Evaluation of lambdacyhalothrin-impregnated bednets in a malaria endemic area of India. Part 1. Implementation and acceptability of the trial

In malaria endemic forested villages in Orissa State, India, a 3-year comparison of nylon nets treated with lambdacyhalothrin at 25 mg/m2, untreated nets, and no nets was carried out. Treated nets retained high insecticidal efficacy for more than 7 months. Nets washed after 3 months of use gave 98% kill in a bioassay with a 3-min exposure. Based on these bioassays during the first year, nets were later reimpregnated at 6-monthly intervals with participation of the users. Compliance with the use of nets was good. Eighty-eight percent of nets were usable even after 3 years. The main benefits perceived by treated net users were reductions in malaria, mosquito bites, head louse infestations, and other nuisance insects. The trial was well accepted by the community. Issues related to social marketing and promotion of nets are discussed.     

Efficacy of PermaNet 2.0 against Anopheles culicifacies and Anopheles stephensi, malaria vectors in India

Bioefficacy of PermaNet was evaluated in both the laboratory and field against Anopheles culicifacies and An. stephensi, major malaria vectors in India. Contact bioassays were carried out after repeated washings and ring net bioassays to determine the median knockdown time of mosquitoes. Three villages were selected for the field trial: in the 1st village PermaNets were distributed, in the 2nd village untreated nets were distributed, and the 3rd village was a control. Entomological data were collected using standard procedures. The PermaNet contact bioassays showed high mortality (>80%) even after 20 washes against both the vector species. The median knockdown time of An. culicifacies and An. stephensi was 392 and 480 sec when exposed to fresh PermaNets and 472 and 986 sec when exposed to PermaNets that had been washed 20 times, respectively. PermaNets showed high efficacy in reducing the person-vector contact as evidenced by reduced person-hour density in the PermaNet village. Long-term field trials are indicated to test the impact of use of PermaNets in controlling malaria.     

Cyfluthrin (EW 050)-impregnated bednets in a malaria control program in Ghassreghand (Baluchistan, Iran)

In a study carried out in the Ghassreghand Division (Baluchistan, Iran) from March through November 1995, efficacy of cyfluthrin-impregnated bednets was compared to that of untreated nets, in relation to malaria control. Ten villages with a total population of 4,572 and 3 villages with a total population of 1,935 were used as treatment and control, respectively. The collection, impregnation (target dosage of 40 mg active ingredient [AI]/m2), and redistribution of the nets (9% nylon, 52% light cotton, 30% medium cotton, and 9% heavy cotton), carried out in mid-April, were done by local health workers, supervised by the senior research staff. Anopheles culicifacies was considered to be the main vector of malaria in the named area. This species is mainly zoophilic, endophilic, and exophagic. The initial uptake of the insecticide was lower than the target dosage, with high variation (nylon, 12.5 +/- 5.4 mg AI/m2; light cotton, 33.3 +/- 26.1 mg AI/m2; medium cotton, 25.9 +/- 20 mg AI/m2; heavy cotton, 17.6 +/- 12.5 mg AI/m2). The use of impregnated mosquito nets (used primarily outside) had no significant effect on the incidence of malaria. No difference was detected in the parasite density of patients with positive slides. No significant effect was observed in the parous rate, human blood index, and sporozoite rate of anopheline vectors. Only the indoor resting densities of An. culicifacies and other malaria vectors were drastically reduced after the introduction of the cyfluthrin-impregnated nets into the treatment villages. The residual activity of cyfluthrin was lower than expected. The mortality of anophelines brought in contact with the treated nets for 3 min in bioassays dropped to less than 55% in 3 months. The loss of chemical activity was greatest for the light cotton nets, followed by the medium cotton nets. Cyfluthrin-treated nets were mildly irritating to host-seeking female anophelines in the laboratory. The protective rate of impregnation (all fabric kinds included) in preventing female mosquitoes from biting through the impregnated nets was initially 5-6 times that of the nonimpregnated nets. The study did not detect any significant difference between the use of untreated versus impregnated bednets in the Ghassreghand area. In planning future medium-scale trials, comparison of new compounds and formulations to the more widely used pyrethroids such as permethrin and deltamethrin is highly recommended.     

