The multiplicity of malaria transmission: a review of entomological inoculation rate measurements and methods across sub-Saharan Africa

Habitat Partitioning of Malaria Vectors in Nchelenge District, Zambia

Nchelenge District in Luapula Province, northern Zambia, experiences holoendemic malaria despite implementation of vector control programs. The major Anopheles vectors that contribute to Plasmodium falciparum transmission in this area had not previously been well defined. Three collections performed during the 2012 wet and dry seasons and the 2013 wet season revealed Anopheles funestus sensu stricto and Anopheles gambiae sensu stricto as the main vectors, where 80-85% of each collection was composed of An. funestus Both vectors were found to be highly anthropophilic, and An. funestus has higher sporozoite infection rates (SIRs) and entomological inoculation rates (EIRs) year-round compared with An. gambiae: SIRs of 1.8-3.0% and 0-2.5%, respectively, and EIRs of 3.7-41.5 infectious bites per 6-month period (ib/p/6mo) and 0-5.9 ib/p/6mo, respectively. Spatial and temporal changes in each vector's dynamics and bionomics were also observed. Anopheles funestus was the predominant vector in the villages near Kenani Stream in both wet and dry seasons, whereas An. gambiae was found to be the main vector in areas near Lake Mweru during the wet season. The vector data illustrate the need for broader temporal and spatial sampling in Nchelenge and present unique opportunities to further our understanding of malarial transmission and implications for malarial control in high-risk areas.

RNAi Trigger Delivery into Anopheles gambiae Pupae

RNA interference (RNAi), a naturally occurring phenomenon in eukaryotic organisms, is an extremely valuable tool that can be utilized in the laboratory for functional genomic studies. The ability to knockdown individual genes selectively via this reverse genetic technique has allowed many researchers to rapidly uncover the biological roles of numerous genes within many organisms, by evaluation of loss-of-function phenotypes. In the major human malaria vector Anopheles gambiae, the predominant method used to reduce the function of targeted genes involves injection of double-stranded (dsRNA) into the hemocoel of the adult mosquito. While this method has been successful, gene knockdown in adults excludes the functional assessment of genes that are expressed and potentially play roles during pre-adult stages, as well as genes that are expressed in limited numbers of cells in adult mosquitoes. We describe a method for the injection of Serine Protease Inhibitor 2 (SRPN2) dsRNA during the early pupal stage and validate SRPN2 protein knockdown by observing decreased target protein levels and the formation of melanotic pseudo-tumors in SRPN2 knockdown adult mosquitoes. This evident phenotype has been described previously for adult stage knockdown of SRPN2 function, and we have recapitulated this adult phenotype by SRPN2 knockdown initiated during pupal development. When used in conjunction with a dye-labeled dsRNA solution, this technique enables easy visualization by simple light microscopy of injection quality and distribution of dsRNA in the hemocoel.

Entomological aspects and the role of human behaviour in malaria transmission in a highland region of the Republic of Yemen

Background

The Republic of Yemen has the highest incidence of malaria in the Arabian Peninsula, yet little is known of its vectors or transmission dynamics.

Methods

A 24-month study of the vectors and related epidemiological aspects of malaria transmission was conducted in two villages in the Taiz region in 2004–2005.

Results

Cross-sectional blood film surveys recorded an overall malaria infection rate of 15.3 % (250/1638), with highest rates exceeding 30 % in one village in May and December 2005. With one exception, Plasmodium malariae, all infections were P.falciparum. Seven Anopheles species were identified among 3407 anophelines collected indoors using light traps (LT) and pyrethrum knockdown catches (PKD): Anopheles arabiensis (86.9 %), An. sergentii (9 %), An. azaniae, An. dthali, An. pretoriensis, An. coustani and An. algeriensis. Sequences for the standard barcode region of the mitochondrial COI gene confirmed the presence of two morphological forms of An. azaniae, the typical form and a previously unrecognized form not immediately identifiable as An. azaniae. ELISA detected Plasmodium sporozoites in 0.9 % of 2921 An. arabiensis (23 P. falciparum, two P. vivax) confirming this species as the primary malaria vector in Yemen. Plasmodium falciparum sporozoites were detected in An. sergentii (2/295) and a single female of An. algeriensis, incriminating both species as malaria vectors for the first time in Yemen. A vector in both wet and dry seasons, An. arabiensis was predominantly anthropophilic (human blood index = 0.86) with an entomological inoculation rate of 1.58 infective bites/person/year. Anopheles sergentii fed on cattle (67.3 %) and humans (48.3; 20.7 % mixed both species), but only 14.7 % were found in PKDs, indicating predominantly exophilic behaviour. A GIS analysis of geographic and socio-economic parameters revealed that An. arabiensis were significantly higher (P < 0.001) in houses with televisions, most likely due to the popular evening habit of viewing television collectively in houses with open doors and windows.

Conclusions

The predominantly indoor human biting vectors recorded in this study could be targeted effectively with LLINs, indoor residual spraying and/or insecticide-treated window/door curtains reinforced by education to instil a perception that effective and affordable malaria prevention is achievable.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1179-8) contains supplementary material, which is available to authorized users.

Transmission-blocking strategies: the roadmap from laboratory bench to the community

Malaria remains one of the most prevalent tropical and infectious diseases in the world, with an estimated more than 200 million clinical cases every year. In recent years, the mosquito stages of the parasite life cycle have received renewed attention with some progress being made in the development of transmission-blocking strategies. From gametocytes to late ookinetes, some attractive antigenic targets have been found and tested in order to develop a transmission blocking vaccine, and drugs are being currently screened for gametocytocidal activity, and also some new and less conventional approaches are drawing increased attention, such as genetically modified and fungus-infected mosquitoes that become refractory to Plasmodium infection. In this review some of those strategies focusing on the progress made so far will be summarized, but also, the challenges that come from the translation of early promising benchwork resulting in successful applications in the field. To do this, the available literature will be screened and all the pieces of the puzzle must be combined: from molecular biology to epidemiologic and clinical data.

Modelling optimum use of attractive toxic sugar bait stations for effective malaria vector control in Africa

BACKGROUND

The development of insecticide resistance and the increased outdoor-biting behaviour of malaria vectors reduce the efficiency of indoor vector control methods. Attractive toxic sugar baits (ATSBs), a method targeting the sugar-feeding behaviours of vectors both indoors and outdoors, is a promising supplement to indoor tools. The number and configuration of these ATSB stations needed for malaria control in a community needs to be determined.

METHODS

A hypothetical village, typical of those in sub-Saharan Africa, 600 × 600 m, consisting of houses, humans and essential resource requirements of Anopheles gambiae (sugar sources, outdoor resting sites, larval habitats) was simulated in a spatial individual-based model. Resource-rich and resource-poor environments were simulated separately. Eight types of configurations and different densities of ATSB stations were tested. Anopheles gambiae population size, human biting rate (HBR) and entomological inoculation rates (EIR) were compared between different ATSB configurations and densities. Each simulated scenario was run 50 times.

RESULTS

Compared to the outcomes not altered by ATSB treatment in the control scenario, in resource-rich and resource-poor environments, respectively, the optimum ATSB treatment reduced female abundance by 98.22 and 91.80 %, reduced HBR by 99.52 and 98.15 %, and reduced EIR by 99.99 and 100 %. In resource-rich environments, n × n grid design, stations at sugar sources, resting sites, larval habitats, and random locations worked better in reducing vector population and HBRs than other configurations (P < 0.0001). However, there was no significant difference of EIR reductions between all ATSB configurations (P > 0.05). In resource-poor environments, there was no significant difference of female abundances, HBRs and EIRs between all ATSB configurations (P > 0.05). The optimum number of ATSB stations was about 25 for resource-rich environments and nine for resource-poor environments.

CONCLUSIONS

ATSB treatment reduced An. gambiae population substantially and reduced EIR to near zero regardless of environmental resource availability. In resource-rich environments, dispersive configurations worked better in reducing vector population, and stations at or around houses worked better in preventing biting and parasite transmission. In resource-poor environments, all configurations worked similarly. Optimum numbers of bait stations should be adjusted according to seasonality when resource availability changes.

Identification of a Major Dimorphic Region in the Functionally Critical N-Terminal ID1 Domain of VAR2CSA

The VAR2CSA protein of Plasmodium falciparum is transported to and expressed on the infected erythrocyte surface where it plays a key role in placental malaria (PM). It is the current leading candidate for a vaccine to prevent PM. However, the antigenic polymorphism integral to VAR2CSA poses a challenge for vaccine development. Based on detailed analysis of polymorphisms in the sequence of its ligand-binding N-terminal region, currently the main focus for vaccine development, we assessed var2csa from parasite isolates infecting pregnant women. The results reveal for the first time the presence of a major dimorphic region in the functionally critical N-terminal ID1 domain. Parasite isolates expressing VAR2CSA with particular motifs present within this domain are associated with gravidity- and parasite density-related effects. These observations are of particular interest in guiding efforts with respect to optimization of the VAR2CSA-based vaccines currently under development.

Novel serologic biomarkers provide accurate estimates of recent Plasmodium falciparum exposure for individuals and communities

Tools to reliably measure Plasmodium falciparum (Pf) exposure in individuals and communities are needed to guide and evaluate malaria control interventions. Serologic assays can potentially produce precise exposure estimates at low cost; however, current approaches based on responses to a few characterized antigens are not designed to estimate exposure in individuals. Pf-specific antibody responses differ by antigen, suggesting that selection of antigens with defined kinetic profiles will improve estimates of Pf exposure. To identify novel serologic biomarkers of malaria exposure, we evaluated responses to 856 Pf antigens by protein microarray in 186 Ugandan children, for whom detailed Pf exposure data were available. Using data-adaptive statistical methods, we identified combinations of antibody responses that maximized information on an individual's recent exposure. Responses to three novel Pf antigens accurately classified whether an individual had been infected within the last 30, 90, or 365 d (cross-validated area under the curve = 0.86-0.93), whereas responses to six antigens accurately estimated an individual's malaria incidence in the prior year. Cross-validated incidence predictions for individuals in different communities provided accurate stratification of exposure between populations and suggest that precise estimates of community exposure can be obtained from sampling a small subset of that community. In addition, serologic incidence predictions from cross-sectional samples characterized heterogeneity within a community similarly to 1 y of continuous passive surveillance. Development of simple ELISA-based assays derived from the successful selection strategy outlined here offers the potential to generate rich epidemiologic surveillance data that will be widely accessible to malaria control programs.

