Impact of lambdacyhalothrin capsule suspension treated bed nets on malaria in tribal villages of Malkangiri district, Orissa, India

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1 Indian J Med Res 128, September 2008, pp Impact of lambdacyhalothrin capsule suspension treated bed nets on malaria in tribal villages of Malkangiri district, Orissa, India S.S. Sahu, T. Vijayakumar, M. Kalyanasundaram, S. Subramanian & P. Jambulingam Vector Control Research Centre, (Indian Council of Medical Research), Puducherry, India Received July 5, 2007 Background & objectives: Insecticide treated mosquito nets are increasingly being used in malaria control programmes. One of the problems with the treatment of bed nets with conventional formulations of insecticides was that regular washing of treated nets diminish insecticidal effect. Lambdacyhalothrin 2.5 capsule suspension (CS) (2.5% a.i., w/v), a new water-based microencapsulated formulation is reported to have wash- resistant property and longer persistence on the netting material than other formulations. We evaluated the impact of the use of nylon bed nets treated with lambdacyhalothrin 2.5 CS at 10 mg (a.i.)/m 2 in comparison to untreated nets and no nets on malaria in tribal villages in Orissa. Methods: Nine foothill villages, highly endemic for falciparum malaria, from the Primary Health Centre (PHC) areas of Khairput and Kudumulugumma of Malkangiri district, Orissa, were divided into three groups, each with a population of about 500 and allocated randomly for treated (TN) and untreated nets (UN) and no nets (NN). Bed nets were distributed in September 2001 and retreatment was done in June The impact was assessed based on the changes in vector density, parous rate, malaria incidence and parasite rates. Indoor-resting collections of Anopheles fluviatilis and An. culicifacies were made at fortnightly intervals from fixed human dwellings. Mass blood surveys before and after distribution of nets and fortnightly active surveillance were carried out to assess the change in parasite rates and malaria incidence. Bioassays were conducted at fortnightly intervals on the bed nets supplied to the villagers. Results: The reductions in indoor resting catches of An. fluviatilis and An. culicifacies were 96 and 38 per cent in villages with treated nets and 2.6 and 23 per cent in villages with untreated nets respectively compared to no net villages. For six months following treatment, 100 per cent mortality of An. fluviatilis was observed on the unwashed nets and on the nets washed once or twice. After re-treatment, 100 per cent mortality of An. fluviatilis or An. culicifacies was observed for nine months even after two washes. Usage rates of treated and untreated nets varied seasonally; 58.9 and 46.3 per cent in rainy season, 48.6 and 37.1 per cent in winter season and 38.1 and 31.6 per cent in summer season respectively. Reductions in malaria parasite rates were about 65 per cent in the treated net villages and 39 per cent in the untreated net villages compared to no net villages. About 75 per cent of treated nets and 60 per cent of untreated nets were in usable condition 19 months after distribution. Interpretation & conclusions: The estimated protection factor based on malaria incidence was 86 per cent for the treated nets during both post-treatment and post-retreatment periods and 34 and 51 per cent for untreated nets for the corresponding periods compared to no nets. The results of the study showed that the use of bed nets treated nets with CS formulation of lambdacyhalothrin at 10 mg (a.i.)/m 2 was acceptable to the community and re-treatment of nets at nine-monthly intervals can significantly reduce density and survival of An. fluviatilis and incidence of falciparum malaria. Key words Anopheles fluviatilis - capsule suspension - insecticide treated bed nets - lambdacyhalothrin - malaria 262

