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Alphamethrin-impregnated bed nets for malaria and mosquito control in China
625
TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE (1994) 88, 625-628
Alphamethrin-impregnated
bed nets for malaria
and mosquito
control
in China
Luo Dapengl, Lu Delingl, Yao Renguo*, Li Peng’, Huo Xueguang 2, Li Aimin’, Wen Lei2, Ge Changyin2, Zhang Shaowed, Huo Hongru2 and Shang Leyuan ’ ‘Henan Provincial Institute of Parasitic Diseases, 47 Wei Wulu, Zhengzhou, Henan, 450003, People’s Republic of China; ‘Health and Anti-epidemic Station of Gusi County, Henan, People’s Republic of China; 3Health and Anti-epidemic Station of Xinyang Prefecture, Henan, People’s Republic of China Abstract
A community-based intervention trial was carried out to evaluate the effectiveness of alphamethrin-impregnated bed nets for control of Plasmodium vivax malaria and its vector in an area of moderate endemicity in southern Henan province, central China in 1990. Malaria incidence was significantly lower in the intervention group than in the comparison group (2.03 vs. 3.57 per 100 person-yearsat risk). The protective efficacy for malaria incidence was 43%. The prevalence of malaria parasitaemiaamong children under 10 years old in the intervention group was about one-quarter of that in the comparison group (0.93% vs. 3.25% and 0.71% vs. 1.96% after one and 4 months use of impregnated nets, respectively). Alphamethrin-impregnated bed nets had a mass killing effect on vector mosquitoes. The outdoor person-biting density of Anopheles anthropophagus and A. sinensis decreased by 70.3% and 29.3% respectively. The density of these 2 mosquito speciesfound resting inside treated nets was close to zero. No side effect was found among users of impregnated bed nets. Impregnation with alphamethrin was more effective on polyester than on cotton netting and residual effectslastedat leastone year. Use of alphamethrin is lessexpensivethan permethrin and deltamethrin. Introduction
The use of bed nets impregnated with pyrethroid insecticides such as permethrin, deltamethrin and lambdacyhalothrin for malaria control has shown very promising results in several countries (GRAVES et al., 1987; SNOW et al., 1988; Lr et al., 1989; NJUNWAet al., 1990; ROBERT & CARNEVALE,1990; SEXTONet al., 1990; ALONSO et al., 1991, 1993; LYIMO et al., 1991; MUYU & CURTIS,
1991; KAMOL-RATANAKUL & PRASITTISUK, 1992; LINDSAY et al., 1993). Alphamethrin, a racemic mixture of the 2 most active isomers of cypermethrin, has been used experimentally in studies of impregnated bed nets in China which suggested that it might be effective (LUO DAPENG, 1992), but large scaletrials with this insecticide have not been reported. This paper describes villagescale trials of alphamethrin-impregnated bed nets against malaria and malaria vectors in Henan, China. Materials and Methods Study area and population
The study was conducted in Liji and Zhangguang townships, Gusi county, Henan province, China in 1990. This is a hilly region, and the inhabitants mainly plant rice. Most houses are very low with thatched roofs, clay walls and poor ventilation. Bed nets were used by most of the inhabitants (98.4%), and most are made of cotton or oolvester netting. The climate is sub-tronical with 4 seasonsper year, and malaria is transmitted from May to October (summer). It is an endemic area for Plasmodium vivax malaria (HUANG, 1987). Anopheles anzhropophagus and A. sinensis are the main vectors (YOU, 1988). Four villages with a total of 11 220 inhabitants ‘in Liji township were selected to receive impregnated bed nets. Liuji village in Zhangguang township, with a population of 2269, 8 km from Liji township, was selected as a comparison area. According to the local malaria surveillance station, malaria incidence rates in the intervention and comparison areaswere 2.64% and 2.38%, respectively, in 1989. Bed net impregnation
Alphamethrin, 5% wettable powder (Fendona@,Shell Corporation, UK), was used at a dosageof 20 mg of active ingredient per square metre for bed net impregnation. Before impregnation in June 1990, public meetings were held to demonstrate the correct use of nets. Villagers were asked to wash their nets and carefully rinse off all traces of soap, as alkaline soap residues could destroy the pyrethroid, and household dust, dirt and soot Addressfor correspondence: Dr Luo Dapeng,HenanProvincial Institute of Parasitic Diseases,47 Wei Wulu, Zhengzhou, Henan,450003,People’sRepublicof China.
