Use of Insecticide-Impregnated Mosquito Nets and Other Impregnated Materials for Malaria Control in the Americas1

Use of insecticide-impregnated mosquito nets and other impregnated materials for malaria control in the Americas1

R. H. Zimmerman 2 and J. Voorham 3

ABSTRACT This article reviews the current status of the use of insecticide-impregnated mosquito nets and other impregnated materials in the Americas. Studies from Brazil, Colombia, Ecuador, Gua- temala, Peru, Suriname, and Venezuela are examined. It is concluded that most studies have suffered from experimental design errors, short duration problems, and/or inadequate measurement of health indicators. The review brings out the great difficulty of conducting scien- tific studies that attempt to measure the impact of insecticide-treated materials on malaria incidence. In particular, the low incidence of malaria in the Americas, the high prevalences of P. vivax and relapsing cases, and the relationship between human activity patterns and the crepuscular biting patterns of certain malaria vectors stand in the way of easy experimental design and execution. The utilization of impregnated mosquito nets or other impregnated materials as a major component of an integrated malaria control program would be premature at this time. However, it is recommended that well-conceived large-scale trials and interventions be considered when they are based on a thorough understanding of the dynamics of malaria transmission in the area of study.

The use of impregnated materials malaria is that they reduce human- prevalence of malaria, and malaria- has become an important topic in vector contact and decrease the num- related mortality among children malaria research and control (1–3). ber of infective and infected mosqui- under five years of age. The review There has been considerable effort in toes by killing some or most of those related the procedure’s vector impact Africa and Asia to conduct large-scale that touch the treated material. The to decreased house entry, changes in mosquito net trials and introduce mos- same reasoning is used to justify feeding and resting behavior, and quito nets into the public health ser- indoor house spraying with insecti- reduced longevity. A general conclu- vice as a component of an integrated cides. However, impregnated materi- sion of the review was that the use of malaria control program (4–7). The als have the advantage that they can impregnated bed nets and curtains basic premise behind using insecti- be deployed at the local level, lend would increase, especially if ongoing cide-impregnated materials to control themselves to community participa- large-scale research in Africa showed a tion efforts, and complement other decrease in child mortality. Recent currently used methods of vector and results of two of the ongoing trials in malaria control. Kenya (6) and Ghana (7) have shown 1 A Spanish version of this article will be published in a forthcoming issue of this journal. Sexton (5) reviewed the use of that child survival did improve when 2 Pan American Health Organization, Brasília, impregnated materials for the control impregnated bed nets were used. In Brazil. Mailing address: Organização Pan-Ameri- cana da Saúde/Organização Mundial da Saúde; of malaria and its vectors outside the addition, the Kenya trial provided the Setor de Embaixadas Norte, Lote 19; Brasília, DF Americas. He concluded that their first evidence that use of impregnated 70800-400 Brazil. 3 Pan American Health Organization, Brasília, use could reduce clinical episodes of bed nets also reduces severe morbidity Brazil. malaria, parasitemia, the incidence and caused by malaria (6).

