- Record: found
- Abstract: found
- Article: found
Evaluation of PermaNet 3.0 a deltamethrin-PBO combination net against Anopheles gambiae and pyrethroid resistant Culex quinquefasciatus mosquitoes: an experimental hut trial in Tanzania
research-article
Author(s):
Patrick Tungu
1
,
2 ,
Stephen Magesa
1
,
2 ,
Caroline Maxwell
1
,
2
,
4 ,
Robert Malima
1
,
2 ,
Dennis Masue
1
,
2 ,
Wema Sudi
1
,
2 ,
Joseph Myamba
1
,
2 ,
Olivier Pigeon
3 ,
Mark Rowland
2
,
4
,
Publication date (Electronic):
19 January 2010
Journal:
Malaria Journal
Publisher:
BioMed Central
Read this article at
There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
Background
Combination mosquito nets incorporating two unrelated insecticides or insecticide plus synergist are designed to control insecticide resistant mosquitoes. PermaNet 3.0 is a long-lasting combination net incorporating deltamethrin on the side panels and a mixture of deltamethrin and synergist piperonyl butoxide (PBO) on the top panel. PBO is an inhibitor of mixed function oxidases implicated in pyrethroid resistance.
Method
An experimental hut trial comparing PermaNet 3.0, PermaNet 2.0 and a conventional deltamethrin-treated net was conducted in NE Tanzania using standard WHOPES procedures. The PermaNet arms included unwashed nets and nets washed 20 times. PermaNet 2.0 is a long-lasting insecticidal net incorporating deltamethrin as a single active.
Results
Against pyrethroid susceptible Anopheles gambiae the unwashed PermaNet 3.0 showed no difference to unwashed PermaNet 2.0 in terms of mortality (95% killed), but showed differences in blood-feeding rate (3% blood-fed with PermaNet 3.0 versus 10% with PermaNet 2.0). After 20 washes the two products showed no difference in feeding rate (10% with 3.0 and 9% with 2.0) but showed small differences in mortality (95% with 3.0 and 87% with 2.0). Against pyrethroid resistant Culex quinquefasciatus, mediated by elevated oxidase and kdr mechanisms, the unwashed PermaNet 3.0 killed 48% and PermaNet 2.0 killed 32% but after 20 washes there was no significant difference in mortality between the two products (32% killed by 3.0 and 30% by 2.0). For protecting against Culex PermaNet 3.0 showed no difference to PermaNet 2.0 when either unwashed or after 20 washes; both products were highly protective against biting. Laboratory tunnel bioassays confirmed the loss of biological activity of the PBO/deltamethrin-treated panel after washing.
Related collections
Most cited references33
- Record: found
- Abstract: found
- Article: not found
Impact of Artemisinin-Based Combination Therapy and Insecticide-Treated Nets on Malaria Burden in Zanzibar
Achuyt Bhattarai, Abdullah S. Ali, S Patrick Kachur … (2007)
Introduction The increased malaria-related morbidity and mortality, especially in children under the age of 5 y (“under five”), due to emerging resistance of Plasmodium falciparum to conventional antimalarial drugs calls for immediate actions to “Roll Back Malaria” in sub-Saharan Africa. This need has been clearly recognized in the Millennium Development Goals “to halt and begin to reverse malaria incidence” [1] as well as in the Abuja Declaration objective to halve malaria mortality in Africa by 2010 through implementation of combined control strategies [2]. In the year 2000, the overall treatment failure of chloroquine was found to be 60% in a 14-d efficacy trial; consequently the Zanzibar Ministry of Health and Social Welfare decided in November 2001 to change both first- and second-line treatment guidelines for uncomplicated malaria from chloroquine and sulfadoxine-pyrimethamine to artemisinin-based combination therapies (ACT) [3]. The ACT policy was implemented in September 2003, when Zanzibar became one of the first regions in sub-Saharan Africa to recommend routine use of ACT. This action was followed by strengthened vector control, culminating in a nation-wide distribution campaign of long-lasting insecticidal nets (LLINs) from early 2006. Both ACT and vector control measures have independently proven to be efficacious malaria control strategies. Ecological studies have credited ACT with enhancing treatment efficacy, reducing malaria transmission, and possibly forestalling drug resistance in low-endemicity areas [4,5]. Moreover, specific African trials have indicated that the use of insecticide-treated nets (ITNs) or indoor residual spraying can reduce mortality of children under five in Africa [6–9]. This is, however, to our knowledge the first study to examine the public health impact of wide-scale deployment of ACTs alone and combined with ITNs through the general health structure/channels on malaria indices and general health parameters in an endemic area in sub-Saharan Africa. Methods Study Site The study was conducted in North A District, Zanzibar, situated just off the coast of mainland Tanzania. The district is rural and has a population of about 85,000. Subsistence farming and fishing are the main occupations. Plasmodium falciparum is the predominant malaria species and Anopheles gambiae complex is considered the main vector. Malaria transmission is stable with seasonal peaks related to rainfall in March–May and October–December. Malaria transmission in the district prior to the interventions has been reported to be high, but specific entomological data are not available to allow a precise characterization of malaria transmission intensity. However, during the screening process of a major antimalarial drug trial conducted in 2002–2003, a P. falciparum prevalence exceeding 30% was observed in febrile children under five [10], suggesting that North A District had been a high transmission area prior to ACT implementation in September 2003. North A District has one Primary Health Care Centre, which includes a hospital with inpatient and laboratory services, e.g., blood transfusion and malaria microscopy services. Basic medical treatment services without laboratory support are provided in 12 Primary Health Care Units located in different shehias (the smallest political administrative unit in Zanzibar). Drugs, including conventional and artemisinin monotherapies, are also available in private shops throughout the district. Malaria Control Interventions Figure 1 illustrates time of implementation of the two malaria control interventions. Figure 1 Malaria Interventions, Cross-Sectional Surveys, Monthly Rainfall, and Reported Clinical Malaria Diagnoses in Children under 5 Years of Age in North A District, Zanzibar (A) Start of the implementation of artemisinin-based combination therapy for treatment of uncomplicated malaria in September 2003. (B) Introduction of LLINs in February 2006. Promotion of ITNs started in January 2004; the use of conventional ITNs, however, remained low, until the introduction of LLINs. Outpatient data for 2006 are up to June. First intervention—ACT. A loose combination of artesunate and amodiaquine (AS+AQ; from various