Reduction in symptomatic malaria prevalence through proactive community treatment in rural Senegal

OBJECTIVES

We piloted a community-based proactive malaria case detection model in rural Senegal to evaluate whether this model can increase testing and treatment and reduce prevalence of symptomatic malaria in target communities.

METHODS

Home care providers conducted weekly sweeps of every household in their village throughout the transmission season to identify patients with symptoms of malaria, perform rapid diagnostic tests (RDT) on symptomatic patients and provide treatment for positive cases. The model was implemented in 15 villages from July to November 2013, the high transmission season. Fifteen comparison villages were chosen from those implementing Senegal's original, passive model of community case management of malaria. Three sweeps were conducted in the comparison villages to compare prevalence of symptomatic malaria using difference in differences analysis.

RESULTS

At baseline, prevalence of symptomatic malaria confirmed by RDT for all symptomatic individuals found during sweeps was similar in both sets of villages (P = 0.79). At end line, prevalence was 16 times higher in the comparison villages than in the intervention villages (P = 0.003). Adjusting for potential confounders, the intervention was associated with a 30-fold reduction in odds of symptomatic malaria in the intervention villages (AOR = 0.033; 95% CI: 0.017, 0.065). Treatment seeking also increased in the intervention villages, with 57% of consultations by home care providers conducted between sweeps through routine community case management.

CONCLUSIONS

This pilot study suggests that community-based proactive case detection reduces symptomatic malaria prevalence, likely through more timely case management and improved care seeking behaviour. A randomised controlled trial is needed to further evaluate the impact of this model.

'A bite before bed': exposure to malaria vectors outside the times of net use in the highlands of western Kenya

Background

The human population in the highlands of Nyanza Province, western Kenya, is subject to sporadic epidemics of Plasmodium falciparum. Indoor residual spraying (IRS) and long-lasting insecticide treated nets (LLINs) are used widely in this area. These interventions are most effective when Anopheles rest and feed indoors and when biting occurs at times when individuals use LLINs. It is therefore important to test the current assumption of vector feeding preferences, and late night feeding times, in order to estimate the extent to which LLINs protect the inhabitants from vector bites.

Methods

Mosquito collections were made for six consecutive nights each month between June 2011 and May 2012. CDC light-traps were set next to occupied LLINs inside and outside randomly selected houses and emptied hourly. The net usage of residents, their hours of house entry and exit and times of sleeping were recorded and the individual hourly exposure to vectors indoors and outdoors was calculated. Using these data, the true protective efficacy of nets (P*), for this population was estimated, and compared between genders, age groups and from month to month.

Results

Primary vector species (Anopheles funestus s.l. and Anopheles arabiensis) were more likely to feed indoors but the secondary vector Anopheles coustani demonstrated exophagic behaviour (p < 0.05). A rise in vector biting activity was recorded at 19:30 outdoors and 18:30 indoors. Individuals using LLINs experienced a moderate reduction in their overall exposure to malaria vectors from 1.3 to 0.47 bites per night. The P* for the population over the study period was calculated as 51% and varied significantly with age and season (p < 0.01).

Conclusions

In the present study, LLINs offered the local population partial protection against malaria vector bites. It is likely that P* would be estimated to be greater if the overall suppression of the local vector population due to widespread community net use could be taken into account. However, the overlap of early biting habit of vectors and human activity in this region indicates that additional methods of vector control are required to limit transmission. Regular surveillance of both vector behaviour and domestic human-behaviour patterns would assist the planning of future control interventions in this region.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0766-4) contains supplementary material, which is available to authorized users.

Applications and limitations of Centers for Disease Control and Prevention miniature light traps for measuring biting densities of African malaria vector populations: a pooled-analysis of 13 comparisons with human landing catches

Background

Measurement of densities of host-seeking malaria vectors is important for estimating levels of disease transmission, for appropriately allocating interventions, and for quantifying their impact. The gold standard for estimating mosquito—human contact rates is the human landing catch (HLC), where human volunteers catch mosquitoes that land on their exposed body parts. This approach necessitates exposure to potentially infectious mosquitoes, and is very labour intensive. There are several safer and less labour-intensive methods, with Centers for Disease Control light traps (LT) placed indoors near occupied bed nets being the most widely used.

Methods

This paper presents analyses of 13 studies with paired mosquito collections of LT and HLC to evaluate these methods for their consistency in sampling indoor-feeding mosquitoes belonging to the two major taxa of malaria vectors across Africa, the Anopheles gambiae sensu lato complex and the Anopheles funestus s.l. group. Both overall and study-specific sampling efficiencies of LT compared with HLC were computed, and regression methods that allow for the substantial variations in mosquito counts made by either method were used to test whether the sampling efficacy varies with mosquito density.

Results

Generally, LT were able to collect similar numbers of mosquitoes to the HLC indoors, although the relative sampling efficacy, measured by the ratio of LT:HLC varied considerably between studies. The overall best estimate for An. gambiae s.l. was 1.06 (95% credible interval: 0.68–1.64) and for An. funestus s.l. was 1.37 (0.70–2.68). Local calibration exercises are not reproducible, since only in a few studies did LT sample proportionally to HLC, and there was no geographical pattern or consistent trend with average density in the tendency for LT to either under- or over-sample.

Conclusions

LT are a crude tool at best, but are relatively easy to deploy on a large scale. Spatial and temporal variation in mosquito densities and human malaria transmission exposure span several orders of magnitude, compared to which the inconsistencies of LT are relatively small. LT, therefore, remain an invaluable and safe alternative to HLC for measuring indoor malaria transmission exposure in Africa.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0761-9) contains supplementary material, which is available to authorized users.

No difference in the incidence of malaria in human-landing mosquito catch collectors and non-collectors in a Senegalese village with endemic malaria

Background

The human landing catches is the gold standard method used to study the vectors of malaria and to estimate their aggressiveness. However, this method has raised safety concerns due to a possible increased risk of malaria or other mosquito-borne diseases among the mosquito collectors. The aim of this study was to evaluate the incidence of malaria attacks among mosquito collectors and to compare these results with those of non-collectors in a Senegalese village.

Methods

From July 1990 to December 2011, a longitudinal malaria study involving mosquito collectors and non-collectors was performed in Dielmo village, Senegal. During the study period, 4 drugs were successively used to treat clinical malaria, and long-lasting insecticide-treated nets were offered to all villagers in July 2008. No malaria chemoprophylaxis was given to mosquito collectors. Incidence of uncomplicated clinical malaria and asymptomatic malaria infection were analyzed among these two groups while controlling for confounding factors associated with malaria risk in random effects negative binomial and logistic regression models, respectively.

Results

A total of 3,812 person-trimester observations of 199 adults at least 15 years of age were analyzed. Clinical malaria attacks accounted for 6.3% both in collectors and non-collectors, and asymptomatic malaria infections accounted for 21% and 20% in collectors and non-collectors, respectively. A non-significant lower risk of malaria was observed in the collector group in comparison with the non-collector group after adjusting for other risk factors of malaria and endemicity level (Clinical malaria: adjusted incidence rate ratio = 0.89; 95% confidence interval = 0.65-1.22; p= 0.47).

Conclusion

Being a mosquito collector in Dielmo was not significantly associated with an increased risk of malaria both under holoendemic, mesoendemic and hypoendemic conditions of malaria epidemiology. This result supports the view that HLC, the most accurate method for evaluating malaria transmission, may be used without health concerns in Dielmo.

Modeling malaria genomics reveals transmission decline and rebound in Senegal

To study the effects of malaria-control interventions on parasite population genomics, we examined a set of 1,007 samples of the malaria parasite Plasmodium falciparum collected in Thiès, Senegal between 2006 and 2013. The parasite samples were genotyped using a molecular barcode of 24 SNPs. About 35% of the samples grouped into subsets with identical barcodes, varying in size by year and sometimes persisting across years. The barcodes also formed networks of related groups. Analysis of 164 completely sequenced parasites revealed extensive sharing of genomic regions. In at least two cases we found first-generation recombinant offspring of parents whose genomes are similar or identical to genomes also present in the sample. An epidemiological model that tracks parasite genotypes can reproduce the observed pattern of barcode subsets. Quantification of likelihoods in the model strongly suggests a reduction of transmission from 2006-2010 with a significant rebound in 2012-2013. The reduced transmission and rebound were confirmed directly by incidence data from Thiès. These findings imply that intensive intervention to control malaria results in rapid and dramatic changes in parasite population genomics. The results also suggest that genomics combined with epidemiological modeling may afford prompt, continuous, and cost-effective tracking of progress toward malaria elimination.

Serological evidence of vector and parasite exposure in Southern Ghana: the dynamics of malaria transmission intensity

BACKGROUND

Seroepidemiology provides robust estimates for tracking malaria transmission when intensity is low and useful when there is no baseline entomological data. Serological evidence of exposure to malaria vectors and parasite contribute to our understanding of the risk of pathogen transmission, and facilitates implementation of targeted interventions. Ab to Anopheles gambiae salivary peptide (gSG6-P1) and merozoite surface protein one (MSP-1(19)) reflect human exposure to malaria vectors and parasites. This study estimated malaria transmission dynamics using serological evidence of vector and parasite exposure in southern Ghana.

