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Comparison of the effect of insecticide-treated bed nets and DDT residual spraying on the prevalence of malaria transmitted by Anopheles anthropophagus in China
TRANSACTIONS
OFTHE
ROYAL
SOCIETY
OFTROPICAL
MEDICINE
AND
HYGIENE
135
(1998) 92,135136
Comparison of the effect of insecticide-treated bed nets and DDT residual spraying on the prevalence of malaria transmitted byAnophe/es anthropophagus in China Xu Bozhaol, Xiao Xianqil, R. H. Webbeg and J. D. Lies2 ‘Hubei Academy of Medical Sciences,Wuhan, Hubei, 430070, China; 2London School of Hygiene and Tropical Medicine, Keppel Street, London, WClE 7HI; UK Abstract In order to improve the control of malaria in a problem part of Hubei Province, China, where Anopheles anthropophagus is the vector of Plasmodium vivax, insecticide treatment of bed nets was introduced. The people were given the choice of DDT residual spraying, which had been used for many years, or deltamethrin treatment of their bed nets.Two counties, in which these 2 different methods had been introduced, and an untreated area were evaluated. DDT house spraying and insecticide treated bed nets were equally effective, but deltamethrin treatment was cheaper and so was considered the method of choice. Keywords: Plasmodium &ax, Anophelesanthropophagus,Anopheles sinensis, control, DDT, malaria, insecticide-treated bed nets, deltamethrin, China
Introduction Hubei province is situated in the centre of China, on the middle reaches of the Yangtze river, with a population of 54.1 million recorded in the census of 1991. Hubei is surrounded bv hills and mountains. while the central part is low lying, with many lakes’along the Yangtze river. The climate is seasonal, with an average rainfall of 800-1600 mm each year and an annual mean temperature of 13-18°C. The lakes, together with a generallv hiah water level and widelv distributed irrination system, irovide optimal conditions not only for gr;wing rice but also for breeding mosquitoes. Malaria (Plasmodium vivax) has existed in Hubei for centuries, especially in rural areas. Since 1974 a malaria control programme has been carried out, resulting in its steady declme (XV et al., 1994). By 1994 there were 4527 cases, an annual incidence of 0.08/1000 @&MSTRY, 1995, 1996). Malaria is focal, with large parts of the province free of infection, but it persists in the northern hilly area where Anophelei anth&pophagus was found in 1985 to be the chief malaria vector; it occurs in 7 of the 60 counties of Hubei province (XU et al., 1994). A. sinensis, the principal vector outside this area, is also present as a secondary vector. The 2 species are closely related and both breed primarily in rice fields, butA. anthropophagus is anthropophilic while A. sine&s is strongly zoophilic. This paper compares control using deltamethrin-treated bed nets and DDT residual spraying in the northern hills, where malaria still occurs. Methods The insecticide-treated bed net (ITBN) areas were chosen in a rather unusual fashion. The county or township was given the choice of DDT residual house spraying or treatment of nets according to the preferences of the people. Meetings were held and, if the people thought that DDT was better, this was chosen; otherwise they opted to have their nets treated with deltamethrin. The 2 areas studied were Shenqiao (Jaingshen county) and Songji (Zhaogyang county) with populations of 33 258 and 32 010 people, respectively, most living in rural villages. An untreated area, with neither Table 1. Malaria prevalence existing bed nets, given DDT Area Bed nets treated DDT Untreated
method practised, was also chosen as a control. Within each area 3 villages were selected for vector studies. In all villages the natural conditions and malaria situation were similar. Most of the population sleep under bed nets, so there was no need to distribute them before the trial. The nets were treated as follows. Nets were measured and the people asked to wash them before treatment. An amount of water just sufficient to wet the whole net, whether it was cotton or nylon, was calculated and the nets were treated with deltamethrin 2.5% wettable powder) at the target dosage of 15 mg/m 1 and then hung up to dry in the shade. A post-treatment survey revealed a coverage rate of over 98%. DDT residual spraying was done in the usual way. All walls inside the houses and cattle sheds were sprayed with DDT (50% emulsion). Spray dilutions were adjusted to give a target dosage of 2 g/m2 on the wall surface (brick, mud, etc.). Before the start of the trial in 1990 at least 50 bloodfed female mosquitoes were collected from nets on l-3 occasions between June and early October 1989. They were placed in specimen tubes with damp cotton wool and the eggs obtained were sent to the local anti-epidemic station for identification. A. sinensis and A. anthropophagus were differentiated by the width of the egg deck (Ho CH’I & CHOU, 1962). Mosquito density was estimated by early morning resting catches of mosquitoes from 50 houses every 10 d by one person in each trial village after control measures-had been started. From Aoril to October in 1990 and 1991 everv fever case (temperature >37*5”C) discovered by the village doctor in the control area was registered and blood samples were collected at the blood examination station, clinic or hospital (Lr, H. et al., 1995). Results The numbers of parasitaemic cases of malaria declined nroaressivelv over the 3 vears of the trial. There was a shghtly greater reduction in the ITBN area than in the DDT area, but the difference was not significant (Table 1). There was a considerable decrease in the population d
in areas of Hubei Province, China, provided residual house spraying, or untreated
d
with insecticide
treatment
1989
Year 1990
1991
281/3658 (7.7%) 169/2158(7.8%)
146/3487 (4.2%) 96/1759 (5.5%)
118/4187(2.8%)b 68/2262 (3.0%)b
181/2108(8.6%)
120/1405
132/1764(7.5%)
aNumber of persons with malaria parasites detected in blood films/number &ignificantly different from value in untreated area (P
(8.5%)
examined.