suppliers with preapproval from WHO) and a fixed combination of artemether–lumefantrine (Coartem; Novartis, Basel, Switzerland), were implemented as first- and second-line treatment, respectively, for uncomplicated malaria in all public health facilities from September 2003. In a pre-implementation assessment of the new treatment policy, partly conducted in North A District 2002–2003, both AS+AQ and artemether–lumefantrine were highly efficacious with PCR-adjusted cure rates by day 28 above 90% [10]. Quinine remained the drug of choice for severe malaria and sulfadoxine-pyrimethamine for intermittent preventive treatment during pregnancy. From September 2003, chloroquine was withdrawn from all health facilities and replaced by free provision of ACT to all malaria patients. Total treatment courses of AS+AQ dispensed in North A 2004 and 2005 were 34,724 and 12,819, respectively. The supply of ACT has been uninterrupted, with no reports of AS+AQ being out of stock from any public health facility in the district during 2003–2006 (unpublished data). ACTs were purchased with support from African Development Bank and Global Fund to Fight AIDS, Tuberculosis and Malaria (GFATM). Second intervention—vector control. A policy to distribute conventional ITNs to the most vulnerable groups—children under five and pregnant women—free of charge through antenatal clinics or local shehia leaders was officially launched in 2004. However, ITN coverage and use remained low in North A District 2004 and 2005 due to limited number of ITNs distributed, 4,026 and 1,550, respectively. A mass campaign was therefore initiated early 2006, with distribution of 23,000 LLINs to the two most vulnerable groups in North A. This campaign was supported by GFATM and the US Agency for International Development. Cross-Sectional Surveys Three cross-sectional surveys with the primary objective to determine P. falciparum prevalences were conducted in North A District between 2003 and 2006. A two-stage cluster sample technique was used. First shehias and then the households were randomly selected from the sampling frame obtained from the Office of Chief Government Statistician, Zanzibar. The sampling frame was updated before each survey. The first exploratory survey, conducted in May 2003, included 625 households and provided baseline data prior to ACT and widespread ITN implementation. Sample size calculations for the follow-up surveys conducted in May 2005 and 2006 were based on the proportion of children under five with malaria parasitemia in 2003, about 9%, and an assumed relative error of 20%. The calculated number of households to be included was 490 after adjusting for a design effect of 2. Trained interviewers visited all selected households. Interviews and blood sample collection were initiated upon written consent from head of each household and proxy consent from the mother or guardian of each child. Information was recorded using a structured questionnaire on recent febrile illness, mosquito net use, and care-seeking behavior from each individual present in the household at the time of the survey. We did not replace households in which residents were not present at time of survey, could not be located, or refused to participate. Thick blood films were collected from all consenting participants, stained with 5% Giemsa for 30 min, and examined by experienced microscopists for presence and density of P. falciparum parasites. If fewer than ten parasites were detected per 200 white blood cells, examinations were extended to 500 white blood cells. Blood slides were considered negative if no asexual parasites were found in 200 high-power fields. High-density parasitemia was defined as presence of ≥ 5,000 parasites/μl [11]. Quality control was conducted for all positive slides and 10% of the negative slides [12]. Health Facility Records Malaria-related indicators, i.e., outpatient attendances, hospital admissions and blood transfusions, from all 13 public health facilities in North A District were obtained from the Health Management and Information System (HMIS) records of the Zanzibar Ministry of Health and Social Welfare. The existing HMIS records were about 90% complete for the period 2000–2004. Data were validated and missing information retrieved by retrospective review of source documents from all 13 health facilities. This confirmed the HMIS records and resolved missing or inconsistent data, which increased the completeness to nearly 100%. A database of malaria-related indicators was created on the basis of this retrospective review. Data from 2005 were abstracted on quarterly basis. Vital Statistics Records of vital events, i.e., births and deaths, for the period 1998–2005 were obtained from the District Commissioner's Office (DCO) in North A. Annual crude mortalities of children under five were estimated from these data. Demographic estimates were obtained from Tanzania National Population and Housing Census 2002. Rainfall Complete records of monthly rainfall during 1999–2005 were obtained from official registers of the Tanzania Metrological Agency of the Ministry of Communications and Transport. On Unguja island, rainfall is centrally measured in one weather station, situated 26 km (radially) from North A District. The mean annual rainfalls recorded in 2003, 2004, 2005, and 2006 were 702, 1,934, 1,231, and 1,214 mm, respectively. The corresponding mean seasonal rainfall (March–May) between 2003 and 2006 was 285, 786, 890, and 613 mm, respectively. During the post-ACT intervention period (2004–2006) the mean annual and seasonal rainfall was 8%–12% lower than the pre-ACT intervention period (2000–2002). However, the only year with a marked reduction in the mean annual and seasonal rainfalls was the year 2003 with two- to three-fold lower rainfall, as compared to both the preceding and subsequent 3 y. Data Processing and Analysis Data were entered and validated using Microsoft Access and Excel. Statistical analyses for cross-sectional surveys, health facility records, vital statistics, and rainfall data were performed using Stata version 8. Analysis for the surveys was corrected for multi-stage sampling errors using the Rao-Scott second order correction [13]. A logistic regression model with robust standard errors (robust cluster) was used to adjust for the effect of age, sex, sleeping under a mosquito-net, and asset index on asexual P. falciparum prevalence and gametocyte carriage across the study years. Households were the primary sampling units in the surveys and were defined as clusters. Wald test was used to assess the fit of the model and interactions between covariates incorporated in the model. Odds ratios were adjusted for the complex sampling design and covariates listed above. Pearson correlation coefficients were calculated to assess the linear relationships between monthly rainfall and outpatient malaria diagnosis, and malaria-attributed