METHODS

Total IgG responses to both antigens in an age stratified cohort (<5, 5-14, >14) were measured from South-eastern Ghana. 295 randomly selected sera were analyzed from archived samples belonging to a cohort study that were followed at 3 consecutive survey months (n = 885); February, May and August 2009. Temporal variations in seroprevalence of both antigens as well as differences between the age-stratified cohorts were determined by χ (2) test with p < 0.05 statistically significant. Non-parametric repeated ANOVA - Friedman's test was used to test differences in antibody levels. Seroprevalence data were fitted to reversible catalytic model to estimate sero-conversion rates.

RESULTS

Whereas parasite prevalence was generally low 2.4%, 2.7% and 2.4% with no apparent trends with season, seroprevalence to both gSG6-P1 and MSP1(19) were high (59%, 50.9%, 52.2%) and 57.6%, 52.3% and 43.6% in respective order from Feb. to August. Repeated measures ANOVA showed differences in median antibody levels across surveys with specific significant differences between February and May but not August by post hoc Dunn's multiple comparison tests for gSG6-P1. For MSP1(19), no differences were observed in antibody levels between February and May but a significant decline was observed from May to August. Seroconversion rates for gSG6-P1 increased by 1.5 folds from February to August and 3 folds for MSP1(19).

CONCLUSION

Data suggests exposure to infectious bites may be declining whereas mosquito bites remains high. Sustained malaria control efforts and surveillance are needed to drive malaria further down and to prevent catastrophic rebound. Operational factors for scaling up have been discussed.

Early biting and insecticide resistance in the malaria vector Anopheles might compromise the effectiveness of vector control intervention in Southwestern Uganda

Background

Southwestern Uganda has high malaria heterogeneity despite moderate vector control and other interventions. Moreover, the early biting transmission and increased resistance to insecticides might compromise strategies relying on vector control. Consequently, monitoring of vector behaviour and insecticide efficacy is needed to assess the effectiveness of strategies aiming at malaria control. This eventually led to an entomological survey in two villages with high malaria prevalence in this region.

Methods

During rainy, 2011 and dry season 2012, mosquitoes were collected in Engari and Kigorogoro, Kazo subcounty, using human landing collection, morning indoor resting collection, pyrethrum spray collection and larval collection. Circumsporozoite protein of Plasmodium falciparum sporozoites in female Anopheles mosquitoes was detected using ELISA assay. Bioassays to monitor Anopheles resistance to insecticides were performed.

Results

Of the 1,021 female Anopheles species captured, 62% (632) were Anopheles funestus and 36% (371) were Anopheles gambiae s.l. The most common species were Anopheles gambiae s.l. in Engari (75%) and A. funestus in Kigorogoro (83%). Overall, P. falciparum prevalence was 2.9% by ELISA. The daily entomological inoculation rates were estimated at 0.17 and 0.58 infected bites/person/night during rainy and dry season respectively in Engari, and 0.81 infected bites/person/night in Kigorogoro during dry season. In both areas and seasons, an unusually early evening biting peak was observed between 6 - 8 p.m. In Engari, insecticide bioassays showed 85%, 34% and 12% resistance to DDT during the rainy season, dry season and to deltamethrin during the dry season, respectively. In Kigorogoro, 13% resistance to DDT and to deltamethrin was recorded. There was no resistance observed to bendiocarb and pirimiphos methyl.

Conclusions

The heterogeneity of mosquito distribution, entomological indicators and resistance to insecticides in villages with high malaria prevalence highlight the need for a long-term vector control programme and monitoring of insecticide resistance in Uganda. The early evening biting habits of Anopheles combined with resistance to DDT and deltamethrin observed in this study suggest that use of impregnated bed nets alone is insufficient as a malaria control strategy, urging the need for additional interventions in this area of high transmission.

Parity-dependent recognition of DBL1X-3X suggests an important role of the VAR2CSA high-affinity CSA-binding region in the development of the humoral response against placental malaria

Plasmodium falciparum multidomain protein VAR2CSA stands today as the leading vaccine candidate against pregnancy-associated malaria (PAM). Most of the studies aiming to decrypt how naturally acquired immunity develops have assessed the immune recognition of individual VAR2CSA Duffy-binding-like (DBL) domains, thus overlooking the presence of conformational epitopes resulting from the overall folding of the full-length protein. In order to characterize the development of humoral immunity toward VAR2CSA, we made use of a large cohort of 293 Senegalese pregnant women to assess the level of recognition by plasma IgG of the full-length VAR2CSA protein of the 3D7 parasite strain (3D7-VAR2CSA), the CSA-binding multidomains 3D7-DBL1X to -DBL3X (3D7-DBL1X-3X), and the CSA nonbinding multidomains 3D7-DBL4ε to -DBL6ε (3D7-DBL4ε-6ε), as well as individual 3D7-DBL domains. Our results revealed a parity-dependent recognition of the full-length 3D7-VAR2CSA and of the CSA-binding region, 3D7-DBL1X-3X. Indeed, multigravid women possess significantly higher levels of antibodies directed against these constructs than primigravidae. Our results suggest an important role of antibodies targeting the CSA-binding region in the development of immunity against PAM, therefore providing new insights on how natural protection might be acquired and further information for the design of VAR2CSA-based vaccines.

The epidemiological impact of HIV antiretroviral therapy on malaria in children

OBJECTIVE

The objective of this study is to determine the epidemiological effectiveness of a first-line antiretroviral regimen with HIV protease inhibitor for preventing recurrent malaria in children under the range of HIV prevalence levels and malaria transmission intensities encountered in sub-Saharan Africa.

DESIGN

A dynamic model of malaria transmission was developed using clinical data on the protease inhibitor extended posttreatment prophylactic effect of the antimalarial treatment, artemether-lumefantrine, in addition to parameter estimates from the literature.

METHODS

To evaluate the benefits of HIV protease inhibitors on the health burden of recurrent malaria among children, we constructed a dynamic model of malaria transmission to both HIV-positive and HIV-negative children, parameterized by data from a recent clinical trial. The model was then evaluated under varying malaria transmission and HIV prevalence settings to determine the health benefits of HIV protease inhibitors in the context of artemether-lumefantrine treatment of malaria in children.

RESULTS

Comparing scenarios of low, intermediate and high newborn HIV prevalence, in a range of malaria transmission settings, our dynamic model predicts that artemether-lumefantrine with HIV protease inhibitor based regimens prevents 0.03-0.10, 5.2-13.0 and 25.5-65.8 annual incidences of malaria per 1000 children, respectively. In addition, HIV protease inhibitors save 0.002-0.006, 0.22-0.8, 1.04-4.3 disability-adjusted life-years per 1000 children annually. Considering only HIV-infected children, HIV protease inhibitors avert between 278 and 1043 annual incidences of malaria per 1000 children.

CONCLUSION

The use of HIV protease inhibitor based regimens as first-line antiretroviral therapy for HIV is an effective measure for reducing recurrent malaria among HIV-infected children in areas where HIV and malaria are coendemic, and artemether-lumefantrine is a first-line antimalarial.

Submicroscopic Plasmodium falciparum Infections Are Associated With Maternal Anemia, Premature Births, and Low Birth Weight

BACKGROUND

Molecular, as opposed to microscopic, detection measures the real prevalence of Plasmodium falciparum infections. Such occult infections are common during pregnancy but their impact on pregnancy outcomes is unclear. We performed a longitudinal study to describe that impact.

METHODS

In a cohort of 1037 Beninese pregnant women, we used ultrasound to accurately estimate gestational ages. Infection with P. falciparum, hemoglobin concentration, use of intermittent preventive treatment during pregnancy (IPTp) for malaria, and other parameters were recorded during pregnancy. Using multivariate analyses, we evaluated the impact of submicroscopic infections on maternal anemia, premature birth, and low birth weight.

RESULTS

At inclusion, polymerase chain reaction (PCR) and microscopy detected infection in 40% and 16% of women, respectively. The proportion infected declined markedly after 2 doses of IPTp but rebounded to 34% (by PCR) at delivery. Submicroscopic infections during pregnancy were associated with lower mean hemoglobin irrespective of gravidity, and with increased anemia risk in primigravidae (odds ratio [OR], 2.23; 95% confidence interval [CI], .98-5.07). Prospectively, submicroscopic infections at inclusion were associated with significantly increased risks of low birth weight in primigravidae (OR, 6.09; 95% CI, 1.16-31.95) and premature births in multigravidae (OR, 2.25; 95% CI, 1.13-4.46).

CONCLUSIONS

In this detailed longitudinal study, we document the deleterious impact of submicroscopic P. falciparum parasitemia during pregnancy on multiple pregnancy outcomes. Parasitemia occurs frequently during pregnancy, but routine microscopic and rapid diagnostic tests fail to detect the vast majority of episodes. Our findings imply caution in any revision of the current strategies for prevention of pregnancy-associated malaria.

Design of a Phase III cluster randomized trial to assess the efficacy and safety of a malaria transmission blocking vaccine

Vaccines interrupting Plasmodium falciparum malaria transmission targeting sexual, sporogonic, or mosquito-stage antigens (SSM-VIMT) are currently under development to reduce malaria transmission. An international group of malaria experts was established to evaluate the feasibility and optimal design of a Phase III cluster randomized trial (CRT) that could support regulatory review and approval of an SSM-VIMT. The consensus design is a CRT with a sentinel population randomly selected from defined inner and buffer zones in each cluster, a cluster size sufficient to assess true vaccine efficacy in the inner zone, and inclusion of ongoing assessment of vaccine impact stratified by distance of residence from the cluster edge. Trials should be conducted first in areas of moderate transmission, where SSM-VIMT impact should be greatest. Sample size estimates suggest that such a trial is feasible, and within the range of previously supported trials of malaria interventions, although substantial issues to implementation exist.

