed acceptable insecticidal activity after 6 months. The PermaNet and A-Z nets both performed significantly better than the Yorkool nets, which were slightly but not significantly worse than the conventionally treated nets. In questionnaire-based interviews, the local householders were found to wash their own (non-study) nets at median and mean frequencies of every 2 and 2.1 weeks, respectively. In conclusion, the PermaNet nets showed better wash resistance than any of the other commercial nets, and were the only commercial nets tested that truly appeared to be LLIN. There still appears to be scope, however, for the impregnation, and thus the wash-resistance, of even the PermaNet nets to be improved.

Entomological evaluation of three brands of manufactured insecticidal nets and of nets conventionally treated with deltamethrin, after repeated washing.

The present study evaluated the efficacy and wash resistance of three types of commercial, deltamethrin-treated nets (PermaNet, Yorkool and A-Z nets) - that their manufacturers claimed to be long-lasting insecticidal nets (LLIN) - and those of nets conventionally treated with deltamethrin, at either 23-27 mg/m(2) (using one K-O Tab tablet/net) or 46-54 mg/m(2) (using two K-O Tab tablets/net). The nets were tested unwashed or after being washed, by hand or machine, five or 15 times. After each wash, the nets were dried vertically on a line, in the shade, for 8 h. Insecticidal activity was assessed, using two types of bio-assay and wild-caught female Anopheles stephensi, as mean median knock-down times and as mortality 24 h after a 3-min exposure. For each type of nets tested, the insecticidal activity of the unwashed nets was greater than that of the nets washed five washes, and that of the nets washed five times was greater than that of the nets washed 15 times, with the latter decline considerably greater than the former. The pattern of decrease, however, differed with net type. The PermaNet and A-Z nets generally had the highest insecticidal activity, and, overall, the Yorkool nets did not perform significantly better than the conventional nets treated with 23-27 mg deltamethrin/m(2). Although washing with detergents can clearly remove insecticide from the fibres of all types of treated nets, the consequent loss of activity varies with net type. After 15 washes, the PermaNet nets showed higher insecticidal activity than any other net type but there is still scope for the manufacturer of the PermaNet nets to improve wash resistance.

Multi-country field trials comparing wash-resistance of PermaNet and conventional insecticide-treated nets against anopheline and culicine mosquitoes.

Insecticide-treated bednets (ITNs) are commonly used as a means of personal protection from malaria transmission by anopheline mosquitoes (Diptera: Culicidae). Long-lasting insecticidal nets (LLINs) have special treatments intended to remain effective after many washes. The present trials assessed the efficacy and wash-resistance of several production batches of PermaNet (polyester net coated with polymer resin containing pyrethroid insecticide deltamethrin 55 mg ai/m2) against malaria vectors in Pakistan, Iran and Tanzania compared to ITNs conventionally treated with alphacypermethrin 15 or 20 mg ai/m2, or deltamethrin 25 or 50 mg ai/m2. Insecticidal efficacy of the nets before and after repeated washing (using W.H.O. recommended and traditional local washing procedures) was monitored through contact bioassays with Anopheles and by experimental hut and outdoor platform tests. Local washing regimes gradually reduced the insecticidal efficacy of conventionally treated nets, but they were not exhausted, even after 21 washes. Using a more rigorous laboratory washing method, insecticide was more readily stripped from conventionally treated nets. PermaNet retained high efficacy after 21 washes, giving more than 97% mortality of Anopheles in contact bioassays with 3-min exposure. Using the more sensitive bioassay criterion of 'median time to knockdown', PermaNet showed no loss of insecticidal activity against Anopheles after washing repeatedly in 2 out of 6 trials; whereas in a further three trials knockdown activity of PermaNet and conventional ITNs declined at comparable rates. Higher mortality levels of Anopheles in contact bioassays did not always translate to superiority in experimental hut or enclosed platform trials. In only one of four comparative field trials did PermaNet out-perform conventional ITNs after washing: this was in the trial of PermaNet 2.0--the product with improved quality assurance. Because PermaNet and conventionally treated nets were both quite tolerant of local washing procedures, it is important in field trials to compare LLINs with conventional ITNs washed an equivalent number of times. Our comparison of PermaNet 2.0 against conventionally treated deltamethrin nets (CTDN) in Pakistan demonstrated superior performance of the LLIN after 20 washes in phase I and phase II bioassays, and this was corroborated by chemical assays of residual deltamethrin. Although PermaNet 2.0 has received WHOPES interim recommendation for malaria control purposes, its performance should be monitored in everyday use throughout its lifespan in various cultural settings to assess its durability and long-term effectiveness for malaria prevention and control. As many millions of conventionally treated nets are already in routine use, and these will require regular re-treatment, programme strategies should be careful to preserve the effectiveness of ITNS before and after establishing the reliability of LLINs in long-term use.

