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

Efficacy of Long-lasting Insecticidal Nets With Declining Physical and Chemical Integrity on Aedes aegypti (Diptera: Culicidae)

Fitting long-lasting insecticidal nets (LLIN) as screens on doors/windows has a significant impact on indoor-adult Aedes aegypti (L.), with entomological reductions measured in a previous study being significant for up to 2 yr post-installation, even in the presence of pyrethroid-resistant Aedes populations. To better understand the mode of LLIN protection, bioassays were performed to evaluate the effects of field deployment (0, 6, and 12 mo) and damage type (none, central, lateral, and multiple) on LLIN efficacy. Contact bioassays confirmed that LLIN residual activity (median knockdown time, in minutes, or MKDT) decreased significantly over time: 6.95 (95% confidence interval [CI]: 5.32-8.58) to 9.24 (95% CI: 8.69-9.79) MKDT at 0- and 12-mo age, respectively, using a pyrethroid-susceptible Aedes strain. Tunnel tests (exposing human forearm for 40 min as attractant) showed that deployment time affected negatively Aedes passage inhibition from 54.9% (95% CI: 43.5-66.2) at 0 mo to 35.7% (95% CI: 16.3-55.1) at 12 mo and blood-feeding inhibition from 65.2% (95% CI: 54.2-76.2) to 48.9% (95% CI: 26.4-71.3), respectively; both the passage/blood-feeding inhibition increased by a factor of 1.8-2.9 on LLINs with multiple and central damages compared with nets with lateral damage. Mosquito mortality was 74.6% (95% CI: 65.3-83.9) at 0 mo, 72.3% (95% CI: 64.1-80.5) at 6 mo, and 59% (95% CI: 46.7-71.3) at 12 mo. Despite the LLIN physical integrity could be compromised over time, we demonstrate that the remaining chemical effect after field conditions would still contribute to killing/repelling mosquitoes.

Field efficacy of a new deltamethrin long lasting insecticidal net (LifeNet©) against wild pyrethroid-resistant Anopheles gambiae in Benin

BACKGROUND

Malaria vector control is mostly based on Long-Lasting Insecticidal Nets (LLIN). To date, all LLINs fully recommended by the World Health Organization Pesticide Scheme (WHOPES) are made of polyester or polyethylene. In this context, a new LLIN named LifeNet©, made of polypropylene fiber is developed. According to the manufacturer, LifeNet©is made of soft filament, has a greater mechanical strength, a superior insecticide wash resistance with a short insecticide regeneration time, a better flammability profile and a better environmental profile compared to polyester or polyethylene nets.

METHODS

Through a WHOPES supervised trial, the efficacy of LifeNet© was evaluated in Benin in experimental huts against free-flying wild mosquitoes.

RESULTS

LifeNet© has equal or better performances in terms of wash resistance, exophily, blood feeding inhibition and mortality compared to conventionally treated nets (CTN) treated with deltamethrin at 25 mg/m2 and washed to just before exhaustion.

CONCLUSIONS

The efficacy of LifeNet© observed in this trial indicates that this net fulfill World Health Organization Pesticide Scheme (WHOPES) requirement for Long Lasting technology in Phase II. Throughout a Phase III trial currently ongoing in Southern Benin, the durability and the acceptability of this long-lasting insecticidal mosquito nets will be assessed under community conditions.

Short and long term evaluation of the efficiency of PermaNet® 2.0 bed net against environmental factors and washing using bioassay tests

The aim of the present study was to examine the resistance of PermaNet® 2.0 bed nets against repeated washing and environmental factors by using bioassay tests. After 5, 15 and 21 washings with detergents and by using bioassay tests, the resistance of 40 PermaNet® 2.0 bed nets was compared with that of 40 bed nets conventionally treated with one K-O tablet. To examine the long-term resistance, 31 PermaNet® 2.0 bed nets were also distributed among villagers, and were re-collected to perform bioassay tests after 1, 2 and 5 years. In the first phase of this study, the insecticidal effect of the conventionally-treated nets significantly decreased due to repeated washings (P < 0.001); however, it was not significant regarding PermaNet® 2.0 bed nets (P = 0.92 in continuous exposure and P = 0.12 in mortality tests). In the long-term phase of this study, the time required for knockdown of PermaNet® 2.0 increased over the first 2 years and then decreased. In addition, the mortality rate decreased over the first 2 years and then increased. In conclusion, it seems that the technique used by the manufacturer for impregnation of PermaNet® 2.0 bed nets has an acceptable efficiency in comparison with conventional techniques.