may affect the performance of alphamethrin. Villagers were asked not to wash their nets again until the end of October. Trials were conducted before dip ing to determine how much water different types oP material absorbed without wastage, whilst ensuring that the whole area was covered. On average, cotton netting absorbed 130 mL of water per square metre of netting, and polyester netting 28 mLim2. Each net was individuallv imnreanated in the owner’s own plastic bowl, and then hung & a line to dry, Morbidity surveillance
Thick and thin blood films were prepared from anyone with a febrile episode (temperature 237.5”C) in the intervention and comparison areas from May to the end of October 1990. Field workers visited all villagers twice weekly to collect blood films from those with current fever or a history of fever in the last 3d, and patients with fever also presented themselves at the field laboratory. Symptomatic cases with a positive blood film were treated with 1.2 mg chloroquine over 3 d and 180mg primaquine over 8 d, the first dosebeing taken under supervision and the remainder at home. The same regime was used to treat any relapsesof P. vivax, one month later. Parasitaemia surveys in children
At the beginning of June 1990, one in every 3 children in both the intervention and comparison areas was randomly selected to provide cohorts of 424 and 155 children under 10 years of age, respectively. Between June and October 1990, 3 cross-sectional malaria parasitaemia surveys (Sl, S2, S3) were carried out among the cohorts in both areas,0, 1, and 4 months after bed net impregnation. Blood films were obtained from all cohort children at each survey. Mosquito density surveillance
Indoor and outdoor densities of Anopheles mosquitoes were recorded. The outdoor density was assessedby the biting rate. A single untreated bed net with a human ‘bait’ was used to catch mosquitoes outdoors every 10 d from 20:00 to 06:OOin both-the intervention and comnarison regions from Tune to Sentember 1990. Anonhelines rest&g inside bed nets weie caught from 05:Oi to 06:OO to estimate indoor density; 30 bed nets were chosen randomly in each area. Two indoor and outdoor catches were carried out in June, before impregnation, and then 8 more, every 10 d, after impregnation until the end of September. Results were expressed as mosquitoes/net/night and bites/person/night, res ectively. Mosquitoes were identified immediately in t Ke field laboratory.
626 Insecticide assay
To evaluate the residual effect of alphamethrin, impregnated bed nets were bioassayed5,47., 115, and 375 d after impregnation. The method was similar to the WHO bioassay. Four glass funnels (c. 15 cm diameter) were attached to the cotton or polyester netting. About 25 adult female A. sinensis, caught in cattle sheds in the morning, were placed in the cones. After 10 min exposure to the netting, the number of dead mosquitoes was recorded and the live mosquitoes were removed and provided with glucose solution in the cold environment of the field laboratory. Mortality was determined after 24 h. The assayswere performed twice on each occasion. Laboratory methods
All blood films were stained with Giemsa’s stain; thin films were used to confirm species identification. All blood films were coded, so that the technicians who examined the films were unaware of whether they came from intervention or comparison groups.
of the differences between groups. All statistical calculations were carried out using EpiInfo version 5. Results At the end of June 1990, a total of 5130 bed nets had been impregnated covering 96.5% of the population in the intervention area. People living between the intervention and comparison areas also used impregnated nets. Inhabitants in the comparison area used their own nets as usual. Malaria morbidity
Malaria incidence rates in the 2 areas before and after net impregnation are shown in Table 1. Before impregnation, incidence rates were broadly similar in the 2 areas. After impregnation, the incidence rate was significantly lower in the intervention group than that in the comparison group, suggesting a protective efficacy of treated nets of 43%. Parasitaemia in children
Statistical methods
Person-years at risk were used in the incidence rate analysis. The rate ratio (RR) was defined as the ratio of the incidence rate of impregnated bed net users to that of the comparison group. Prevalence ratio (PR) was used to compare the prevalence of malaria parasitaemia in children under 10 years old between the 2 groups. 95% confidence intervals were calculated for each ratio, and ~2 tests were carried out to determine statistical significance Table 1. Malaria incidence rates in intervention and comparison areas l&impregnation Post-impregnation No. of Incidence/ No. of Incidence/ PYAR” cases 100PYAR” PYAR” cases 100PYAR” Intervention Comparison Rate ratioh 2 ;
1870 378
4.27 80 13 3.44 1.24 (0.7-2.21) 0.56 0.46
3740 756
76 2.03 27 3.57 0.57 (0.36-0.89) 6.66 0.009
There was a roughly similar prevalence of malaria among children under 10 years old in the intervention and comparison groups before impregnation (Table 2). One month after bed net impregnation, the prevalence of malaria parasitaemia among children in the intervention group had fallen to about one-quarter of that in the comparison group. Mosquito density
The outdoor density of Anopheles (biting rate) markedly decreasedafter impregnation, that of A. anthropophagus more than that of A. sinensis (Table 3). Impregnation of bed nets with alphamethrin greatly decreasedthe indoor density of both A. anthropophagus and A. sinensis (Table 3). Alphamethrin bioassay
The lethal effect of bed nets impregnated with alphamethrin on A. sinensis lasted at least one year, and polyester nets treated with alphamethrin performed slightly better than cotton nets (Table 4).