How reliable are light traps in estimating biting rates of adult Anopheles gambiae s.l. (Diptera: Culicidae) in the presence of treated bed nets?

The sampling efficiency of light trap catches relative to human bait catches in estimating biting rates of the mosquito Anopheles gambiae Giles was investigated in two types of community in southern Sierra Leone: (i) where most of the inhabitants slept under treated bed nets; and (ii) where most of the inhabitants slept without bed nets. The number of female A. gambiae mosquitoes caught in these communities by light trap was strongly correlated (r > or = 0.72) with those from corresponding human biting catches performed either on the same or adjacent nights. It was found that the relative sampling efficiency of light traps varied slightly but significantly with mosquito abundance in villages with treated bed nets, but not in those without them. Nevertheless, the relationship between relative sampling efficiency and mosquito abundance did not differ significantly between the two types of village. Overall, there was insufficient evidence to show that the presence of treated nets altered the relative efficiency of light traps and any bias was only slight, and unlikely to be of any practical importance. Hence, it was concluded that light traps can be used as a surrogate for human bait catches in estimating biting rates of A. gambiae mosquitoes in the two communities.

Spatial and temporal distribution of Anopheles gambiae s.l. (Diptera: Culicidae) in two Tanzanian villages: implication for designing mosquito sampling routines.

This paper describes the spatial and temporal distribution of Anopheles gambiae s.l. Giles in two Tanzanian villages based on data collected from a five-month intensive mosquito sampling programme and analysed using Taylor's power law. The degree of spatial aggregation of female A. gambiae in each village was similar to its corresponding temporal aggregation, indicating that in designing sampling routines for estimating the abundance of mosquitoes, sampling effort should be allocated equally to houses (spatial) and nights (temporal). The analysis also showed that for a given amount of sampling effort, estimates of village-level mosquito abundance are more precise when sampling is carried out in randomly selected houses, than when the same houses are used on each sampling occasion. Also, the precision of estimating parous rates does not depend on whether mosquito sampling is carried out in the same or a random selection of houses. The implications of these findings for designing sampling routines for entomological evaluation of vector control trials are discussed.

Diversion of Anopheles gambiae from children to other hosts following exposure to permethrin-treated bednets.

Permethrin-treated bednets reduce mortality and morbidity from malaria in Gambian children. However, it is not certain how this effect is achieved, as neither mosquito numbers nor the human blood index of indoor-resting female Anopheles gambiae Giles sensu lato (Diptera: Culicidae) mosquitoes have been reduced when treated bednets were introduced into a community. One possibility is that insecticide-treated bednets divert mosquitoes from children to adults. To investigate this hypothesis, a cross-over trial with insecticide-treated bednets was undertaken in two small Gambian villages. To differentiate mosquitoes that had fed on children from those that had fed on adults, all children in the study villages were immunized with rabies vaccine before the trial. Using the detection of rabies antibody in a bloodmeal as an indicator that a mosquito had bitten a child, it was found that the percentage of blood-fed mosquitoes caught indoors that had bitten a child fell significantly from 30.8% to 9.2% and from 28.0% to 6.9% in each village after insecticide-treated bednets were introduced. To investigate the possibility that some diversion to animals had occurred, a PCR analysis for human beta-globin DNA was undertaken on selected samples. The results of this investigation were confusing, as some rabies-antibody positive bloodmeals were negative for human DNA. This may have been due to cross-reacting antibodies in animal sera and/or DNA degradation by digestion in the mosquito. Although good evidence for diversion of mosquitoes away from children was obtained, it remains uncertain whether diversion was mainly to adult humans, to animals or to both.

Should DDT be banned by international treaty?

The insecticide DDT has been an effective and affordable means of malaria control in many countries, but pressure for its use to be banned is mounting. Here, Chris Curtis and Jo Lines take a critical look at evidence that links house spraying by DDT with harm to the environment and human health, and stress the need for resources for alternatives to DDT to be made available to countries that would be affected by a DDT ban.

A new strategy for treating nets. Part 1: formulation and dosage.