18 Rev Panam Salud Publica/Pan Am J Public Health 2(1), 1997 In the Americas, several studies ination. The mean incidence of Plas- predominant parasite was Plasmodium have been conducted using insecti- modium falciparum malaria in the gen- vivax (present in 63.0% of the infec- cide-impregnated materials to control eral population had decreased signifi- tions). Clinical, parasitologic, and ento- malaria (8–20). The question now is cantly one year after introduction of mologic evaluations were performed whether or not enough convincing evi- the impregnated curtains (from 40% to bimonthly for one year. The number of dence exists to recommend broad use 4%, P < 0.01). At the same time that mosquitoes recorded by human land- of impregnated materials in the Amer- there was this decrease in malaria inci- ing counts inside the houses with icas, or if specific large-scale trials such dence at Lourenço, other mining sites impregnated bed nets fluctuated over as those conducted in Africa and Asia in the area did not exhibit decreased time, as did mosquito collections made should be promoted. malaria incidence. in the houses with untreated nets and This article reviews the current More recently, Cavalcante (9) has in the control houses. No statistical research on use of impregnated mate- experimented with use of impreg- comparison was made of the results rials in the Americas and recommends nated curtains to surround the un- obtained in the three groups of houses, the latter course. walled shacks or barracas used by min- but general comparisons showed no ers. She examined only entomologic observable differences between the factors and used both synthetic fiber groups using treated and untreated STUDIES CONDUCTED IN cloth (raffia) and jute cloth (sarrapilha). nets. Also, standard epidemiologic THE AMERICAS Her results demonstrated that DDT (2 indicators (malaria prevalence, splen- mg a.i./m2) and deltamethrin (25 mg omegaly, and serology) revealed no Brazil a.i./m2) had a repellent effect for up to consistent effect on malaria morbidity 120 days. Mosquito mortality from by age group. The author suggests that Various unpublished trials have DDT-treated synthetic fiber curtains the main factor contributing to the lack been conducted using impregnated dropped from 100% at the beginning of significant differences between the curtains made of jute in shelters with- of the experiment to 23% 240 days control and the intervention groups out walls. One study, done in Utinga later (WHO bioassay tests). The mor- was a general decline in malaria inci- (Belém, Pará State) over a 22-week tality of anophelines exposed to DDT- dence throughout the state. period in 1983–1984, showed a 95% treated jute dropped from 100% to reduction in anophelines biting inside 51% over the same time period. In the when only 5% of the space was not case of deltamethrin, the mosquito Colombia protected and an 80% reduction when mortality dropped from 100% to 84% 17% of the space was not protected. with the synthetic-fiber curtains and In the region of the San Juan River, Similar results were recorded for cur- from 100% to 56% with the jute cur- along the Pacific Coast of Colombia in tains treated with insecticide as com- tains after 240 days. The effects of the the locality of Potedó, a malaria con- pared to curtains without insecticide. insecticides persisted longer on the trol study was conducted in 1993–1994 The insecticides used, malathion and sides of the huts away from the sun with cotton and synthetic bed nets propoxur, were considered effective and in areas that were higher up. An impregnated with lambda-cyhalothrin against the mosquitoes for six months, additional trial conducted later in (10–30 mg a.i./m2) as part of a larger according to WHO bioassay tests. Manaus using deltamethrin (25 mg malaria control investigation (12). The Similar results were obtained from a a.i./m2) to impregnate raffia found bed net intervention was conducted in trial conducted in the Cumaru mining that mosquito mortality was still 87% 11 paired localities (11 where treated area of Pará State in 1984–1985 using after 360 days (10). nets were used and 11 controls). The DDT and malathion (unpublished In the municipality of Costa Mar- number of persons residing in the data, PAHO/SUCAM 4). ques, Rondônia State, an investigation intervention settlements was 1 225 In Amapá State, studies were con- of impregnated mosquito-net effec- before the intervention and 1 418 after ducted in a mining camp named tiveness was carried out in 1992–1993 it. In the control localities there were Lourenço in 1986–1987 (8; unpub- (11). This study compared three 1 220 inhabitants before the interven- lished data, PAHO/SUCAM). The groups of 20 houses each where cotton tion and 1 032 after it. Malaria inci- study was conducted using delta- bed nets impregnated with delta- dence was assessed by household methrin at a strength of 20 mg of active methrin (20 mg a.i./m2), bed nets interviews two weeks before the in- ingredient (a.i.) per square meter on without insecticide, and no bed nets (a tervention and four months after it. jute curtains. The malaria incidence control group) were employed. The The predominant Plasmodium parasite was determined by passive and active treated bed nets were reimpregnated found was P. falciparum (present in case detection and blood smear exam- with insecticide after six months. Bed 68.5% of the infections); the major vec- net use during the study was irregular, tor was An. neivai. Two intervention ranging from 48.3% to 78.8%. The prin- localities showed an increased malaria

4 SUCAM = Superintendência de Campanhas de cipal vectors in the area were Anopheles incidence and nine showed a decline Saúde Pública. darlingi and An. deaneorum, and the four months after the intervention,