of
XU BOZHAO ET&..
136
Table 2. Numbers of Anopheles anthropophagus and A. sinensis collected in July, August and September 1991 in areas of Hubei Province, China, provided with insecticide treatment of existing bed nets, given DDT residual house spraying, or untreated Area
A. anthropophagus
A. sinensis
6 9 180
27 24 91
Bed nets treated DDT Untreated
of A. anthropophugus and A. sinensis in both the DDT and ITBN areas, while the number of vectors in the untreated area remained high (Table 2). Discussion A. anthropophagus was not known to be a serious vector until 1985 when it was found to be breeding in rice fields. It had however been present before 1962 and the start of the malaria programme (LJ, M. B. & YI, 1989). It disappeared once control measures were started, but then made an unexpected reappearance in 1985. Since then it has progressively increased to occupy the central part of the province (XU et al., 1994). The reason for this recurrence of A. anthropophugus is uncertain, but it coincided with a change in the national agricultural policy from producing 2 crops of rice per ye% to only one. This resulted in longer neriods of standing water in the rice fields together with shade from the “growing rice, conditions more favourable to A. anthropophagus than to A. sinensis (see XU et al., 1994). The population of A. anthropophagus reaches a peak later than that of A. sinensis (Figure). Another possible reason is a reduction in the agricultural use of pesticides, which would favourd. anthropophagus.
1 Jun.
3
1
Jul.
3 Month
1
Aug.
3
1
Sep.
3
1
Oct.
(thirds)
Figure. Seasonal abundance of indoor-resting Anopheles anthropophagus (hatched bars) and A. sine&s (solid black bars) in Hubei Province, China. (Reproduced with permission from Xu et al., 1994: Journal of Tropical Medicine and Hygiene, 97, 277-28 1.)
Due to the success of control using ITBN in other parts of China (LI, Z. et al., 1987, 1989) it was decided to introduce them in Hubei. There were complaints about the continuing use of DDT, so people were offered this alternative. Malaria control was the prime objective, but it was also possible to evaluate their introduction by studying one county in each area. While not an ideal study design, this has provided valuable in-
formation on the effectiveness of ITBN compared to DDT in controlling malaria predominantly transmitted by A. anthropophagus. The results of the trial showed a good reduction, both of parasitaemic cases of malaria and of the indoor-resting density of vectors, in both the DDT and ITBN areas. Either of these methods would have been satisfactory, but, due to the ease of application and lower cost, it was preferable to treat nets. This was becoming the preferred method of the people as they were tired of preparing their houses for DDT spraying, having to leave the door open, remove or cover foodstuffs and pull the furniture into the middle of the room. Some DDT resistance ofA. sinensis was also appearing, so the method was unlikely to remain effective for much longer. Continued use of DDT is also now environmentally unacceptable. Treating bed nets is easily carried out, involves no labour cost and the supervision required is minimal. Although deltamethrin is 10 times more expensive than DDT, only 0.5 g is required to treat 3 nets (the average number per household). This compares with 2-3 kg of DDT per house plus the salaries of spraymen, making the cost of spraying 3 times as great. Based on the results of this trial, the current strategy in the area is to treat all nets with deltamethrin. Acknowledgements
We thank the British Council for funding an Academic Links with China Scheme, under which this collaborative research was possible. Dr Webber and Dr Lines are members of the Malaria Programme funded by the Department for International Development (DfID) of the British Government, whose assistance is gratefully acknowledged. References