A spatial individual-based model predicting a great impact of copious sugar sources and resting sites on survival of Anopheles gambiae and malaria parasite transmission

Background

Agent-based modelling (ABM) has been used to simulate mosquito life cycles and to evaluate vector control applications. However, most models lack sugar-feeding and resting behaviours or are based on mathematical equations lacking individual level randomness and spatial components of mosquito life. Here, a spatial individual-based model (IBM) incorporating sugar-feeding and resting behaviours of the malaria vector Anopheles gambiae was developed to estimate the impact of environmental sugar sources and resting sites on survival and biting behaviour.

Methods

A spatial IBM containing An. gambiae mosquitoes and humans, as well as the village environment of houses, sugar sources, resting sites and larval habitat sites was developed. Anopheles gambiae behaviour rules were attributed at each step of the IBM: resting, host seeking, sugar feeding and breeding. Each step represented one second of time, and each simulation was set to run for 60 days and repeated 50 times. Scenarios of different densities and spatial distributions of sugar sources and outdoor resting sites were simulated and compared.

Results

When the number of natural sugar sources was increased from 0 to 100 while the number of resting sites was held constant, mean daily survival rate increased from 2.5% to 85.1% for males and from 2.5% to 94.5% for females, mean human biting rate increased from 0 to 0.94 bites per human per day, and mean daily abundance increased from 1 to 477 for males and from 1 to 1,428 for females. When the number of outdoor resting sites was increased from 0 to 50 while the number of sugar sources was held constant, mean daily survival rate increased from 77.3% to 84.3% for males and from 86.7% to 93.9% for females, mean human biting rate increased from 0 to 0.52 bites per human per day, and mean daily abundance increased from 62 to 349 for males and from 257 to 1120 for females. All increases were significant (P < 0.01). Survival was greater when sugar sources were randomly distributed in the whole village compared to clustering around outdoor resting sites or houses.

Conclusions

Increases in densities of sugar sources or outdoor resting sites significantly increase the survival and human biting rates of An. gambiae mosquitoes. Survival of An. gambiae is more supported by random distribution of sugar sources than clustering of sugar sources around resting sites or houses. Density and spatial distribution of natural sugar sources and outdoor resting sites modulate vector populations and human biting rates, and thus malaria parasite transmission.

Underestimation of foraging behaviour by standard field methods in malaria vector mosquitoes in southern Africa

BACKGROUND

Defining the anopheline mosquito vectors and their foraging behaviour in malaria endemic areas is crucial for disease control and surveillance. The standard protocol for molecular identification of host blood meals in mosquitoes is to morphologically identify fed mosquitoes and then perform polymerase chain reaction (PCR), precipitin tests, or ELISA assays. The purpose of this study was to determine the extent to which the feeding rate and human blood indices (HBIs) of malaria vectors were underestimated when molecular confirmation by PCR was performed on both visually fed and unfed mosquitoes.

METHODS

In association with the Southern Africa International Centers of Excellence in Malaria Research (ICEMR), mosquito collections were performed at three sites: Choma district in southern Zambia, Nchelenge district in northern Zambia, and Mutasa district in eastern Zimbabwe. All anophelines were classified visually as fed or unfed, and tested for blood meal species using PCR methods. The HBIs of visually fed mosquitoes were compared to the HBIs of overall PCR confirmed fed mosquitoes by Pearson's Chi-Square Test of Independence.

RESULTS

The mosquito collections consisted of Anopheles arabiensis from Choma, Anopheles funestus s.s., Anopheles gambiae s.s. and Anopheles leesoni from Nchelenge, and An. funestus s.s. and An. leesoni from Mutasa. The malaria vectors at all three sites had large human blood indices (HBI) suggesting high anthropophily. When only visually fed mosquitoes tested by PCR for blood meal species were compared to testing those classified as both visually fed and unfed mosquitoes, it was found that the proportion blooded was underestimated by up to 18.7%. For most Anopheles species at each site, there was a statistically significant relationship (P < 0.05) between the HBIs of visually fed mosquitoes and that of the overall PCR confirmed fed mosquitoes.

CONCLUSION

The impact on HBI of analysing both visually fed and unfed mosquitoes varied from site to site. This discrepancy may be due to partial blood feeding behaviour by mosquitoes, digestion of blood meals, sample condition, and/or expertise of entomology field staff. It is important to perform molecular testing on all mosquitoes to accurately characterize vector feeding behaviour and develop interventions in malaria endemic areas.

Serological markers for monitoring historical changes in malaria transmission intensity in a highly endemic region of Western Kenya, 1994-2009

Background

Monitoring local malaria transmission intensity is essential for planning evidence-based control strategies and evaluating their impact over time. Anti-malarial antibodies provide information on cumulative exposure and have proven useful, in areas where transmission has dropped to low sustained levels, for retrospectively reconstructing the timing and magnitude of transmission reduction. It is unclear whether serological markers are also informative in high transmission settings, where interventions may reduce transmission, but to a level where considerable exposure continues.

Methods

This study was conducted through ongoing KEMRI and CDC collaboration. Asembo, in Western Kenya, is an area where intense malaria transmission was drastically reduced during a 1997–1999 community-randomized, controlled insecticide-treated net (ITN) trial. Two approaches were taken to reconstruct malaria transmission history during the period from 1994 to 2009. First, point measurements were calculated for seroprevalence, mean antibody titre, and seroconversion rate (SCR) against three Plasmodium falciparum antigens (AMA-1, MSP-1₁₉, and CSP) at five time points for comparison against traditional malaria indices (parasite prevalence and entomological inoculation rate). Second, within individual post-ITN years, age-stratified seroprevalence data were analysed retrospectively for an abrupt drop in SCR by fitting alternative reversible catalytic conversion models that allowed for change in SCR.

Results

Generally, point measurements of seroprevalence, antibody titres and SCR produced consistent patterns indicating that a gradual but substantial drop in malaria transmission (46-70%) occurred from 1994 to 2007, followed by a marginal increase beginning in 2008 or 2009. In particular, proportionate changes in seroprevalence and SCR point estimates (relative to 1994 baseline values) for AMA-1 and CSP, but not MSP-1₁₉, correlated closely with trends in parasite prevalence throughout the entire 15-year study period. However, retrospective analyses using datasets from 2007, 2008 and 2009 failed to detect any abrupt drop in transmission coinciding with the timing of the 1997–1999 ITN trial.

Conclusions

In this highly endemic area, serological markers were useful for generating accurate point estimates of malaria transmission intensity, but not for retrospective analysis of historical changes. Further investigation, including exploration of different malaria antigens and/or alternative models of population seroconversion, may yield serological tools that are more informative in high transmission settings.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-451) contains supplementary material, which is available to authorized users.

Modelling the contribution of the hypnozoite reservoir to Plasmodium vivax transmission

Plasmodium vivax relapse infections occur following activation of latent liver-stages parasites (hypnozoites) causing new blood-stage infections weeks to months after the initial infection. We develop a within-host mathematical model of liver-stage hypnozoites, and validate it against data from tropical strains of P. vivax. The within-host model is embedded in a P. vivax transmission model to demonstrate the build-up of the hypnozoite reservoir following new infections and its depletion through hypnozoite activation and death. The hypnozoite reservoir is predicted to be over-dispersed with many individuals having few or no hypnozoites, and some having intensely infected livers. Individuals with more hypnozoites are predicted to experience more relapses and contribute more to onwards P. vivax transmission. Incorporating hypnozoite killing drugs such as primaquine into first-line treatment regimens is predicted to cause substantial reductions in P. vivax transmission as individuals with the most hypnozoites are more likely to relapse and be targeted for treatment.

DOI:http://dx.doi.org/10.7554/eLife.04692.001

Malaria is one of the world's most deadly infections, causing 100s of 1000s of deaths each year despite being both preventable and curable. Malaria is caused by Plasmodium parasites, which are transmitted between humans by mosquitoes. When a mosquito bites a human, Plasmodium is injected into the bloodstream with the mosquito's saliva. The parasite then travels through the bloodstream to the liver, infects liver cells and multiplies within those cells without causing any noticeable symptoms. After remaining silent in the liver for weeks or months, the now abundant parasite ruptures the host liver cell, re-enters the bloodstream, and begins infecting red blood cells. If another mosquito bites the infected individual and takes a blood meal, the parasite moves into the mosquito and the cycle of transmission continues.

There are several species of Plasmodium that are known to cause malaria. The most widely studied species is P. falciparum, which also causes one of the deadliest types of malaria. However, another Plasmodium species called P. vivax is the most widely distributed species and, despite being less virulent than P. falciparum, is particularly dangerous because it causes recurring malaria.

In contrast to P. falciparum, P. vivax has the ability to form hypnozoites: a dormant form of the parasite that can remain inside liver cells for long periods of time, sometimes for years. The reservoir of P. vivax hypnozoites can regularly populate the bloodstream with the infectious form of the parasite, triggering relapses of malaria. Even if an individual suffering a relapse receives prompt treatment to clear parasites in the blood, more parasites may emerge from the liver and cause new blood-stage infections.

White et al. developed a mathematical model to help understand how P. vivax is transmitted. Unlike many of the established models of malaria transmission, the new model accounts for the reservoir of P. vivax hypnozoites in the liver, and assumes that hypnozoites in the reservoir either die, or are activated and enter the bloodstream, at a constant rate. This produces patterns that closely match how often relapses occur in patients. White et al. go on to predict that although many infected people have few or no hypnozoites in their liver, some have many hypnozoites, and these people are more likely to suffer from malaria relapses. This suggests that if the initial treatments given to malaria sufferers incorporate additional drugs that kill the hypnozoites in the liver, then it may be possible to substantially reduce the extent of P. vivax transmission.