Rev Panam Salud Publica/Pan Am J Public Health 2(1), 1997 19 resulting in a change in the average abundance of anophelines during the 1 225 before the intervention and 1 418 incidence from 6.5% to 2.3%. Five con- intervention year that greatly in- after it, while the population of the trol localities showed an increased creased vector-human contact. An- control communities was 1 220 before malaria incidence and six showed a other was a suspension of regular the intervention and 1 032 after it. The decline, resulting in a change in the indoor house spraying for malaria predominant vector in the area was average incidence from 9.1% to 7.5%. prevention just before the test, a sus- An. albimanus, and the predominant Only 58.9% of the houses in the inter- pension that may have kept the parasite (detected in 68.6% of the vention localities had treated bed nets malaria incidence high among the malaria infections) was P. falciparum. at the end of the study in 1994. Limited intervention group and may also have Bed net coverage increased from 82% bed net use in the intervention locali- raised the incidence in the control at the start of the study to 96.2% at ties appears to have been the main group slightly. It should also be noted the end. The mean malaria incidence drawback of this study (12). that the participants washed their nets decreased from 9.6% around the frequently, and that the 80% bed net start of the intervention to 3.7% four coverage attained may not have been months post-intervention in the com- Ecuador sufficient. munities using the impregnated bed Another study using cotton and nets and from 7.9% to 4.6% in the con- In the north coastal province of synthetic bed nets impregnated with trol communities. One factor limiting Esmeraldas, an interdisciplinary ma- permethrin (200–300 mg a.i./m2) was the intervention’s success could have laria and leishmaniasis project was conducted in Esmeraldas Province been a high rate of P. vivax infection conducted jointly in 1989–1990 by the in 1993–1994 (12). In the Canton of (50%) with poor case detection and Ecuadorian Ministry of Public Health Bordón, nine paired localities were presumed overlapping of malaria and the University of Heidelberg, Ger- selected. P. falciparum was present in attacks in the control group (14). many (13). Part of this project con- 85.7% of the malaria infections de- sisted of evaluating the effectiveness tected in these localities; the princi- of cotton (85%) and synthetic (15%) pal vector was An. albimanus. Despite Guatemala bed nets treated with permethrin (300 pairing of the localities, no controls mg a.i./m2). Three groups of subjects were used. Instead, localities where The only study in the area of Central were compared. One group consisting DDT was used as an indoor house America and Mexico was conducted in of 225 families (1 561 persons) received spray were compared with the locali- northern Guatemala in 1990–1991 (15, treated bed nets plus additional edu- ties using the impregnated bed nets. 16). Malaria’s incidence and point cation about malaria; this group had The study localities with the treated prevalence were measured through- 80% coverage during the study period. bed nets had 2 093 inhabitants before out the study. Results were compared A second group consisting of 235 fam- the intervention and 1 806 after it, for separate localities using polyester ilies (1 455 persons) received treated while the localities where DDT house permethrin-treated nets (500 mg a.i./ bed nets but no additional education; spraying was employed had 1 711 m2), untreated nets, and no nets. In coverage in this group was 45%. The inhabitants before the intervention two additional localities, 25 houses third group, a control group consisting and 1 422 after it. Bed net coverage employed treated nets and 25 houses of 916 families (5 599 persons), re- ranged from 81.8% to 88.6% during the served as controls. The dominant ceived neither treated nets nor addi- trial. The mean incidence of malaria in malaria parasite during the study was tional education. P. vivax was present the nine intervention localities four P. vivax (present in 90% of the cases) in most (75.7%) of the malaria cases months before treatment of the bed (15), and the principal malaria vectors detected; the major vector was An. nets was 21.6%, as compared to 5.3% were An. albimanus and An. vestitipen- albimanus. There were no significant four months after the intervention. By nis (16). Bed net coverage during the differences between the intervention comparison, the pre-intervention ma- trial was 99%. and control groups with regard to laria incidence in the DDT-treated After 13 months the incidence den- malaria incidence, as measured by localities averaged 16.4%, while the sity (cases per 1 000 person-years of active case detection two weeks before average post-treatment malaria inci- observation) was significantly lower in the intervention and eight months dence was 5.8%. Overall, the treated the treated locality (86 cases per 1 000 afterward. However, eight months bed nets appeared to lower the inhabitants) and the untreated net- after the intervention the more inten- malaria incidence; however, without using locality (106 cases per 1 000 sive intervention group showed a control data this is uncertain. inhabitants) than it was in the control decrease in malaria incidence of 1.0% Another similar study had been con- locality (200 cases per 1 000 inhabi- (from 12.7% to 11.7%), while the conducted in the Canton of Muísne, tants). Also, the episodes of clinical trol group exhibited a 2.2% increase Esmeraldas Province, in 1991–1992. malaria were fewer among people (from 13.5% to 15.7%). Seven paired intervention and control using treated versus untreated nets, Several reasons were given for the localities were selected (12). The popu- but this difference was not statistically poor results. One was an increased lation of the former communities was significant.

