Large-scale spraying of bednets to control mosquito vectors and malaria in Sichuan, China.

Large-scale spraying of bednets to control mosquito vectors and malaria in Sichuan, China* Cheng Huailu,1 Yang Wen,2 Kang Wuanmin,3 & Liu Chongyi4 Since 1987, up to 2.42 million bednets owned by rural householders in over 40 counties in seven pre- fectures of Sichuan Province, China, have been sprayed annually with deltamethrin at a dose of about 10 mg/li2. Data for the years 1987-89 indicate that there were marked reductions in the biting popula- tions and survival of the two vector species Anopheles anthropophagus and A. sinensis. Extensive tests in 1992 in areas where bednet spraying had been carried out for 5 years showed that mortality was 100% with the WHO-recommended discriminating dose of deltamethrin, i.e., there was no indication of resistance. Malaria data obtained by passive surveillance of reported cases, mass blood surveys of schoolchildren, and active surveillance of blood slides from fever cases all indicated marked reductions after introduction of the net spraying. In contrast, in the control areas, where the nets were not treated, the situation remained static or deteriorated slightly. Introduction WHO has recommended the use of bednets impreg- nated with pyrethroid insecticides for malaria vector controla and trials of a few thousand such nets have been carried out in many malarious countries (1). However, in Sichuan Province, China, up to 2.42 million treated nets have been used annually for malaria vector control since 1987. The results have been reported in Chinese (2) but have previously only been summarized in English (3, 4). This article presents for the first time a detailed report of the results of the work in Sichuan Province over its first three years (1987-89), together with data on testing for deltamethrin resistance in the vector species 5 years after the operation began. Malaria and its vectors in Sichuan Province A total of 27% of cases of malaria in China occur in Sichuan Province, in the south-west of the country. Sichuan has a total area of 570 000 km2 and a popu- lation of 107 million; large parts are mountainous and non-malarious. However, the eastem lowlands * From: Sichuan Institute of Parasitic Diseases, 10 University Road, Chengdu 610041, Sichuan, China. ' Head, Malaria Department. Requests for reprints should be sent to this author. 2 Deputy Head, Malaria Department. 3 Entomologist. 4Director. a The use of impregnated bednets and other materials for vector-borne disease control. Unpublished document WHO/VBC/ 89.981, 1989. Reprint No. 5604 have a climate influenced by the south-east monsoon with a rainfall of 1000-1200 mm per year; from May to October the temperature is above 16 °C and these months are suitable for Plasmodium vivax transmis- sion and for breeding of the vectors, Anopheles anthropophagus and A. sinensis. In 1954 there were 87.4 cases of malaria per 10 000 population in Sichuan Province, and even after 40 years of activ- ities to combat vivax malaria its importance is ex- ceeded among infectious diseases only by hepatitis and diarrhoeal diseases. Of the two malaria vector species in Sichuan, A. anthropophagus is the more efficient and, where it is present, the disease is more difficult to control than in areas where only A. sinensis occurs (5). The greater vectorial capacity of A. anthropophagus re- sults from its strong tendency to bite humans and, hence, its relatively high sporozoite rate. A. anthropophagus larvae are difficult to control because they occur in numerous small puddles in rice fields, ponds, etc. The adults, however, come into houses and animal sheds to rest and feed. Feed- ing takes place late at night when most people are asleep; thus bednets, especially if made more effec- tive by pyrethroid impregnation, should offer ef- fective protection against malaria transmission in Sichuan. The nets should also kill large numbers of vectors that rest on them, and thus reduce their vectorial capacity. Before 1987 about 95% of the rural population in Sichuan already owned