deaths. Ethical Approval Protocols for the household surveys were reviewed and approved by the Medical Research Coordinating Committee of the Tanzanian Commission on Science and Technology, the Zanzibar Health Research Council and the institutional review board of US Centers for Disease Control and Prevention. Results Cross-Sectional Surveys The timings of the cross-sectional surveys in relation to start of each malaria control intervention and seasonal rainfalls are presented in Figure 1. The number of households enrolled and participant characteristics in the respective surveys are shown in Table 1. Over 95% of all participants agreed to both answer questionnaires and provide blood samples in the respective surveys. Table 1 Number of Households Surveyed and Characteristics of Survey Participants The parasite prevalences and odds ratios (ORs) of asexual P. falciparum parasitemia and gametocyte carriage at the time of cross-sectional surveys are shown in Table 2. Between 2003 and 2005 the parasite prevalence was reduced by about 50% in children under five. A further 10-fold decrease in P. falciparum prevalence was observed between 2005 and 2006, following mass distribution of LLINs specifically targeting this age group. Concomitant reductions of parasite prevalence were observed in children over the age of 5 y, although only by about 3-fold, between 2005 and 2006 (OR 0.41, 95% confidence interval [CI] 0.13–1.21), p = 0.08). Table 2 Parasite Prevalence and ORs of P. falciparum Asexual Parasitemia and Gametocytemia in Children 0–14 Years of Age in North A District, Zanzibar, in May 2003, 2005, and 2006 High-density parasitemia (≥5,000/μl) was found in 14 (2.7%) and 2 (0.6%) children under five in 2003 and 2005, respectively. No child carried high-density parasitemia in 2006. Reported fever within 14 d prior to the survey was similar in 2003 and 2006 among children under five (2003, 13% [95% CI 11–17]; 2006, 12% [95% CI 9–16]), whereas care-seeking at public health facilities by recently febrile children under five increased significantly (2003 was reference year; 2005, OR 3.91 [95% CI 0.85–17.9]; 2006, OR 5.5 [95% CI 2.3–13.3]; p-value for trend < 0.001). The proportions of children under five sleeping under effective ITNs were below 10% in both 2003 and 2005 (Table 1), whereas in 2006, 90% were reported sleeping under an LLIN on the night before survey. Health Facility Surveillance All reported clinical outpatient malaria diagnoses in North A District between January 1999 and June 2006 among children under five are shown by month in Figure 1 and by year in Table 3. Between 2002 and 2005 the total number of out-patient malaria diagnoses decreased by 77%. The annual incidences of malaria diagnoses standardized per 1,000 children under five in North A District were 843, 786, and 233 in 2003, 2004, and 2005, respectively. The total number of children under five attending public health facilities for any cause during 1999 and 2005 remained relatively constant, ranging from 31,069 to 39,374 annually. Up to 2003 malaria accounted for about 50% of all outpatient diagnoses in this age group, whereas in 2005 this proportion had decreased to 13%. Table 3 Outpatient Malaria Diagnoses, Hospital Admissions, Blood Transfusions, and Malaria-Attributed Deaths in North A District, Zanzibar, between 2000 and 2005 Malaria-related hospital admissions, non-malaria admissions, and blood transfusions in children under five between 2000 and 2005 are also shown in Table 3. From 2002 to 2005, malaria-related admissions, blood transfusions, and malaria-attributed mortality decreased by 77%, 67%, and 75%, respectively. Crude Mortality Data A total of 23,200 live births and 1,032 deaths in children under five (49% females) were registered between January 1998 and December 2005. The annual mortality figures for children under five, children (1–4 y), and infants (0–1 y) are shown in Table 4. Between 2002 and 2005, crude under five, infant, and child mortality decreased by 52%, 33%, and 71%, respectively. Table 4 Mortality of Children under 5 Years of Age in North A District, Zanzibar between 1998 and 2005 Relationships between Rainfall and Malaria Diagnosis and Deaths In the pre-ACT intervention period (2000–2002), significant positive correlations were found between monthly rainfall and both outpatient malaria diagnoses (Pearson correlation coefficient [r p] = 0.59, p < 0.001) and malaria-attributed deaths (r p = 0.75, p < 0.001), when data were adjusted to allow for a 1-mo lag between rainfall and malaria diagnoses and deaths. However, in the post-ACT intervention period (2003–2005), no significant correlations were found between monthly rainfall and outpatient malaria diagnosis (r p = −0.05; p = 0.75) or malaria-attributed deaths (rp = 0.23; p = 0.20). Discussion Malaria burden in Zanzibar, as in most parts of sub-Saharan Africa, has remained high and in many areas even increased during the last 10–20 y, a major reason being rapid spread of resistance to commonly used monotherapies against malaria. This problem has necessitated urgent implementation of new and effective control strategies to “Roll Back Malaria.” Two main cornerstones in this effort are the introduction of ACTs for treatment of uncomplicated malaria and the promotion of ITN use. The targets for the implementation of these new strategies have been defined by the UN Millennium Development Goals [1] and the Abuja Declaration [2], to be achieved by the years 2015 and 2010, respectively. Deployment of ACTs The ACTs were dispensed free of charge to all patients in the study area through public health facilities from September 2003 onwards. The ACT implementation and deployment was very rapid, effective, and with high coverage. Monitoring of drug supplies confirmed that ACTs were available throughout the study period in all 13 public health care settings in North A District. This outcome also indicates that estimates were adequate of the needed and thus deployed numbers of ACT treatments in the district. This result was accomplished despite an apparent two-fold increase in care seeking among children under the age of 5 y at public health facilities as observed in the cross-sectional surveys. We believe that the observed shift in treatment-seeking behavior at public facilities may be related to availability of free, effective ACTs. A previous study in Zanzibar showed that people's attitudes towards health seeking at public health facilities (biomedical practices) are negatively influenced by the distribution of ineffective antimalarial drugs [14]. High ACT coverage was rapidly achieved in malaria patients despite availability of other drugs in the private sector. This achievement was probably influenced both by comprehensive information to the public and health care staff and by the strong commitment of the Zanzibar government to rapidly ensure free coverage of the ACTs. Also, in North A