20 Zimmerman and Voorham • Insecticide-impregnated materials for malaria control The results of the entomologic evalu- control communities from 15.1% to Suriname ation were as follows: (1) Fewer 30.3%, the increase in the latter being anophelines were found resting inside over twice as great. In the second year Three studies of treated mosquito the houses with treated bed nets than the mean malaria incidence declined nets have been conducted in Suriname. were found in the other houses; (2) the in the intervention communities to One study published in 1989 examined average number of mosquitoes col- 10.5% and in the control communities permethrin-impregnated cotton cloth lected with human bait in the houses to 11.3%. The authors questioned hammock nets and their effect on mor- with treated bed nets did not differ sig- whether the change in insecticide tality among anophelines that entered nificantly from the number collected in might have had an effect on malaria a hut where these nets were deployed this manner at other houses; and (3) incidence. The high prevalence of (19). Results obtained from exit trap twenty times more dead mosquitoes P. vivax, the early biting activity of mosquito collections indicated 58% were recovered from the horizontal An. albimanus, and the low degree of mortality among mosquitoes leaving panels of the treated bed nets than bed net coverage were the probable the hut with impregnated nets versus from those of the untreated nets (16). reasons why no significant differences 27% mortality among those leaving a This greater abundance of dead mos- were found between the average control hut. Although these results quitoes on the treated nets could be an malaria incidences in the intervention were encouraging, the high control indicator of the treated nets’ impact on and control communities (14). mortality prevented meaningful statis- infected and infective mosquitoes. A second permethrin-impregnated tical comparison. bed net study was conducted in the From 1989 to 1991, hammock nets Peruvian Amazon, in the Department impregnated with permethrin (500 mg Peru of Madre de Dios, in 1991–1992 (18). a.i./m2) were tested in all Amerindian Eighteen paired communities were villages of southern Suriname. Ham- In the north coastal province of selected for study, the pre-intervention mock net coverage was 100%. After Piura, in the community of San Juan and post-intervention populations of 36 months the malaria prevalence Bautista de Catacaos, an intense epi- the intervention communities being decreased from approximately 15–20% demiologic study of malaria was 3 018 and 2 993 respectively, and the to less than 1%.5 This success was conducted in 1991–1993 (17). Part pre-intervention and post-intervention attributed to customary cultural use of of this study included the testing populations of the control communi- hammock nets and a low washing fre- of pyrethroid-impregnated mosquito ties being 4 285 and 2 716 respectively. quency of one to four months (J. nets. Some of the nets were made of Pre-intervention bed net coverage was Voorham, unpublished data). cotton, others of synthetic fiber. The 90%. At the start of the study, 67.2% of Voorham (20) suggested that be- insecticide used in the first year of the the nets in the intervention communi- cause the Bushnegro inhabitants of test was lambda-cyhalothrin (9 mg ties were impregnated with perme- Suriname typically wash their bed nets a.i./m2), while that used in the second thrin (225–497 mg a.i./m2); a second weekly (unlike the Amerindians) the year was permethrin (550 mg a.i./m2). treatment was applied to 42.4% of the relative effectiveness of treated nets Six localities were selected for inter- bed nets 2–6 months after the first. among this population would be vention, and six others served as con- P. vivax was present in all the detected low. Therefore, he experimented with trols. Only random samples of the malaria cases; the most abundant lambda-cyhalothrin-impregnated ny- study populations (308 residents of the anophelines were An. evansae, An. lon wide-mesh gauze that covered the intervention communities and 766 res- nuñeztovari, and An. rangeli. The mean large openings in the wooden walls of idents of the control communities) malaria incidence in all the inter- huts. Exit traps and landing counts were tested for malaria before the vention communities taken together were used to evaluate the biting activ- intervention. However, the general decreased from 8.3% before the trial to ity of anophelines. Results showed population (2 693 inhabitants in the 3.7% after the trial, while in the control that indoor biting declined consider- intervention communities, 4 117 in the communities it remained essentially ably in experimental huts relative to control communities) was tested after unchanged, rising from 5.4% before control huts. As no insecticide activity the intervention. The predominant the trial to 5.5% after it. However, the was recorded after 11 months, a reim- malaria parasite found (in 99% of the authors pointed out that significant pregnation method using brushes was cases) was P. vivax; the malaria vector changes in malaria incidence were employed. Subsequently, the use of was An. albimanus. The percentages of only detected in those intervention impregnated wide-mesh gauze was the intervention community popula- communities that had a pre-interven- introduced into the entire eastern inte- tion using bed nets for personal pro- tion incidence exceeding 10%. In those rior region of Suriname, replacing tection ranged from 62.9% in the first intervention communities where the DDT house spraying. The prevalence year to 67.2% in the second year. In the pre-intervention malaria incidence of P. falciparum and P. malariae cases first year the mean malaria incidence was below 10%, no significant change increased in the intervention commu- in malaria incidence was found rela- nities from 19.5% to 27.4% and in the tive to the control communities. 5 Voorham J, personal communication.