DOI:http://dx.doi.org/10.7554/eLife.04692.002

EMIRA: Ecologic Malaria Reduction for Africa--innovative tools for integrated malaria control

BACKGROUND

Malaria control is based on early treatment of cases and on vector control. The current measures for malaria vector control in Africa are mainly based on long-lasting insecticide treated nets (LLINs) and to a much smaller extent on indoor residual spraying (IRS). A third pillar in the fight against the malaria vector, larval source management (LSM), has virtually not been used in Africa since the ban of DDT in the 1960s. Within the light of recent WHO recommendations for Bacillus thuringiensis israelensis (Bti) use against malaria and other vector species, larval source management could see a revival in the upcoming years. In this project we analyze the ecologic and health impacts as well as the cost effectiveness of larval source management under different larviciding scenarios in a health district in Burkina Faso.

METHODS

The project is designed as prospective intervention study with duration of three years (2013-2015). Its spatial scale includes three arms of interventions and control, comprising a total of 127 villages and the district capital Nouna in the extended HDSS (Health Demographic Surveillance System) of the Kossi province. Baseline data on mosquito abundance, parasitemia in U5 children, and malaria related morbidity and mortality are gathered over the project duration. Besides the outcome on ecologic and health parameters, the economic costs are seized and valued against the achieved health benefits.

CONCLUSIONS

Risk map based, guided larvicide application might be a possibility to further decrease economic cost of LSM and facilitate its faster incorporation to integrated malaria control programs. Given the limited resources in many malaria endemic countries, it is of utmost importance to relate the costs of novel strategies for malaria prevention to their effect on the burden of the disease. Occurring costs and the impact on the health situation will be made comparable to other, existing intervention strategies, allowing stakeholders and policymakers decision making.

Association between malaria control and paediatric blood transfusions in rural Zambia: an interrupted time-series analysis

Background

Blood transfusions can reduce mortality among children with severe malarial anaemia, but there is limited evidence quantifying the relationship between paediatric malaria and blood transfusions. This study explores the extent to which the use of paediatric blood transfusions is affected by the number of paediatric malaria visits and admissions. It assesses whether the scale-up of malaria control interventions in a facility catchment area explains the use of paediatric blood transfusions.

Methods

The study was conducted at a referral hospital for 13 rural health centres in rural Zambia. Data were used from facility and patient records covering all paediatric malaria admissions from 2000 to 2008. An interrupted time series analysis using an autoregression-moving-average model was conducted to assess the relationship between paediatric malaria outpatient visits and admissions and the use of paediatric blood transfusions. Further investigation explored whether the use of paediatric blood transfusions over time was consistent with the roll out of malaria control interventions in the hospital catchment area.

Results

For each additional paediatric malaria outpatient visit, there were 0.07 additional paediatric blood transfusions (95% CI 0.01-0.13; p < 0.05). For each additional paediatric admission for severe malarial anaemia, there were 1.09 additional paediatric blood transfusions (95% CI 0.95-1.23; p < 0.01). There were 19.1 fewer paediatric blood transfusions per month during the 2004–2006 malaria control period (95% CI 12.1-26.0; p < 0.01), a 50% reduction compared to the preceding period when malaria control was relatively limited. During the 2007–2008 malaria control period, there were 27.5 fewer paediatric blood transfusions per month (95% CI 14.6-40.3; p < 0.01), representing a 72% decline compared to the period with limited malaria control.

Conclusions

Paediatric admissions for severe malarial anaemia largely explain total use of paediatric blood transfusions. The reduction in paediatric blood transfusions is consistent with the timing of the malaria control interventions. Malaria control seems to influence the use of paediatric blood transfusions by reducing the number of paediatric admissions for severe malarial anaemia. Reduced use of blood transfusions could benefit other areas of the health system through greater blood availability, particularly where supply is limited.

Mosquito magnet® liberty plus trap baited with octenol confirmed best candidate for Anopheles surveillance and proved promising in predicting risk of malaria transmission in French Guiana

BACKGROUND

In French Guiana, Mosquito Magnet® Liberty Plus trap baited with octenol (MMoct) has been proposed for sampling Anopheles darlingi after comparison with CDC light trap and Human landing catch (HLC). However, other available lures were not tested. The current study compared MMoct and MM baited with Lurex™ (MMlur) to HLC, and analysed entomological data from MMoct collection with malaria cases to facilitate malaria surveillance.

METHODS

Two independent experiments were conducted during 2012 and 2013 in Saint-Georges town, French Guiana. The first experiment used Latin square design to compare MMoct and MMlur to HLC between 18:30 to 22:30 and 05:00 to 07:00. Parity rate was determined for An. darlingi from each sampling system. In the second experiment, a 24:00 hour collection was done for four consecutive days during the first week of each month and every four days for the rest of the month using MMoct. Portion of the 24 hour collection was dissected for parity rate. All anophelines were screened for Plasmodium infection by PCR. Data for number of malaria cases was analysed for association with density of An. darlingi.

RESULTS

In the first experiment, 3,721 anopheline mosquitoes were collected over 21 nights. Of these, 95.7% was identified morphologically to five species and An. darlingi contributed 98.4%, mainly from HLC (75.1%, CI 95% [73.2-77.0]) than MMoct (14.1%, CI 95% [12.6-15.7]) and MMlur (10.8%, CI 95% [9.4-12.2]). Species richness was highest in HLC meanwhile species diversity index was greatest in MMoct. MMoct collected more parous An. darlingi than HLC (p<0.0001) and MMlur (p=0.0021). The second experiment amounted to 2035 females, 60.8% belonging to 10 species. Anopheles darlingi constituted 85.0% of the species and had parity rate of 52.3%. Specimens were uninfected with Plasmodium. Density of An. darlingi best correlated with malaria cases observed six weeks later (p=0.0016; r=0.4774).

CONCLUSION

Though MMoct and MMlur performed well in sampling An. darlingi, MMoct captured more species and, therefore, would be useful for surveillance. Even if it collected mostly parous mosquitoes, MMoct proved useful in collecting entomological data required for predicting malaria emergence. It is a potential replacement for HLC.

Increasing role of Anopheles funestus and Anopheles arabiensis in malaria transmission in the Kilombero Valley, Tanzania

Background

In order to sustain the gains achieved by current malaria control strategies, robust surveillance systems that monitor dynamics of vectors and their roles in malaria transmission over time are essential. This longitudinal study demonstrates the trends in malaria vector dynamics and their relative contribution to malaria transmission in hyperendemic transmission settings in Tanzania.

Methods

The study was conducted in two villages within the Kilombero Valley, in rural Tanzania for five consecutive years (2008–2012). Seventy-two houses were selected per village and each house was sampled for mosquitoes monthly using a CDC light trap. Collected mosquitoes were assessed for species identity and sporozoite infection status using PCR and ELISA, respectively. Anopheles funestus and Anopheles arabiensis susceptibility to insecticides was assessed using WHO guidelines.

Results

A total of 100,810 malaria vectors were collected, of which 76% were Anopheles gambiae s. l. and 24% were An. funestus. Of all An. funestus samples that amplified with PCR (n = 2,737), 97% were An. funestus s.s., 2% were Anopheles rivorulum and 1% Anopheles leesoni. Whereas for An. gambiae s.l. (n = 8,117), 93% were An. arabiensis and 7% were Anopheles gambiae s.s. The proportion of An. gambiae s.s. identified by PCR (2,924) declined from 0.2% in the year 2008 to undetectable levels in 2012. Malaria transmission intensity significantly decreased from an EIR of 78.14 infectious bites/person/year in 2008 to 35 ib/p/yr in 2011 but rebounded to 226 ib/p/yr in 2012 coinciding with an increased role of An. funestus in malaria transmission. Insecticide susceptibility tests indicated high levels of resistance in An. funestus against deltamethrin (87%), permethrin (65%), lambda cyhalothrin (74%), bendiocarb (65%), and DDT (66%). Similarly, An. arabiensis showed insecticide resistance to deltamethrin (64%), permethrin (77%) and lambda cyhalothrin (42%) in 2014.

Conclusion

The results indicate the continuing role of An. arabiensis and the increasing importance of An. funestus in malaria transmission, and pyrethroid resistance development in both species. Complementary vector control and surveillance tools are needed that target the ecology, behaviour and insecticide resistance management of these vector species, in order to preserve the efficacy of LLINs.

Is there an efficient trap or collection method for sampling Anopheles darlingi and other malaria vectors that can describe the essential parameters affecting transmission dynamics as effectively as human landing catches? - A Review

Distribution, abundance, feeding behaviour, host preference, parity status and human-biting and infection rates are among the medical entomological parameters evaluated when determining the vector capacity of mosquito species. To evaluate these parameters, mosquitoes must be collected using an appropriate method. Malaria is primarily transmitted by anthropophilic and synanthropic anophelines. Thus, collection methods must result in the identification of the anthropophilic species and efficiently evaluate the parameters involved in malaria transmission dynamics. Consequently, human landing catches would be the most appropriate method if not for their inherent risk. The choice of alternative anopheline collection methods, such as traps, must consider their effectiveness in reproducing the efficiency of human attraction. Collection methods lure mosquitoes by using a mixture of olfactory, visual and thermal cues. Here, we reviewed, classified and compared the efficiency of anopheline collection methods, with an emphasis on Neotropical anthropophilic species, especially Anopheles darlingi, in distinct malaria epidemiological conditions in Brazil.

Modelling heterogeneity in malaria transmission using large sparse spatio-temporal entomological data

Background

Malaria transmission is measured using entomological inoculation rate (EIR), number of infective mosquito bites/person/unit time. Understanding heterogeneity of malaria transmission has been difficult due to a lack of appropriate data. A comprehensive entomological database compiled by the Malaria Transmission Intensity and Mortality Burden across Africa (MTIMBA) project (2001–2004) at several sites is the most suitable dataset for studying malaria transmission–mortality relations. The data are sparse and large, with small-scale spatial–temporal variation.