District, as well as in Zanzibar generally, the entire population has relatively easy access to public health facilities, which are located within 5 km from any community and are served by good transport links. However, the absence of co-formulation or even of co-blistering of the two compounds in the first-line treatment, artesunate and amodiaquine, may have resulted in some degree of monotherapy with either compound. Mortality Impact Our study provides the first, to our knowledge, observation of a reduction in mortality of children under five following introduction of ACTs solely in a stable malaria-endemic setting. The highly significant reduction of 52% in crude under-five mortality according to vital statistics between 2002 and 2005 also highlights the importance of malaria as a major cause of death among children in malaria-endemic areas. The 71% reduction among children aged 1–4 y indicates that the relative contribution of malaria to crude mortality is particularly important in this age group. Major reductions in crude under-five mortality has also been observed in previous randomized intervention studies with ITNs [6,7] and community-based malaria treatment [15,16], but the reduction rates (between 25% and 40%) have been less pronounced than those in our study in Zanzibar. We believe our findings are valid and represent a true picture of the effects of ACT deployment in North A District, Zanzibar. No other major political, socioeconomic, or health-care change with the potential to halve mortality in children under five occurred in Zanzibar after 2002. This includes Expanded Programme on Immunization coverage, which remained constantly above 80% in the district during 1999–2005. Furthermore, there was no significant change in rainfall that may have contributed to the observed reduction in malaria transmission. Indeed, the only year with reduced rainfall with potential influence on vector capacity occurred before the introduction of ACTs—in 2003. Increased use of ITNs may also represent a potential confounding factor in our study. However, the ITN use was below 10% during 2004 and 2005 as reported and observed during the cross-sectional surveys. A significant improvement in ITN coverage was only achieved in 2006 after the introduction of LLINs (see further below) and only affected the 2006 cross-sectional results. We chose 2002 as reference year in our analyses of health facility surveillance and under-five mortality, because 2002 represents the last complete year before ACT introduction in September 2003. Routinely collected mortality statistics may underestimate the true values. However, such data have been shown to provide valid mortality trends [17,18]. Morbidity Impact A significant reduction was found with regard to hospitalization of malaria patients and incidence of blood transfusions, which may be considered proxy indicators of severe malaria. The reduction of severe malaria showing a similar pattern thus supports the under-five mortality trends. This health impact probably represents effects of improved case management of uncomplicated malaria with ACT, thus preventing the development of severe manifestations of the disease. The decrease in malaria morbidity (and mortality) at health facilities between 2003 and 2005 confirms the therapeutic efficacy of ACT [10], but the reduction in outpatient malaria diagnoses may also reflect some transmission blocking effect of artemisinin derivatives through its gametocytocidal activity. Reduction in transmission potential has been suggested after the introduction of artemisinin derivatives (before vector control) for routine treatment in a low and seasonal malaria transmission setting in Thailand [4]. Data obtained from routine health facility records have inherent potential pitfalls and need to be interpreted cautiously. However, the fact that they all show the same downward trend after improved coverage of malaria prevention and treatment interventions, and with no change in the climatic conditions that are favorable for malaria transmission, supports the plausible conclusion that enhanced malaria control interventions contributed to the observed public health benefits. Deployment of ITNs The deployment of LLINs in early 2006 provided a high coverage, i.e., over 90% reported use in children under five in the cross-sectional survey in May 2006. Importantly, this high mosquito-net use was observed after strong government commitment and after free LLIN distribution to children under five and pregnant women. The most significant decrease in prevalence of asymptomatic parasitemia was achieved in 2006, when LLINs were widely used by the children under five, whereas the major impact on the under-five mortality was achieved earlier with ACT use only. Strengthened vector control and the use of ACT also resulted in marked and sustained malaria control in South Africa [5]. The similar public health benefits observed in North A supports the concomitant use of vector control and ACT for malaria control. However, it should be emphasized that our study captures short-term trends in malaria control in North A, which may be too short to generalize long-term trends in the burden of malaria. Sustained coverage and use of LLINs by vulnerable groups is yet to be demonstrated, especially under declining malaria endemicity and if the free LLIN distribution scheme were to be changed. Conclusions The declining under-five mortality, malaria morbidity, and malaria prevalence observed in our study is the first comprehensive evidence supporting the major public health benefits of ACT and ITNs in a stable endemic malaria transmission setting in sub-Saharan Africa. The findings suggest that ACTs with high coverage of ITN use may potentially even eliminate malaria as a public health problem in highly endemic areas of sub-Saharan Africa. High community uptake of the two interventions is probably required but indeed achievable if, as in our study, they are easily available free of charge. The UN Millennium Development Goals to alleviate malaria as a major public health problem and substantially reduce the under-five mortality in sub-Saharan Africa are thus achievable even in settings with historically intense malaria transmission. The sustainability of these efforts as well as surveillance to prevent resurgence of malaria represent key research and programmatic follow-up issues of malaria control in Africa.
- Record: found
- Abstract: found
- Article: found
Reduced Efficacy of Insecticide-treated Nets and Indoor Residual Spraying for Malaria Control in Pyrethroid Resistance Area, Benin
Raphael N’Guessan, Vincent Corbel, Martin Akogbeto … (2007)
