Nabie Bayoh

Profile: The KEMRI/CDC Health and Demographic Surveillance System—Western Kenya

The KEMRI/Centers for Disease Control and Prevention (CDC) Health and Demographic Surveillance System (HDSS) is located in Rarieda, Siaya and Gem Districts (Siaya County), lying northeast of Lake Victoria in Nyanza Province, western Kenya. The KEMRI/CDC HDSS, with approximately 220 000 inhabitants, has been the foundation for a variety of studies, including evaluations of insecticide-treated bed nets, burden of diarrhoeal disease and tuberculosis, malaria parasitaemia and anaemia, treatment strategies and immunological correlates of malaria infection, and numerous HIV, tuberculosis, malaria and diarrhoeal disease treatment and vaccine efficacy and effectiveness trials for more than a decade. Current studies include operations research to measure the uptake and effectiveness of the programmatic implementation of integrated malaria control strategies, HIV services, newly introduced vaccines and clinical trials. The HDSS provides general demographic and health information (such as population age structure and density, fertility rates, birth and death rates, in- and out-migrations, patterns of health care access and utilization and the local economics of health care) as well as disease- or intervention-specific information. The HDSS also collects verbal autopsy information on all deaths. Studies take advantage of the sampling frame inherent in the HDSS, whether at individual, household/compound or neighbourhood level.

Evaluation of long-lasting insecticidal nets after 2 years of household use.

Development of long‐lasting insecticidal nets (LLINs) may eliminate the need for insecticide retreatment of ITNs. While two LLINs (Olyset®, Sumitomo Chemical Co., Japan; and PermaNet® 1.0, Vestergaard‐Frandsen, Denmark) have received recommendations from the World Health Organization Pesticide Evaluation Scheme, field‐testing under normal use has been limited. We used a survival analysis approach to compare time to net failure of conventional polyester bednets treated only with deltamethrin to two LLINs and two candidate LLINs (Olyset®; PermaNet®; Insector, Athanor, France; and Dawa®, Siamdutch Mosquito Netting Co., Thailand). Additionally, we evaluated nets treated with a process designed to increase the wash‐durability of permethrin‐treated nets through the addition of cyclodextrin (a starch) in the treatment process. Houses in western Kenya were randomly assigned to one of the six net types and nets were distributed to cover all sleeping spaces. Households were visited monthly to assess reported side effects in inhabitants and washing frequency. Nets were evaluated for insecticidal activity by periodic WHO cone bioassays with mortality assessed at 24 h. Nets with bioassay mortality <70% were assayed monthly until failure, defined as the first of two consecutive bioassay mortality rates <50%. Time to failure was analyzed using an extended Cox Proportional Hazards model controlling for the cumulative number of washes. We distributed 314 nets to 177 households in June–July 2002; 22 nets (7.0%) were lost to follow‐up and 196 (62.4%) failed during the first 2 years of the evaluation. Controlling for cumulative number of washes, PermaNet® 1.0 [Hazard Ratio (HR) 0.14, 95% Confidence Interval (CI) 0.06–0.31] had a significantly lower risk of failure than conventional nets while Insector had a significantly higher risk of failure (HR 2.57, 95% CI 1.06–4.15). The risks of failure of the remaining nets (Olyset®: HR 1.29, 95% CI 0.79–2.10; Dawa®: HR 0.58, 95% CI 0.32–1.18; cyclodextrin: HR 0.65, 95% CI 0.40–1.1) were not significantly different from that of a conventional net. PermaNet® 1.0 performed significantly better than conventional nets and should be recommended to malaria control programs.