Objective

This work demonstrates a rigorous approach for analysing large and highly variable entomological data for the study of malaria transmission heterogeneity, measured by EIR, within the Rufiji Demographic Surveillance System (DSS), MTIMBA project site in Tanzania.

Design

Bayesian geostatistical binomial and negative binomial models with zero inflation were fitted for sporozoite rates (SRs) and mosquito density, respectively. The spatial process was approximated from a subset of locations. The models were adjusted for environmental effects, seasonality and temporal correlations and assessed based on their predictive ability. EIR was calculated using model-based predictions of SR and density.

Results

Malaria transmission was mostly influenced by rain and temperature, which significantly reduces the probability of observing zero mosquitoes. High transmission was observed at the onset of heavy rains. Transmission intensity reduced significantly during Year 2 and 3, contrary to the Year 1, pronouncing high seasonality and spatial variability. The southern part of the DSS showed high transmission throughout the years. A spatial shift of transmission intensity was observed where an increase in households with very low transmission intensity and significant reduction of locations with high transmission were observed over time. Over 68 and 85% of the locations selected for validation for SR and density, respectively, were correctly predicted within 95% credible interval indicating good performance of the models.

Conclusion

Methodology introduced here has the potential for efficient assessment of the contribution of malaria transmission in mortality and monitoring performance of control and intervention strategies.

Comparing decisions for malaria testing and presumptive treatment: a net health benefit analysis

BACKGROUND

Rapid tests for malaria are being distributed through vendors to individual patients, presenting the dilemma of determining how individuals are incentivized to pursue testing for malaria, versus the traditional approach of presumptively treating fevers with antimalarial drugs.

METHODS AND FINDINGS

We incorporated testing and treatment data from 6 African countries into a dynamic model of malaria transmission and nonmalarial causes of fever to investigate how variations in the epidemiologic risk of malaria and the prices of rapid diagnostic tests (RDTs) and treatments affect testing and treatment choices from the perspective of febrile patients, public health officials, and drug shop owners. In environments falling below a critical threshold infection rate (entomological inoculation rate) of 282 for patients older than 5 years (95% confidence interval [CI]: 275-289) or 300 for 0- to 5-year-olds (95% CI: 203-307), testing was more beneficial than presumptive therapy in terms of health and financial costs to patients. Infection and cost conditions generally aligned the best patient-level strategy with the best public health strategy to minimize an overall population's morbidity and mortality from both malaria and nonmalarial causes of fever. However, the infection and cost conditions of very high malaria transmission settings did not align patient interests or public health interests with the interests of private drug shop owners. In such settings, a further lowering of testing prices may realign the interests of all 3 parties.

CONCLUSIONS

A threshold transmission rate exists under which malaria testing confers more health and financial benefits to patients than presumptive treatment. Studying local transmission rates and testing and treatment costs may facilitate an approach to align the interests of individual patients, public health officials, and distributors of tests and therapies.

Recasting the theory of mosquito-borne pathogen transmission dynamics and control

Mosquito-borne diseases pose some of the greatest challenges in public health, especially in tropical and sub-tropical regions of the world. Efforts to control these diseases have been underpinned by a theoretical framework developed for malaria by Ross and Macdonald, including models, metrics for measuring transmission, and theory of control that identifies key vulnerabilities in the transmission cycle. That framework, especially Macdonald's formula for R₀ and its entomological derivative, vectorial capacity, are now used to study dynamics and design interventions for many mosquito-borne diseases. A systematic review of 388 models published between 1970 and 2010 found that the vast majority adopted the Ross–Macdonald assumption of homogeneous transmission in a well-mixed population. Studies comparing models and data question these assumptions and point to the capacity to model heterogeneous, focal transmission as the most important but relatively unexplored component in current theory. Fine-scale heterogeneity causes transmission dynamics to be nonlinear, and poses problems for modeling, epidemiology and measurement. Novel mathematical approaches show how heterogeneity arises from the biology and the landscape on which the processes of mosquito biting and pathogen transmission unfold. Emerging theory focuses attention on the ecological and social context for mosquito blood feeding, the movement of both hosts and mosquitoes, and the relevant spatial scales for measuring transmission and for modeling dynamics and control.

Impact of climate change on global malaria distribution

Malaria is an important disease that has a global distribution and significant health burden. The spatial limits of its distribution and seasonal activity are sensitive to climate factors, as well as the local capacity to control the disease. Malaria is also one of the few health outcomes that has been modeled by more than one research group and can therefore facilitate the first model intercomparison for health impacts under a future with climate change. We used bias-corrected temperature and rainfall simulations from the Coupled Model Intercomparison Project Phase 5 climate models to compare the metrics of five statistical and dynamical malaria impact models for three future time periods (2030s, 2050s, and 2080s). We evaluated three malaria outcome metrics at global and regional levels: climate suitability, additional population at risk and additional person-months at risk across the model outputs. The malaria projections were based on five different global climate models, each run under four emission scenarios (Representative Concentration Pathways, RCPs) and a single population projection. We also investigated the modeling uncertainty associated with future projections of populations at risk for malaria owing to climate change. Our findings show an overall global net increase in climate suitability and a net increase in the population at risk, but with large uncertainties. The model outputs indicate a net increase in the annual person-months at risk when comparing from RCP2.6 to RCP8.5 from the 2050s to the 2080s. The malaria outcome metrics were highly sensitive to the choice of malaria impact model, especially over the epidemic fringes of the malaria distribution.

Measuring changes in Plasmodium falciparum transmission: precision, accuracy and costs of metrics

As malaria declines in parts of Africa and elsewhere, and as more countries move towards elimination, it is necessary to robustly evaluate the effect of interventions and control programmes on malaria transmission. To help guide the appropriate design of trials to evaluate transmission-reducing interventions, we review 11 metrics of malaria transmission, discussing their accuracy, precision, collection methods and costs and presenting an overall critique. We also review the nonlinear scaling relationships between five metrics of malaria transmission: the entomological inoculation rate, force of infection, sporozoite rate, parasite rate and the basic reproductive number, R0. Our chapter highlights that while the entomological inoculation rate is widely considered the gold standard metric of malaria transmission and may be necessary for measuring changes in transmission in highly endemic areas, it has limited precision and accuracy and more standardised methods for its collection are required. In areas of low transmission, parasite rate, seroconversion rates and molecular metrics including MOI and mFOI may be most appropriate. When assessing a specific intervention, the most relevant effects will be detected by examining the metrics most directly affected by that intervention. Future work should aim to better quantify the precision and accuracy of malaria metrics and to improve methods for their collection.

Needs for monitoring mosquito transmission of malaria in a pre-elimination world

As global efforts to eliminate malaria intensify, accurate information on vector populations and transmission dynamics is critical for directing control efforts, developing new control tools, and predicting the effects of these interventions under various conditions. Currently available sampling tools for mosquito population monitoring suffer from well-recognized limitations. As reported in this workshop summary, a recent gathering of medical entomologists, modelers, and malaria experts reviewed these issues and agreed that efforts are needed to improve methods to monitor key transmission parameters. Identified needs include standardized methods for sampling of both mosquito adults and larvae, improved tools for mosquito species identification and age-grading, and a better means for determining the entomological inoculation rate.

Relative importance of climatic, geographic and socio-economic determinants of malaria in Malawi

Background

Malaria transmission is influenced by variations in meteorological conditions, which impact the biology of the parasite and its vector, but also socio-economic conditions, such as levels of urbanization, poverty and education, which impact human vulnerability and vector habitat. The many potential drivers of malaria, both extrinsic, such as climate, and intrinsic, such as population immunity are often difficult to disentangle. This presents a challenge for the modelling of malaria risk in space and time.

Methods

A statistical mixed model framework is proposed to model malaria risk at the district level in Malawi, using an age-stratified spatio-temporal dataset of malaria cases from July 2004 to June 2011. Several climatic, geographic and socio-economic factors thought to influence malaria incidence were tested in an exploratory model. In order to account for the unobserved confounding factors that influence malaria, which are not accounted for using measured covariates, a generalized linear mixed model was adopted, which included structured and unstructured spatial and temporal random effects. A hierarchical Bayesian framework using Markov chain Monte Carlo simulation was used for model fitting and prediction.

Results

Using a stepwise model selection procedure, several explanatory variables were identified to have significant associations with malaria including climatic, cartographic and socio-economic data. Once intervention variations, unobserved confounding factors and spatial correlation were considered in a Bayesian framework, a final model emerged with statistically significant predictor variables limited to average precipitation (quadratic relation) and average temperature during the three months previous to the month of interest.

Conclusions

When modelling malaria risk in Malawi it is important to account for spatial and temporal heterogeneity and correlation between districts. Once observed and unobserved confounding factors are allowed for, precipitation and temperature in the months prior to the malaria season of interest are found to significantly determine spatial and temporal variations of malaria incidence. Climate information was found to improve the estimation of malaria relative risk in 41% of the districts in Malawi, particularly at higher altitudes where transmission is irregular. This highlights the potential value of climate-driven seasonal malaria forecasts.

Assessing malaria transmission in a low endemicity area of north-western Peru

BACKGROUND

Where malaria endemicity is low, control programmes need increasingly sensitive tools for monitoring malaria transmission intensity (MTI) and to better define health priorities. A cross-sectional survey was conducted in a low endemicity area of the Peruvian north-western coast to assess the MTI using both molecular and serological tools.

METHODS

Epidemiological, parasitological and serological data were collected from 2,667 individuals in three settlements of Bellavista district, in May 2010. Parasite infection was detected using microscopy and polymerase chain reaction (PCR). Antibodies to Plasmodium vivax merozoite surface protein-119 (PvMSP1₁₉) and to Plasmodium falciparum glutamate-rich protein (PfGLURP) were detected by ELISA. Risk factors for exposure to malaria (seropositivity) were assessed by multivariate survey logistic regression models. Age-specific antibody prevalence of both P. falciparum and P. vivax were analysed using a previously published catalytic conversion model based on maximum likelihood for generating seroconversion rates (SCR).