2 SAHU et al: IMPACT OF LAMBDACYHALOTHRIN (CS) TREATED BED NETS ON MALARIA 263 A large number of trials using bed nets treated with different pyrethroids have been carried out since 1990 in various countries in Asia 1,2, Africa 3-5, Latin America 6 and Central America 7. In India, field trials with bed nets treated with either emulsifiable concentrate (EC) or flowable concentrate (FC) formulation of synthetic pyrethroids, such as deltamethrin, permethrin, alphacypermethrin, lambdacyhalothrin, have been conducted for malaria control with variable results Insecticide treated mosquito nets are now being increasingly used in malaria control programmes as a component of the new Global Malaria Control Programme 14. One of the problems of treatment of bed nets with conventional formulations (EC or FC) was that regular washing of treated nets diminish the insecticidal effect and nets had to be re-treated every six months. Since villagers wash their nets at least once in two to three months, it is desirable to have a formulation that resists the effects of washing. As an alternative, three brands of WHO recommended long lasting insecticidal nets are already in the market and ready for operational use 15. Lambdacyhalothrin 2.5 capsule suspension (CS) (2.5% a.i. w/v), a new water-based microencapsulated formulation used to impregnate nets and other fabrics, is reported to have a longer persistence on the netting material, and demonstrates wash resistant properties compared to other formulations 16. The CS formulation of lambdacyhalothrin has not been tested for its efficacy against malaria vectors and acceptability to the users under Indian conditions. We undertook this study to assess the impact of the use of lambdacyhalothrin 2.5 CS treated bed-nets compared to untreated nets and no nets on malaria in the tribal villages of Malkangiri district, Orissa State, which are highly endemic for falciparum malaria. Material & Methods Study area: The trial area, Malkangiri district is situated in the southernmost part of Orissa State (17 o 50' and 20 o 30 N and 81 o 27' and 84 o 10 E), India, and is inhabited by tribal populations. The details of the study area have been described elsewhere 13. The district has a population of 5,20,000 with an annual parasite incidence (API) varying from per thousand and number of deaths due to malaria ranging from 8 to 54 during (Chief District Medical Office, Malkangiri, Orissa). While all four species of human malaria parasite have been recorded, Plasmodium falciparum was the predominant species, contributing to >90 per cent of the total malaria cases 13,17. Malaria transmission occurs perennially with peak incidence during July-August and November-December 17. Anopheles fluviatilis is the primary vector transmitting malaria throughout the year and is endophilic, endophagic and anthropophagic 18. An. culicifacies sensu lato is a secondary vector, prevalent during summer and rainy seasons and is also endophilic but highly zoophagic 18. Nine foothill villages Rauliguda, Majhiguda Rengamguda (no net villages), Pakhnaguda, Bonoguda, Machaguda (untreated net villages) and Pusuriguda, Sindhiguda, Nandiniguda (treated net villages) were selected for the trial from the Primary Health Centre (PHC) areas of Khairput and Kudumulugumma of Malkangiri district (Fig. 1). The villages were classified into three groups with comparable pre-treatment vector abundance. The three groups were then allocated randomly for each type of intervention or for control namely, lambdacyhalothrin treated (TN) and untreated nets (UN) and no nets (NN). The total population of each treatment group was about 500. The male to female human ratio was 1: The human to cattle ratio was 1: 0.8. The majority of the population (over 80%) had no formal education and their major occupations were agriculture and collection of forest products. Generally people have no habit of using mosquito nets, other mosquito repellents and there was no privately owned bed nets in these villages. However, they used to generate smoke when the mosquito nuisance was very high. The study was carried out from October 2000 to June Ethical clearance for this study was obtained from the Ethics Committee of Vector Control Research Center, Puducherry. Treatment and distribution of bed nets: Three sizes of nylon bed nets - single ( cm), double Fig. 1. Scaled plan of the area indicating the study villages.