The conventional dosages of pyrethroid insecticides on mosquito nets assume that nets will be retreated at 6-12 month intervals. However, dosage should be related to washing of nets; if nets are only washed once or twice a year, their dosage requirements will be different to those which are washed fortnightly. A 'low-dose, frequent-wash' retreatment system might be technically more appropriate and more affordable where nets are washed frequently, as they are in Dar es Salaam. Moreover, for use as a domestic insecticide, water-based formulations of pyrethroid are preferable to the more commonly used emulsifiable concentrates (ECs). This paper reports laboratory evaluations of three formulations (ECs, Flowable, CS) of three pyrethroids (deltamethrin, lambdacyhalothrin, permethrin). Insecticidal activity was tested using serial bioassays at a range of dosages using Anopheles gambiae. The water-based formulations were no less effective than the ECs, even at the lowest dosages. Nets treated with 3 mg/m2 and then repeatedly washed and retreated after each wash with either 3 mg/m2 or 1 mg/m2 were subjected to gas chromatography analysis. This showed that the amounts of pyrethroid in the nets accumulated rapidly over the first few wash-retreatment cycles and then remained fairly stable over subsequent cycles. These nets gave consistently high bioassay mortalities throughout the experiment, while the mortality declined rapidly after several washes with the nets that were treated at 3 mg/m2 but not retreated. Experimental huts were used to compare the effectiveness of these 2 net retreatment regimes and nets which were not retreated. All nets caused high mortality rates amongst Anopheles females, but had negligible effects on culicines; either in killing them or in preventing feeding. Therefore use of a high 'loading' dose for initial treatment with lower 'maintenance' doses for retreatment may be preferable to ensure that net users promptly perceive the benefits of the insecticide against culicines.

Comparison of the effect of insecticide-treated bed nets and DDT residual spraying on the prevalence of malaria transmitted by Anopheles anthropophagus in China.

In order to improve the control of malaria in a problem part of Hubei Province, China, where Anopheles anthropophagus is the vector of Plasmodium vivax, insecticide treatment of bed nets was introduced. The people were given the choice of DDT residual spraying, which had been used for many years, or deltamethrin treatment of their bed nets. Two counties, in which these 2 different methods had been introduced, and an untreated area were evaluated. DDT house spraying and insecticide treated bed nets were equally effective, but deltamethrin treatment was cheaper and so was considered the method of choice.

Comparison of bednets treated with alphacypermethrin, permethrin or lambdacyhalothrin against Anopheles gambiae in the Gambia.

In the Gambian village of Saruja, where malaria is transmitted mainly by mosquitoes of the Anopheles gambiae complex, a trial was undertaken of the acceptability and efficacy of bednets treated with one of three pyrethroid insecticides--alphacypermethrin 40 mg/m2, permethrin 500 mg/m2 and lambdacyhalothrin 10 mg/ m2. Fewer mosquitoes were found alive under nets treated with insecticide than under control nets. Significantly more dead mosquitoes were found under nets treated with alphacypermethrin than under nets treated with permethrin or lambdacyhalothrin. Side-effects were reported by a proportion of the users of nets treated with each of the insecticides, but none were severe and their prevalence was similar between treatment groups. Unwashed nets treated with alphacypermethrin were more effective at killing anopheline mosquitoes in bioassays than nets treated with permethrin or lambdacyhalothrin. Killing activity was reduced when nets were washed, irrespective of which insecticide was used. Bednets treated with alphacypermethrin are well accepted, effectively killed anopheline mosquitoes and should therefore be evaluated for personal protection against malaria transmission.

Anopheles gambiae gonotrophic cycle duration, biting and exiting behaviour unaffected by permethrin-impregnated bednets in The Gambia.

Permethrin-impregnated bednets protect children against malaria in The Gambia, where Anopheles gambiae complex mosquitoes are the main vectors of malaria. However, no effect has been found on mosquito density, parous rates or sporozoite rates in An.gambiae sensu lato populations; only a reduction in the numbers of mosquitoes resting indoors in rooms with treated bednets. A possible explanation for this paradox is that exposure to treated bednets leads to changed vector behaviour such as a shift in biting time, a diversion to biting outdoors instead of indoors, to biting animals instead of humans, or to increased duration of the gonotrophic cycle. To investigate these possibilities, we observed the biting and existing behaviour of An.gambiae in ten pairs of villages, in half of which the residents used permethrin-treated bednets. The possible influence of treated bednets on the gonotrophic cycle length was evaluated by mark-release-recapture experiment. No significant difference was found between villages with treated and untreated bednets in the indoor/outdoor ratio of human biting, in mean biting times or in human blood indices of An.gambiae females found resting indoors in the mornings. The proportions of unfed, fed or gravid An.gambiae females collected in exit traps, and the number of females exiting showed no significant differences between rooms with treated and untreated bednets. Indications for a gonotrophic cycle length of 2 days were found. No evidence for any change in duration of the gonotrophic cycle in relation to exposure to treated bednets was found, although the number of recaptures was low in the villages with treated bednets. Since equal numbers of infective An.gambiae were found in villages with treated or untreated bednets, and no changes in mosquito behaviour were detected, we cannot account for how children are protected against malaria by treated bednets. One possibility is that mosquitoes divert to bite other hosts, including adults.