Wash resistance and efficacy of Olyset net and Permanet 2.0 against Anopheles stephensi in India

Long-lasting insecticidal nets (LLIN) have been developed for wash resistance and long-lasting effects against mosquito vectors. In this study we evaluated 2 LLIN products, Olyset net and Permanet 2.0, washed for 0, 5, 10, 15, and 20 times, against Anopheles stephensi, an urban malaria vector in India. We assessed the wash resistance and efficacy of these nets in relation to bloodfeeding inhibition and percent mortality in cone and tunnel test bioassays. Both Olyset and Permanet showed >80% mortality of An. stephensi in cone bioassays after 20 washes. In tunnel tests there was no significant difference between Olyset and Permanet 2.0 in causing total mosquito mortality (immediate and delayed) up to 10 washes and bloodfeeding inhibition and entry rate up to 15 washes. After the 20th wash, Permanet 2.0 was significantly more effective than the Olyset net in causing total mosquito mortality, whereas Olyset net showed less bloodfeeding and entry of mosquitoes as compared to Permanet 2.0. There was a gradual decline in efficacy of both LLIN products as the number of washings increased. Cone bioassays indicated a strong wash resistance in both the LLIN products after 20 washes. However, the tunnel tests demonstrated a gradual decline in efficacy of both products with the number of washings.     

Prevention of malaria with pyrethroid treated bednets

Malaria parasites (Plasmodium) are carried from person to person by female mosquitoes of the genus Anopheles. 80-90% of malaria incidence worldwide occurs in Africa. The main Anopheles species in Africa which bite humans are A. gambiae and A. funestus. Since these animals feed upon humans mainly during late night when most people are in bed, the consistent use of bednets can block the transmission of malaria. However, bednets should be treated with synthetic pyrethroid insecticides, for the physical barrier of bednets is often inadequate to prevent the entry of Anopheles. In addition, the mosquitoes are drawn to the bednets by the carbon dioxide and body odor emitted by the occupant. The insecticide-treated nets therefore double as mosquito traps. Bednets are treated by dipping them into an aqueous emulsion of a pyrethroid, wringing them out, then laying them out to dry. A considerable number of mosquitoes can be killed with a relatively small amount of insecticide. A treated net's insecticidal power can be tested by wrapping part of it around a wire frame, then introducing some Anopheles mosquitoes inside the frame. A median mosquito knockdown time of less than 10 minutes inside the net indicates a good pyrethroid deposit. The persistence of a net's insecticidal power is considerably reduced if the net is vigorously washed. People vary in the frequency they wash their nets. Most villagers in Tanzania were found to wash and retreat their nets every 6 months.     

Evaluation of lambdacyhalothrin-impregnated bednets in a malaria endemic area of India. Part 2. Impact on malaria vectors

In malaria endemic villages of the Indian State of Orissa, the impact of bednets treated with lambdacyhalothrin at 25 mg/m2 on malaria vectors was assessed during a 3-year intervention trial beginning in May 1990. The main malaria vector was Anopheles culicifacies with a small contribution from Anopheles fluviatilis. The impregnated bednets caused a significant reduction in vector density as assessed by morning indoor resting catches, man-biting rate, light trapping, the proportion of females engorged with human blood, and the parity rate as compared with villages with untreated or no nets. No statistically significant difference was observed in these parameters between the villages with untreated nets or no nets. The trial demonstrated that the lambdacyhalothrin-treated nets were highly effective against the malaria vectors.     

Comparative efficacy and persistency of permethrin in Olyset net and conventionally treated net against Aedes aegypti and Anopheles stephensi

The efficacy of the Olyset net was compared to a net treated conventionally with permethrin 10% emulsifiable concentrate at the World Health Organization recommended dose. The nets were assessed under laboratory conditions against the mosquito vectors Aedes aegypti and Anopheles stephensi. Knockdown efficacy for the Olyset net was assessed after the first 10 washings and then after the 15th and 20th washings. Data were not collected for the conventionally treated netting after 5 washings because the conventional treatment was no longer effective. The results of the study indicate permethrin effectively persisted on Olyset net for at least 20 washings, confirming the regeneration of pesticide after each wash.     