3 264 INDIAN J MED RES, SEPTEMBER 2008 ( cm) and family sizes ( cm) commonly available in the market with a mesh size of 2 x 2 mm were used. The nets were treated with lambdacyhalothrin 2.5 CS (M/s. Syngenta Crop Protection Pvt. Ltd., Chennai, India) at the recommended application rate of 10 mg (ai)/m 2 following the standard procedure 19. The treated bed nets were dried in the shade 2-3 days prior to the distribution to the villagers. The requirement of treated and untreated nets was assessed based on a population census carried out in August A total of 235 and 230 untreated and treated bed nets were distributed in two groups of three villages each with a population of 528 and 497 respectively. The nets were distributed in September 2001 (before the annual peak transmission period) to the villagers according to the size of the family, their relationship and their sleeping habits. Based on the bioassay of treated nets against field collected An. fluviatilis, insecticide re-treatment was carried out in June 2002, just before the rainy peak transmission period. Since the villagers had never used bed nets before, the method of use was demonstrated. The use rate was assessed by nighttime spot surveys conducted monthly with the help of local volunteers. Surveys covered 197 to 642 net users per season. The washing frequency, community attitude and acceptability were also assessed based on interviews. Entomological evaluation: Entomological data were collected from October 2000 to September 2001 prior to distribution of bed nets and from October 2001 to June 2003 after distribution. Indoor-resting collections of An. fluviatilis and An. culicifacies were made at fortnightly intervals from fixed catching stations (18 human dwellings in each group). Adult mosquitoes resting indoors were collected between 0700 and 0800 h using an aspirator and flashlight. The anophelines collected were identified, counted and classified according to their abdominal conditions and manually dissected for gut/gland infection and ovary dilatations. Blood meals of An. fluviatilis were identified using the agar-gel diffusion method 20. Man biting collections were not carried out for ethical reasons. Bioassay: To evaluate the persistence of residual efficacy of lambdacyhalothrin on the treated bed nets, bioassays were conducted at fortnightly intervals on the bed nets supplied to the villagers. Wild-caught fully fed female mosquitoes (13-15 in numbers) were exposed in WHO bioassay cones to the treated nets for 3 min and the mortality recorded after 24 h. Three replicates were exposed and untreated nets served as controls. An. fluviatilis females were used in the bioassay tests following the first treatment during October March 2002 and again from September 2002 to March From April - August 2002, An. culicifacies females were used, as this species was generally more abundant during this season. Bioassays were also conducted on the washed bed nets. Mass blood surveys and fever surveillance: Mass blood surveys were conducted for the population of 1-14 years of age in August 2001 before the distribution of bed nets and in August 2002 and June 2003 after the distribution of bed nets. Fortnightly active case detection was carried out from house to house in all villages throughout the study period (Pre treatment: October September 2001; Post-treatment: October June 2002; Post-retreatment: July June 2003). Blood samples, both thin and thick smears were collected by finger prick from people reporting fever. All the blood samples were stained with Giemsa and examined for malaria parasites in 100 fields of thick smears at 500 magnifications under a compound microscope (Model CHS, Olympus Co. Ltd., made in Japan). If a slide was found to be positive, the species was confirmed by examining the thin smear at 500 magnifications. The average monthly parasite incidence (MPI) was calculated using the data from active case detection. Data from mass blood surveys were used to estimate the overall parasite rate for each group of villages. All malaria positive cases were promptly treated with chloroquine and primaquine according to the National Anti-Malaria Programme guidelines 21. Effect on other household pests: Samples of floor sweepings from randomly selected houses one day after the distribution of the treated and untreated bed nets and subsequently at fortnight intervals were collected. The sweepings were examined for any dead household pests such as bedbugs, moths and cockroaches. Physical condition of the nets: At the end of one year and 7 months after the distribution of nets, checks on the nets and enquiries were made door to door to find out the condition of the nets. Statistical analysis: All analyses on entomological data were carried out for the pre-treatment and entire posttreatment period (October 2001 to June 2003) because of smaller samples collected during the latter period. One-way analysis of variance (ANOVA) was used to compare the changes in the indoor resting catches of anophelines between village groups during pre-