RESULTS

The overall parasite prevalence by microscopy and PCR were extremely low: 0.3 and 0.9%, respectively for P. vivax, and 0 and 0.04%, respectively for P. falciparum, while seroprevalence was much higher, 13.6% for P. vivax and 9.8% for P. falciparum. Settlement, age and occupation as moto-taxi driver during previous year were significantly associated with P. falciparum exposure, while age and distance to the water drain were associated with P. vivax exposure. Likelihood ratio tests supported age seroprevalence curves with two SCR for both P. vivax and P. falciparum indicating significant changes in the MTI over time. The SCR for PfGLURP was 19-fold lower after 2002 as compared to before (λ1 = 0.022 versus λ2 = 0.431), and the SCR for PvMSP1₁₉ was four-fold higher after 2006 as compared to before (λ1 = 0.024 versus λ2 = 0.006).

CONCLUSION

Combining molecular and serological tools considerably enhanced the capacity of detecting current and past exposure to malaria infections and related risks factors in this very low endemicity area. This allowed for an improved characterization of the current human reservoir of infections, largely hidden and heterogeneous, as well as providing insights into recent changes in species specific MTIs. This approach will be of key importance for evaluating and monitoring future malaria elimination strategies.

Plasmodium falciparum: genetic diversity and complexity of infections in an isolated village in western Thailand

Genetic diversity of Plasmodium falciparum is intimately associated with morbidity, mortality and malaria control strategies. It is therefore imperative to study genetic makeup and population structure of this parasite in endemic areas. In Kong Mong Tha, an isolated village in western Thailand, the majority of P. falciparum infections are asymptomatic. In this study we investigated complexity of infections and single nucleotide polymorphisms (SNPs) in the P. falciparum population of Kong Mong Tha, and compared results with those previously obtained from Mae Sod, in northwestern Thailand, where the majority of infections were symptomatic. Using PCR-based determination of the 5' merozoite surface protein 1 gene (msp1) recombinant types, we found that 39% of 59 P. falciparum isolates from Kong Mong Tha had multiple 5' recombinant types with a mean number of 1.54. These values were much lower than those obtained from Mae Sod: 96% for multiple infections and with a mean number of 3.61. Analysis of full-length sequences of two housekeeping genes, the P-type Ca(2+)-transporting ATPase gene (n=33) plus adenylosuccinate lyase gene (n=33), and three vaccine candidate antigen genes, msp1 (n=26), the circumsporozoite protein gene, csp (n=30) and the apical membrane antigen 1 gene, ama 1 (n=32), revealed that in all of these genes within-population SNP diversity was at similar levels between Kong Mong Tha and Mae Sod, suggesting that the extent of MOI and clinical manifestations of malaria are not strongly associated with genetic diversity. Additionally, we did not detect significant genetic differentiation between the two parasite populations, as estimated by the Wright's fixation index of inter-population variance in allele frequencies, suggesting that gene flow prevented the formation of population structuring. Thus, this study highlights unique features of P. falciparum populations in Thailand. The implications of these finding are discussed.

Estimating malaria transmission through mathematical models

Evaluating the effectiveness of malaria control interventions on the basis of their impact on transmission is increasingly important as countries move from malaria control to pre-elimination programs. Mathematical modeling can examine relationships between malaria indicators, allowing translation of easily measured data into measures of transmission, and addressing key concerns with traditional methods for quantifying transmission. Simulations show these indicators are statistically correlated, allowing direct comparisons of malaria transmission using data collected using different methods across a range of transmission intensities and seasonal patterns. Results from such models can provide public health officials with accurate estimates of transmission, by seasonal pattern, that are necessary for assessing and tailoring malaria control and elimination programs to specific settings.

Plasmodium falciparum mitochondrial genetic diversity exhibits isolation-by-distance patterns supporting a sub-Saharan African origin

The geographical distribution of single nucleotide polymorphism (SNP) in the mitochondrial genome of the human malaria parasite Plasmodium falciparum was investigated. We identified 88 SNPs in 516 isolates from seven parasite populations in Africa, Southeast Asia and Oceania. Analysis of the SNPs postulated a sub-Saharan African origin and recovered a strong negative correlation between within-population SNP diversity and geographic distance from the putative African origin over Southeast Asia and Oceania. These results are consistent with those previously obtained for nuclear genome-encoded housekeeping genes, indicating that the pattern of inheritance does not substantially affect the geographical distribution of SNPs.

Entomologic inoculation rates of Anopheles arabiensis in southwestern Ethiopia

We collected anophelines every second week for one year from randomly selected houses in southwestern Ethiopia by using Centers for Disease Control (CDC) light traps, pyrethrum spray catches, and artificial pit shelter constructions to detect circumsporozoite proteins and estimate entomologic inoculation rates (EIRs). Of 3,678 Anopheles arabiensis tested for circumsporozoite proteins, 11 were positive for Plasmodium falciparum and three for P. vivax. The estimated annual P. falciparum EIR of An. arabiensis was 17.1 infectious bites per person per year (95% confidence interval = 7.03-34.6) based on CDC light traps and 0.1 infectious bites per person per year based on pyrethrum spray catches. The P. falciparum EIRs from CDC light traps varied from 0 infectious bites per person per year (in 60% of houses) to 73.2 infectious bites per person per year in the house nearest the breeding sites. Risk of exposure to infectious bites was higher in wet months than dry months, with a peak in April (9.6 infectious bites per person per month), the period of highest mosquito density.

High rates of parasite recrudescence following intermittent preventive treatment with sulphadoxine-pyrimethamine during pregnancy in Benin

BACKGROUND

Despite widespread parasite resistance to sulphadoxine-pyrimethamine (SP) its use for intermittent preventative treatment during pregnancy remains the policy in Benin and throughout most of sub-Saharan Africa.

METHODS

In a prospective study, 982 pregnant women were recruited in Benin and followed until delivery. The prevalence of point mutations in the pfdhfr and pfdhps genes associated with Plasmodium falciparum resistance to SP during consecutive antenatal visits was determined. Parasites clearance among women infected at SP intake was assessed by microscopy and PCR. Association between the persistence of parasites and malaria consequences, were investigated. Recurrent parasites were genotyped to identify recrudescences from re-infections.

RESULTS

The prevalence of pfdhfr/pfdhps quadruple mutants (triple pfdhfr + single pfdhps) was consistently above 80% while quintuple and sextuple mutants remained low. Importantly the higly mutated parasites apparently never included the two key mutations, pfdhfr 164 L or pfdhps 540E. Based on PCR results, SP failed to clear existing parasitaemia in half (48%) of the women who were infected at IPTp schedule. The frequency of recrudescence reached 76% after the second dose. Women with persistent parasitaemia had an increased prevalence of anaemia (P = 0.03).

CONCLUSION

The data presented here, highlight the inability of SP to ensure optimal antiplasmodial protection in late pregnancy, and invite urgent consideration of an alternative drug or strategy.

Ivermectin to reduce malaria transmission: a research agenda for a promising new tool for elimination

Background

The heterogeneity of malaria transmission makes widespread elimination a difficult goal to achieve. Most of the current vector control measures insufficiently target outdoor transmission. Also, insecticide resistance threatens to diminish the efficacy of the most prevalent measures, indoor residual spray and insecticide treated nets. Innovative approaches are needed. The use of endectocides, such as ivermectin, could be an important new addition to the toolbox of anti-malarial measures. Ivermectin effectively targets outdoor transmission, has a novel mechanism of action that could circumvent resistance and might be distributed over the channels already in place for the control of onchocerciasis and lymphatic filariasis.

Methods

The previous works involving ivermectin and Anopheles vectors are reviewed and summarized. A review of ivermectin’s safety profile is also provided. Finally three definitive clinical trials are described in detail and proposed as the evidence needed for implementation. Several smaller and specific supportive studies are also proposed.

Conclusions

The use of ivermectin solves many challenges identified for future vector control strategies. It is an effective and safe endectocide that was approved for human use more than 25 years ago. Recent studies suggest it might become an effective and complementary strategy in malaria elimination and eradication efforts; however, intensive research will be needed to make this a reality.

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

Background

Operational 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.

Methods

Mosquitoes 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.

Results

When 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.

Conclusions

ITT, 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.

Entomological surveillance of behavioural resilience and resistance in residual malaria vector populations

Background

The most potent malaria vectors rely heavily upon human blood so they are vulnerable to attack with insecticide-treated nets (ITNs) and indoor residual spraying (IRS) within houses. Mosquito taxa that can avoid feeding or resting indoors, or by obtaining blood from animals, mediate a growing proportion of the dwindling transmission that persists as ITNs and IRS are scaled up.

Presentation of the hypothesis

Increasing frequency of behavioural evasion traits within persisting residual vector systems usually reflect the successful suppression of the most potent and vulnerable vector taxa by IRS or ITNs, rather than their failure. Many of the commonly observed changes in mosquito behavioural patterns following intervention scale-up may well be explained by modified taxonomic composition and expression of phenotypically plastic behavioural preferences, rather than altered innate preferences of individuals or populations.

Testing the hypothesis

Detailed review of the contemporary evidence base does not yet provide any clear-cut example of true behavioural resistance and is, therefore, consistent with the hypothesis presented.