Rev Panam Salud Publica/Pan Am J Public Health 2(1), 1997 21 (P. vivax did not occur in the study were similar (12, 13, 17, 18). In the outcome of trials studying the effi- area) was reduced significantly, from course of reporting these studies, the ciency and efficacy of impregnated 35–40% to 5–10%. Since the initial authors discuss their experimental mosquito nets or curtains. These good coverage could not be main- design limitations and the difficulties include the vectors’ biting behavior, tained easily, the malaria prevalence of conducting bed net studies in the seasonal variations in vector abun- went up after one year; nevertheless, region. In addition, Kroeger et al. (14) dance, shifts in vector behavior due to the prevalence remained below previ- reviewed the results of four of these insecticides, and vector competence. ous values. trials and concluded that the main lim- If we look at just two factors, human iting factors were a high prevalence of activity patterns and mosquito biting P. vivax, low community bed net cov- activity, we can get an idea of the dif- Venezuela erage, and variable vector biting rates. ficulty of measuring the effects of In Guatemala, the authors con- interventions with impregnated mate- In 1986 a preliminary trial of treated cluded that a larger sample size was rials and comparing results between bed nets was conducted at Las needed in order to determine whether areas. Figure 1 shows measurements Majadas in Bolivar State.6 Las Majadas treated nets lowered the incidence of of An. darlingi’s cumulative biting then had 350 inhabitants, of which 55 malaria more than untreated nets (15). activity from 7 p.m. to 6 a.m. in Suri- adults used bed nets. For the study, 33 They also concluded that further epi- name and Mato Grosso, Brazil. It can of the 55 nets were sprayed with per- demiologic and sociologic studies be seen that depending on the region, methrin. Six months later, 22 of the 33 were needed to confirm the value of the biting activity differs—and there- persons who contracted malaria were impregnated bed nets for malaria con- fore the intensity of human-vector asked if they had used the impreg- trol in northern Guatemala. contact differs. In Suriname, 88% of nated nets. Only one had used an Recently, the effectiveness of insec- the mosquito biting activity occurs impregnated mosquito net. The inves- ticide-impregnated bed nets in reduc- inside houses after 9 p.m. and before 5 tigators concluded that this one person ing malaria infections was reviewed in a.m., when most of the people are probably contracted malaria outside a meta-analysis of published results asleep (23). However, in Mato Grosso the area enclosed by the net. This was (22). The only study included from the only 32% of the biting activity occurs the first experience with testing bed Americas was the Guatemalan trial, inside houses at these hours (24). nets in Venezuela, and the investiga- and even this study was excluded Reduced human-vector contact during tors suggested that more effort would from the final analysis because too few hours of peak mosquito biting activity be necessary to properly evaluate this studies had been carried out in areas may have contributed to the success of approach for malaria control. Since of P. vivax malaria to permit a meta- the impregnation programs in the then, one or two small studies have analysis. (Where P. vivax malaria is Surinamese Amerindian and Bushne- been attempted with inconclusive prevalent, case relapses may confound gro populations. It is not known, how- results; no large-scale trial has been the analysis—22.) ever, whether impregnated mosquito conducted. Mosquito net and curtain trials are nets would significantly lower the extremely difficult to perform under malaria incidence in Mato Grosso, ideal conditions, let alone when there where 68% of the An. darlingi biting CONCLUSIONS are multiple factors directly or indi- activity occurs when people are not rectly affecting the results that cannot under the nets. The situation is even The highly variable results of the be easily separated. Besides experi- more complicated when more than foregoing studies (Table 1) are hard to mental design problems involving one vector is involved and when interpret. Most, except for the such things as randomization, con- human activity patterns vary. Guatemalan study, appear to have suf- trols, sample size, and systematic mon- An. albimanus was the principal vec- fered from experimental design errors, itoring of health indicators, there are tor involved in the studies conducted short duration, and/or inadequate other factors related to the disease in Ecuador (12, 13), Peru (17, 18), and measurement of health indicators (21). itself that put any study at risk of being Guatemala (16). It exhibits crepuscular The mosquito-net studies conducted inconclusive. These disease factors biting behavior (Figure 2), which in Colombia, Ecuador, and Peru were include low overall incidence, yearly means that its principal biting activity part of larger epidemiologic and edu- fluctuations in incidence, low mortal- occurs when much of the human pop- cational projects. They were con- ity, and a high prevalence of P. vivax ulation is outside and most people are ducted by the same core group of sci- cases (30–100%) in the region. These not protected by bed nets. Overall, entists, and the experimental methods are the same factors that have impeded some 82% of the An. albimanus in the the study and success of other inter- Peru study areas were found to bite vention strategies in the Americas, before 11 p.m. and after 5–6 a.m. (17). including study of malaria vaccines. Even though slightly lower percent- 6 Y. Rubio-Palis, Escuela de Malariología y Sanea- miento Ambiental Dr. Arnoldo Gabaldón, Maracay, In this same vein, there are impor- ages of An. albimanus were collected Venezuela. Personal communication. tant entomologic factors that affect the when people were active outside

22 Zimmerman and Voorham • Insecticide-impregnated materials for malaria control TABLE 1. Summary of malaria control studiesa using insecticide-impregnated materials in the Americas