4 SAHU et al: IMPACT OF LAMBDACYHALOTHRIN (CS) TREATED BED NETS ON MALARIA 265 treatment and post-treatment periods. The data were normalized by log transformation before carrying out the ANOVA. Pair-wise comparison of treatment effects was made by Duncan s multiple range tests or Dunnett s C-test, respectively, depending on whether the variances were equal or unequal between treatment groups 22. The heterogeneity 2 -test was used to compare the relative changes in parous rates and human blood indices of vector mosquitoes between villages with no nets and villages with treated nets. Odds ratio was calculated to assess the protective effect of nets with or without treatment over villages without nets. The log-odds ratio interaction test was used to compare the malaria parasite rate before and after treatments for each group of villages. The relative changes in monthly parasite incidence from pre- to post-treatment or postretreatment period between villages with no net (NN) and untreated (UN) or treated net (TN) were compared using a Poisson regression model. The observed monthly parasite incidence was used as the dependent variable with person-month as exposure variable. The treatment group (0=NN, 1=UN and 2=TN) and period of treatment (0=pre, 1=post-treatment, 2=postretreatment) and their interactions (treatment period) were included as independent variables. The estimated coefficients were used to compare the relative changes in incident rate ratio (IRR) for the NN villages with UN or TN villages from pre to post-treatment or postretreatment period. All statistical analyses were carried out using the stata/se software version 9.2 (Stata CorpLP, Texas). Results Resting catches of malaria vectors: A total of 15,950 anopheline females comprising 16 species were collected during the study period. Out of 7311 and 8639 anophelines collected during pre-and post-treatment periods, 2481, 2483 and 2347 during pre-treatment and 3362, 3300 and 1977 during post-treatment were from villages with no nets, untreated nets and treated nets respectively. The percentage of An. culicifacies collected ranged from 42.1 to 47.5 per cent during pretreatment and 40.5 to 44.9 per cent during post-treatment in different groups of villages. Similarly, the percentage of An. fluviatilis ranged from 13.0 to 16.4 per cent during pre-treatment and 1.7 to 16.7 per cent during posttreatment periods in different village groups. None of the percentages differed significantly between pre- and post-treatment periods for any group of villages except in treated net villages (16.4 vs. 1.7%; P<0.05). Pretreatment monthly indoor resting catches of An. fluviatilis did not differ significantly between the three groups of villages [(F = 0.12; df = (2, 213); P = 0.9)] and the seasonal variation was similar in all the villages. For villages without nets and untreated nets, the An. fluviatilis populations were relatively low during February-May, with an increase in June-July and a peak in November-December (Fig. 2A). After treated nets were distributed, there was about 96.2 per cent (95% CI: ) reduction in the indoor resting catches of An. fluviatilis in villages with treated nets and only a 2.6 per cent (95% CI: ) reduction in villages with untreated nets compared to villages with no nets. Oneway ANOVA revealed significant difference among the three groups of villages [(F = 43.9, df = (2, 374); P < 0.001)]. The indoor resting catches were significantly lower for villages with treated nets when compared to villages with no nets or villages with untreated nets (Dunnett s C test, P < 0.05). Pre-treatment sporozoite infections were 0.66 per cent (2 out of 304), 0.33 per cent (1 out of 300) and 0.32 per cent (1 out of 316) from villages that later had treated, untreated and Fig. 2. Indoor resting catches of (A) An. fluviatilis and (B) An. culicifacies in villages with no nets, untreated nets and treated nets.

5 266 INDIAN J MED RES, SEPTEMBER 2008 no nets respectively. During post-treatment the sporozoite rate was only 0.23 per cent (1 out of 443) in the villages with untreated nets. No sporozoite positive was found among 21 and 529 dissected from the treated and no net villages. The indoor resting catches of An. culicifacies peaked in July and August and were low during September-January in all the three groups of villages during the pre-treatment period (Fig. 2B). The indoor resting catches of An. culicifacies did not differ significantly between the three groups of villages [(F = 2.3; df = (2, 215); P = 0.11)] before treatment. There was 38.0 per cent (95% CI: ) and 23.0 per cent (95% CI: ) reduction in the indoor resting catches of An. culicifacies in villages with treated and untreated nets respectively [(F = 8.2, df = (2, 374); P < 0.001)] compared to villages with no nets. The difference in indoor resting catches between villages with and without nets as well as between villages with treated and untreated nets was significant (P < 0.05). Out of 456, 627, and 528 An. culicifacies dissected during pre-treatment and 490, 907, and 934 during posttreatment from villages with treated, untreated and no nets respectively, no oocyst or sporozoite was found. Bioassay: After the initial treatment of the nets in September 2001, 100 per cent mortality of