Consistently high estimates for the proportion of human exposure to malaria vector populations occurring indoors in rural Africa

Background Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are highly effective tools for controlling malaria transmission in Africa because the most important vectors, from the Anopheles gambiae complex and the A. funestus group, usually prefer biting humans indoors at night. Methods Matched surveys of mosquito and human behaviour from six rural sites in Burkina Faso, Tanzania, Zambia, and Kenya, with ITN use ranging from 0.2% to 82.5%, were used to calculate the proportion of human exposure to An. gambiae sensu lato and An. funestus s.l. that occurs indoors (πi), as an indicator of the upper limit of personal protection that indoor vector control measures can provide. This quantity was also estimated through use of a simplified binary analysis (πiB) so that the proportions of mosquitoes caught indoors (Pi), and between the first and last hours at which most people are indoors (Pfl) could also be calculated as underlying indicators of feeding by mosquitoes indoors or at night, respectively. Results The vast majority of human exposure to Anopheles bites occurred indoors (πiB = 0.79–1.00). Neither An. gambiae s.l. nor An. funestus s.l. strongly preferred feeding indoors (Pi = 0.40–0.63 and 0.22–0.69, respectively), but they overwhelmingly preferred feeding at times when most humans were indoors (Pfl = 0.78–1.00 and 0.86–1.00, respectively). Conclusions These quantitative summaries of behavioural interactions between humans and mosquitoes constitute a remarkably consistent benchmark with which future observations of vector behaviour can be compared. Longitudinal monitoring of these quantities is vital to evaluate the effectiveness of ITNs and IRS and the need for complementary measures that target vectors outdoors.

An online tool for mapping insecticide resistance in major Anopheles vectors of human malaria parasites and review of resistance status for the Afrotropical region

BackgroundMalaria control programmes across Africa and beyond are facing increasing insecticide resistance in the major anopheline vectors. In order to preserve or prolong the effectiveness of the main malaria vector interventions, up-to-date and easily accessible insecticide resistance data that are interpretable at operationally-relevant scales are critical. Herein we introduce and demonstrate the usefulness of an online mapping tool, IR Mapper.MethodsA systematic search of published, peer-reviewed literature was performed and Anopheles insecticide susceptibility and resistance mechanisms data were extracted and added to a database after a two-level verification process. IR Mapper (http://www.irmapper.com) was developed using the ArcGIS for JavaScript Application Programming Interface and ArcGIS Online platform for exploration and projection of these data.ResultsLiterature searches yielded a total of 4,084 susceptibility data points for 1,505 populations, and 2,097 resistance mechanisms data points for 1,000 populations of Anopheles spp. tested via recommended WHO methods from 54 countries between 1954 and 2012. For the Afrotropical region, data were most abundant for populations of An. gambiae, and pyrethroids and DDT were more often used in susceptibility assays (51.1 and 26.8% of all reports, respectively) than carbamates and organophosphates. Between 2001 and 2012, there was a clear increase in prevalence and distribution of confirmed resistance of An. gambiae s.l. to pyrethroids (from 41 to 87% of the mosquito populations tested) and DDT (from 64 to 91%) throughout the Afrotropical region. Metabolic resistance mechanisms were detected in western and eastern African populations and the two kdr mutations (L1014S and L1014F) were widespread. For An. funestus s.l., relatively few populations were tested, although in 2010–2012 resistance was reported in 50% of 10 populations tested. Maps are provided to illustrate the use of IR Mapper and the distribution of insecticide resistance in malaria vectors in Africa.ConclusionsThe increasing pyrethroid and DDT resistance in Anopheles in the Afrotropical region is alarming. Urgent attention should be afforded to testing An. funestus populations especially for metabolic resistance mechanisms. IR Mapper is a useful tool for investigating temporal and spatial trends in Anopheles resistance to support the pragmatic use of insecticidal interventions.

The combination of indoor residual spraying and insecticide-treated nets provides added protection against malaria compared with insecticide-treated nets alone.

Both insecticide-treated bed nets (ITNs) and indoor residual spraying (IRS) reduce malaria in high malaria transmission areas. The combined effect of these interventions is unknown. We conducted a non-randomized prospective cohort study to determine protective efficacy of IRS with ITNs (ITN + IRS) compared with ITNs alone (ITN only) in preventing Plasmodium falciparum parasitemia. At baseline, participants provided blood samples for malaria smears, were presumptively treated for malaria, and received ITNs. Blood smears were made monthly and at sick visits. In total, 1,804 participants were enrolled. Incidence of P. falciparum parasitemia in the ITN + IRS and ITN only groups was 18 and 44 infections per 100 persons-years at risk, respectively (unadjusted rate ratio = 0.41; 95% confidence interval [CI] = 0.31-0.56). Adjusted protective efficacy of ITN + IRS compared with ITN only was 62% (95% CI = 0.50-0.72). The combination of IRS and ITN might be a feasible strategy to further reduce malaria transmission in areas of persistent perennial malaria transmission.