Item used and % coverage Predominant Predominant Location Duration Insecticide attained Plasmodium sp. vector Resultsb

Brazil: Lourenço, 1986–87 24 months Deltamethrin Curtains (jute), P. falciparum An. darlingi Decrease in malaria 20 mg a.i./m2 100% incidence Brazil: Costa Marques, 12 months Deltamethrin Bed nets, 63% An. darlingi, No difference in malaria 1992–93 20 mg a.i./m2 48.3–78.8% P. vivax An. deaneorum incidence Colombia: Potedó, San Juan 8 months Lambda- Bed nets, 68.5% An. neivai Decrease in malaria River, 1993–94 cyhalothrin 58.9% P. falciparum incidence (pre/post) 10–30 mg a.i./m2 Ecuador: Las Esmeraldas, 12 months Permethrin Bed nets, 75.7% An. albimanus No significant difference 1989–90 300 mg a.i./m2 45–80% P. vivax in malaria incidence Ecuador: Las Esmeraldas, 8 months Permethrin Bed nets, 85.7% An. albimanus No significant difference Bordón, 1993–94 200–300 mg a.i./m2 81.8–88.6% P. falciparum in malaria incidence demonstrated Ecuador: Muísne, 1991–92 8 months Permethrin Bed nets, 68.6% An. albimanus No difference in malaria 200–300 mg a.i./m2 82.0–96.2% P. falciparum incidence Guatemala: Los Amates, 13 months Permethrin Bed nets, 90% An. albimanus, Malaria incidence 1990–91 500 mg a.i./m2 99.0% P. vivax An. vestitipennis decreased significantly Peru: Piura, Catacaos, 24 months Lambda-cyhalothrin, Bed nets, 99% An. albimanus No significant difference 1992–93 9 mg a.i./m2, Year 1; 62.9–67.2% P. vivax between intervention and Permethrin, control communities 550 mg a.i./m2, Year 2 Peru: Madre de Dios, 8 months Permethrin Bed nets, 100% An. evansae, Malaria decreased in 1991–92 225–497 mg a.i./m2 60.8% P. vivax An. nuñeztovari, localities with over 10% An. rangeli incidence Suriname: 1989–91 36 months Permethrin Hammock nets, Predominantly An. darlingi Decrease in malaria 500 mg a.i./m2 100% P. falciparum incidence Suriname: 1989–92 36 months Lambda-cyhalothrin Wide-mesh 95% An. darlingi Decrease in malaria 25 mg a.i./m2 gauze, 100% P. falciparum incidence Venezuela: Las Majadas, 6 months Synthetic Bed nets 70% An. darlingi Preliminary study 1986 pyrethroid P. falciparum a Lourenço, Brazil; Las Majadas, Venezuela; and Suriname were before/after studies without controls. All the other trials were case-control studies. In Bordón, Ecuador, localities employing house spraying with DDT were used to compare with the localities using treated bed nets. b See text for specific results and Kroeger et al. (14) for more details on the studies conducted in Muísne, Potedó, Catacaos, and Madre de Dios.

houses in Ecuador (13), this pattern of development, patterns of economic However, the information currently biting activity alone could have had a activity, gender, age composition, available is sufficient to support a rec- significant negative impact on the malaria immunity, cultural practices, ommendation for well-conceived, results of these trials. In conclusion, and migration patterns also constitute large-scale trials at the community or the overlap of anopheline and human essential a priori information needed regional level. These studies should be activity during hours when people are for the design and implementation of based on a thorough understanding of not protected by the mosquito nets treated mosquito net or curtain trials. the dynamics of malaria transmission could significantly influence the out- in the areas involved. come of insecticide-impregnated mos- Specifically, the recommendations quito net or curtain trials. Of course, if RECOMMENDATIONS of this review are as follows: infective females tend to bite later in the night and noninfective females It would be premature to use insec- • Promote a large-scale trial using tend to bite earlier, this influence ticide-impregnated mosquito nets or impregnated raffia in “stable” min- could be reduced (25). other materials as a major component ing areas of Brazil. The entomologic Data on human factors such as the of an integrated malaria control pro- data collected by Cavalcante (9) and population’s level of socioeconomic gram in the Americas at this time. Figueirdo (10), and malaria case