Wash resistance of insecticide-treated materials

The effectiveness of insecticide-treated materials (ITMs) for malaria control is reduced by washing them. This research in Colombia and Bolivia investigated the resistance of different insecticide formulations and, in particular, a commercially available impregnated bednet (PermaNet) which provides chemical protection for the insecticide. The fabrics studied were all polyester; the pyrethroids used for impregnation were deltamethrin (tablet and suspension concentrate both at 25 mg/m2 target dose), lambdacyhalothrin (capsule suspension at 15 mg/m2; laboratory study only), alphacypermethrin (suspension concentrate at 40 mg/m2) and, in the case of PermaNet, deltamethrin (55 mg/m2). The indicator of wash resistance was Anopheles spp. mortality (using the bioassay cone method) before and after different numbers and intensities of washing. When the fabrics were washed under controlled conditions, gently with water and a bar of soap, the wash resistance of all formulations was good (100% Anopheles mortality after 3 washes). However, when the impregnated nets were soaked for 30-60 min and washed with soap powder and tap water by local women in the usual way, the mortality after 4 washes declined considerably (43.5% and 41.3% for deltamethrin tablets and liquid respectively when washing every second day). Alphacypermethrin showed slightly better results after 3 washes every 7th day compared to deltamethrin tablets (63.8% and 43.3% mortality, respectively). The wash resistance offered by PermaNet was much better and longer lasting: Anopheles mortality after 4 washes was 92.6%, after 10 washes 83.7% and after 20 washes 87.1%. The limitations of commercially available wash-resistant nets are, however, their limited accessibility and the difficulty of replacing all existing bednets with a new product.     

Deltamethrin and permethrin residue on long-lasting insecticidal nets after 18 months of use in a visceral leishmaniasis-endemic area in Nepal

The insecticide residue on two types of long-lasting insecticidal nets (LLIN), Olyset Net and PermaNet 2.0, used in a visceral leishmaniasis-endemic village in eastern Nepal was quantified using HPLC. After two washes during 18 months of use the mean insecticide residues on PermaNet 2.0 and Olyset Net were 53.5 mg/m(2) (97.3% of the target dose) of deltamethrin and 911.8 mg/m(2) (91.2% of the target dose) of permethrin, respectively. These residues were close to the insecticide loads specified by the manufacturers of the two LLINs. The use of LLINs has been postulated as an alternative or complementary method to indoor residual spraying. Our results suggest that LLINs should be washed 4-5 times each year throughout their lifespan by specifically requesting villagers to wash nets on certain dates. The insecticide residue on the nets and their bioefficacy against sand fly vectors should be monitored after each wash, in various cultural settings, to assess their durability and long-term retention of the insectide.     

Combined field and laboratory evaluation of a long-term impregnated bednet, PermaNet

For how long does the insecticidal effect of long-lasting nets achieve high Anopheles mortality? Four PermaNets tested previously in Colombia for wash resistance were bioassayed again after 3 years of use and 23 washes. The tests were done both in Colombia and a reference laboratory in Indonesia. Additionally the residual concentration of deltamethrin on the nets was chemically analysed. The results showed high Anopheles mortality and a mean deltamethrin concentration of 9.6 mg/m2. Discrepancies with other studies are discussed.     

Wash resistance and residual efficacy of long-lasting polyester netting coated with α-cypermethrin (Interceptor) against malaria-transmitting mosquitoes in Assam, northeast India

