Child mortality and malaria transmission intensity in Africa

Transmission-Blocking Strategies Against Malaria Parasites During Their Mosquito Stages

Malaria is still the most widespread parasitic disease and causes the most infections globally. Owing to improvements in sanitary conditions and various intervention measures, including the use of antimalarial drugs, the malaria epidemic in many regions of the world has improved significantly in the past 10 years. However, people living in certain underdeveloped areas are still under threat. Even in some well-controlled areas, the decline in malaria infection rates has stagnated or the rates have rebounded because of the emergence and spread of drug-resistant malaria parasites. Thus, new malaria control methods must be developed. As the spread of the Plasmodium parasite is dependent on the part of its life cycle that occurs in mosquitoes, to eliminate the possibility of malaria infections, transmission-blocking strategies against the mosquito stage should be the first choice. In fact, after the gametocyte enters the mosquito body, it undergoes a series of transformation processes over a short period, thus providing numerous potential blocking targets. Many research groups have carried out studies based on targeting the blocking of transmission during the mosquito phase and have achieved excellent results. Meanwhile, the direct killing of mosquitoes could also significantly reduce the probability of malaria infections. Microorganisms that display complex interactions with Plasmodium, such as Wolbachia and gut flora, have shown observable transmission-blocking potential. These could be used as a biological control strategy and play an important part in blocking the transmission of malaria.

Mosquito Net Use in Early Childhood and Survival to Adulthood in Tanzania

BACKGROUND

It has been hypothesized that in high-transmission settings, malaria control in early childhood (<5 years of age) might delay the acquisition of functional immunity and shift child deaths from younger to older ages.

METHODS

We used data from a 22-year prospective cohort study in rural southern Tanzania to estimate the association between early-life use of treated nets and survival to adulthood. All the children born between January 1, 1998, and August 30, 2000, in the study area were invited to enroll in a longitudinal study from 1998 through 2003. Adult survival outcomes were verified in 2019 through community outreach and mobile telephones. We used Cox proportional-hazards models to estimate the association between the use of treated nets in early childhood and survival to adulthood, adjusting for potential confounders.

RESULTS

A total of 6706 children were enrolled. In 2019, we verified information on the vital status of 5983 participants (89%). According to reports of early-life community outreach visits, approximately one quarter of children never slept under a treated net, one half slept under a treated net some of the time, and the remaining quarter always slept under a treated net. Participants who were reported to have used treated nets at half the early-life visits or more had a hazard ratio for death of 0.57 (95% confidence interval [CI], 0.45 to 0.72) as compared with those who were reported to have used treated nets at less than half the visits. The corresponding hazard ratio between 5 years of age and adulthood was 0.93 (95% CI, 0.58 to 1.49).

CONCLUSIONS

In this long-term study of early-life malaria control in a high-transmission setting, the survival benefit from early-life use of treated nets persisted to adulthood. (Funded by the Eckenstein-Geigy Professorship and others.).

Key Characteristics of Residual Malaria Transmission in Two Districts in South-Eastern Tanzania-Implications for Improved Control

After 2 decades of using insecticide-treated nets (ITNs) and improved case management, malaria burden in the historically-holoendemic Kilombero valley in Tanzania has significantly declined. We review key characteristics of the residual transmission and recommend options for improvement. Transmission has declined by >10-fold since 2000 but remains heterogeneous over small distances. Following the crash of Anopheles gambiae, which coincided with ITN scale-up around 2005-2012, Anopheles funestus now dominates malaria transmission. While most infections still occur indoors, substantial biting happens outdoors and before bed-time. There is widespread resistance to pyrethroids and carbamates; An. funestus being particularly strongly-resistant. In short and medium-term, these challenges could be addressed using high-quality indoor residual spraying with nonpyrethroids, or ITNs incorporating synergists. Supplementary tools, eg, spatial-repellents may expand protection outdoors. However, sustainable control requires resilience-building approaches, particularly improved housing and larval-source management to suppress mosquitoes, stronger health systems guaranteeing case-detection and treatment, greater community-engagement and expanded health education.