‘PYAR=person-years at risk. h95% Confidence interval in parentheses.
Table 2. Prevalence of malaria parasitaemia in intervention and comparison groups Survey no.
Intervention No. No. of children parasitaemic
Comparison No. of No. parasitaemic children
Prevalence ratioa
P
::
424
154 155
0.24 1.10 (0.07-0.85) (0.363.35)
0.016 0.87
s3
423
153
0.22 (0~05-090)
0.02
"95% Confidence interval in parentheses.
Table 3. Person-biting rate and indoor density of Anopheles sinensis and A. anthropophagus in intervention and comparison areas Intervention June A JulyTSept.
Comparison June July-djepr. C
51.5 103 143.5 287
18.4 147 27.0 216
7.0 14 10.3 21
8.4 67 10.3 82
:
0.36 22 0.38 23
0.51 122 0.48
Corrected percentage reduction=
Bites/person/night (outdoor) A. anthropophagus
No. caught
A. sinensis
No. caught Mosquitoes/net/night (indoor) A. anthropophagus
No. caught A. sinensis
No. caught
1.01 61 0.15 8
0.004
1
115
-70.3 -29.2
-
-100 -57.8
-
“[lo0 (BxC)/(AxD)loo]- i.e., the percentage reduction after bed net impregnation in the intervention area corrected for the different values m the comparison area before (June) and after (July-Sept.) bed net impregnation.
627 Table 4. Mortality of Anopheles sinensis 24 h after 10 min exposure to alphamethrin-impregnated bed net fabric” Cotton Polyester Days since impregnation Treated Not treated Treated Not treated 100% (51) 4% (49) 100% 153) 6% (50‘1 5 8% (52j 6% (52j 82% (5Oj ioo% (soj 88% (52) 0 (50) 79% (48) 0 (51) 1;‘: 78% (58) 11% (53) 82% (44) 6% (50) 375 “The numbers of mosquitoes tested are shown in parentheses. Discussion
We found a 43% reduction in malaria incidence rate, and a reduction of over 75% in prevalence of malaria parasitaemia, among children under 10 years old following the introduction of bed nets impregnated with alphamethrin. The reduction in malaria incidence was much greater than that seen in recent studies in Mali (RANQUE et al., 1984), Malaysia (HII et al., 1987), Kenya (SEXTON et al., 1990), Tanzania (LYIMO et al., 1991; MSUYU & CURTIS, 1991), and The Gambia (SNOW et al., 1988; ALONSOet al., 1993), using bed nets impregnated with deltamethrin, permethrin or lambdacyhalothrin. However, the protective efficacy was much lower than that reported in southern China (LI et al., 1989). The study in southern China gave only limited information on methods and, as the authors-used historical controls, it is difficult to exclude the effects of general trends in malaria incidence, as suggested by ~BERMEJO& VEEKEN (1992). The other studies used child morbidity to assess the effect of intervention. Bed nets impregnated with pyrethroid insecticide probably provided more protection to young children than adults, since children go to bed earlier (SHRECK & SELF, 1985; GRAVESet al., 1987). A reason why alphamethrin-impregnated bed nets provided more protection in the present study might be because our study was very large scale, resulting in mass killing of mosquitoes. Also, the present study was carried out in an area of moderate endemicity for malaria: according to the models of MACDONALD(1957) and MosKOVSKiJ(1967), and the actual observations made in the Garki oroiect (MOLINEAUX & GRAMICCIA.1980: ROZENDAAL,’ 1989),‘with a high degree of endemicity, large changes in vectorial capacity may produce little apparent effect, whereas, in areas with low or moderate endemicity as in the present trial area, very small changes in vectorial capacity may decreasethe incidence of malaria greatly and even bring about its eradication. Alphamethrin-impregnated bed nets dramatically decreasedthe outdoor mosquito densities. They affected A. anthropophagus, an endophilic, anthropophilic mosquito, more than A. sinensis, an exophilic and zoophilic species. Bed nets impregnated with alphamethrin were more effective against A. anthropophagus than A. sine&s, probably because the former has a greater preference for human blood and tends to stay in houses after blood feeding, whereas A. sinensis is a zoophilic species more easily diverted to feeding on cattle or other animals. The reduction in numbers of mosquitoes resting inside bed nets can be attributed to irritating and excito%epellent effects, as well as to the killing effects of the impregnated bed nets. Although the