Malaria is endemic in Assam, northeast India, with low-to-moderate transmission of the causative parasites, mostly by Anopheles minimus. Plasmodium falciparum is the predominant parasite (>60%), with remaining cases being due to P. vivax. As an alternative intervention for malaria control, long-lasting insecticidal nets [Interceptor coated with α-cypermethrin 10% suspension concentrate (SC), 0.667% w/w, 0.2 g/m²] underwent field evaluation for laboratory wash resistance and residual efficacy in field conditions against malaria-transmitting mosquitoes. Based on entomological observations, the Interceptor net intervention was the most effective, corresponding to the lowest mosquito vector density in experimental villages. There was virtual disappearance of A. minimus in Interceptor net villages in contrast to the untreated net intervention and the no-net control. Contact cone bioassay tests revealed 100% mortality in the A.minimus group of mosquito species in the community using the Interceptor net, which was consistent during the follow-up monitoring period (October 2006 to April 2007) in field conditions. Similar levels of mortality were observed in laboratory-washed nets compared with unwashed nets, and wash resistance was consistent even after the 20th serial wash at fortnightly intervals. Community compliance and acceptance of the Interceptor net was high, with decreased nuisance due to biting mosquitoes and other household insect pests being reported.     

Anopheline vectors and malaria transmission in eastern Afghanistan

Anopheline vectors and malaria transmission were studied in 2 river-irrigated, rice-growing districts of eastern Afghanistan from May 1995 to December 1996. Clinical malaria was monitored in 12 rural villages (population 14,538) by passive case detection at local clinics. Adult mosquitoes were collected by space-spraying of living quarters and stables and by cattle bait catches. Mosquito head-thoraces (17,255 specimens) were tested for Plasmodium falciparum and P. vivax circumsporozoite protein (CSP) using enzyme-linked immunosorbent assay. The recorded incidence of P. vivax and P. falciparum was 199 and 41 episodes per 1000 person years, respectively. Twelve species of anopheline were recorded; Anopheles stephensi comprised 82% and A. culicifacies 5%. Eight species tested positive for CSP: A. stephensi, A. culicifacies, A. fluviatilus, A. annularis, A. pulcherrimus, A. maculatus, A. splendidus and A. superpictus. Among infected mosquitoes 46% were positive for P. falciparum, 45% for P. vivax VK-247, and 9% for P. vivax PV-210. Estimates of the feeding rates of infective vectors on humans indicated that A. stephensi would contribute 76% of infective bites, A. fluviatilis and A. pulcherrimus 7% each, and A. culicifacies and A. superpictus 3% each. The overall infective vector feeding rate correlated with the P. vivax incidence rate in the human population. The conventional view of A. culicifacies being the main rural vector and A. stephensi important only in urban settings needs to be reconsidered in western outreaches of the Indo-Pakistan subcontinent.     

A preliminary test of the protective efficacy of permethrin-treated bed nets in an area of Anopheles gambiae metabolic resistance to pyrethroids in north Cameroon

A trial of permethrin-treated nets (PTNs) versus untreated nets (UTNs) was conducted in Pitoa (north Cameroon), where the main malaria vectors, Anopheles gambiae s.l. and Anopheles arabiensis, show metabolic-based permethrin resistance. The deterrent effect of permethrin greatly reduced A. gambiae biting rate inside rooms where PTNs were installed. After 3 months of net use, malaria reinfection rate was significantly lower in children sleeping under a PTN, but no such effect was observed after 6 months. Parasitaemia was not significantly different between the two arms. These findings suggest good, although transitory, personal protection against malaria conferred by PTNs in an area of metabolic-based permethrin resistance.     

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.     

Relative efficacy of synthetic pyrethroid-impregnated fabrics against mosquitoes under laboratory conditions

The efficacy of synthetic pyrethroid-impregnated fabrics was evaluated against Anopheles stephensi, Aedes aegypti, and Culex quinquefasciatus, under laboratory conditions. Results revealed that delta-methrin was significantly superior in comparison to lambdacyhalothrin and cyfluthrin. Results of bioassay tests revealed that deltamethrin was 1.5 and 1.9 times more effective than lambdacyhalothrin and cyfluthrin, respectively, against An. stephensi exposed to cotton fabric treated at 100 g/m2. Deltamethrin was 3.9 and 4.6 times more effective against Ae. aegypti and 3.53 and 4.0 times more effective against Cx. quinquefasciatus. Of cotton, nylon, polyethylene, and jute fabrics, the cotton was the best on the basis of median lethal dose (LD50) and 95% lethal dose (LD90) values and persistence of insecticide.     