During the last decade, pyrethroid-treated mosquito nets have become the main method of malaria prevention in many malaria-endemic African countries ( 1 , 2 ). In a few notable exceptions, usually those with a more developed health infrastructure, such as South Africa, a longstanding practice of applying indoor residual spraying (IRS) has been successful ( 3 ). The 2 approaches to malaria prevention, insecticide-treated nets (ITNs) and spraying (IRS), are not mutually exclusive, and in malaria-endemic areas where ITN coverage is still limited, the feasibility of introducing IRS to reduce transmission is being considered, for example, by the President’s Initiative Fund ( 4 ). Trials of IRS and ITNs have shown that in areas with pyrethroid-susceptible Anopheles gambiae the effectiveness of the 2 methods in controlling malaria does not differ ( 5 ). This comparability may not hold true for areas with pyrethroid-resistant populations. In southern Africa, for example, IRS with pyrethroid failed to control pyrethroid-resistant An. funestus and necessitated a switch to an alternative class of insecticide to which there was no resistance ( 6 ). During the last decade, pyrethroid resistance caused by the kdr mechanism has become widespread in An. gambiae in West Africa and is common in some areas ( 7 ). Whether kdr undermines the effectiveness of ITN in areas of high prevalence is unclear. An early experimental hut trial of ITNs in Côte d’Ivoire demonstrated a survival advantage of homozygotes for kdr resistance ( 8 ), whereas subsequent hut trials in adjacent resistant and susceptible populations showed no apparent difference in the effectiveness of ITNs between the 2 localities ( 9 ). Village randomized trials in Côte d’Ivoire showed that ITNs continued to prevent malaria despite a vector population that was kdr resistant ( 10 ). Whether kdr would undermine the effectiveness of IRS in the same way as resistance due to oxidases did against An. funestus in southern Africa ( 6 ) is unknown. To assess the practicability of applying IRS with pyrethroid in West Africa, we need to examine the effectiveness of this approach against a kdr-resistant population of An. gambiae is important. To get a clearer understanding of the influence of kdr resistance on the effectiveness of ITN, further experimental hut trials of ITNs against kdr-resistant populations need to be conducted. We describe 2 experimental hut trials in Benin. One compares the impact of IRS and ITN against a kd- resistant population in the southern part of the country; the other compares IRS and ITNs against a pyrethroid-resistant population several hundred kilometers to the north. Material and Methods Study Sites Ladji is a large village on the outskirts of Cotonou, the capital of Benin. The village floods during the rainy season. An. gambiae comprises the Mopti (M) cytotype and shows resistance to pyrethroids and DDT; kdr is present at high frequency ( 11 ). The nuisance mosquito Culex quinquefasciatus is also present and shows resistance to pyrethroids. Five experimental huts belonging to the Centre de Recherche Entomologique de Cotonou (CREC) are situated in the village. Malanville is in northern Benin, 800 km from Cotonou, in an irrigated rice-growing valley. The local An. gambiae comprises the M cytotype, but the kdr gene is almost absent and mosquitoes are susceptible to lambdacyalothrin and deltamethrin. Six experimental huts are present at Malanville. Experimental Huts The treated nets, residual spray treatments, and their respective untreated controls were evaluated in 4 experimental huts at each field site. Experimental huts are specially designed to test vector control product against freely entering mosquitoes under natural but controlled conditions. Huts were typical of the region. Each was made from concrete bricks, with a corrugated iron roof and a ceiling of thick polyethylene sheeting lined with hessian sackcloth on the interior surface, and each was built on a concrete base surrounded by a water-filled moat to exclude ants ( 12 ). Mosquito access was through 4 window slits, constructed from pieces of plywood fixed at an angle to create a funnel with a 1-cm gap, present on 3 sides of the huts. Mosquitoes had to fly upward to enter through the gaps and downwards to exit; this precluded or limited exodus through the aperture and enabled us to account for most entering mosquitoes. A veranda trap projected from the back wall of each hut. Movement of mosquitoes between a room and the veranda was unimpeded. Mosquito Net Treatments The nets were made of white, 100-denier polyester (SiamDutch Mosquito Netting Co., Bangkok, Thailand). Nets measured 2.0-m long, 1.6-m wide, and 1.8-m tall and had a surface area of 16.9 m2. To simulate badly torn nets, 80 holes, each measuring 2 × 2 cm, were cut in the sides and ends of each net. Insecticides used were formulations of lambdacyhalothrin (Icon, Syngenta, Switzerland): lambdacyhalothrin 2.5% CS, a microencapsulated suspension designed for ITNs, and lambdacyhalothrin 10% WP, a wettable powder designed for IRS. The lambdacyhalothin application rates of 18 mg/m2 for ITNs and 30 mg/m2 for IRS were within the ranges recommended by the manufacturer. Indoor residual treatments were applied with a hand-operated compression sprayer equipped with a flat fan nozzle. The cement walls and sackcloth ceilings were sprayed uniformly after masking the veranda and window slits with protective coverings. The control hut was sprayed with water only. The treated huts were left for 1 week before evaluations were started. Sleepers and Mosquito Collections Preliminary experiments showed the huts to be evenly attractive to mosquitoes. The treatments were randomly allocated to the 4 experimental huts at each site. The main trials were conducted from April to June 2005 at the Ladji site and from September to November 2005 at the Malanville site. Eight adult men employed by CREC slept overnight in the huts and collected mosquitoes from the huts in the mornings. Informed consent to participate in the study was given beforehand, and chemoprophylaxis was provided during the trial. Ethical approval was granted by the London School of Hygiene and Tropical Medicine (LSHTM) and Benin national ethics committees. The trial ran for 50 nights for 8 weeks at each site. The sleepers were rotated between huts to correct for possible variation in individual attractiveness. Each morning, mosquitoes were collected from the floors, walls, and ceilings of rooms, verandas, and nets with aspirators and torches. Mosquitoes were identified and scored as blood-fed or unfed and dead or live. Live mosquitoes were held in netted plastic cups and supplied with 10% honey solution for 24 h before delayed death was recorded. Male mosquitoes were not recorded. The entomologic impact of each treatment on mosquitoes was expressed relative to the control in terms of the following: deterrence, the proportional reduction in the number of mosquitoes entering a treated hut relative to that entering the control hut; induced exophily, the proportion of mosquitoes collected from the veranda trap of the treatment hut relative to the proportion