Rev Panam Salud Publica/Pan Am J Public Health 2(1), 1997 23 FIGURE 1. Cumulative biting activity of An. darlingi inside houses in Suriname (23) and enced the trials’ outcomes. These Mato Grosso, Brazil (24 ) trials did not sufficiently meet the evaluation criteria presented by 100 Bermejo and Veeken (21), nor did 90 they consider the overlap of vector 80 and human activities. A thorough 70 review of the epidemiologic data 60 collected by these studies needs to 50 be made and a better understanding of the malaria transmission dynam- Percent Suriname 40 ics involved needs to be obtained 30 Mato Grosso before conducting other large-scale 20 trials in these areas. 10 • Region-wide interventions using appropriate health indicators for 0 special populations such as the Suri- 19 20 21 22 23 0 1 2 3 4 5 6 namese Amerindians (impregnated Hourly interval hammock nets) and Bushnegros (impregnated wide-mesh gauze) are warranted. • With indigenous groups like the data from the gold mines of for this trial, consideration should Yanomami Indians, questions re- Lourenço, support this idea. No be given to vector behavior in this garding decreasing immunity and doubt such work would need to be region; in addition, mosquito-net whether a vector control measure supplemented with targeted clinical use by the local communities in- such as use of treated mosquito nets treatment. In addition, a flexible volved should be measured and the or curtains can be sustained need to management system must be de- acceptability of such use assessed; be seriously considered before con- signed that would ensure the integ- and a determination should be ducting a trial. In many cases the rity of the intervention trial and, if made regarding the extent of local principal question is not whether it proves successful, provide the government commitment to sustain the method works but whether it groundwork for its future integra- the effort. would be sustainable. tion into the regional health services. • The inconclusive results of the • Implement a large-scale trial using treated bed net trials in Colombia, None of this should be taken to in- impregnated mosquito nets in rural Ecuador, and Peru suggest that fac- dicate that further experimentation in northern Guatemala. In preparation tors other than those assessed influ- other regions or alternatives such as impregnated screens and curtains are not warranted. For example, the feasi- bility of using impregnated wide-mesh FIGURE 2. Cumulative biting activity of An. albimanus inside houses in Catacaos, Peru (17 ) gauze in stable agricultural coloniza- tion areas and of using impregnated 100 hammocks in areas with highly mobile 90 populations should be evaluated. 80 Equally important, regional guide- 70 lines need to be established for impregnated mosquito net and curtain 60 Catacaos, Peru trials in the Americas. The guidelines 50 should be based on a sound knowl-

Percent 40 edge of vector and human activity, 30 disease incidence, cultural practices, and method sustainability. This infor- 20 mation needs to be reviewed before a 10 project is implemented or impreg- 0 nated materials are recommended as a 1830 1930 2030 2130 2230 2330 0030 0130 0230 0330 0430 0530 major alternative for malaria control in Hourly interval the Americas.