thick cotton bednets, which suggested that a large-scale impregnation pro- gramme would be feasible. The motives for net own- ership are not protection against malaria, however, but protection from nuisance insects breeding in the Bulletin of the World Health Organization, 1995, 73 (3): 321-328 © World Health Organization 1995 321 Cheng Huallu et al. extensive rice fields, as well as protection against dust and cold draughts. The nets are not well main- tained or carefully closed over beds and thus do not constitute an effective physical barrier against malar- ia vectors, but they are an appropriate and available medium for the pyrethroid insecticide deltamethrin, which is harmless to humans at the low doses required, and lasting but biodegradable in the en- vironment (6). Materials and methods Bioassays To determine what dose of deltamethrin would pro- duce an insecticidal effect that would persist for over 1 year, two types of bioassay were carried out on laboratory-reared 3-4-day-old A. anthropophagus mosquitos, using cotton netting that had been im- pregnated with deltamethrin 431 days previously. The first bioassay, in which the exposure time was constant (10 minutes) and a range of deltameth- rin doses was used (3-15 mg per m2 (Table 1)) resulted in a high mean mortality level (87%) even at the lowest dose. The second bioassay, in which a fixed dose of deltamethrin was used (10 mg active ingredient per m2) with various exposure times (5-30 minutes) resulted in a high mean mortality (90%) even at the shortest exposure time; there was no rela- tionship between mortality level and the exposure time (Table 1). These results suggest that knockdown occurred quickly and that the mosquitos fell off the nets, i.e., at the longer exposure times the insects did not remain in contact with the nets. On the basis of these findings, it was decided to adopt an annual dose of 10 mg active ingredient per m2 as the target for operational net treatment, which is less than the 25-mg/M2 dose used in several trials in West Africa (7-9). An appropriate dilution in water of a 2.5% concentrate of deltamethrinb was used; the target dosage was achieved by saturating the cotton nets with the solution and then allowing them to dry. Net treatment The bednets were sprayed with the deltamethrin solu- tion, as in some of the West African trials (7, 8); this differs from most other trials of pyrethroid-treated nets, where the nets have been dipped in the insecti- cide. Spraying of nets in situ can be performed more rapidly than taking them down, dipping, and re- hanging them. Speed was particularly important since the treatment is carried out in Sichuan in May, at b Obtained from Agrovet, Roussel-Uclaf, Romainville, France. the beginning of the malaria season. This is a busy month for farmers, who would have been unwilling to spare the time to participate in net dipping. Tests showed no difference between the effectiveness and persistence of the insecticide on dipped or sprayed nets throughout the 6-month malaria season. Spray pumps, which were already available in the villages for applying agricultural pesticides, were used to treat the bednets with deltamethrin. The spray teams were volunteers from the village that were to have their nets treated and they were super- vised by the local village health workers. The spray- men were taught the properties of deltamethrin, how to dispose of waste insecticide safely, and how to avoid spraying food or clothing and contaminating themselves or silkworms, which are frequently reared in houses and are vulnerable to deltamethrin. It was decided that a feasible target was to spray 95% of the nets in each community. Radio broad- casts, public meetings, and schools were used to popularize net spraying and explain what it was intended to achieve in terms of malaria vector con- trol. The resources of both local government and the primary health care network were employed to ensure high coverage and good quality spraying. In instances where checking spray team's records showed that coverage had been inadequate, the team