Patterns and predictive factors of long-lasting insecticidal net usage in a previously high malaria endemic area in Sri Lanka: a cross-sectional survey

BACKGROUND

Long-lasting insecticidal nets (LLINs) have been widely distributed in Sri Lanka for malaria control. Their effectiveness depends on proper utilisation and maintenance at the household level.

METHODS

A cross-sectional study was performed to examine the patterns and predictive factors of LLIN maintenance and use in Anuradhapura district. Data was collected and analysed from 530 LLIN-owning households, selected by a multi-stage cluster sampling technique. Multivariable logistic regression identified factors associated with proper maintenance at the household level. Hierarchical linear modelling identified factors associated with LLIN use the previous night.

RESULTS

Almost 75% (377/504) of households had used all their LLINs the previous night, while 82.9% (418/504) had used at least one. Only 3.2% (15/474) were maintaining the LLIN in such a way as to maximise its insecticidal efficacy. Six variables were significantly associated (p<0.05) with use the previous night: more residents, fewer plain nets, reporting practical benefits of LLINs, conical shape, newer nets and lack of side effects. Two variables were significantly associated with proper maintenance: increasing level of education and taking safety precautions while washing.

CONCLUSIONS

Results suggest LLIN practices could improve in settings of low malaria transmission if distribution programmes took into account recipient preferences, promoted LLIN use over plain nets, and emphasised the techniques and significance of proper net maintenance.

Entomological evaluation of PermaNet 2.0® and K-O Tab 1-2-3® treated nets in comparison to nets conventionally treated with deltamethrin, after repeated washing

The residual insecticidal power of two types of ITNs (PermaNet 2.0® (PN2) and K-O Tab 1-2-3® (KO 123)), compared to K-O Tab® (KO) treated nets, was assessed. The nets were tested unwashed, and after being washed, by hand 5, 15 and 21 times, respectively. After each wash, the nets were dried vertically on a line, in the shade. Two types of bioassays (mean median knock down times (MMKDT) and mortality 24 hours after a 3-minute exposure (%mortality)) were used, along with reared female Anopheles stephensi. The number of washes had a great impact on MMKDT and %mortality of all types of nets. This impact was greater for conventionally treated nets, indicating that PN2 and KO 123 nets are significantly more wash resistant than KO nets after 21 washes. There was no significant difference between PN2 and KO 123 with respect to %mortality 24 hours after a 3-minute exposure at 0, 15 and 21 washes. Similarly, the same results were obtained for MMKDT, and the differences between PN2 and KO 123 were not statistically significant. This study demonstrates that the efficacy of KO 123 nets is as beneficial as the efficacy of PN2 nets up to 21 washes.

Optimization of pyrethroid and repellent on fabrics against Stegomyia albopicta (=Aedes albopictus) using a microencapsulation technique

A new approach employing a combination of pyrethroid and repellent is proposed to improve the protective efficacy of conventional pyrethroid-treated fabrics against mosquito vectors. In this context, the insecticidal and repellent efficacies of commonly used pyrethroids and repellents were evaluated by cone tests and arm-in-cage tests against Stegomyia albopicta (=Aedes albopictus) (Diptera: Culicidae). At concentrations of LD50 (estimated for pyrethroid) or ED50 (estimated for repellent), respectively, the knock-down effects of the pyrethroids or repellents were further compared. The results obtained indicated that deltamethrin and DEET were relatively more effective and thus these were selected for further study. Synergistic interaction was observed between deltamethrin and DEET at the ratios of 5 : 1, 2 : 1, 1 : 1 and 1 : 2 (but not 1 : 5). An optimal mixing ratio of 7 : 5 was then microencapsulated and adhered to fabrics using a fixing agent. Fabrics impregnated by microencapsulated mixtures gained extended washing durability compared with those treated with a conventional dipping method. Results indicated that this approach represents a promising method for the future impregnation of bednet, curtain and combat uniform materials.

Laboratory evaluation of Fendona 6SC treated bednets and Interceptor long-lasting nets against Anopheles gambiae s.l. in Burkina Faso