number of mosquitoes resting inside bed nets is often used in China as an indicator of ‘indoor’ mosquito density (XV et al., 1988), it might not be appropriate here in evaluating the effectiveness of treated bed nets becauseof irritant and excite-repellent effects of the pyrethroid insecticide. The methods used by SEXTON et al. (19901 and LINDSAY et al. (19931 would be more appropriate in a future study. The reduction of ‘outdoor’ density, on the other hand, must have been due to mass killing by treated bed nets used on a large scale. These results are very similar to those of other studies using deltamethrin-impregnated bed nets in other parts of China (LI et al., 1989; CURTIS,1992). Bioassay showed that the effectiveness of bed nets im-
pregnated with alphamethrin at a dosageof 20 mgim2 of active ingredient lasted at least one year. Impregnation with alphamethrin is more effective on polyester netting than on cotton netting. These results are similar to most of those reported using permethrin, although there is little difference between the fabrics when deltamethrin is used (CURTIS et al., 1990; MILLER, 1990). The better performance of alphamethrin on polyester netting compared with that on cotton netting might be due to the difference in surface texture of the fabrics. Most of the pyrethroid remains on the surface of synthetic fibres and is therefore easily available to mosquitoes, whereas it seemsto be less accessiblewhen in the crevices of rough cotton fibres (HOSSAIN& CURTIS, 1989; MILLER, 1990). No side effect was seen among people who used the treated nets. Only one person in the impregnation teams, who did not use rubber gloves during impregnation, complained of itching and burning of face and neck. His symptoms disappearedwithin 24 h. In this study, the intervention and comparison areas were 8 km apart, and people living between the 2 areas used treated nets, so it is unlikely that the mosquitoes could have flown from one area to the other. Bias was unlikely as the technicians who examined the slides were ‘blinded’. Malaria incidence was similar in the 2 areasin 1989, and the incidence rate and prevalence of malaria parasitaemia in the areas were similar before the introduction of bed net impregnation in 1990. We are thus confident that reductions in malaria incidence rate and parasite prevalence were due to the intervention described and not to other factors. The very low dose necessary makes alphamethrin cheaper to use than other pyrethroids. The average dose of alphamethrin at a dosage of 20 mg of active ingredientimz is US$O.O6 per net, whereas permethrin at the normal dosage of 500 mg of active ingredient/m2 costs about US$O.8 per net (Xu et al., 1988; KAMOL-RATANKUL & PRATTISUK,1992), and deltamethrin at a dose of 15 mg of active ingredient/m2 costs US$O.O97 (CURTIS, 1990). Thus, alphamethrin is a suitable insecticide for malaria control through bed net impregnation. Acknowledgements We are grateful to MS J. Armstrong Schellenberg and Dr J. Lines at the London School of Hygiene and Tropical Medicine for critical comments on drafts of this paper. This project received financial sup ort from the UNDlYWorld BankWHO Special Programme 4or Research and Training in Tropical Diseases. References Alonso, I’. L., Lindsay, S. W., Armstrong, J. R. M., Konten, M., Hill, A. S. G., David, P. H., Fegan, G., Franciso, A. E., Hall, A. J., Shenton, F. C., Cham, K. & Greenwood, B. M. (1991). The effect of insecticide-treated bednets on mortality of Gambian children. Lancet, 337,1499-1502. Alonso, I’. L., Lindsay, S. W., Armstrong Schellenberg, J. R. M., Keita, K., Gomez,P., Shenton, P. C., Hill, A. G., David, P. H., Fegan, G., Cham, K. & Greenwood, B. M. (1993). A malaria control trial using insecticide-treated bed
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You, X. (1988). Relationships between geographic distribution of Anopheles lesteri anthropophagus and endemicity of malaria in southern Henan. ChineseJournal of Parasitology and Parasitic Diseases, 6, 144-145. Received 3 August 1993; revised 2SJanua I6 Februa y I994
for publication
y 1994; accepted
TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE (1994) 88, 625-628
Alphamethrin-impregnated
bed nets for malaria