Estimation of the primary, secondary and composite effects of malaria vaccines using data on multiple clinical malaria episodes

BACKGROUND

An effective malaria vaccine affects the risk of malaria directly, through the vaccine-induced immune response (the primary effect), and indirectly, as a consequence of reduced exposure to malaria infection and disease, leading to slower acquisition of natural immunity (the secondary effect). The beneficial primary effect may be offset by a negative secondary effect, resulting in a smaller or nil composite effect. Reports of malaria vaccine trials usually present only the composite effect. We aimed to demonstrate how the primary and secondary effects can also be estimated from trial data.

METHODS

We propose an enhancement to the conditional frailty model for the estimation of primary effect using data on disease episodes. We use the Andersen-Gill model to estimate the composite effect. We consider taking the ratio of the hazard ratios to estimate the secondary effect. We used directed acyclic graphs and data from a randomized trial of the RTS,S/AS02 malaria vaccine to illustrate the problems and solutions. Time-varying effects were estimated by partitioning the follow-up into four time periods.

RESULTS

The primary effect estimates from our proposed model were consistently stronger than the conditional frailty model in the existing literature. The primary effect of the vaccine was consistently stronger than the composite effect across all time periods. Both the primary and composite effects were stronger in the first three months, with hazard ratios (95% confidence interval) 0.62 (0.49-0.79) and 0.68 (0.54-0.84), respectively; the hazard ratios weakened over time. The secondary effect appeared mild, with hazard ratio 1.09 (1.02-1.16) in the first three months.

CONCLUSIONS

The proposed analytic strategy facilitates a more comprehensive interpretation of trial data on multiple disease episodes. The RTS,S/AS02 vaccine had modest primary and secondary effects that waned over time, but the composite effect in preventing clinical malaria remained positive up to the end of the study.

CLINICAL TRIALS REGISTRATION

ClinicalTrials.gov NCT00197041.

Cerebral Plasmodium falciparum malaria: The role of PfEMP1 in its pathogenesis and immunity, and PfEMP1-based vaccines to prevent it

Malaria, a mosquito-borne infectious disease caused by parasites of the genus Plasmodium continues to be a major health problem worldwide. The unicellular Plasmodium-parasites have the unique capacity to infect and replicate within host erythrocytes. By expressing variant surface antigens Plasmodium falciparum has evolved to avoid protective immune responses; as a result in endemic areas anti-malaria immunity develops gradually over many years of multiple and repeated infections. We are studying the role of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed by asexual stages of P. falciparum responsible for the pathogenicity of severe malaria. The immunopathology of falciparum malaria has been linked to cyto-adhesion of infected erythrocytes to specific host receptors. A greater appreciation of the PfEMP1 molecules important for the development of protective immunity and immunopathology is a prerequisite for the rational discovery and development of a safe and protective anti-disease malaria vaccine. Here we review the role of ICAM-1 and EPCR receptor adhering falciparum-parasites in the development of severe malaria; we discuss our current research to understand the factors involved in the pathogenesis of cerebral malaria and the feasibility of developing a vaccine targeted specifically to prevent this disease.

Modelling the relationship between malaria prevalence as a measure of transmission and mortality across age groups

Background

Parasite prevalence has been used widely as a measure of malaria transmission, especially in malaria endemic areas. However, its contribution and relationship to malaria mortality across different age groups has not been well investigated. Previous studies in a health and demographic surveillance systems (HDSS) platform in western Kenya quantified the contribution of incidence and entomological inoculation rates (EIR) to mortality. The study assessed the relationship between outcomes of malaria parasitaemia surveys and mortality across age groups.

Methods

Parasitological data from annual cross-sectional surveys from the Kisumu HDSS between 2007 and 2015 were used to determine malaria parasite prevalence (PP) and clinical malaria (parasites plus reported fever within 24 h or temperature above 37.5 °C). Household surveys and verbal autopsy (VA) were used to obtain data on all-cause and malaria-specific mortality. Bayesian negative binomial geo-statistical regression models were used to investigate the association of PP/clinical malaria with mortality across different age groups. Estimates based on yearly data were compared with those from aggregated data over 4 to 5-year periods, which is the typical period that mortality data are available from national demographic and health surveys.