concerned was reprimanded and made to repeat the operation. The number of nets treated in each area was publicized on posters, which commended those spray teams that had performed well and achieved high coverage. Cost of spraying The cost of spraying the bednets was borne mainly by the provincial, municipal, and county govern- ments, but money for the spraymen was collected from the communities concerned. The average pro- portion of the total cost borne directly by the com- munities was 30%, but ranged from 2.8% to 57.7% in the seven districts involved. The total cost of the treatment over 3 years was 2.15 million yuan (US$ 373 913), i.e., 0.26 yuan (US$ 0.045) per person protected per year. This compares with an estimated cost of 0.74 yuan (US$ 0.13) per person per year if mass pyrimetha- mine prophylaxis had been used, or 1.05 yuan (US$ 0.18) per person per year if DDT house spraying had been used. Number of nets treated and people protected Table 2 shows the total number of nets treated and of people protected in seven prefectures of Sichuan (Leshan, Yibin, Luzhou, Chengdu, Chongquing, Ya'an, and Mianyang) over the period 1987-92. The cover- age was less in 1989 because of a shortage of 322 WHO Bulletin OMS. Vol 73 1995 Spraying of bednets to control malaria in Sichuan, China Table 1: Mortality levels of Anopheles anthropophagus mosquitos exposed for various times and doses to cotton netting Impregnated with deltamethrln 431 days previouslya Deltamethrin Mortality level (%) at exposure times of: dose (mg/M2) 5 min 10 min 15 min 20 min 25 min 30 min Mean (%) 3 86.6 87.0 90.9 87.7 86.9 82.4 86.9 6 88.7 90.4 89.9 90.2 86.7 89.0 89.1 9 98.7 91.1 93.4 29.2 95.2 90.7 92.2 12 92.1 96.9 95.6 92.9 90.4 90.2 93.0 15 91.3 92.7 93.6 95.6 95.0 94.2 93.0 Mean (%) 90.5 91.6 92.7 91.1 90.8 89.3 a The number of mosquitos in each test group was 19-213 and the control number was 25. Also, deaths occurred in the control group. imported deltamethrin. In certain areas (e.g., Xinwen county) the number of townships covered has been reduced because they are now considered to be non- endemic for malaria (4). Mosquito collection Mosquitos were collected as described below. Experimental huts. Collections were made from 19:00 to 06:00. A man slept on a bed inside an experimental hut, where the ground was covered with white paper to facilitate counting the mosquitos, dead and a live, at 07:00 using a glass breath tube. Mosquito density. Mosquitos inside or resting on the nets were collected from 50 nets between 08:00 and 10:00 with a breath tube. Mosquito landing rate. A bed hung with a net, but which was not closed at the front, was set indoors; a man then sat on the bed to catch and count the mos- quitos that landed on him from 20:00 to 06:00. Monitoring and malaria treatment Entomological and malariological monitoring was carried out at 35 sites where annual net treatment began in 1987 and at 10 sites with hyperendemic malaria where annual net treatment began in 1988. Around each treatment site, control sites were chosen where nets were not treated. Typically these were sited about 2km from the treated villages and in some cases there were natural barriers (mountains) between the treated and control villages. In both the sprayed and control sites, cases of malaria that had been confirmed by blood slide were treated, adults receiving 1.2 g chloroquine + 90 mg primaquine each day for 4 days and children, 20 mg chloroquine + 3 mg primaquine per kg body weight. Infants were not given primaquine. Inhabitants of houses that ajoined a house with a malaria case were given pyrimethamine as prophy- laxis. Cases of fever, but in which parasites could not be found, received a single adult dose of 0.6 g chloroquine as a presumptive treatment, followed by 100 mg pyrimethamine and 90 mg primaquine to interfere with gametocyte production and reduce the chances of transmission. Self-treatment or use of prophylaxis obtained other than through the medical service is not usual in Sichuan. Results Entomological data Experimental huts. Regular mosquito collections were