and mosquito
control
in China
Luo Dapengl, Lu Delingl, Yao Renguo*, Li Peng’, Huo Xueguang 2, Li Aimin’, Wen Lei2, Ge Changyin2, Zhang Shaowed, Huo Hongru2 and Shang Leyuan ’ ‘Henan Provincial Institute of Parasitic Diseases, 47 Wei Wulu, Zhengzhou, Henan, 450003, People’s Republic of China; ‘Health and Anti-epidemic Station of Gusi County, Henan, People’s Republic of China; 3Health and Anti-epidemic Station of Xinyang Prefecture, Henan, People’s Republic of China Abstract
A community-based intervention trial was carried out to evaluate the effectiveness of alphamethrin-impregnated bed nets for control of Plasmodium vivax malaria and its vector in an area of moderate endemicity in southern Henan province, central China in 1990. Malaria incidence was significantly lower in the intervention group than in the comparison group (2.03 vs. 3.57 per 100 person-yearsat risk). The protective efficacy for malaria incidence was 43%. The prevalence of malaria parasitaemiaamong children under 10 years old in the intervention group was about one-quarter of that in the comparison group (0.93% vs. 3.25% and 0.71% vs. 1.96% after one and 4 months use of impregnated nets, respectively). Alphamethrin-impregnated bed nets had a mass killing effect on vector mosquitoes. The outdoor person-biting density of Anopheles anthropophagus and A. sinensis decreased by 70.3% and 29.3% respectively. The density of these 2 mosquito speciesfound resting inside treated nets was close to zero. No side effect was found among users of impregnated bed nets. Impregnation with alphamethrin was more effective on polyester than on cotton netting and residual effectslastedat leastone year. Use of alphamethrin is lessexpensivethan permethrin and deltamethrin. Introduction
The use of bed nets impregnated with pyrethroid insecticides such as permethrin, deltamethrin and lambdacyhalothrin for malaria control has shown very promising results in several countries (GRAVES et al., 1987; SNOW et al., 1988; Lr et al., 1989; NJUNWAet al., 1990; ROBERT & CARNEVALE,1990; SEXTONet al., 1990; ALONSO et al., 1991, 1993; LYIMO et al., 1991; MUYU & CURTIS,
1991; KAMOL-RATANAKUL & PRASITTISUK, 1992; LINDSAY et al., 1993). Alphamethrin, a racemic mixture of the 2 most active isomers of cypermethrin, has been used experimentally in studies of impregnated bed nets in China which suggested that it might be effective (LUO DAPENG, 1992), but large scaletrials with this insecticide have not been reported. This paper describes villagescale trials of alphamethrin-impregnated bed nets against malaria and malaria vectors in Henan, China. Materials and Methods Study area and population
The study was conducted in Liji and Zhangguang townships, Gusi county, Henan province, China in 1990. This is a hilly region, and the inhabitants mainly plant rice. Most houses are very low with thatched roofs, clay walls and poor ventilation. Bed nets were used by most of the inhabitants (98.4%), and most are made of cotton or oolvester netting. The climate is sub-tronical with 4 seasonsper year, and malaria is transmitted from May to October (summer). It is an endemic area for Plasmodium vivax malaria (HUANG, 1987). Anopheles anzhropophagus and A. sinensis are the main vectors (YOU, 1988). Four villages with a total of 11 220 inhabitants ‘in Liji township were selected to receive impregnated bed nets. Liuji village in Zhangguang township, with a population of 2269, 8 km from Liji township, was selected as a comparison area. According to the local malaria surveillance station, malaria incidence rates in the intervention and comparison areaswere 2.64% and 2.38%, respectively, in 1989. Bed net impregnation
Alphamethrin, 5% wettable powder (Fendona@,Shell Corporation, UK), was used at a dosageof 20 mg of active ingredient per square metre for bed net impregnation. Before impregnation in June 1990, public meetings were held to demonstrate the correct use of nets. Villagers were asked to wash their nets and carefully rinse off all traces of soap, as alkaline soap residues could destroy the pyrethroid, and household dust, dirt and soot Addressfor correspondence: Dr Luo Dapeng,HenanProvincial Institute of Parasitic Diseases,47 Wei Wulu, Zhengzhou, Henan,450003,People’sRepublicof China.