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

The effect of community-wide use of bednets treated with lambdacyhalothrin 10 mg/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.

Promoting operational research on insecticide-treated netting: a joint TDR/IDRC initiative and call for research proposals.

The evidence in favour of insecticide-treated nets for reducing mortality from malaria among Africa children is becoming compelling. In order to support the next step in the development of this promising intervention the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR) and the International Development Research Centre (IDRC) decided to launch a joint initiative on operational research. In the first step, 3 reviews on the essential aspects of net technology, programme implementation and programme promotion were commissioned and will be published soon as a book. Secondly, expert opinion was sought regarding priority topics for research. Finally, a first round of research projects was funded and a call for more proposals is being widely advertised.

Mosquito control in Dar es Salaam. I. Assessment of Culex quinquefasciatus breeding sites prior to intervention.

In preparation for a trial polystyrene beads and pyriproxyfen for the control of Culex quinquefasciatus mosquitoes, surveys of their breeding were carried out in two contrasting areas of Dar es Salaam, Mikocheni and Ilala, during the dry season. Sanitation structures (latrines, soakage pits, septic tanks and cess pits) were the most profilic breeding places, totalling 780 in Mikocheni and 1544 in Ilala. Those in Mikocheni were estimated to contain about 1.4 times more mosquito pupae, per site, than in such structures in Ilala. This was both because a higher proportion of sites contained visible water and because sites with water were more likely to contain pupae in Mikocheni. The relative importance of the different types of structure, in terms of productivity, was the same in both areas. Although septic tanks and cess pits made up only 10.5% of the on-site sanitation structures in Ilala, they contained 53% of the total number of pupae in enclosed sites; they were therefore particularly important targets for treatment with polystyrene beads. A survey during the rainy season of sites in Ilala revealed little change in the proportion that were wet, or in the frequency of breeding in those with visible water. The number, type and area of open breeding sites varied greatly between the two study areas. In Mikocheni the area of open breeding sites was 100 times greater than in Ilala, with 97% of the 13,000 m2 being flooded grassland. In Ilala all but four of the sixty-six open breeding sites were puddles of sullage water derived from bathrooms.(ABSTRACT TRUNCATED AT 250 WORDS)

Mosquito control in Dar es Salaam. II. Impact of expanded polystyrene beads and pyriproxyfen treatment of breeding sites on Culex quinquefasciatus densities.

In two contrasting areas of Dar es Salaam (Ilala and Mikocheni) all enclosed breeding sites of Culex quinquefasciatus, such as latrines and septic tanks, were treated with a floating layer of expanded polystyrene beads. 7 months later checks in both study areas revealed only one site (from which the polystyrene had been removed during emptying) containing immature stages of Cx quinquefasciatus. Open breeding sites such as areas of flooded land and blocked drains were treated with pyriproxyfen (an insect growth regulator) at a concentration of 0.1 ppm. Emergence of Cx quinquefasciatus adults from these sites was inhibited for 4 weeks during the rainy season and for up to 11 weeks during the dry season. The problem of mosquito breeding sites caused by bathroom sullage water was addressed through a combination of health education and indirect pressure from the Urban Malaria Control Project (UMCP) via local community leaders. Households responsible for these sites were encouraged to eliminate them by diverting the water into an enclosed drainage structure, usually a pit latrine. After two weekly visits 64.7% of households had complied and 93.4% had complied after five visits. 5 months later, only 15.7% had reverted to allowing sullage water to collect into puddles. Densities of Cx quinquefasciatus adults dropped by 76.7% in Mikocheni and by 46.2% in Ilala following intervention, but increased by 84.9% and 25.6% in two untreated comparison areas. The reasons for differential success of the combined interventions in the two treated areas are discussed.