Results

Using 5-year aggregated data, associations were established between parasite prevalence and malaria-specific mortality in the whole population (RR_malaria = 1.66; 95% Bayesian Credible Intervals: 1.07–2.54) and children 1–4 years (RR_malaria = 2.29; 1.17–4.29). While clinical malaria was associated with both all-cause and malaria-specific mortality in combined ages (RR_all-cause = 1.32; 1.01–1.74); (RR_malaria = 2.50; 1.27–4.81), children 1–4 years (RR_all-cause = 1.89; 1.00–3.51); (RR_malaria = 3.37; 1.23–8.93) and in older children 5–14 years (RR_all-cause = 3.94; 1.34–11.10); (RR_malaria = 7.56; 1.20–39.54), no association was found among neonates, adults (15–59 years) and the elderly (60+ years). Distance to health facilities, socioeconomic status, elevation and survey year were important factors for all-cause and malaria-specific mortality.

Conclusion

Malaria parasitaemia from cross-sectional surveys was associated with mortality across age groups over 4 to 5 year periods with clinical malaria more strongly associated with mortality than parasite prevalence. This effect was stronger in children 5–14 years compared to other age-groups. Further analyses of data from other HDSS sites or similar platforms would be useful in investigating the relationship between malaria and mortality across different endemicity levels.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2869-9) contains supplementary material, which is available to authorized users.

A Multi-Stage Plasmodium vivax Malaria Vaccine Candidate Able to Induce Long-Lived Antibody Responses Against Blood Stage Parasites and Robust Transmission-Blocking Activity

Malaria control and interventions including long-lasting insecticide-treated nets, indoor residual spraying, and intermittent preventative treatment in pregnancy have resulted in a significant reduction in the number of Plasmodium falciparum cases. Considerable efforts have been devoted to P. falciparum vaccines development with much less to P. vivax. Transmission-blocking vaccines, which can elicit antibodies targeting Plasmodium antigens expressed during sexual stage development and interrupt transmission, offer an alternative strategy to achieve malaria control. The post-fertilization antigen P25 mediates several functions essential to ookinete survival but is poorly immunogenic in humans. Previous clinical trials targeting this antigen have suggested that conjugation to a carrier protein could improve the immunogenicity of P25. Here we report the production, and characterization of a vaccine candidate composed of a chimeric P. vivax Merozoite Surface Protein 1 (cPvMSP1) genetically fused to P. vivax P25 (Pvs25) designed to enhance CD4+ T cell responses and its assessment in a murine model. We demonstrate that antibodies elicited by immunization with this chimeric protein recognize both the erythrocytic and sexual stages and are able to block the transmission of P. vivax field isolates in direct membrane-feeding assays. These findings provide support for the continued development of multi-stage transmission blocking vaccines targeting the life-cycle stage responsible for clinical disease and the sexual-stage development accountable for disease transmission simultaneously.

Dramatic decreases of malaria transmission intensities in Ifakara, south-eastern Tanzania since early 2000s

BACKGROUND

Ongoing epidemiological transitions across Africa are particularly evident in fast-growing towns, such as Ifakara in the Kilombero valley, south-eastern Tanzania. This town and its environs (population ~ 70,000) historically experienced moderate to high malaria transmission, mediated mostly by Anopheles gambiae and Anopheles funestus. In early 2000s, malaria transmission [Plasmodium falciparum entomological inoculation rate (PfEIR)] was estimated at ~ 30 infectious bites/person/year (ib/p/yr). This study assessed the PfEIR after 15 years, during which there had been rapid urbanization and expanded use of insecticide-treated nets (ITNs).

METHODS

Randomly-selected 110 households were sampled across Ifakara town and four adjacent wards. Mosquitoes were trapped nightly or monthly (June.2015-May.2016) using CDC-light-traps indoors, Suna^® traps outdoors and human landing catches (HLC) indoors and outdoors. All Anopheles mosquitoes were morphologically identified and analysed by ELISA for Plasmodium circumsporozoite proteins. Mosquito blood meals were identified using ELISA, and sub-samples of An. gambiae and An. funestus examined by PCR to distinguish morphologically-similar siblings. Insecticide resistance was assessed using WHO-susceptibility assays, and some Anopheles were dissected to examine ovariole tracheoles for parity.