Ho Ch’I, & Chou,T. C. (1962). [TheAnopheZes hycranus group and its relation to malaria in East China.] Chinese Medical Journal, 2, 72-77. [In Chinese, with English abstract.] Li. H.. Xu. B. & Webber, R. H. (199%. Primarv health care; ;he basis for malaria ‘control ‘in Hbbei, China. Southern; Asian .7ournal of Tropical Medicine and Public Health, 26, 29-33: Li, M. B. &Yi,Y. F. (1989). Studies on the ecology ofAnopheles hycrunus group and its relation to malaria in the northern part of Hubei. Medical Zoolonv, 5,3640. Li; Z., Xu, J., Li, B., Zhu, T. &Li,-M. (1987). Mosquito nets impregnated with deltamethrin against malaria vectors in China. Geneva: World Health Organization, mimeographed documentWHONBCl87.939. Li, Z., Zhang, M., Wu, Y., Zhang, B., Lin, G. & Huang, H. (1989). Trial of deltamethrin impregnated bed nets for the control of malaria transmitted by Anopheles sine&s and A. anthropophagus. American Journal of Tropical Medicine and Hygiene, 40,356-359. Ministry [of Public Health, Advisory Committee on Malaria] (1995). [Malaria situation in the People’s Republic of China in 1994.1 Journal of Chinese Parasitology and parasitic Diseases, 13, 161-164. [m Chinese, with English abstract.] Ministry [of Public Health, Advisory Committee on Malaria] (1996). [Malaria situation in the People’s Republic of China in 1995.1 Journal of Chinese Parasitology and Parasitic Diseases, 14, 169-172. [In Chinese, with English abstract.] Xu, B., Li, H. &Webber, R. H. (1994). Malaria in Hubei Province, China: approaching eradication. Journal of Tropical Medicine and Hygiene, 97,277-28 1.
Received 16 October 1997; revised 12 November 1997; acceptedfor publication 12 November 1997
OFTHE
ROYAL
SOCIETY
OFTROPICAL
MEDICINE
AND
HYGIENE
135
(1998) 92,135136
Comparison of the effect of insecticide-treated bed nets and DDT residual spraying on the prevalence of malaria transmitted byAnophe/es anthropophagus in China Xu Bozhaol, Xiao Xianqil, R. H. Webbeg and J. D. Lies2 ‘Hubei Academy of Medical Sciences,Wuhan, Hubei, 430070, China; 2London School of Hygiene and Tropical Medicine, Keppel Street, London, WClE 7HI; UK Abstract In order to improve the control of malaria in a problem part of Hubei Province, China, where Anopheles anthropophagus is the vector of Plasmodium vivax, insecticide treatment of bed nets was introduced. The people were given the choice of DDT residual spraying, which had been used for many years, or deltamethrin treatment of their bed nets.Two counties, in which these 2 different methods had been introduced, and an untreated area were evaluated. DDT house spraying and insecticide treated bed nets were equally effective, but deltamethrin treatment was cheaper and so was considered the method of choice. Keywords: Plasmodium &ax, Anophelesanthropophagus,Anopheles sinensis, control, DDT, malaria, insecticide-treated bed nets, deltamethrin, China
Introduction Hubei province is situated in the centre of China, on the middle reaches of the Yangtze river, with a population of 54.1 million recorded in the census of 1991. Hubei is surrounded bv hills and mountains. while the central part is low lying, with many lakes’along the Yangtze river. The climate is seasonal, with an average rainfall of 800-1600 mm each year and an annual mean temperature of 13-18°C. The lakes, together with a generallv hiah water level and widelv distributed irrination system, irovide optimal conditions not only for gr;wing rice but also for breeding mosquitoes. Malaria (Plasmodium vivax) has existed in Hubei for centuries, especially in rural areas. Since 1974 a malaria control programme has been carried out, resulting in its steady declme (XV et al., 1994). By 1994 there were 4527 cases, an annual incidence of 0.08/1000 @&MSTRY, 1995, 1996). Malaria is focal, with large parts of the province free of infection, but it persists in the northern hilly area where Anophelei anth&pophagus was found in 1985 to be the chief malaria vector; it occurs in 7 of the 60 counties of Hubei province (XU et al., 1994). A. sinensis, the principal vector outside this area, is also present as a secondary vector. The 2 species are closely related and both breed primarily in rice fields, butA. anthropophagus is anthropophilic while A. sine&s is strongly zoophilic. This paper compares control using deltamethrin-treated bed nets and DDT residual spraying in the northern hills, where malaria still occurs. Methods The insecticide-treated bed net (ITBN) areas were chosen in a rather unusual fashion. The county or township was given the choice of DDT residual house spraying or treatment of nets according to the preferences of the people. Meetings were held and, if the people thought that DDT was better, this was chosen; otherwise they opted to have their nets treated with deltamethrin. The 2 areas studied were Shenqiao (Jaingshen county) and Songji (Zhaogyang county) with populations of 33 258 and 32 010 people, respectively, most living in rural villages. An untreated area, with neither Table 1. Malaria prevalence existing bed nets, given DDT Area Bed nets treated DDT Untreated