24 Zimmerman and Voorham • Insecticide-impregnated materials for malaria control REFERENCES

1. World Health Organization. Aplicación de la Brazil: Instituto Nacional de Pesquisas da 19. Rozendaal JA, Voorham J, Van Hoof JPM, estratégia mundial de lucha contra el Paludismo. Amazônia; 1993. Oostburg BFJ. Efficacy of mosquito nets Geneva: WHO; 1993. (Technical report series, 10. Figueirdo RMP. Controle da malaria: poder treated with permethrin in Suriname. Med Vet 839). residual da deltametrina observado em diferente Entomol 1989;3:353–365. 2. Lengeler C, Lines JD, Cattani J, Feilden R, tipos de paredes [master’s thesis in biological 20. Voorham J. The use of impregnated wide- Zimicki S, Savigny D. Promoting operational sciences]. Manaus, Brazil: Instituto Nacional mesh gauze covering wall openings in huts as research on insecticide-treated netting: a de Pesquisas da Amazônia; 1996. a vector control method in Suriname. Rev joint TDR/IDRC initiative and call for re- 11. Santos JB. Estudo sobre o uso de mosquiteiros Saude Publica (São Paulo) 1997;31:9–14. search proposals. Trop Med Int Health 1996;1: impregnados com deltametrina em una área en- 21. Bermejo A, Veeken H. Insecticide-impreg- 273–276. dêmica de malária da Amazônia Brasileira [post- nated bed nets for malaria control: a review of 3. United Nations Development Program, graduate thesis]. Brasília: Federal University the field trials. Bull World Health Organ World Bank, World Health Organization. of Minas Gerais; 1995. 1992;70:293–296. TDR news. Switzerland: WHO; 1996 (No. 50, 12. Kroeger A, Mancheno M, Pesse K. Métodos para 22. Choi HW, Breman JG, Teutsch SM, Liu S, June 1996). mejorar el control de la malaria en Ecuador y Hightower AW, Sexton JD. The effectiveness 4. Greenwood BM, Baker JR. A malaria control Colombia. Cayambe, Ecuador: Abya-Yala; 1995. of insecticide-impregnated bed nets in reduc- trial using insecticide-treated bed nets and 13. Kroeger A, Mancheno M, Ruiz W, Estrella E. ing cases of malaria infection: a meta-analysis targeted chemoprophylaxis in a rural area of Malaria y leishmaniasis cutanea en Ecuador: un of published results. Am J Trop Med Hyg the Gambia, West Africa. Trans R Soc Trop estudio interdisciplinario; aspectos históricos, epi- 1995;52:377–382. Med Hyg 1993;87(Suppl 2):1–60. demiológicos, antropológicos, entomológicos y 23. Rozendaal JA, Van Hoof JPM, Voorham J, 5. Sexton JD. Impregnated bed nets for malaria métodos de control. Quito: Abya-Yala; 1991. Oostburg BFJ. Behavioral responses of Ano- control: biological success and social responsi- 14. Kroeger A, Mancheno M, Alarcón J, Pesse K. pheles darlingi in Suriname to DDT residues on bility. Am J Trop Med Hyg 1994;50(suppl): Insecticide-impregnated bed nets for malaria house walls. J Am Mosq Control Assoc 72–81. control: varying experiences from Ecuador, 1989;5:339–350. 6. Nevill CG, Some ES, Mung’ala VO, et al. Colombia, and Peru concerning acceptability 24. Charlwood JD, Wilkes TJ. Studies on the age Insecticide-treated bed nets reduce mortality and effectiveness. Am J Trop Med Hyg 1995; composition of samples of Anopheles darlingi and severe morbidity from malaria among 53:313–323. Root (Diptera: Culicidae) in Brazil. Bull Ento- children on the Kenyan coast. Trop Med Int 15. Richards FO, Klein RE, Zea Flores R, et al. Per- mol Res 1979;69:337–342. Health 1996;1:139–146. methrin-impregnated bed nets for malaria 25. Bockarie MJ, Alexander N, Bockaire F, Ibam 7. Binka FN, Kubaje A, Adjuik M, et al. Impact control in northern Guatemala: epidemiologi- E, Barnish G, Alpers M. The late biting habit of permethrin impregnated bed nets on cal impact and community acceptance. Am J of parous Anopheles mosquitoes and pre-bed- child mortality in Kassena-Nankana district, Trop Med Hyg 1993;49:410–418. time exposure of humans to infective female Ghana: a randomized controlled trial. Trop 16. Richards FO, Zea Flores R, Sexton JD, et al. mosquitos. Trans R Soc Trop Med Hyg Med Int Health 1996;1:147–154. Effects of permethrin-impregnated bed nets 1996;90:23–25. 8. Xavier PA, Lima JENS. O uso de cortinas on malaria vectors of northern Guatemala. impregnadas com deltametrina no control da Bull Pan Am Health Organ 1994;28:112–121. malária em garimpos no Território Federal do 17. Pesse K, Castro C. Control de la malaria en la Amapá: nota prévia, 1986. Rev Bras Malariol comunidad campesina San Juan Bautista de Cata- Doencas Trop 1986;38:137–139. caos. Report; 1995. 9. Cavalcante ETS. Avaliação da deltrametrina para 18. Kroeger A, Alarcón J. Malaria en Ecuador y o controle da malaria em areas de garimpos [mas- Peru y estrategias alternativas de control. Quito: Manuscript received on 13 February 1996. Revised ver- ter’s thesis in biological sciences]. Manaus, Abya-Yala; 1993. sion accepted for publication on 17 October 1996.

RESUMEN En este artículo se revisa el uso actual en las Américas de mosquiteros y otros materiales impregnados con insecticida. Se examinan diversos estudios efectuados en el Brasil, Colombia, Ecuador, Guatemala, Perú, Suriname y Venezuela y se llega a la Uso de mosquiteros y conclusión de que en su mayor parte estos estudios adolecen de graves deficiencias otros materiales impregnados de diseño, problemas de excesiva brevedad, o de una medición inadecuada de los para el control de la malaria indicadores de salud. La revisión resalta la gran dificultad de llevar a cabo estudios que buscan medir el impacto de los materiales tratados con insecticidas sobre la inci- en las Américas dencia de malaria. En particular, la baja incidencia de malaria en las Américas, las altas prevalencias de Plasmodium vivax y de casos recurrentes y la relación entre los patrones de actividad de los seres humanos y los hábitos de picadura crepusculares de los mosquitos impiden hacer experimentos de fácil diseño y ejecución. Por ahora sería prematuro usar mosquiteros u otros materiales impregnados como compo- nentes principales de un programa integral para el control de la malaria. No obstante, se recomienda que se considere llevar a cabo ensayos bien diseñados a gran escala siempre que se basen en un conocimiento profundo de la dinámica de la transmisión de la malaria en la zona del estudio.

Rev Panam Salud Publica/Pan Am J Public Health 2(1), 1997 25