RESULTS

After 3572 trap-nights, one Plasmodium-infected Anopheles was found (an An. funestus caught outdoors in Katindiuka-ward by HLC), resulting in overall PfEIR of 0.102 ib/p/yr. Nearly 80% of malaria vectors were from Katindiuka and Mlabani wards. Anopheles gambiae densities were higher outdoors (64%) than indoors (36%), but no such difference was observed for An. funestus. All An. funestus and 75% of An. gambiae dissected were parous. Anopheles gambiae complex consisted entirely of Anopheles arabiensis, while An. funestus included 84.2% An. funestus s.s., 4.5% Anopheles rivulorum, 1.4% Anopheles leesoni and 9.9% with unamplified-DNA. Anopheles gambiae were susceptible to bendiocarb and malathion, but resistant to pyrethroids, DDT and pirimiphos-methyl. Most houses had brick walls and/or iron roofs (> 90%), and 52% had screened windows.

CONCLUSION

Malaria transmission in Ifakara has decreased by > 99% since early-2000s, reaching levels nearly undetectable with current entomological methods. These declines are likely associated with ITNs use, urbanization and improved housing. Remaining risk is now mostly in peri-urban wards, but concerted efforts could further decrease local transmission. Parasitological surveys are required to assess actual prevalence, incidence and importation rates.

Quantification of anti-parasite and anti-disease immunity to malaria as a function of age and exposure

Fundamental gaps remain in our understanding of how immunity to malaria develops. We used detailed clinical and entomological data from parallel cohort studies conducted across the malaria transmission spectrum in Uganda to quantify the development of immunity against symptomatic P. falciparum as a function of age and transmission intensity. We focus on: anti-parasite immunity (i.e. ability to control parasite densities) and anti-disease immunity (i.e. ability to tolerate higher parasite densities without fever). Our findings suggest a strong effect of age on both types of immunity, not explained by cumulative-exposure. They also show an independent effect of exposure, where children living in moderate/high transmission settings develop immunity faster as transmission increases. Surprisingly, children in the lowest transmission setting appear to develop immunity more efficiently than those living in moderate transmission settings. Anti-parasite and anti-disease immunity develop in parallel, reducing the probability of experiencing symptomatic malaria upon each subsequent P. falciparum infection.

Infant and child mortality in relation to malaria transmission in KEMRI/CDC HDSS, Western Kenya: validation of verbal autopsy

Background

Malaria transmission reduction is a goal of many malaria control programmes. Little is known of how much mortality can be reduced by specific reductions in transmission. Verbal autopsy (VA) is widely used for estimating malaria specific mortality rates, but does not reliably distinguish malaria from other febrile illnesses. Overall malaria attributable mortality includes both direct and indirect deaths. It is unclear what proportion of the deaths averted by reducing malaria transmission are classified as malaria in VA.

Methods

Both all-cause, and cause-specific mortality reported by VA for children under 5 years of age, were assembled from the KEMRI/CDC health and demographic surveillance system in Siaya county, rural Western Kenya for the years 2002–2004. These were linked to household-specific estimates of the Plasmodium falciparum entomological inoculation rate (EIR) based on high resolution spatio-temporal geostatistical modelling of entomological data. All-cause and malaria specific mortality (by VA), were analysed in relation to EIR, insecticide-treated net use (ITN), socioeconomic status (SES) and parameters describing space–time correlation. Time at risk for each child was analysed using Bayesian geostatistical Cox proportional hazard models, with time-dependent covariates. The outputs were used to estimate the diagnostic performance of VA in measuring mortality that can be attributed to malaria exposure.