method practised, was also chosen as a control. Within each area 3 villages were selected for vector studies. In all villages the natural conditions and malaria situation were similar. Most of the population sleep under bed nets, so there was no need to distribute them before the trial. The nets were treated as follows. Nets were measured and the people asked to wash them before treatment. An amount of water just sufficient to wet the whole net, whether it was cotton or nylon, was calculated and the nets were treated with deltamethrin 2.5% wettable powder) at the target dosage of 15 mg/m 1 and then hung up to dry in the shade. A post-treatment survey revealed a coverage rate of over 98%. DDT residual spraying was done in the usual way. All walls inside the houses and cattle sheds were sprayed with DDT (50% emulsion). Spray dilutions were adjusted to give a target dosage of 2 g/m2 on the wall surface (brick, mud, etc.). Before the start of the trial in 1990 at least 50 bloodfed female mosquitoes were collected from nets on l-3 occasions between June and early October 1989. They were placed in specimen tubes with damp cotton wool and the eggs obtained were sent to the local anti-epidemic station for identification. A. sinensis and A. anthropophagus were differentiated by the width of the egg deck (Ho CH’I & CHOU, 1962). Mosquito density was estimated by early morning resting catches of mosquitoes from 50 houses every 10 d by one person in each trial village after control measures-had been started. From Aoril to October in 1990 and 1991 everv fever case (temperature >37*5”C) discovered by the village doctor in the control area was registered and blood samples were collected at the blood examination station, clinic or hospital (Lr, H. et al., 1995). Results The numbers of parasitaemic cases of malaria declined nroaressivelv over the 3 vears of the trial. There was a shghtly greater reduction in the ITBN area than in the DDT area, but the difference was not significant (Table 1). There was a considerable decrease in the population d
in areas of Hubei Province, China, provided residual house spraying, or untreated
d
with insecticide
treatment
1989
Year 1990
1991
281/3658 (7.7%) 169/2158(7.8%)
146/3487 (4.2%) 96/1759 (5.5%)
118/4187(2.8%)b 68/2262 (3.0%)b
181/2108(8.6%)
120/1405
132/1764(7.5%)
aNumber of persons with malaria parasites detected in blood films/number &ignificantly different from value in untreated area (P
(8.5%)
examined.
of
XU BOZHAO ET&..
136
Table 2. Numbers of Anopheles anthropophagus and A. sinensis collected in July, August and September 1991 in areas of Hubei Province, China, provided with insecticide treatment of existing bed nets, given DDT residual house spraying, or untreated Area
A. anthropophagus
A. sinensis
6 9 180
27 24 91
Bed nets treated DDT Untreated
of A. anthropophugus and A. sinensis in both the DDT and ITBN areas, while the number of vectors in the untreated area remained high (Table 2). Discussion A. anthropophagus was not known to be a serious vector until 1985 when it was found to be breeding in rice fields. It had however been present before 1962 and the start of the malaria programme (LJ, M. B. & YI, 1989). It disappeared once control measures were started, but then made an unexpected reappearance in 1985. Since then it has progressively increased to occupy the central part of the province (XU et al., 1994). The reason for this recurrence of A. anthropophugus is uncertain, but it coincided with a change in the national agricultural policy from producing 2 crops of rice per ye% to only one. This resulted in longer neriods of standing water in the rice fields together with shade from the “growing rice, conditions more favourable to A. anthropophagus than to A. sinensis (see XU et al., 1994). The population of A. anthropophagus reaches a peak later than that of A. sinensis (Figure). Another possible reason is a reduction in the agricultural use of pesticides, which would favourd. anthropophagus.