may affect the performance of alphamethrin. Villagers were asked not to wash their nets again until the end of October. Trials were conducted before dip ing to determine how much water different types oP material absorbed without wastage, whilst ensuring that the whole area was covered. On average, cotton netting absorbed 130 mL of water per square metre of netting, and polyester netting 28 mLim2. Each net was individuallv imnreanated in the owner’s own plastic bowl, and then hung & a line to dry, Morbidity surveillance
Thick and thin blood films were prepared from anyone with a febrile episode (temperature 237.5”C) in the intervention and comparison areas from May to the end of October 1990. Field workers visited all villagers twice weekly to collect blood films from those with current fever or a history of fever in the last 3d, and patients with fever also presented themselves at the field laboratory. Symptomatic cases with a positive blood film were treated with 1.2 mg chloroquine over 3 d and 180mg primaquine over 8 d, the first dosebeing taken under supervision and the remainder at home. The same regime was used to treat any relapsesof P. vivax, one month later. Parasitaemia surveys in children
At the beginning of June 1990, one in every 3 children in both the intervention and comparison areas was randomly selected to provide cohorts of 424 and 155 children under 10 years of age, respectively. Between June and October 1990, 3 cross-sectional malaria parasitaemia surveys (Sl, S2, S3) were carried out among the cohorts in both areas,0, 1, and 4 months after bed net impregnation. Blood films were obtained from all cohort children at each survey. Mosquito density surveillance
Indoor and outdoor densities of Anopheles mosquitoes were recorded. The outdoor density was assessedby the biting rate. A single untreated bed net with a human ‘bait’ was used to catch mosquitoes outdoors every 10 d from 20:00 to 06:OOin both-the intervention and comnarison regions from Tune to Sentember 1990. Anonhelines rest&g inside bed nets weie caught from 05:Oi to 06:OO to estimate indoor density; 30 bed nets were chosen randomly in each area. Two indoor and outdoor catches were carried out in June, before impregnation, and then 8 more, every 10 d, after impregnation until the end of September. Results were expressed as mosquitoes/net/night and bites/person/night, res ectively. Mosquitoes were identified immediately in t Ke field laboratory.
626 Insecticide assay
To evaluate the residual effect of alphamethrin, impregnated bed nets were bioassayed5,47., 115, and 375 d after impregnation. The method was similar to the WHO bioassay. Four glass funnels (c. 15 cm diameter) were attached to the cotton or polyester netting. About 25 adult female A. sinensis, caught in cattle sheds in the morning, were placed in the cones. After 10 min exposure to the netting, the number of dead mosquitoes was recorded and the live mosquitoes were removed and provided with glucose solution in the cold environment of the field laboratory. Mortality was determined after 24 h. The assayswere performed twice on each occasion. Laboratory methods
All blood films were stained with Giemsa’s stain; thin films were used to confirm species identification. All blood films were coded, so that the technicians who examined the films were unaware of whether they came from intervention or comparison groups.
of the differences between groups. All statistical calculations were carried out using EpiInfo version 5. Results At the end of June 1990, a total of 5130 bed nets had been impregnated covering 96.5% of the population in the intervention area. People living between the intervention and comparison areas also used impregnated nets. Inhabitants in the comparison area used their own nets as usual. Malaria morbidity
Malaria incidence rates in the 2 areas before and after net impregnation are shown in Table 1. Before impregnation, incidence rates were broadly similar in the 2 areas. After impregnation, the incidence rate was significantly lower in the intervention group than that in the comparison group, suggesting a protective efficacy of treated nets of 43%. Parasitaemia in children
Statistical methods
Person-years at risk were used in the incidence rate analysis. The rate ratio (RR) was defined as the ratio of the incidence rate of impregnated bed net users to that of the comparison group. Prevalence ratio (PR) was used to compare the prevalence of malaria parasitaemia in children under 10 years old between the 2 groups. 95% confidence intervals were calculated for each ratio, and ~2 tests were carried out to determine statistical significance Table 1. Malaria incidence rates in intervention and comparison areas l&impregnation Post-impregnation No. of Incidence/ No. of Incidence/ PYAR” cases 100PYAR” PYAR” cases 100PYAR” Intervention Comparison Rate ratioh 2 ;
1870 378
4.27 80 13 3.44 1.24 (0.7-2.21) 0.56 0.46
3740 756
76 2.03 27 3.57 0.57 (0.36-0.89) 6.66 0.009
There was a roughly similar prevalence of malaria among children under 10 years old in the intervention and comparison groups before impregnation (Table 2). One month after bed net impregnation, the prevalence of malaria parasitaemia among children in the intervention group had fallen to about one-quarter of that in the comparison group. Mosquito density
The outdoor density of Anopheles (biting rate) markedly decreasedafter impregnation, that of A. anthropophagus more than that of A. sinensis (Table 3). Impregnation of bed nets with alphamethrin greatly decreasedthe indoor density of both A. anthropophagus and A. sinensis (Table 3). Alphamethrin bioassay
The lethal effect of bed nets impregnated with alphamethrin on A. sinensis lasted at least one year, and polyester nets treated with alphamethrin performed slightly better than cotton nets (Table 4).