in the veranda of the control hut; blood-feeding inhibition, the reduction in blood-feeding rate relative to the control hut; and mortality, the proportions of mosquitoes found dead in the hut at the time of collection and after a 24-h holding period. If a treatment deters a considerable number of mosquitoes from entering the hut, the values given by proportion blood-feeding or proportion killed in the treatment hut may underestimate the full personal protective effect and overestimate the full insecticidal efficacy of the treatment. The personal protective effect of a treatment is better described by the reduction in the number of blood-fed mosquitoes in the treatment hut relative to the number blood-fed in the control hut: % personal protection = 100 (Bu – Bt)/Bu where Bu = is the total number of blood-fed mosquitoes in the untreated control huts and Bt is the total number blood-fed mosquitoes in the huts with insecticide treatment. The overall insecticidal effect of a treatment needs to take into account that a considerable number of mosquitoes might be deterred from entering the hut and hence not be killed by the treatment. A mass killing effect is desirable to reduce transmission. The overall insecticidal effect of a treatment relative to the number of mosquitoes that would ordinarily enter an untreated hut can be estimated by using the following formula and expressed as a percentage: Overall insecticidal effect (%) = 100 (Kt – Ku)/(Tu – Ku) where Kt is the number killed in the treated hut, Ku is the number dying in the untreated control hut, and Tu is the total number collected from the control hut. Residual Activity of Insecticide Treatments To evaluate residual activity, World Health Organization (WHO) cone bioassays were undertaken monthly in the Ladji huts and bimonthly in the Malanville huts with a laboratory-susceptible strain of An. gambiae (Kisumu). An. gambiae females, 3–5 days old, were exposed within the cones to nets for 3 min or to sprayed walls and ceilings for 30 min. Approximately 50 mosquitoes in 5 replicates of 10 mosquitoes were tested on each substrate. Honey solution was provided during the 24-h holding period, and the temperature was kept at 25°C. Biochemical Assays Biochemical tests on individual mosquitoes were conducted to determine the activity of mixed function oxidases and nonspecific esterases present in pyrethroid-resistant and -susceptible samples of An. gambiae from the Ladji and Malanville sites. Tests were conducted on 3-day-old adult females (initially collected as larvae) in microtiter plates ( 13 ). Susceptible (Kisumu) and pyrethroid-resistant (Vkper) An. gambiae served as controls. Genotyping of An. gambiae was carried out to assess kdr frequency at both field stations ( 14 ). Adult Bioassay Data To determine whether a stronger pyrethroid resistance mechanism was present in the Ladji population than in the standard kdr strain Vkper, bioassays with 0.05% lambdacyalothrin-treated papers (18 mg/m2) were conducted in WHO resistance test kits by using a range of exposure times on batches of 25 unfed An. gambiae females 2–5 days of age. One hundred mosquitoes per exposure period were tested. Deaths were scored 24 h later. Log-time mortality curves were generated, and lethal time to kill 50% (LT50), estimated by using probit analysis. Data Analysis Proportional data from the hut trial (exophily, blood-feeding, deaths) were analyzed by using logistic regression (STATA 6 software, Stata Corporation, College Station, TX, USA). Deterrence rates were analyzed by comparing the number of mosquitoes entering each hut by using the Wilcoxon rank sum test. Biochemical activity was analyzed with Kruskal–Wallis and Wilcoxon rank sum tests. The level of resistance to lambdacyalothrin in insecticide bioassays was analyzed by using probit analysis. Results Insecticide Residual Activity Residual activity on ITN as measured by cone bioassay tests showed no decline during the 8 weeks of the trial. Activity of the IRS wettable powder formulation on sackcloth and cement showed a decline in performance by week 4. This trend continued until the end of the trial (Table 1). Table 1 Residual activity of lambdacyalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying over 3 mo in experimental huts, Malanville and Ladji field stations* When and where substrate tested ITNs at 18 mg/m2 Indoor residual spraying at 30 mg/m2 Sides + top of net Ceiling Walls No. tested % Corrected mortality No. tested % Corrected mortality No. tested % Corrected mortality Wk 0 Malanville 77 100 33 100 60 100 Ladji 51 100 30 93.3 54 100 Wk 2 Ladji 52 100 22 100 41 100 Wk 4 Ladji 54 100 21 52.4 47 42.5 Wk 6 Ladji 57 100 25 80.0 45 31.1 Wk 8 Malanville 52 100 29 41.4 54 2.6 Ladji 44 97.7 8 25.0 39 18.5 *As determined by using World Health Organization cone bioassays and susceptible Anopheles gambiae (Kisumu). Efficacy of Treatments in Huts Over the 2-month trial, 1,395 An. gambiae, 3,070 Cx. quinquefasciatus, and small numbers of Mansonia uniformis, An. pharoensis, and Aedes aegypti were collected at Ladji. At Malanvile, 1,523 An. gambiae, 2,804 Mansonia sp., and smaller numbers of An. funestus and Ae. aegypti were collected. Only the malaria vector An. gambiae and the nuisance mosquito Cx. quinquefasciatus were analyzed further. Fewer An. gambiae entered the ITN- and IRS-treated huts than the respective control huts. The treatment induced reduction in hut entry was more evident in the resistance area than in the susceptible area (Table 2). The proportion deterred at each site did not differ between ITN or ITS treatments. Table 2 Experimental hut results of lambdacyhalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying (IRS) against Anopheles gambiae, Ladji (pyrethroid resistance) and Malanville (pyrethroid susceptibility) field stations* Ladji (pyrethroid resistance) Malanville (pyrethroid susceptibility) ITN Untreated net Lambdacyhalothrin 18 mg/m2 Untreated net Lambdacyhalothrin 18 mg/m2 Total collected 689a 386b 363a 267b Deterred, % _ 44.0 ._ 26.4 Exiting, % (CI) 25.0†
(21.7–28.2) 29.0†
(24.5–33.5) 36.1
(31.1–41.0) 46.8‡
(40.8–52.8) Blood-fed, % (CI) 82.0†
(79.1–84.9) 82.1†
(78.3–85.9) 77.7†
(73.4–81.9) 3.0‡