1 Jun.
3
1
Jul.
3 Month
1
Aug.
3
1
Sep.
3
1
Oct.
(thirds)
Figure. Seasonal abundance of indoor-resting Anopheles anthropophagus (hatched bars) and A. sine&s (solid black bars) in Hubei Province, China. (Reproduced with permission from Xu et al., 1994: Journal of Tropical Medicine and Hygiene, 97, 277-28 1.)
Due to the success of control using ITBN in other parts of China (LI, Z. et al., 1987, 1989) it was decided to introduce them in Hubei. There were complaints about the continuing use of DDT, so people were offered this alternative. Malaria control was the prime objective, but it was also possible to evaluate their introduction by studying one county in each area. While not an ideal study design, this has provided valuable in-
formation on the effectiveness of ITBN compared to DDT in controlling malaria predominantly transmitted by A. anthropophagus. The results of the trial showed a good reduction, both of parasitaemic cases of malaria and of the indoor-resting density of vectors, in both the DDT and ITBN areas. Either of these methods would have been satisfactory, but, due to the ease of application and lower cost, it was preferable to treat nets. This was becoming the preferred method of the people as they were tired of preparing their houses for DDT spraying, having to leave the door open, remove or cover foodstuffs and pull the furniture into the middle of the room. Some DDT resistance ofA. sinensis was also appearing, so the method was unlikely to remain effective for much longer. Continued use of DDT is also now environmentally unacceptable. Treating bed nets is easily carried out, involves no labour cost and the supervision required is minimal. Although deltamethrin is 10 times more expensive than DDT, only 0.5 g is required to treat 3 nets (the average number per household). This compares with 2-3 kg of DDT per house plus the salaries of spraymen, making the cost of spraying 3 times as great. Based on the results of this trial, the current strategy in the area is to treat all nets with deltamethrin. Acknowledgements
We thank the British Council for funding an Academic Links with China Scheme, under which this collaborative research was possible. Dr Webber and Dr Lines are members of the Malaria Programme funded by the Department for International Development (DfID) of the British Government, whose assistance is gratefully acknowledged. References
Ho Ch’I, & Chou,T. C. (1962). [TheAnopheZes hycranus group and its relation to malaria in East China.] Chinese Medical Journal, 2, 72-77. [In Chinese, with English abstract.] Li. H.. Xu. B. & Webber, R. H. (199%. Primarv health care; ;he basis for malaria ‘control ‘in Hbbei, China. Southern; Asian .7ournal of Tropical Medicine and Public Health, 26, 29-33: Li, M. B. &Yi,Y. F. (1989). Studies on the ecology ofAnopheles hycrunus group and its relation to malaria in the northern part of Hubei. Medical Zoolonv, 5,3640. Li; Z., Xu, J., Li, B., Zhu, T. &Li,-M. (1987). Mosquito nets impregnated with deltamethrin against malaria vectors in China. Geneva: World Health Organization, mimeographed documentWHONBCl87.939. Li, Z., Zhang, M., Wu, Y., Zhang, B., Lin, G. & Huang, H. (1989). Trial of deltamethrin impregnated bed nets for the control of malaria transmitted by Anopheles sine&s and A. anthropophagus. American Journal of Tropical Medicine and Hygiene, 40,356-359. Ministry [of Public Health, Advisory Committee on Malaria] (1995). [Malaria situation in the People’s Republic of China in 1994.1 Journal of Chinese Parasitology and parasitic Diseases, 13, 161-164. [m Chinese, with English abstract.] Ministry [of Public Health, Advisory Committee on Malaria] (1996). [Malaria situation in the People’s Republic of China in 1995.1 Journal of Chinese Parasitology and Parasitic Diseases, 14, 169-172. [In Chinese, with English abstract.] Xu, B., Li, H. &Webber, R. H. (1994). Malaria in Hubei Province, China: approaching eradication. Journal of Tropical Medicine and Hygiene, 97,277-28 1.
Received 16 October 1997; revised 12 November 1997; acceptedfor publication 12 November 1997