An. fluviatilis was observed up to six months on the unwashed nets and on the nets washed once or twice. Beyond this period, insufficient numbers of mosquitoes were available for assay. Therefore, the test was continued with An. culicifacies. The percentage of mortality for An. culicifacies ranged from 88 to 90 per cent for unwashed nets, 86 to 64 per cent for nets washed once and 70 to 53 per cent for nets washed twice for further two months. After re-treatment of the bed nets in June 2002, 100 per cent mortality was observed for An. culicifacies for 3 months (July-September 2002) and thereafter against An. fluviatilis for six months up to March 2003, for a total period of nine months. Bioassay results were similar on both unwashed and washed nets (twice washings) up to nine months. Beyond this period, insufficient number of mosquitoes was available for assay. Human blood index: Based on the tests on An. fluviatilis from each of the three groups of villages, the human blood index (HBI) ranged from 0.81 to 0.91 during the pre-treatment period and there were no significant differences between the three groups of villages. The HBI values for the post-treatment period were 0.87 (n=608), 0.82 (n=518), and 0.69 (n=84) in the NN, UN and TN villages respectively. The values did not significantly differ between the three groups. Parity: During the pre-treatment period, the proportion of parous mosquitoes did not vary significantly between the three groups of villages for both An. fluviatilis and An. culicifacies (Table I). During the post-treatment period in villages with treated nets, the parity rates for both the species were significantly lower than those in villages with no nets and untreated nets (P < 0.02 for all comparisons). However, the rates for An. fluviatilis did not differ significantly between villages with untreated and no nets and the rate for An. culicifacies was significantly lower in the untreated villages compared to no-net villages (P = 0.009). It may be noted that only 21 An. fluviatilis could be caught and dissected in the villages with treated nets during the post-treatment period. Malaria incidence: Table II shows the MPI and IRR based on the fitting of Poisson regression model to the observed parasite incidence data. The IRR was significantly higher in the UN (1.55 times) and TN (1.46 times) villages than that in NN (1.0) villages during pre-treatment period (P<0.05). In the NN villages, the IRR increased significantly during the post-treatment (2.7 times) and post-retreatment (2.8 times) periods. In contrast, in the UN villages, the IRR declined to 0.34 and 0.51 times of the IRR in the NN villages during post-treatment and post-retreatment periods respectively. In the TN villages, the IRR declined to 0.14 time of the IRR in the NN during both the posttreatment and post-retreatment periods. The declines both in UN and TN villages were significant (P<0.0001). The relative decline in TN villages when compared to UN villages was significant during both Table I. Parity of vectors during pre- and post-treatment in the three groups of villages Village groups Pre-treatment period Post-treatment period Number Proportion Number Proportion dissected parous dissected parous* An. fluviatilis: NN a a UN a a TN a b An. culicifacies: NN a a UN a b TN a c *Proportion parous between pre- and post-treatment periods with same superscript letters do not differ significantly using heterogeneity 2 -test NN, not net; UN, untreated net; TN, treated net

6 SAHU et al: IMPACT OF LAMBDACYHALOTHRIN (CS) TREATED BED NETS ON MALARIA 267 post-treatment (P<0.001) and post-retreatment (P<0.001) periods. The age-wise malaria incidence during the pretreatment and the post-treatment periods in villages with treated nets, untreated nets and no nets is given in Fig. 3. The results showed that there was a significant reduction in incidence in all age groups in treated net villages compared to villages with untreated and no nets (P< 0.05). In the infant age group, there was a reduction of 63.5 per cent in treated net villages and 46.3 per cent in untreated villages compared to no net villages during the post-treatment period (post-treatment and post-retreatment periods combined due to small sample size). The reduction was, however, significant only in treated net villages (P<0.05). Malaria prevalence: Among the population below 15 yr, per cent was examined in the mass blood surveys. The reductions in parasite rate were 70.7 and 65.4 per cent during post-treatment and post-retreatment periods respectively in villages with treated bed nets when compared to the pre-treatment period. In the villages with untreated nets, the corresponding figures were 39.5 and 39.2 per cent. In contrast, there was an increase in parasite rate by 55.2 and 47.9 per cent in villages with no nets during post-treatment and postretreatment periods. The reductions in parasite rate in the villages with treated and untreated nets were significant compared to villages with no nets (Table III, log-odds ratio interaction test, P<0.05). Though there was a significant reduction (P=0.03) in parasite rate in treated net villages compared to villages with untreated