Spatial and temporal dynamics of malaria transmission in rural Western Kenya

BackgroundUnderstanding the relationship between Plasmodium falciparum malaria transmission and health outcomes requires accurate estimates of exposure to infectious mosquitoes. However, measures of exposure such as mosquito density and entomological inoculation rate (EIR) are generally aggregated over large areas and time periods, biasing the outcome-exposure relationship. There are few studies examining the extent and drivers of local variation in malaria exposure in endemic areas.MethodsWe describe the spatio-temporal dynamics of malaria transmission intensity measured by mosquito density and EIR in the KEMRI/CDC health and demographic surveillance system using entomological data collected during 2002–2004. Geostatistical zero inflated binomial and negative binomial models were applied to obtain location specific (house) estimates of sporozoite rates and mosquito densities respectively. Model-based predictions were multiplied to estimate the spatial pattern of annual entomological inoculation rate, a measure of the number of infective bites a person receive per unit of time. The models included environmental and climatic predictors extracted from satellite data, harmonic seasonal trends and parameters describing space-time correlation.ResultsAnopheles gambiae s.l was the main vector species accounting for 86 % (n = 2309) of the total mosquitoes collected with the remainder being Anopheles funestus. Sixty eight percent (757/1110) of the surveyed houses had no mosquitoes. Distance to water bodies, vegetation and day temperature were strongly associated with mosquito density. Overall annual point estimates of EIR were 6.7, 9.3 and 9.6 infectious bites per annum for 2002, 2003 and 2004 respectively. Monthly mosquito density and EIR varied over the study period peaking in May during the wet season each year. The predicted and observed densities of mosquitoes and EIR showed a strong seasonal and spatial pattern over the study area.ConclusionsSpatio-temporal maps of malaria transmission intensity obtained in this study are not only useful in understanding variability in malaria epidemiology over small areas but also provide a high resolution exposure surface that can be used to analyse the impact of transmission on malaria related and all-cause morbidity and mortality.

The impact of hotspot-targeted interventions on malaria transmission: study protocol for a cluster-randomized controlled trial.

BackgroundMalaria transmission is highly heterogeneous in most settings, resulting in the formation of recognizable malaria hotspots. Targeting these hotspots might represent a highly efficacious way of controlling or eliminating malaria if the hotspots fuel malaria transmission to the wider community.Methods/designHotspots of malaria will be determined based on spatial patterns in age-adjusted prevalence and density of antibodies against malaria antigens apical membrane antigen-1 and merozoite surface protein-1. The community effect of interventions targeted at these hotspots will be determined. The intervention will comprise larviciding, focal screening and treatment of the human population, distribution of long-lasting insecticide-treated nets and indoor residual spraying. The impact of the intervention will be determined inside and up to 500 m outside the targeted hotspots by PCR-based parasite prevalence in cross-sectional surveys, malaria morbidity by passive case detection in selected facilities and entomological monitoring of larval and adult Anopheles populations.DiscussionThis study aims to provide direct evidence for a community effect of hotspot-targeted interventions. The trial is powered to detect large effects on malaria transmission in the context of ongoing malaria interventions. Follow-up studies will be needed to determine the effect of individual components of the interventions and the cost-effectiveness of a hotspot-targeted approach, where savings made by reducing the number of compounds that need to receive interventions should outweigh the costs of hotspot-detection.Trial registrationNCT01575613. The protocol was registered online on 20 March 2012; the first community was randomized on 26 March 2012.

Design of a study to determine the impact of insecticide resistance on malaria vector control: a multi-country investigation.