(0.9–5.0) Blood-feeding inhibition, % _ 0 _ 96.1 Personal protection, % (no. bloodfed) – (572) 44.6 (317) – (282) 97.2 (8) % Dead (CI) 13.6† (11.1–16.2) 29.8‡ 25.2–34.4) 3.6† (1.7–5.5) 98.5‡ (97.0–99.9) Insecticidal effect, % (no. dead) – (94) 3.0 (115) – (13) 68.9 (263) IRS Unsprayed hut Lambdacyhalothrin 30 mg/m2 Unsprayed hut Lambdacyhalothrin 30 mg/m2 Total collected 203† 117‡ 498† 395‡ Deterred, % _ 42.4 _ 20.7 Exiting, % (CI) 45.8† (38.9–52.7) 58.1† (49.2–67.1) 54.4† (50.0–58.8) 63.3† (58.5–68.0) Blood-fed, % (CI) 87.7† (83.2–92.2) 73.5‡ (65.5–81.5) 93.8† (91.6–95.9) 69.6‡ (65.1–74.2) Blood-feeding inhibition, % _ 16.2 _ 25.8 Personal protection, % (no. bloodfed) – (178) 51.7 (86) – (467) 41.1 (275) Dead, % (CI) 12.3† (7.8–16.8) 30.8† (22.4–39.1) 1.4† (0.4–2.4) 72.1‡ (67.7–76.6) Insecticidal effect, % (no. dead) – (25) 5.4 (36) – (7) 55.8 (285) *For each untreated–treated pair, values not sharing the same superscript are significantly different at the 5% level.. CI, 95% confidence interval. The untreated net was little or no barrier to blood-feeding of An. gambiae at either field site owing to the large number of holes cut in each net. Treating the holed net with pyrethroid led to a 96% reduction in the number of mosquitoes blood-feeding at the susceptible site (Malanville) but to no reduction in blood-feeding at the resistant site (Ladji). Inhibition of blood-feeding by IRS at either the resistant or susceptible site was limited (Table 2). Natural mortality of An. gambiae occurred in both types of control huts but was notably higher at Ladji than at Malanville. Both modes of treatment were highly insecticidal at Malanville: ITNs treated with 18 mg/m2 lambdacyhalothrin killed 99%, and IRS applied at 30 mg/m2 killed 72% of An. gambiae that entered the huts. At Ladji, the proportions of An. gambiae killed in either the ITN- or IRS-treated hut did not exceed 30% (Table 2). The proportion of An. gambiae collected from the veranda traps in the mornings was greater at Malanville than at Ladji and greater in the huts with untreated nets than in the unsprayed control huts. Relative to the controls, lambdacyalothrin-treated nets and IRS induced little or no exophily of the pyrethroid-resistant An. gambiae into the verandas of the Ladji huts, despite high survival rate of mosquitoes in huts. At Malanville, pyrethroid-induced exophily by ITN or IRS hut was not evident and may have been obscured by the high death rates among the mosquitoes. The personal protection derived from ITN was almost 100% in the susceptible area. Despite the low mortality rate and high rate of blood-feeding observed with ITN in the resistance area, the level of personal protection there was almost 50% because of the deterrent effect of lambdacyhalothin on mosquito entry into huts. The personal protective effect of IRS was low in both areas, and IRS was no barrier to blood-feeding. The overall insecticidal effect of pyrethroid-treated nets and IRS was negligible in the resistance area ( 55.8%). Table 3 breaks down the mortality data into 2-week blocks. Mortality associated with IRS treatments decreased week by week at both sites but started at a lower rate at the Ladji site because of the expression of resistance. Mortality associated with ITN treatments also showed a downward trend over time at Ladji but not at Malanville, where mosquitoes showed high susceptibility throughout the study. Table 3 Mortality rate of free-flying, naturally entering mosquitoes in huts, first 8 weeks of trial Ladji (pyrethroid-resistant Anopheles gambiae) Malanville (pyrethroid-susceptible An. gambiae) ITN IRS ITN IRS Wk No. % Corrected mortality No. % Corrected mortality No. % Corrected mortality No. % Corrected mortality 1–2 41 43.2 15 53.3 67 100 91 100 3–4 83 50.5 42 47.6 93 100 108 88.7 5–6 209 28.7 39 24.2 54 92.6 78 57.8 7–8 53 5.7 21 23.8 53 98.8 118 39.0 *ITN, insecticide-treated net; IRS, indoor residual spraying. Both ITN and IRS treatments at Ladji showed poor efficacy against Cx. quinquefasciatus (this species was not encountered in Malanville). Insecticide-induced deterrence was greater for ITN than for IRS (Table 4). Neither method killed many Culex nor stimulated repellency into verandas. The IRS treatment produced an unusually high level of blood-feeding inhibition. Table 4 Experimental hut results of lambdacyalothrin (insecticide)-treated nets (ITNs) and indoor residual spraying (IRS) against Culex quinquefasciatus, Ladji (pyrethroid resistance) field station* Results Treatments ITNs IRS Untreated net Lambdacyhalothrin 18 mg/m2 Unsprayed hut Lambdacyhalothrin 30 mg/m2 Total entered 845 598 858 769 Deterred, % _ 29.2 _ 10.4 Exiting, % (CI) 29.8 (26.7–32.9) 35.9 (32.1–39.8) 52.7 (49.3–56.0) 54.6 (51.1–58.1) Blood-fed, % (CI) 62.8 (59.6–56.1) 59.5 (55.6–63.5) 85.1 (82.7–87.5) 42.9 (39.4–46.4) Blood-feeding inhibition – NS – 49.6 Personal protection, % (no. blood-fed – (531) 33.1 (355) – (730) 54.8 (330) Dead, % (CI) 4.3 (2.9–5.6) 8.5 (6.3–10.8) 3.4 (2.2–4.6) 16.3 (13.7–18.9) Insecticidal effect, % (no. dead) – (36) 1.9 (51) – (29) 11.6 (125) *CI, 95% confidence interval. Biochemical Assays and kdr Genotyping An. gambiae from Ladji expressed a significantly higher level of oxidase activity than the standard susceptible (Kisumu) and the laboratory kdr (Vkper) strains, which had a similar level of oxidase activity. However, the pyrethroid-susceptible strain from Malanville showed a level of oxidase activity that was not significantly different from that of the Ladji strain. This finding would appear to rule out any contribution from oxidases to the pyrethroid resistance observed in An. gambiae from Ladji. The level of α-esterase activity in An. gambiae from Ladji was significantly higher than that expressed in Malanville or Kisumu strains, whereas the level of β-esterase activity in Ladji, Vkper, and Kisumu strains was similar and clearly played no part in resistance (Table 5). Overall, the mean level of esterase activity at Malanville was significantly lower than that of the susceptible reference strain (p 95%, a degree of personal protection associated with ITNs and IRS was still evident (45%–50%) relative to the untreated net or unsprayed hut owing to a partial deterrent effect of treatments on entry of mosquitoes rather than to any inhibition of blood-feeding once the insects were inside the huts. Indeed, on entering the huts, most mosquitoes did go on to blood-feed, and the deliberately holed ITN was no barrier to resistant mosquitoes. By contrast, in northern Benin, only 4% of the insecticide-susceptible mosquitoes that entered the hut fed through the holed ITN. The loss of personal protection and loss of mosquito mortality associated with resistance would presumably combine to make ITNs unattractive from the perspective of both the individual user and the malaria control manager. Incision of 80 holes per net is the standard for ITN trials in West Africa ( 8 , 9 , 12 ), and such nets have given a degree of personal protection in earlier trials. An ITN with no or few holes might be expected to give some protection against resistant mosquitoes from Ladji, but there were insufficient huts available to test this idea. These experimental hut results from southern Benin stand in contrast to results from an area of Côte d’Ivoire (Yaokoffikro) that had a comparable frequency of kdr (78%) to that of Ladji (83%) ( 15 ) and where lambdacyhalothrin-treated nets and other ITN showed continuing efficacy, with mortality rates of 45%–68% ( 8 , 16 – 19 ). We sought evidence that other resistance mechanisms than kdr might be contributing to the reduced efficacy of pyrethroids at