‘PYAR=person-years at risk. h95% Confidence interval in parentheses.
Table 2. Prevalence of malaria parasitaemia in intervention and comparison groups Survey no.
Intervention No. No. of children parasitaemic
Comparison No. of No. parasitaemic children
Prevalence ratioa
P
::
424
154 155
0.24 1.10 (0.07-0.85) (0.363.35)
0.016 0.87
s3
423
153
0.22 (0~05-090)
0.02
"95% Confidence interval in parentheses.
Table 3. Person-biting rate and indoor density of Anopheles sinensis and A. anthropophagus in intervention and comparison areas Intervention June A JulyTSept.
Comparison June July-djepr. C
51.5 103 143.5 287
18.4 147 27.0 216
7.0 14 10.3 21
8.4 67 10.3 82
:
0.36 22 0.38 23
0.51 122 0.48
Corrected percentage reduction=
Bites/person/night (outdoor) A. anthropophagus
No. caught
A. sinensis
No. caught Mosquitoes/net/night (indoor) A. anthropophagus
No. caught A. sinensis
No. caught
1.01 61 0.15 8
0.004
1
115
-70.3 -29.2
-
-100 -57.8
-
“[lo0 (BxC)/(AxD)loo]- i.e., the percentage reduction after bed net impregnation in the intervention area corrected for the different values m the comparison area before (June) and after (July-Sept.) bed net impregnation.
627 Table 4. Mortality of Anopheles sinensis 24 h after 10 min exposure to alphamethrin-impregnated bed net fabric” Cotton Polyester Days since impregnation Treated Not treated Treated Not treated 100% (51) 4% (49) 100% 153) 6% (50‘1 5 8% (52j 6% (52j 82% (5Oj ioo% (soj 88% (52) 0 (50) 79% (48) 0 (51) 1;‘: 78% (58) 11% (53) 82% (44) 6% (50) 375 “The numbers of mosquitoes tested are shown in parentheses. Discussion
We found a 43% reduction in malaria incidence rate, and a reduction of over 75% in prevalence of malaria parasitaemia, among children under 10 years old following the introduction of bed nets impregnated with alphamethrin. The reduction in malaria incidence was much greater than that seen in recent studies in Mali (RANQUE et al., 1984), Malaysia (HII et al., 1987), Kenya (SEXTON et al., 1990), Tanzania (LYIMO et al., 1991; MSUYU & CURTIS, 1991), and The Gambia (SNOW et al., 1988; ALONSOet al., 1993), using bed nets impregnated with deltamethrin, permethrin or lambdacyhalothrin. However, the protective efficacy was much lower than that reported in southern China (LI et al., 1989). The study in southern China gave only limited information on methods and, as the authors-used historical controls, it is difficult to exclude the effects of general trends in malaria incidence, as suggested by ~BERMEJO& VEEKEN (1992). The other studies used child morbidity to assess the effect of intervention. Bed nets impregnated with pyrethroid insecticide probably provided more protection to young children than adults, since children go to bed earlier (SHRECK & SELF, 1985; GRAVESet al., 1987). A reason why alphamethrin-impregnated bed nets provided more protection in the present study might be because our study was very large scale, resulting in mass killing of mosquitoes. Also, the present study was carried out in an area of moderate endemicity for malaria: according to the models of MACDONALD(1957) and MosKOVSKiJ(1967), and the actual observations made in the Garki oroiect (MOLINEAUX & GRAMICCIA.1980: ROZENDAAL,’ 1989),‘with a high degree of endemicity, large changes in vectorial capacity may produce little apparent effect, whereas, in areas with low or moderate endemicity as in the present trial area, very small changes in vectorial capacity may decreasethe incidence of malaria greatly and even bring about its eradication. Alphamethrin-impregnated bed nets dramatically decreasedthe outdoor mosquito densities. They affected A. anthropophagus, an endophilic, anthropophilic mosquito, more than A. sinensis, an exophilic and zoophilic species. Bed nets impregnated with alphamethrin were more effective against A. anthropophagus than A. sine&s, probably because the former has a greater preference for human blood and tends to stay in houses after blood feeding, whereas A. sinensis is a zoophilic species more easily diverted to feeding on cattle or