Insecticide-treated bednets play a cornerstone role in the efforts to control malaria. Bednets entomological efficacy is the determinant factor of their use to control malaria. In this study, we compared under laboratory conditions, the efficacy of two long-lasting nets (PermaNet versus Interceptor) and two treatments kits K-O TAB (deltamethrin) versus Fendona 6SC (alpha-cypermethrin) against Anopheles gambiae s.l. malaria vectors. The efficacy of washed and unwashed bednets was assessed by contact bioassays using World Health Organization (WHO) cones. Three to five-days-old mosquitoes were exposed to the netting for 3 min; the median and 95% knockdown time, the after 24 h mortality was recorded for each type of bednet. The mortality after 24 h was equivalent for the Fendona 6SC treated bednets and the K-O TAB treated bednets [79.4% confidence limits (CL) (73.9-84.6) and 74% CL (68.3-80.0), respectively]. However, the Fendona 6SC treated bednets were superior in 50% knockdown time to the K-O TAB treated bednets [7.8 min, CL (6.5-9.0) and 15. 2 min, CL (14.0-16.4), respectively]. Washed Interceptor and PermaNet bednets showed similar efficacy in terms of 50% knockdown times. Mortality after 24 h was similar from the fifth to the twentieth wash, but PermaNet performed better than Interceptor for the first four washes and for unwashed bednets. This study showed that Fendona 6SC kit and the Interceptor bednets have exhibited consistent comparable efficacy in the laboratory compared to the well known and in use K-O TAB kit and PermaNet bednets.

Synergistic insecticidal and repellent effects of combined pyrethroid and repellent-impregnated bed nets using a novel long-lasting polymer-coating multi-layer technique

New and improved strategies for malaria control and prevention are urgently needed. As a contribution to an optimized personal protection strategy, a novel long-lasting insecticide and repellent-treated net (LLIRN) has been designed by binding combinations of permethrin plus N,N-diethyl-m-toluamide (DEET), or insect repellent 3535 (IR3535), and etofenprox plus DEET, onto fibres of bed net fabric employing a new multi-layer polymer-coating technique. Protective repellent efficacy, toxicological effectiveness and residual activity of 12 LLIRN types have been evaluated by laboratory testing against adult Aedes aegypti. The novel multi-layer LLIRN design allowed simultaneous embedding at concentrations up to 5,930 mg/m(2) for DEET, 3,408 mg/m(2) for IR3535, 2,296 mg/m(2) for permethrin and 2,349 mg/m(2) for etofenprox, respectively. IR3535 layers prevented co-binding of additional pyrethroid-containing polymer layers, thus making pyrethroids plus DEET LLIRNs an ideal combination. All LLIRNs revealed synergistic insecticidal effects which, when measured against concentration controls of the isolated compounds, were significant in all LLIRN types designed. DEET in DEET plus permethrin LLIRNs significantly (p < 0.0001) reduced the concentration-dependent permethrin 100 % knockdown (KD) time from 55 to 75 %, the corresponding 100 % kill time (p < 0.0001) from 55 to 64 %. DEET in DEET plus etofenprox LLIRNs reduced the dose-specific 100 % knockdown (KD) time of etofenprox from 42 to 50 % (p = 0.004), the 100 % kill time from 25 to 38 % (p < 0.0001). Permethrin or etofenprox did not influence spatial repellency of DEET or IR3535 on LLIRNs. Vice versa, DEET and IR3535 increased spatial and excitatory repellency and reduced landing and probing frequency on LLIRNs resulting in strongly enhanced biting protection, even at low concentrations. One hundred percent biting and probing protection of stored LLIRNs was preserved for 83 weeks with the 5,930 mg/m(2) DEET and 2,139 mg/m(2) etofenprox LLIRN, for 72 weeks with the 5,002 mg/m(2) DEET and 2,349 mg/m(2) etofenprox LLIRN, for 63 weeks with the 3,590 mg/m(2) DEET and 1,208 mg/m(2) permethrin LLRN, and for 61 weeks with the 4,711 mg/m(2) DEET and 702 mg/m(2) etofenprox LLIRN. Because 100 % bite protection with up to 75 % quicker contact toxicity of pyrethroids were documented, synergistic toxicological and repellent effects of multi-layer polymer-coating LLIRNs may overcome LLIN-triggered selection pressure for development of new kdr- and metabolic pyrethroid resistances while simultaneously increasing protective efficacy also against kdr- and metabolic pyrethroid-resistant mosquitoes substantially due to the repellent-induced effects of LLIRNs thus indicating that this approach is a promising new candidate for future bed net, curtain, and window screen impregnation aiming at optimized prevention from mosquito-borne diseases.