Ladji. Metabolic resistance due to mixed function oxidases (MFO) has, for example, undermined attempts at malaria control with deltamethrin residual spraying in southern Africa caused by An. funestus ( 6 ), and elevated MFO activity in a strain of An. gambiae from Cameroon reduced the efficacy of permethrin-treated netting in laboratory tests ( 20 ). The combined elevated activity of MFOs, glutathione S–transferase, and esterases resulted in a failure of the S. Mexican IRS program against An. albimanus ( 21 ). Our examination of enzymatic activity in An. gambiae showed no evidence that MFO activity is any greater in mosquitoes from Ladji than in mosquitoes from Malanville, nor did esterase activity differ between Ladji and Vkper (kdr) strains. Thus, there was no evidence of metabolic resistance enhancing the resistance already caused by kdr in mosquitoes from Ladji. Lambdacyhalothin bioassay tests showed no evidence of resistance level differing between Ladji and Vkper strains, and we conclude that metabolic mechanisms made no contribution to the observations in Ladji. In East Africa a different type of kdr based on a leucine-to-serine mutation, which confers resistance to permethrin and DDT ( 22 ), has been detected in several countries. However, no mosquitoes of this genotype were detected in tests on samples of An. gambiae from Ladji ( 23 ). The complete absence of efficacy of lambdacyalothrin against Cx. quinquefasciatus in Ladji merely confirms earlier findings involving other types of pyrethroid in experimental huts in West Africa ( 6 , 9 , 16 , 18 ). The contribution of kdr to pyrethroid resistance in An. gambiae needs to be reappraised. While lambdacyhalothin-treated nets (reported here) and permethrin-treated nets reported earlier ( 24 ) were less effective in hut trials in the kdr area of Benin (Ladji) than in a corresponding area of Côte d’Ivoire (Yaokoffikro), pyrethroid-treated nets were more effective in the susceptible area of Benin (Malanville) than in the corresponding susceptible area of Côte d’Ivoire (M’Be) ( 9 ) for reasons that are presently unknown. Other differences between the biology of An. gambiae from Côte d’Ivoire and Benin exist. Ivorian An. gambiae with kdr is mainly of the S molecular form, whereas Beninoise An. gambiae is of the M form (V. Corbel, unpub. data). M and S forms differ in ecologic distribution and habitat. While mosquitoes of the M form with kdr might behave differently from those of the S form with kdr when exposed to pyrethroids, this is mere speculation. Moreover, the M form in Malanville showed higher vulnerability to ITN than did the corresponding S form in Côte d’Ivoire, a finding that seems at odds with a behavioral hypothesis. Our study provides persuasive evidence that pyrethroid resistance in Benin is capable of undermining control measures based on ITN. Nor is there reassurance to be taken from IRS, and any attempt to switch vector control strategy would seem doomed to fail. Whereas the earlier phase 3 malaria control trials of ITN in Côte d’Ivoire showed continuing effectiveness despite kdr at high levels ( 10 ), our phase 2 results from Benin give no grounds for optimism. However, only phase 3 can provide a definitive answer. Further phase 3 trials using pyrethroid-treated nets and IRS need to be undertaken in Benin in an area of pyrethroid resistance. The normal practice with phase 3 is to aim at complete community coverage. Coverage in real life is usually less than total, and the danger with the type of pyrethroid resistance found in Benin is that at lower levels of coverage the important mass protective effect of ITNs ( 25 , 26 ) may be lost and transmission may continue unabated among those who do not have ITNs. To establish whether this is true, phase 3 trials on resistant mosquito populations should ideally set the coverage level at <100%. If it is considered unacceptable to deny a section of the trial population access to ITNs, an alternative but much less rigorous approach would be to monitor malaria incidence among users and non-users of long-lasting insecticide nets (LLIN) during the proposed scaling up of LLIN coverage in Benin currently being considered. Pyrethroid resistance in Benin is far from homogeneous, and LLIN should give good protection wherever mosquito populations are susceptible. Use of LLIN should be encouraged but scale-up of treated nets may ultimately select for further resistance. The need to develop alternative insecticides to replace or supplement pyrethroids on nets is urgent and should be put on a par with the seeking of new antimalarial drugs or vaccines that have received far greater attention and resources in recent years.
- Record: found
- Abstract: found
- Article: not found
Artemisinin-based combination treatment of falciparum malaria.
François Nosten, Nicholas J. White (2007)
Artemisinin-based combination treatments (ACTs) are now generally accepted as the best treatments for uncomplicated falciparum malaria. They are rapidly and reliably effective. Efficacy is determined by the drug partnering the artemisinin derivative and, for artesunate-mefloquine, artemether-lumefantrine, and dihydroartemisinin-piperaquine, this usually exceeds 95%. Artesunate-sulfadoxine-pyrimethamine and artesunate-amodiaquine are effective in some areas, but in other areas resistance to the partner precludes their use. There is still uncertainty over the safety of artemisinin derivatives in the first trimester of pregnancy, when they should not be used unless there are no effective alternatives. Otherwise, except for occasional hypersensitivity reactions, the artemisinin derivatives are safe and remarkably well tolerated. The adverse effect profiles of the artemisinin-based combination treatments are determined by the partner drug. Most malaria endemic countries have now adopted artemisinin-based combination treatments as first-line treatment of falciparum malaria, but in most of these only a minority of the patients that need artemisinin-based combination treatments actually receive them.
Author and article information
Journal
Journal ID (nlm-ta): Malar J
Title:
Malaria Journal
Publisher:
BioMed Central
ISSN
(Electronic):
1475-2875
Publication date Collection: 2010
Publication date
(Electronic):
19
January
2010
Volume: 9
Page: 21
Affiliations
[1
]Amani Medical Research Centre, National Institute for Medical Research, PO Box 81,
Muheza, Tanzania
Article
Publisher ID: 1475-2875-9-21DOI: 10.1186/1475-2875-9-21
PMC ID: 2817703
PubMed ID: 20085631
SO-VID: 5c00d640-902f-455c-9471-cb4ebbb013ca
Copyright © Copyright ©2010 Tungu et al; licensee BioMed Central Ltd.
License:
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
History
Date
received
: 9
June
2009
Date
accepted
: 19
January
2010
Categories
Subject:
Research
ScienceOpen disciplines: Infectious disease & Microbiology
Data availability:
ScienceOpen disciplines: Infectious disease & Microbiology