nets during post-treatment, the same was not significant (P = 0.09) during post-retreatment period. Usage pattern of distributed nets: Usage rates of treated and untreated nets varied seasonally; 58.9 and 46.3 per cent in the rainy season, 48.6 and 37.1 per cent in the Fig. 3. Age-specific malaria incidence in the study villages during pre-, post-treatment and post- re-treatment periods. Table II. Malaria monthly parasite incidence (MPI) per 1000 person-months, incident rate ratio (IRR) and its 95 per cent confidence interval (CI) based on the fitting of Poisson regression model Village groups Pop BSE Pf Pv Pm Mixed Total MPI IRR (95% CI) Pre-treatment: NN UN ( ) a TN ( ) a Post-treatment: NN ( ) a UN ( ) b TN ( ) c Post-retreatment: NN ( ) a UN ( ) b TN ( ) c Pop, population; BSE, blood smear examined; Pf, P. falciparum; Pv, P. vivax; Pm, P. malariae IRR with same superscript letters, within pre, post or retreatment periods do not differ significantly (P = 0.05)

7 268 INDIAN J MED RES, SEPTEMBER 2008 Table III. Malaria parasite rate in the experimental villages during pre- and post-treatment periods Village groups Population BSE Pf Pv Pm Mixed Parasite Change in Odds ratio < 15 yr rate (%) parasite (95% CI)* rate (%) Pre-treatment: NN UN TN Post-treatment: NN ( ) a UN ( ) b TN ( ) c Post-retreatment: NN ( ) a UN ( ) b TN ( ) b NN, no nets; UT, untreated nets; TN, treated nets: BSE, blood smears examined; Pf, Plasmodium falciparum; Pv, P. vivax; Pm, P. malariae *Odds ratios with same superscript letters do not differ significantly at P = 0.05 winter season, and 38.1 and 31.6 per cent in the summer season, respectively. By May 2002, 64 per cent of the treated nets were washed once, 6 per cent of the nets twice and 3 per cent of the nets thrice after the first treatment. By February 2003, 40 per cent of the nets were washed once, 26 per cent of the nets twice and 13 per cent of the nets thrice after the second treatment. One year and 7 months after distribution, 75 per cent of the treated and 60 per cent of the untreated nets were in usable conditions. The remaining were either severely damaged ( %) or found missing (10-13%). Attitude and acceptance of the nets by the community: Out of 497 treated net users, 12 complained of a burning sensation on the face (skin irritation) during the first seven days after the distribution of treated nets, but not thereafter. There were no other complaints of any discomfort in using the treated nets. The villagers who used treated nets also benefited from other pests as infestations of head-lice, ants, flies, etc., were greatly reduced. Their acceptance was evident from their readiness to get their nets re-treated in June Women actively participated in the re-treatment of bed nets. Household pests: The treatment was effective in killing household pests for 50 days post-treatment and the sweepings in the houses with treated nets contained as many as 13 species of household pests, including cockroaches, houseflies and wasps. After this period, no pests were found in the sweepings. Discussion An. fluviatilis is predominantly endophilic with peak biting rates in the middle of the night 23. The use of bed nets treated with lambdacyhalothrin 2.5 CS at 10 mg (ai)/m 2 significantly reduced the indoor resting catches of An. fluviatilis for a period of nine months. This reduction in the indoor resting population could be partly due to the widespread use of lambdacyhalothrin treated nets in the villages and partly due to the excito-repellent effect of the insecticide in driving mosquitoes out of treated houses 13,24. In longitudinal trials carried out elsewhere in India, reductions in the indoor resting catches of An. fluviatilis were also reported in residences that had bed nets treated with lambdacyhalothrin EC 9,25, cyfluthrin 26, and alphacypermethrin SC 13. Curtis et al 27 reported significant reduction in the populations of An. gambiae and An. funestus in the villages of Tanzania with nets treated with lambdacyhalothrin CS at 20 mg (ai)/m 2. An. culicifacies is a zoophilic species 23 and the probability of its coming into contact with the treated nets was relatively low. As a result, there was relatively lesser impact of the treated nets on the indoor resting catches of An. culicifacies. An earlier study in the same area with lambdacyhalothrin (EC) treated bed nets, demonstrated similar observations 9. The observed reduction in the villages using bed nets might be the result of diversion of mosquitoes to animal feeding and resting in other shelters. The excess reduction in treated net villages could be due to the killing effect. An. culicifacies in Orissa (study area) consists of sibling species B (63.7%) and C (36.3%) and both of them are zoophagic in nature and poor vectors 28. Since the manbiting rate and the malaria infection rate of An. culicifacies were low in the study area 18, the marginal