BackgroundProgress in reducing the malaria disease burden through the substantial scale up of insecticide-based vector control in recent years could be reversed by the widespread emergence of insecticide resistance. The impact of insecticide resistance on the protective effectiveness of insecticide-treated nets (ITN) and indoor residual spraying (IRS) is not known. A multi-country study was undertaken in Sudan, Kenya, India, Cameroon and Benin to quantify the potential loss of epidemiological effectiveness of ITNs and IRS due to decreased susceptibility of malaria vectors to insecticides. The design of the study is described in this paper.MethodsMalaria disease incidence rates by active case detection in cohorts of children, and indicators of insecticide resistance in local vectors were monitored in each of approximately 300 separate locations (clusters) with high coverage of malaria vector control over multiple malaria seasons. Phenotypic and genotypic resistance was assessed annually. In two countries, Sudan and India, clusters were randomly assigned to receive universal coverage of ITNs only, or universal coverage of ITNs combined with high coverage of IRS. Association between malaria incidence and insecticide resistance, and protective effectiveness of vector control methods and insecticide resistance were estimated, respectively.ResultsCohorts have been set up in all five countries, and phenotypic resistance data have been collected in all clusters. In Sudan, Kenya, Cameroon and Benin data collection is due to be completed in 2015. In India data collection will be completed in 2016.DiscussionThe paper discusses challenges faced in the design and execution of the study, the analysis plan, the strengths and weaknesses, and the possible alternatives to the chosen study design.

Standardizing operational vector sampling techniques for measuring malaria transmission intensity: evaluation of six mosquito collection methods in western Kenya.

BackgroundOperational vector sampling methods lack standardization, making quantitative comparisons of malaria transmission across different settings difficult. Human landing catch (HLC) is considered the research gold standard for measuring human-mosquito contact, but is unsuitable for large-scale sampling. This study assessed mosquito catch rates of CDC light trap (CDC-LT), Ifakara tent trap (ITT), window exit trap (WET), pot resting trap (PRT), and box resting trap (BRT) relative to HLC in western Kenya to 1) identify appropriate methods for operational sampling in this region, and 2) contribute to a larger, overarching project comparing standardized evaluations of vector trapping methods across multiple countries.MethodsMosquitoes were collected from June to July 2009 in four districts: Rarieda, Kisumu West, Nyando, and Rachuonyo. In each district, all trapping methods were rotated 10 times through three houses in a 3 × 3 Latin Square design. Anophelines were identified by morphology and females classified as fed or non-fed. Anopheles gambiae s.l. were further identified as Anopheles gambiae s.s. or Anopheles arabiensis by PCR. Relative catch rates were estimated by negative binomial regression.ResultsWhen data were pooled across all four districts, catch rates (relative to HLC indoor) for An. gambiae s.l (95.6% An. arabiensis, 4.4% An. gambiae s.s) were high for HLC outdoor (RR = 1.01), CDC-LT (RR = 1.18), and ITT (RR = 1.39); moderate for WET (RR = 0.52) and PRT outdoor (RR = 0.32); and low for all remaining types of resting traps (PRT indoor, BRT indoor, and BRT outdoor; RR < 0.08 for all). For Anopheles funestus, relative catch rates were high for ITT (RR = 1.21); moderate for HLC outdoor (RR = 0.47), CDC-LT (RR = 0.69), and WET (RR = 0.49); and low for all resting traps (RR < 0.02 for all). At finer geographic scales, however, efficacy of each trap type varied from district to district.ConclusionsITT, CDC-LT, and WET appear to be effective methods for large-scale vector sampling in western Kenya. Ultimately, choice of collection method for operational surveillance should be driven by trap efficacy and scalability, rather than fine-scale precision with respect to HLC. When compared with recent, similar trap evaluations in Tanzania and Zambia, these data suggest that traps which actively lure host-seeking females will be most useful for surveillance in the face of declining vector densities.

Geospatial distribution and determinants of child mortality in rural western Kenya 2002–2005

Objective  To describe local geospatial variation and geospatial risk factors for child mortality in rural western Kenya.