Evaluation of a prototype long-lasting insecticide-treated mosquito net under field conditions in rural Burkina Faso

Insecticide measurements and standard World Health Organization bioassays on random samples of new unwashed, traditionally washed and up to 18 months field-used 'long-lasting' deltamethrin treated mosquito nets demonstrated a rapid reduction of efficacy under field conditions. The technology of 'long-lasting' insecticide-treatment needs much improvement.     

拜虫杀对嗜人按蚊的实验室生物学测定

目的：在实验室用嗜人按蚊对拜虫杀ＲesponsarSC125新剂型进行敏感性持效观察。方法：浸泡法。结果：拜虫杀对嗜人按蚊的杀灭效果取决于使用的有效剂量、接触时间及处理方法,加大剂量或增加杀虫剂与按蚊接触时间均可提高效果。结论：相同的实验阶段、同样长的接触时间、相同的材料、水洗蚊帐比不水洗效果差。     

Larvicidal activity of Hibiscus abelmoschus Linn. (Malvaceae) against mosquitoes

The larvicidal activity of roots of Hibiscus abelmoschus was evaluated against the larvae of mosquitoes in the genera Anopheles and Culex. Mean median lethal concentration values of the aqueous extract from the roots of H. abelmoschus against the larvae of Anopheles culicifacies, An. stephensi, and Culex quinquefasciatus were 52.3, 52.6, and 43.8 ppm, respectively. Efficacy of fraction code HAM-4 decreased with an increase in water depth when sprayed at a dose calculated by surface area. Fraction code HAM-4 at the rate of 82 ppm showed 91.1% reduction of larval An. stephensi in a tank, whereas 87.4% reduction of larval Cx. quinquefaisciatus occurred in a blocked drain 24 h after application of HAM-4 under field conditions.     

Comparative susceptibility of different members of the Anopheles culicifacies complex to Plasmodium vivax

Three colonized species of the Anopheles culicifacies complex (species A, B and C) were compared with Anopheles stephensi (control) for susceptibility to Plasmodium vivax. In feeding experiments involving over 400 paired comparisons, the mean number of oocysts, oocyst rate and sporozoite rate were found to be significantly different. Of the test groups, species A had the highest percentage of mosquitoes with oocysts (> 60%) and sporozoites (> 50%). An. culicifacies species B were least susceptible, less than 5% had oocysts and less than 2% had sporozoites in the salivary glands. The results are discussed in the light of the vectorial potential of the members of the An. culicifacies complex observed in field studies.     

Insecticidal activity of Valeriana jatamansi (Valerianaceae) against mosquitoes

A root extract of Valeriana jatamansi (code BAL-O) exhibited larvicidal and adulticidal activity against different mosquito species. The median lethal concentration (LC50) of BAL-O against larvae of Anopheles stephensi, Anopheles culicifacies, Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus were 68.1, 42.8, 51.2, 53.8, and 80.6 mg/liter, respectively. The LC50 and the 90% lethal concentration against adult An. stephensi, An. culicifacies, Ae. aegypti, Ae. albopictus, and Cx. quinquefasciatus were 0.14, 0.16, 0.09, 0.08, and 0.17 and 0.24, 0.34, 0.25, 0.21, and 0.28 mg/cm2, respectively. The median knock-down time and 90% knock-down time of the fraction were 13, 13, 12, 13, and 18 and 24, 25, 21, 20, and 42 min against An. stephensi, An. culicifacies, Ae. aegypti, An. albopictus and Cx. quinquefasciatus, respectively, using 0.28 mg/cm2 impregnated papers. The median effective time and 90% effective time against An. stephensi at 4 degrees C were 46.6 and 8.7 days, and at 29 degrees C, 25.5 and 5.6 days, respectively. Gas chromatographic-mass spectrometric analysis of BAL-O showed 2-butanone,4-(2,6,6-trimethyl-2-cyclohexen-l-yl) (10.11%), patchouli alcohol (8.55%), cubenol (5.97%), caryophyllene oxide (5.46%), cadinol (5.23%), and aristolene (5.19%).