Novel long-lasting impregnation technique transferred from clothing to bednets: extended efficacy and residual activity of different pyrethroids against Aedes aegypti as shown by EN ISO 6330-standardized machine laundering

A novel long-lasting insecticide-treated net (LLIN) has been designed by embedding the pyrethroids deltamethrin, cyfluthrin, permethrin, or etofenprox, onto the fibres of bed net cotton fabric by transferring a new polymer-coating technique from clothing to netting material. EN ISO 6330, the more stringent European Norm for domestic washing and drying procedures for textile testing, has been newly employed to monitor and validate more precise wash durability and residual bioactivity of LLINs. Wash durability, residual insecticidal activity and mosquito landing/biting protection has been investigated and compared with four commercially available LLINs. The pyrethroid-embedding impregnation technique allows binding of high concentrations exceeding 8,000 mg permethrin/m(2) within one single-polymer layer. Recovery rates of 95.7 ± 5.8%, 92.4 ± 14.0%, 70.2 ± 9.1%, and 64.2 ± 6.3% for cyfluthrin; 32.4 ± 11.4%, 32.4 ± 12%, 35.1 ± 16.2%, and 35.8 ± 15.7% for deltamethrin; 75.3 ± 12.9%, 57.1 ± 15.8%, 48.5 ± 4.0%, and 35.6 ± 4.7% for etofenprox; and 95.7 ± 5.8%, 80.2 ± 8.6%, 39.1 ± 7.9%, and 34.1 ± 8.8% were measured after 1, 5, 10, and 20 launderings. Laundering resistance was highest with cyfluthrin > deltamethrin ≈ etofenprox ≈ permethrin. After one and five launderings, commercial LLINs revealed percentage pyrethroid recovery rates of 26.3 ± 11.8% and 0.9 ± 1.2% for the Care Plus(®) net; 31.8% and 28.9% for the Permanet(®) 2.0; 61.4% and 45.6% for the Net Protect(®); and 80.4% and 68.3% for the Conmanet(®). Recovery rates reveal that the polymer-coating method resulted in extended wash durability. Dose-dependent 100% knockdown rates were most effective with deltamethrin > cyfluthrin > permethrin ≈ etofenprox. LLINs tested at concentrations up to 8,000 mg permethrin/m(2) did not protect from mosquito bites. Results on long-lasting efficacy and bioactivity of the polymer-coating technique show that this new LLIN technique is a highly promising potential candidate for future malaria control strategies. Standardized machine laundering and drying according to EN ISO 6330 is highly recommended to monitor and validate wash durability and residual activity of LLINs.

Residual activity and integrity of PermaNet® 2.0 after 24 months of household use in a community randomised trial of long lasting insecticidal nets against visceral leishmaniasis in India and Nepal

The World Health Organization (WHO) recommends several brands of long lasting insecticidal net (LN) for protection against insect vectors but also advises national programmes to monitor and evaluate performance under local conditions to help them select the most suitable LN for their setting. During the course of a community randomised trial of LNs against visceral leishmaniasis in northern India and Nepal, opportunity arose to assess the efficacy of PermaNet 2.0 (Vestergaard-Frandsen, Denmark) after two years of use against sandfly vectors. Between 63% (India) and 78% (Nepal) of LNs became holed over the course of two years, deltamethrin residues fell from 55 mg/m(2) to an average of 11.6 mg/m(2) (India) and 27.9 mg/m(2) (Nepal), but on the basis of bioassay criteria all LNs tested still met the WHO Pesticide Evaluation Scheme standard for LN effectiveness. Nets had on average only been washed 2.5 times (India) and 0.6 times (Nepal) by householders over the course of two years. The loss of insecticide was attributed to factors which had little or nothing to do with washing, such as handling, friction and torsion during daily use. Under conditions pertaining in this region of south Asia, and for two years at least, this brand of net continues to meet the criteria established by WHO for LNs.

Evaluation of the wash resistance of three types of manufactured insecticidal nets in comparison to conventionally treated nets

The present study evaluated the efficacy and wash resistance of several commercial deltamethrin-treated nets (PermaNet, from factory (PN-F) and market (PN-M), Yorkool (Y) and AZ net) that were claimed by the manufacturers to be Long-Lasting Insecticide Treated Nets (LLITNs), compared to ITNs conventionally treated with deltamethrin (23-27 mg/m(2), using one K-O Tab tablet (KO) per net). Montpellier washing technique was used for washing the pieces of the nets. Insecticidal activity was assessed on dried pieces of nets after 0, 2, 5, 8, 11, 15, 18 and 21 washes, using two types of bioassay (mean median knock down times and mortality 24 h after a 3-min exposure) and reared female Anopheles stephensi. To evaluate the effect of heat on diffusion of insecticide from inside of the nets to the surface of them, some Permanet nets were heated. For all the types of nets tested the median knock down time (MKDT) increased approximately linearly with number of washes. The slopes of the lines (increase of MKDT per wash) were low with the PN-F and PN-M, intermediate with Y and equally high with KO and AZ. No significant differences can be claimed with the 3-min exposure tests. The slopes of the regression lines did not differ significantly between the heated and unheated samples. It is concluded that diffusion at ambient temperature is fast enough to rapidly compensate for the loss of insecticide on the surface with no need to artificially stimulate diffusion by heating.

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.