Results

The overall under-five mortality rate was 80 per 1000 person-years during the study period. Eighty-one percent of the total deaths were assigned causes of death by VA, with malaria assigned as the main cause of death except in the neonatal period. Although no trend was observed in malaria-specific mortality assessed by VA, ITN use was associated with reduced all-cause mortality in infants (hazard ratio 0.15, 95% CI 0.02, 0.63) and the EIR was strongly associated with both all-cause and malaria-specific mortality. 48.2% of the deaths could be attributed to malaria by analysing the exposure–response relationship, though only 20.5% of VAs assigned malaria as the cause and the sensitivity of VAs was estimated to be only 26%. Although VAs assigned some deaths to malaria even in areas where there was estimated to be no exposure, the specificity of the VAs was estimated to be 85%.

Conclusion

Interventions that reduce P. falciparum transmission intensity will not only significantly reduce malaria-diagnosed mortality, but also mortality assigned to other causes in under-5 year old children in endemic areas. In this setting, the VA tool based on clinician review substantially underestimates the number of deaths that could be averted by reducing malaria exposure in childhood, but has a reasonably high specificity. This suggests that malaria transmission-reducing interventions such as ITNs can potentially reduce overall child mortality by as much as twice the total direct malaria burden estimated from VAs.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2184-x) contains supplementary material, which is available to authorized users.

Functional characterization of Plasmodium berghei PSOP25 during ookinete development and as a malaria transmission-blocking vaccine candidate

Background

Plasmodium ookinete surface proteins as post-fertilization target antigens are potential malaria transmission-blocking vaccine (TBV) candidates. Putative secreted ookinete protein 25 (PSOP25) is a highly conserved ookinete surface protein, and has been shown to be a promising novel TBV target. Here, we further investigated the TBV activities of the full-length recombinant PSOP25 (rPSOP25) protein in Plasmodium berghei, and characterized the potential functions of PSOP25 during the P. berghei life-cycle.

Methods

We expressed the full-length P. berghei PSOP25 protein in a prokaryotic expression system, and developed polyclonal mouse antisera and a monoclonal antibody (mAb) against the recombinant protein. Indirect immunofluorescence assay (IFA) and Western blot were used to test the specificity of antibodies. The transmission-blocking (TB) activities of antibodies were evaluated by the in vitro ookinete conversion assay and by direct mosquito feeding assay (DFA). Finally, the function of PSOP25 during Plasmodium development was studied by deleting the psop25 gene.

Results

Both polyclonal mouse antisera and anti-rPSOP25 mAb recognized the PSOP25 proteins in the parasites, and IFA showed the preferential expression of PSOP25 on the surface of zygotes, retorts and mature ookinetes. In vitro, these antibodies significantly inhibited ookinetes formation in an antibody concentration-dependent manner. In DFA, mice immunized with the rPSOP25 and those receiving passive transfer of the anti-rPSOP25 mAb reduced the prevalence of mosquito infection by 31.2 and 26.1%, and oocyst density by 66.3 and 63.3%, respectively. Genetic knockout of the psop25 gene did not have a detectable impact on the asexual growth of P. berghei, but significantly affected the maturation of ookinetes and the formation of midgut oocysts.

Conclusions

The full-length rPSOP25 could elicit strong antibody response in mice. Polyclonal and monoclonal antibodies against PSOP25 could effectively block the formation of ookinetes in vitro and transmission of the parasites to mosquitoes. Genetic manipulation study indicated that PSOP25 is required for ookinete maturation in P. berghei. These results support further testing of the PSOP25 orthologs in human malaria parasites as promising TBV candidates.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1932-4) contains supplementary material, which is available to authorized users.

Assessing the effects of malaria interventions on the geographical distribution of parasitaemia risk in Burkina Faso

BACKGROUND

Burkina Faso conducted its first nationally representative household malaria survey in 2010/2011. The survey collected among others, information on malaria interventions, treatment choices and malaria parasite prevalence in children aged 6-59 months.

METHODS

In this study, Bayesian geostatistical models were employed to assess the effects of health interventions related to insecticide-treated bed nets (ITN), indoor residual spray (IRS), artemisinin-based combination therapy (ACT) coverage associated with childhood malaria parasite risk at national and sub-national level, after taking into account geographical disparities of climatic/environmental and socio-economic factors. Several ITN coverage measures were calculated and Bayesian variable selection was used to identify the most important ones. Parasitaemia risk depicting spatial patterns of infections were estimated.