made in buffalo-baited experimental huts with exit traps on the windows of the type described by Li Table 2: Coverage of deltamethrin bednet spraying in Sichuan, China, 1987-92 No. of No. of No. of No. of Year counties houses bednets sprayed people 1987 37 553 401 1 489 929 2 239 092 (95.1)a 1988 42 897 182 2 259 286 3 852 664 (96.7) 1989 43 562 859 1 343 770 2 275 665 (94.9) 1990 38 663 641 1 592 740 2 916 594 (95.3) 1991 41 1 023 146 2 424 855 3 702 589 (95.0) 1992 35 721 892 1 682 009 2 693 390 (94.8) a Figures in parentheses are the percentages of the population in the treated areas. WHO Bulletin OMS. Vol 73 1995 323 Cheng Huailu et al. Table 3: Data from 12 mosquito collections in an experimental hut at approximately 10-day Intervals over a period of 113 days starting on the day of bednet treatment, Sichuan, Chinaa Window traps: Inside hut: Total number Total number Anopheles species Area collected % mortality collected % mortality A. anthropophagus Treated 79 (0-12)b 89.9 (72.9-100) 63 (0-12) 92.1 (77.9-100) A. sinensis Control 167 (2-26) 1.8 (0-7.7) 427 (6-55) 5.9 (0-11.1) a Data are for experimental huts with window exit traps in an area where nets were treated and in a control area. b Figures in parentheses are the range of values from individual collections. et al.c These collections were made in both the treat- ment and control areas over a 113-day period follow- ing net treatment. The data in Table 3 show that fewer mosquitos were collected in an experimental hut and its window traps in the treatment area than in a similar hut in the control area. The mortality level in the hut with treated nets was highly significantly greater than that in the control hut, as indicated by the lack of overlap between the results for individual collections. Mosquitos inside or resting on nets. Table 4 shows the mean numbers of vector mosquitos found inside nets or caught landing on nets attracted by the sleep- ers inside them. The catches were made in the month preceding net treatment and in the six following months in both the treated and control areas. The catches were carried out in 35 houses in each of seven prefectures. There were major reductions in the numbers of mosquitos after net treatment. After correcting for upward or downward seasonal trends in the control areas, the numbers caught in the treat- ed areas were estimated to have been reduced to 0.6-21.5% of those before treatment, i.e., the reduc- tions attributable to the treatments were 78.5-99.4%. These reductions were comparable for both the more endophilic and anthropophilic A. anthropophagus and for the more zoophilic A. sinensis. These reduc- tions could be due to decreases in the mosquito populations after net treatment and/or to the excito- repellency of deltamethrin, which drove the mosqui- tos away from the nets. This is indicated by the data in Table 3 and by the findings reported by Li et al., showing higher proportions of total catches in exit traps than inside huts when deltamethrin treated nets were used, compared with the results for untreated nets.c Mosquito landing rates. Data for landing rates on human subjects between dusk and dawn inside the c Li Zuzi et al. Mosquito nets impregnated with deltamethrin against malaria vectors in China. Unpublished document WHONBC/87.939, 1987. study houses are shown in Table 5. The reductions attributed to net treatment were 52.3-57.5% in 1987 for the two species of mosquitos, but were much larger in 1988, amounting to 93.7-98.3%. Because the catches were made in houses with treated nets, these reductions in indoor biting rate could be due to the following factors: reductions in the local mosqui- to populations, and/or the killing or excito-repellency effect of the treated nets in the rooms where the catches were made. It is unlikely that the populations of the more zoophilic A. sinensis would have been as affected by the nets as much as the more anthropo- philic A. anthropophagus. We therefore consider that the second of these factors was more important for A. sinensis and that both factors were important for