8 SAHU et al: IMPACT OF LAMBDACYHALOTHRIN (CS) TREATED BED NETS ON MALARIA 269 impact of insecticide treated bed nets on this species may not greatly affect malaria transmission. Impact observed on the population of An. culicifacies in other studies with lambdacyhalothrin (EC) and deltamethrin (EC) treated bed nets in Sundargarh district 12, 25 might be due to the use of higher application rate, 25 mg (ai)/ m 2 and different sibling species composition in the area. The proportion parous of An. fluviatilis and An. culicifacies in treated net villages was significantly reduced in comparison with villages with no nets as observed in trials with EC formulation 12,13,29. Nearly 40 per cent reduction in proportion parous in this study coincided with the significant reduction of malaria incidence and parasite rate in treated villages. Some degree of diversion of surviving population of An. fluviatilis was also evident from the reduction in the HBI in the villages with treated nets. Reduction in HBI in villages with untreated nets without reduction in parous rate might be due to the physical protection of individuals from vectors. In the present study, the residual efficacy of unwashed or washed treated nets lasted for 8-9 months. The duration was slightly higher than that reported for 4-7 months in Tanzania 16 with the same dosage as well as for 6-7 months in India 9,25. In the laboratory, nets treated with lambdacyhalothrin CS at 10 mg (ai)/ m 2 killed per cent of An. gambiae up to 15 months 30. The polyethylene and polyester nets treated with lambdacyhalothrin 2.5 CS could sustain the insecticidal activity up to five washes with water and two washes with soap and water 31. Although apparently, lambdacyhalothrin CS gave a longer duration of residual activity than EC against malaria vectors, the variation observed could partly be due to the difference in the susceptibility of the vector species. Significant levels of vector mortality observed in the bioassay and the reduction in indoor resting An. fluviatilis population for a period of nine months in the villages with treated nets compared to those in untreated and no net villages are indications of possible impact of treated nets on the two-peak transmission seasons. Such impact was evident from the reductions of malaria incidence in all age groups including infants and parasite rate in treated net villages compared to villages with no nets as observed elsewhere 11, 12. Though untreated nets also reduced the risk in malaria incidence and parasite rate compared to no nets, such reductions by treated nets were significantly greater than the untreated nets. The treated nets were acceptable to the community, may be because of the perceptible reduction in mosquito biting rates, knockdown effect of the insecticide on household pests and minimal side effects as observed earlier in trials with lambdacyhalothrin CS or EC treated nets 16,25. Aerating the nets for a week prior to usage could alleviate even the transient discomfort. However, interruption of transmission could not be achieved during the study period, which could be due to the low level of net usage during winter season coinciding with the peak transmission period. Although use rate was the lowest during summer season, as people prefer to sleep outdoors for aeration, the risk of transmission and mosquito nuisance are at the lowest during the period. While the use rate can be further enhanced, particularly during the rainy season and the early part of winter season, through behaviour change communication, one has to keep in mind the prevailing environmental conditions in the area and the cultural habit of the tribes sleeping around a fire during winter season 13. Our assessment of impact parameters was based on non-blinded study and we purposely avoided blinding because field staff could easily identify villages with or without nets. However, since separate teams were engaged for the collection of field samples and for laboratory examination, the bias due to non-blinding was expected to be minimal. In conclusion, the results of the study showed that the use of lambdacyhalothrin 2.5 CS treated bed nets at 10 mg (ai)/m 2 was acceptable to the community and at 9 monthly intervals of treatment could significantly reduce the vector abundance and incidence of malaria. In this setting, about per cent of the nets were in usable condition at the end of 19 months after the supply of nets. Acknowledgment Authors thank Dr P.K. Das, Formerly Director, Vector Control Research Centre (VCRC) (ICMR), Puducherry, India, for the facilities provided, and the staff of VCRC field station at Malkangiri district, Orissa, for technical assistance. Authors acknowledge M/s Syngenta Crop Protection Pvt. Ltd., Chennai for providing the insecticide formulation and sponsoring the evaluation. References 1. Dolan G, ter Kuile FO, Jacoutot V, White NJ, Luxemburger C, Malankirii L, et al. Bed nets for the prevention of malaria and anaemia in pregnancy. Trans R Soc Trop Med Hyg 1993; 87 : Rowland M, Bouma M, Ducornez D, Durrani N, Rozendaal J, Schapira A, et al. Pyrethroid impregnated bed nets for personal protection against malaria for Afghan refugees. Trans R Soc Trop Med Hyg 1996; 90 :

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Kalyanasundaram, Deputy Director (Senior Grade), Vector Control Research Centre (ICMR) Medical Complex, Indira Nagar, Puducherry , India kannananthi@yahoo.com