other animals. The reduction in numbers of mosquitoes resting inside bed nets can be attributed to irritating and excito%epellent effects, as well as to the killing effects of the impregnated bed nets. Although the number of mosquitoes resting inside bed nets is often used in China as an indicator of ‘indoor’ mosquito density (XV et al., 1988), it might not be appropriate here in evaluating the effectiveness of treated bed nets becauseof irritant and excite-repellent effects of the pyrethroid insecticide. The methods used by SEXTON et al. (19901 and LINDSAY et al. (19931 would be more appropriate in a future study. The reduction of ‘outdoor’ density, on the other hand, must have been due to mass killing by treated bed nets used on a large scale. These results are very similar to those of other studies using deltamethrin-impregnated bed nets in other parts of China (LI et al., 1989; CURTIS,1992). Bioassay showed that the effectiveness of bed nets im-
pregnated with alphamethrin at a dosageof 20 mgim2 of active ingredient lasted at least one year. Impregnation with alphamethrin is more effective on polyester netting than on cotton netting. These results are similar to most of those reported using permethrin, although there is little difference between the fabrics when deltamethrin is used (CURTIS et al., 1990; MILLER, 1990). The better performance of alphamethrin on polyester netting compared with that on cotton netting might be due to the difference in surface texture of the fabrics. Most of the pyrethroid remains on the surface of synthetic fibres and is therefore easily available to mosquitoes, whereas it seemsto be less accessiblewhen in the crevices of rough cotton fibres (HOSSAIN& CURTIS, 1989; MILLER, 1990). No side effect was seen among people who used the treated nets. Only one person in the impregnation teams, who did not use rubber gloves during impregnation, complained of itching and burning of face and neck. His symptoms disappearedwithin 24 h. In this study, the intervention and comparison areas were 8 km apart, and people living between the 2 areas used treated nets, so it is unlikely that the mosquitoes could have flown from one area to the other. Bias was unlikely as the technicians who examined the slides were ‘blinded’. Malaria incidence was similar in the 2 areasin 1989, and the incidence rate and prevalence of malaria parasitaemia in the areas were similar before the introduction of bed net impregnation in 1990. We are thus confident that reductions in malaria incidence rate and parasite prevalence were due to the intervention described and not to other factors. The very low dose necessary makes alphamethrin cheaper to use than other pyrethroids. The average dose of alphamethrin at a dosage of 20 mg of active ingredientimz is US$O.O6 per net, whereas permethrin at the normal dosage of 500 mg of active ingredient/m2 costs about US$O.8 per net (Xu et al., 1988; KAMOL-RATANKUL & PRATTISUK,1992), and deltamethrin at a dose of 15 mg of active ingredient/m2 costs US$O.O97 (CURTIS, 1990). Thus, alphamethrin is a suitable insecticide for malaria control through bed net impregnation. Acknowledgements We are grateful to MS J. Armstrong Schellenberg and Dr J. Lines at the London School of Hygiene and Tropical Medicine for critical comments on drafts of this paper. This project received financial sup ort from the UNDlYWorld BankWHO Special Programme 4or Research and Training in Tropical Diseases. References Alonso, I’. L., Lindsay, S. W., Armstrong, J. R. M., Konten, M., Hill, A. S. G., David, P. H., Fegan, G., Franciso, A. E., Hall, A. J., Shenton, F. C., Cham, K. & Greenwood, B. M. (1991). The effect of insecticide-treated bednets on mortality of Gambian children. Lancet, 337,1499-1502. Alonso, I’. L., Lindsay, S. W., Armstrong Schellenberg, J. R. M., Keita, K., Gomez,P., Shenton, P. C., Hill, A. G., David, P. H., Fegan, G., Cham, K. & Greenwood, B. M. (1993). A malaria control trial using insecticide-treated bed
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