A. anthropophagus. Parous rate. Further evidence that the community- wide presence of treated nets killed considerable numbers of mosquitos is provided by the parous rates determined by observation of the ovarian tracheoles; these rates were highly significantly reduced after treatment, compared with the control areas (Table 6). The parous rates in the different areas did not differ significantly before treatment. The effect on A. sinensis was as great as that on A. anthropophagus, even though the latter would be expected to be more frequently attracted to nets in houses. Susceptibility to deltamethrin. The susceptibility to deltamethrin of vector anophelines was assessed in 1989 based on the LT50- and LT95-values in A. anthropophagus populations from areas where nets had been treated for different lengths of time (Table 7). The values were highest for areas where delta- methrin use had been longest, suggesting some selec- tion for tolerance. These tests were conducted with papers impregnated with 0.01% deltamethrin-a dose that has been recommended for detecting resis- tance in China (4). However, in 1992, tests conduct- ed according to WHO recommendations (0.025% deltamethrin test papers and 1 hour's exposure) found 100% mortality levels in blood-fed A. anthro- 324 WHO Bulletin OMS. Vol 73 1995 Spraying of bednets to control malaria in Sichuan, China Table 4: Mean numbers of Anopheles mosquitos found in or on each bednet from May to October after net treat- ment In May 1987 and In newly treated areas In May 1988, compared with the control areas, Sichuan, China A. anthropophagus A. sinensis: Mean number in: % reduction Mean number in: % reductionYear/treat- No. of attributed attributed ment status sites Treated area Control area to treatmenta Treated area Control area to treatmenta 1987 Before 35 0.83 (1 017)b 1.23 (1 057) 1.93 (2 364) 4.05 (4 961) After 35 0.08 (98) 0.61 (747) 80.6 0.09 (110) 0.88 (1 078) 78.5 1988 Before 10 46.86 (16 401) 62.01 (21 704) 143.74 (50 309) 61.17 (21 410) After 10 1.16 (406) 257.63 (90 171) 99.4 1.37 (480) 37.62 (13 167) 98.4 a Corrected for seasonal trends in the control area; calculated as follows: 100 (1 Treated (after) x Control (before)Ii. 1 l Treated (before) Control (after) Il b Figures in parentheses are the total number of mosquitos. Table 5: Numbers of Anopheles mosquitos landing on humans each night Inside houses from May to October before and after bednet treatment and the corresponding trend in control areas, Sichuan, China A. anthropophagus A. sinensis: Mean number in: % reduction Mean number in: % reductionYear/treat- No. of attributed attributed ment status sites Treated area Control area to treatmenta Treated area Control area to treatmenta 1987 Before 35 16.4 (574)b 23.7 (830) 36.2 (1 267) 38.3 (1 341) After 35 3.1 (109) 9.4 (329) 52.3 4.9 (172) 12.2 (427) 57.5 1988 Before 10 14.5 (145) 13.0 (130) 37.9 (379) 2.8 (228) After 10 1.6 (16) 22.8 (228) 93.7 2.4 (24) 10.4 (104) 98.3 a Corrected for seasonal trends in the control area. See footnote a, Table 4, for details. b Figures in parentheses are the total number of mosquitos. Table 6: Parous rates of Anopheles mosquitos 2 months before and 4 months after bednet treatment and at the cor- responding times in the control areas, Sichuan, China Parous rate (%) for: A. anthropophagus A. sinensisTreatment status Treated area Control area x2 test Treated area Control area x2 test Before 61.2 (317)a 67.1 (246) 2.0; NSb 64.1 (206) 64.1 (206) 0 After 33.3 (156) 64.5 (795) 52.6c 31.5 (146) 64.7 (493) 50.6c a Figures in parentheses are the number of mosquitos that were dissected. b Not significant. C p <0.001 pophagus and A. sinensis from eight different counties where annual net treatment had been prac- tised for at least 5 years (Table 8). Even after 15 and 30 minutes' exposure to the test papers mortality was high, indicating no evidence for the evolution of resistance, despite prolonged and extensive selection. Malaria infection Total cases detected. Table 9 shows the total num- ber of malaria cases detected through the primary health care