Implications of the hypothesis

Caution should be exercised before over-interpreting most existing reports of increased frequency of behavioural traits which enable mosquitoes to evade fatal contact with insecticides: this may simply be the result of suppressing the most behaviourally vulnerable of the vector taxa that constituted the original transmission system. Mosquito taxa which have always exhibited such evasive traits may be more accurately described as behaviourally resilient, rather than resistant. Ongoing national or regional entomological monitoring surveys of physiological susceptibility to insecticides should be supplemented with biologically and epidemiologically meaningfully estimates of malaria vector population dynamics and the behavioural phenotypes that determine intervention impact, in order to design, select, evaluate and optimize the implementation of vector control measures.

Evaluation of alternative mosquito sampling methods for malaria vectors in Lowland South--East Zambia

Background

Sampling malaria vectors and measuring their biting density is of paramount importance for entomological surveys of malaria transmission. Human landing catch (HLC) has been traditionally regarded as a gold standard method for surveying human exposure to mosquito bites. However, due to the risk of human participant exposure to mosquito-borne parasites and viruses, a variety of alternative, exposure-free trapping methods were compared in lowland, south-east Zambia.

Methods

Centres for Disease Control and Prevention miniature light trap (CDC-LT), Ifakara Tent Trap model C (ITT-C), resting boxes (RB) and window exit traps (WET) were all compared with HLC using a 3 × 3 Latin Squares design replicated in 4 blocks of 3 houses with long lasting insecticidal nets, half of which were also sprayed with a residual deltamethrin formulation, which was repeated for 10 rounds of 3 nights of rotation each during both the dry and wet seasons.

Results

The mean catches of HLC indoor, HLC outdoor, CDC-LT, ITT-C, WET, RB indoor and RB outdoor, were 1.687, 1.004, 3.267, 0.088, 0.004, 0.000 and 0.008 for Anopheles quadriannulatus Theobald respectively, and 7.287, 6.784, 10.958, 5.875, 0.296, 0.158 and 0.458, for An. funestus Giles, respectively. Indoor CDC-LT was more efficient in sampling An. quadriannulatus and An. funestus than HLC indoor (Relative rate [95% Confidence Interval] = 1.873 [1.653, 2.122] and 1.532 [1.441, 1.628], respectively, P < 0.001 for both). ITT-C was the only other alternative which had comparable sensitivity (RR = 0.821 [0.765, 0.881], P < 0.001), relative to HLC indoor other than CDC-LT for sampling An. funestus.

Conclusions

While the two most sensitive exposure-free techniques primarily capture host-seeking mosquitoes, both have substantial disadvantages for routine community-based surveillance applications: the CDC-LT requires regular recharging of batteries while the bulkiness of ITT-C makes it difficult to move between sampling locations. RB placed indoors or outdoors and WET had consistently poor sensitivity so it may be useful to evaluate additional alternative methods, such as pyrethrum spray catches and back packer aspirators, for catching resting mosquitoes.

Can malaria vector control accelerate the interruption of lymphatic filariasis transmission in Africa; capturing a window of opportunity?

Background

The Global Programme to Eliminate Lymphatic Filariasis (GPELF) was launched in 2000, and nearly all endemic countries in the Americas, Eastern Mediterranean and Asia-Pacific regions have now initiated the WHO recommended mass drug administration (MDA) campaign to interrupt transmission of the parasite. However, nearly 50% of the LF endemic countries in Africa are yet to implement the GPELF MDA strategy, which does not include vector control. Nevertheless, the recent scale up in insecticide treated /long lasting nets (ITNs/LLINs) and indoor residual spraying (IRS) for malaria control in Africa may significantly impact LF transmission because the parasite is transmitted mainly by Anopheles mosquitoes. This study examined the magnitude, geographical extent and potential impact of vector control in the 17 African countries that are yet to or have only recently started MDA.

Methods

National data on mosquito bed nets, ITNs/LLINs and IRS were obtained from published literature, national reports, surveys and datasets from public sources such as Demographic Health Surveys, Malaria Indicator Surveys, Multiple Indicator Cluster Surveys, Malaria Report, Roll Back Malaria and President’s Malaria Initiative websites. The type, number and distribution of interventions were summarised and mapped at sub-national level. and compared with known or potential LF distributions, and those which may be co-endemic with Loa loa and MDA is contraindicated.

Results

Analyses found that vector control activities had increased significantly since 2005, with a three-fold increase in ITN ownership and IRS coverage. However, coverage varied dramatically across the 17 countries; some regions reported >70% ITNs ownership and regular IRS activity, while others had no coverage in remote rural populations where the risk of LF was potentially high and co-endemic with high risk L.loa.

Conclusions

Despite many African countries being slow to initiate MDA for LF, the continued commitment and global financial support for NTDs, and the concurrent expansion of vector control activities for malaria, is promising. It is not beyond the capacity of GPELF to reach its target of global LF elimination by 2020, but monitoring and evaluating the impact of these activities over the next decade will be critical to its success.

A regional-scale, high resolution dynamical malaria model that accounts for population density, climate and surface hydrology

BACKGROUND

The relative roles of climate variability and population related effects in malaria transmission could be better understood if regional-scale dynamical malaria models could account for these factors.

METHODS

A new dynamical community malaria model is introduced that accounts for the temperature and rainfall influences on the parasite and vector life cycles which are finely resolved in order to correctly represent the delay between the rains and the malaria season. The rainfall drives a simple but physically based representation of the surface hydrology. The model accounts for the population density in the calculation of daily biting rates.

RESULTS

Model simulations of entomological inoculation rate and circumsporozoite protein rate compare well to data from field studies from a wide range of locations in West Africa that encompass both seasonal endemic and epidemic fringe areas. A focus on Bobo-Dioulasso shows the ability of the model to represent the differences in transmission rates between rural and peri-urban areas in addition to the seasonality of malaria. Fine spatial resolution regional integrations for Eastern Africa reproduce the malaria atlas project (MAP) spatial distribution of the parasite ratio, and integrations for West and Eastern Africa show that the model grossly reproduces the reduction in parasite ratio as a function of population density observed in a large number of field surveys, although it underestimates malaria prevalence at high densities probably due to the neglect of population migration.

CONCLUSIONS

A new dynamical community malaria model is publicly available that accounts for climate and population density to simulate malaria transmission on a regional scale. The model structure facilitates future development to incorporate migration, immunity and interventions.

Spatio-temporal malaria transmission patterns in Navrongo demographic surveillance site, northern Ghana

BACKGROUND

The relationship between entomological measures of malaria transmission intensity and mortality remains uncertain. This is partly because transmission is heterogeneous even within small geographical areas. Studying this relationship requires high resolution, spatially structured, longitudinal entomological data. Geostatistical models that have been used to analyse the spatio-temporal heterogeneity have not considered the uncertainty in both sporozoite rate (SR) and mosquito density data. This study analysed data from Kassena-Nankana districts in northern Ghana to obtain small area estimates of malaria transmission rates allowing for this uncertainty.

METHODS

Independent Bayesian geostatistical models for sporozoite rate and mosquito density were fitted to produce explicit entomological inoculation rate (EIR) estimates for small areas and short time periods, controlling for environmental factors.

RESULTS

Mosquitoes were trapped from 2,803 unique locations for three years using mainly CDC light traps. Anopheles gambiae constituted 52%, the rest were Anopheles funestus. Mean biting rates for An. funestus and An. gambiae were 32 and 33 respectively. Most bites occurred in September, the wettest month. The sporozoite rates were higher in the dry periods of the last two years compared with the wet period. The annual EIR varied from 1,132 to 157 infective bites. Monthly EIR varied between zero and 388 infective bites. Spatial correlation for SR was lower than that of mosquito densities.

CONCLUSION

This study confirms the presence of spatio-temporal heterogeneity in malaria transmission within a small geographical area. Spatial variance was stronger than temporal especially in the SR. The estimated EIR will be used in mortality analysis for the area.

How much vector control is needed to achieve malaria elimination?

Roll Back Malaria's ambitious goals for global malaria reduction by 2015 represent a dilemma for National Malaria Control Programs (NMCPs) that are still far from malaria elimination. Current vector control efforts by NMCPs generally fall short of their potential, leaving many NMCPs wondering how much vector control it will take to achieve malaria elimination. We believe the answer is detailed in the relationships between the entomological inoculation rate (EIR) and four epidemiological measures of malaria in humans. To achieve adequate vector control, NMCPs must evaluate EIRs to identify problematic foci of transmission and reduce annual EIRs to less than one infectious bite per person.

Geographic coincidence of increased malaria transmission hazard and vulnerability occurring at the periphery of two Tanzanian villages

BACKGROUND

The goal of malaria elimination necessitates an improved understanding of any fine-scale geographic variations in transmission risk so that complementary vector control tools can be integrated into current vector control programmes as supplementary measures that are spatially targeted to maximize impact upon residual transmission. This study examines the distribution of host-seeking malaria vectors at households within two villages in rural Tanzania.

METHODS

Host-seeking mosquitoes were sampled from 72 randomly selected households in two villages on a monthly basis throughout 2008 using CDC light-traps placed beside occupied nets. Spatial autocorrelation in the dataset was examined using the Moran's I statistic and the location of any clusters was identified using the Getis-Ord Gi* statistic. Statistical associations between the household characteristics and clusters of mosquitoes were assessed using a generalized linear model for each species.

RESULTS

For both Anopheles gambiae sensu lato and Anopheles funestus, the density of host-seeking females was spatially autocorrelated, or clustered. For both species, houses with low densities were clustered in the semi-urban village centre while houses with high densities were clustered in the periphery of the villages. Clusters of houses with low or high densities of An. gambiae s.l. were influenced by the number of residents in nearby houses. The occurrence of high-density clusters of An. gambiae s.l. was associated with lower elevations while An. funestus was also associated with higher elevations. Distance from the village centre was also positively correlated with the number of household occupants and having houses constructed with open eaves.

CONCLUSION

The results of the current study highlight that complementary vector control tools could be most effectively targeted to the periphery of villages where the households potentially have a higher hazard (mosquito densities) and vulnerability (open eaves and larger households) to malaria infection.