system, based on clinical symptoms and usually confirmed by a blood slide. The criteria for clinical diagnosis of malaria were those customarily used by primary health care staff in Sichuan; no attempt was made to separate cases that may have been imported into the areas where they were detec- ted. Most cases involved adults. In the areas where nets were treated, a steady decline in the number of cases occurred over the 3 years of treatment com- pared with the pre-treatment situation (in 1986), from 12.9 per 1000 to 2.8 per 1000. In the control WHO Bulletin OMS. Vol 73 1995 325 Cheng Huailu et al. Table 7: LT50 and LT-values to deltamethrin of Anophe- les anthropophagus, according to duration of bednet treatment, Sichuan, Chinaa Treatment duration: Untreated 1 year 2 years 3 years LT50 (minutes) 4.36 2.53 2.96 5.92 LT95 (minutes) 14.17 11.0 17.0 17.99 a Values were determined using 0.01% deltamethrin test papers. areas, the rate was steady (ca. 4.0 per 1000), with some sign of an increase in the final year (1989). Mass blood surveys of children. Table 10 shows the results of mass blood surveys for malaria infection among schoolchildren aged 7-12 years in the months preceding and 5 months after nets were treated in 35 areas in 1988. Comparable data are also shown for the control areas. The slide positivity rate declined in the treated areas but increased slightly in the control areas. In 1988, a similar difference in trend was observed in 10 areas where net treatment was newly introduced. The reduction attributed to the treatment Table 9: Number of malaria cases diagnosed In the treatment and control areas in 1986 (pre-treatment) and 1987-89 (treatment), Sichuan, China No. of cases in: Area 1986 1987 1988 1989 Treatment 60 264 29 788 14 577 13 179 (12.94)a (6.39) (3.12) (2.78) Control 1 896 1 653 1 727 2 487 (3.60) (3.30) (3.42) (4.72) a Figures in parentheses are the incidence per 10 000 population. (calculated in the same way as the entomological data) was 61.6-71.7%. Active surveillance of fever cases. Table 11 shows the slide positivity rates among fever cases in all age groups, as determined by surveys. The coverage of these surveys varied according to area and season. The trend in slide positivity rate was downward after net treatment each year, whereas the controls showed rises in the slide positivity rate over the correspond- ing months. The reductions attributable to the treat- ment were 53-64%. Table 8: Susceptibility of Anopheles mosquitos to deltamethrin in 1992 in areas where bednets had been treated annually since 1987 or earlier, Sichuan, China, Exposed to test papers for: 15 minutes 30 minutes 60 minutes Treated area Control area Treated area Control area Treated area Control area A. anthropophagusb % Mortality 94.3 (247)C 1.2 (250) 98.6 (215) 1.9 (214) 100 (293) 0 (275) A. sinensisb % Mortality 96.4 (337) 1.0 (310) 97.9 (396) 1.1 (355) 100 (443) 0.7 (426) a Susceptibility to WHO 0.025% deltamethrin test papers. b Collected from dwelling houses and cattle sheds in 8 countries. c Figures in parentheses are the number of mosquitos. Table 10: Results of mass blood surveys for malaria infection among schoolchildren aged 7-12 years, Sichuan, China, 1988-88 1987 (35 sites): 1988 (10 sites): % reduction % reduction Before net attributed Before net attributed Area: treatment Octobera to treatmentb treatment Octobera to treatmentb Treatment No. sampled 6 708 (3.65)c 7 113 (1.07) 5 426 (2.2) 4 369 (0.78) 71.7 61.6 Control No. sampled 6 533 (2.98) 6 181 (3.09) 2 844 (2.25) 2 043 (2.08) a 5 months after treatment. b Corrected for seasonal trends in the control area. See footnote a, Table 4, for details. c Figures in parentheses are the percentage of children who were slide-positive. WHO Bulletin OMS. Vol 73 1995326 Spraying of bednets to control malaria in Sichuan, China Table 11: Results of active surveillance of fever cases In all age groups in the months before and after bednet treat- ment, Sichuan, China, 1987-88 1987 (35 sites): 1988 (10 sites): % reduction % reduction attributed attributed Area: Jan.-May" June-Dec.b to treatmentc Jan.-Maya June-Dec.b to treatmentc Treatment No. sampled 8 525 (7.5)d 31 085 (2.7) 13 372 (7.3) 29 168 (3.4) 64.0 53.4 Control No. sampled 25 589 (3.1) 5 582 (5.2) 1 674 (5.5) 4 324 (6.6) a Before treatment. b After treatment. c Corrected for seasonal trends in the control area. See footnote a, Table 4, for details. d Figures in parentheses are the percentage of children who were positive. Discussion and conclusions The entomological, malariological, and economic data, collected mostly during the first 2-3 years after the start of the net treatment operation, indicate that it is effective, having substantially reduced the hypo- endemic levels of malaria, and that in Sichuan, where net ownership is the norm, it is more cost- effective than alternative methods of malaria control. In Sichuan net spraying is more appropriate than dipping. After 5 years of the operation, we have found no convincing evidence for the development of resistance among the vectors. Acknowledgements We thank our many colleagues in Sichuan for their conscientious work in this large operation. Professor C.F. Curtis is thanked for assistance in preparing the manu- script. R6sume/ Traitement A grande 6chelle des moustiquaires de lit par pulv6risation dans la lutte contre les moustiques vecteurs et le paludisme dans la province de Sichuan, Chine Depuis 1987, jusqu'a 2,42 millions de mousti- quaires de lit appartenant a des menages ruraux de plus de 40 departements de sept pr6fectures de la province de Sichuan, en Chine, sont traitees chaque annee par pulv6risation de deltam6thrine a la dose d'environ 10 mg/M2. Les donnees pour 1987-1989 montrent une r6duction marqu6e des populations d'anopheles piqueurs et de la survie des deux especes vectrices, Anopheles anthropo- phagus et A. sinensis. En 1992, d'importantes series d'essais effectu6es dans des regions ou le traitement des moustiquaires de lit 6tait effectu6 depuis cinq ans ont montre chez les vecteurs une mortalit6 de 100% avec la dose discriminante de deltam6thrine recommandee par l'OMS, et donc une absence apparente de resistance. Les don- n6es parasitologiques obtenues par surveillance passive des cas notifies, enquetes hematolo- giques chez les ecoliers, et surveillance active des frottis sanguins de cas febriles montrent dans 1'ensemble une reduction marquee apres introduc- tion du traitement des moustiquaires. En re- vanche, dans les zones temoins, ou les mousti- quaires n'6taient pas trait6es, la situation est rest6e stationnaire ou s'est l6gerement deterior6e. References 1. Curtis CF. Workshop on impregnated bednets at the International Congress of Tropical Medicine. Japanese journal of sanitary zoology, 1993, 44: 65-68. 2. Cheng Huailu et al. [Application of deltamethrin on a large scale for controlling malaria and its mosquito vector]. In: Papers on controlling mosquito vectors and malaria with deltamethrin in Sichuan, China. Chengdu, China, Chengdu University of Science of Technology Press, 1991: 3-8 (in Chinese). 3. Li Zuzi, Lu Baolin. Deltamethrin treatment of nets in Sichuan Province. In: Curtis CF, ed. Control of disease vectors in the community. London, Wolfe, 1991: 36. 4. Curtis CF. Spraying bednets with deltamethrin in Sichuan, China. Tropical disease bulletin, 1992, 89(8): R1-R6. WHO Bulletin OMS. Vol 73 1995 327 Cheng Huailu et al. 5. Lu Bao Lin. [The malaria vectors and their control in China]. Chinese journal of epidemiology, 1982, 3: 379-395 (in Chinese). 6. Lhoste J. ed. Deltamethrin monograph. Avignon, Roussel Uclaf, 1982. 7. Robert V, Carnevale P. Influence of deltamethrin treatment of bednets on malaria transmission in the Kou Valley, Burkina Faso. Bulletin of the World Health Organization, 1991, 69: 735-740. 8. Le Goff G et al. Efficacy of insecticide impregnated bednets to control malaria in a rural forested area in southern Cameroon. Memorias Instituto Oswaldo Cruz, 1992, 87 (suppl. III): 355-359. 9. Karch et al. Moustiquaires impregnees contre le paludisme au Zaire. Annales de la Societ6 belge de M6decine tropicale, 1993, 73: 37-53. 328 WHO Bulletin OMS. Vol 73 1995