RESULTS

The results show that the predicted population-adjusted parasitaemia risk ranges from 4.04 % in Kadiogo province to 82 % in Kompienga province. The effect of ITN coverage was not important at national level; however ITNs have an important protective effect in Ouagadougou as well as in three districts in the western part of the country with high parasitaemia prevalence and low to moderate coverage. There is a large variation in ACT coverage between the districts. Although at national level the ACT effects on parasitaemia risk was not important, at sub-national level 18 districts around Ouagadougou deliver effective treatment.

CONCLUSION

The produced maps show great variations in parasitaemia risk across the country and identify the districts where interventions are being effective. These outputs are valuable tools that can help improve malaria control in Burkina Faso.

Narrative review of current context of malaria and management strategies in Uganda (Part I)

In accordance with international targets, the Uganda National Malaria Control Strategic Plan established specific targets to be achieved by 2010. For children under five, this included increasing the number of children sleeping under mosquito nets and those receiving a first-line antimalarial to 85%, and decreasing case fatality to 2%. This narrative review offers contextual information relevant to malaria management in Uganda since the advent of artemisinin combination therapy (ACT) as first-line antimalarial treatment in 2004. A comprehensive search using key words and phrases was conducted using the web search engines Google and Google Scholar, as well as the databases of PubMed, ERIC, EMBASE, CINAHL, OvidSP (MEDLINE), PSYC Info, Springer Link, Cochrane Central Register of Controlled Trials (CENTRAL), and Cochrane Database of Systematic Reviews were searched. A total of 147 relevant international and Ugandan literature sources meeting the inclusion criteria were included. This review provides an insightful understanding on six topic areas: global and local priorities, malarial pathology, disease burden, malaria control, treatment guidelines for uncomplicated malaria, and role of the health system in accessing antimalarial medicines. Plasmodium falciparum remains the most common cause of malaria in Uganda, with children under five being most vulnerable due to their underdeveloped immunity. While international efforts to scale up malaria control measures have resulted in considerable decline in malaria incidence and mortality in several regions of sub-Saharan Africa, this benefit has yet to be substantiated for Uganda. At the local level, key initiatives have included implementation of a new antimalarial drug policy in 2004 and strengthening of government health systems and programs. Examples of such programs include removal of user fees, training of frontline health workers, providing free ACT from government systems and subsidized ACT from licensed private outlets, and introduction of the integrated community case management program to bring diagnostics and treatment for malaria, pneumonia and diarrhea closer to the community. However despite notable efforts, Uganda is far from achieving its 2010 targets. Several challenges in the delivery of care and treatment remain, with those most vulnerable and living in rural settings remaining at greatest risk from malaria morbidity and mortality.

Plasmodium vivax gametocyte proteins, Pvs48/45 and Pvs47, induce transmission-reducing antibodies by DNA immunization

Malaria transmission-blocking vaccines (TBV) aim to interfere with the development of the malaria parasite in the mosquito vector, and thus prevent spread of transmission in a community. To date three TBV candidates have been identified in Plasmodium vivax; namely, the gametocyte/gamete protein Pvs230, and the ookinete surface proteins Pvs25 and Pvs28. The Plasmodium falciparum gametocyte/gamete stage proteins Pfs48/45 and Pfs47 have been studied as TBV candidates, and Pfs48/45 shown to induce transmission-blocking antibodies, but the candidacy of their orthologs in P. vivax, Pvs48/45 (PVX_083235) and Pvs47 (PVX_083240), for vivax TBV have not been tested. Herein we investigated whether targeting Pvs48/45 and Pvs47 can inhibit parasite transmission to mosquitoes, using P. vivax isolates obtained in Thailand. Mouse antisera directed against the products from plasmids expressing Pvs48/45 and Pvs47 detected proteins of approximately 45- and 40-kDa, respectively, in the P. vivax gametocyte lysate, by Western blot analysis under non-reducing conditions. In immunofluorescence assays Pvs48/45 was detected predominantly on the surface and Pvs47 was detected in the cytoplasm of gametocytes. Membrane feeding transmission assays demonstrated that anti-Pvs48/45 and -Pvs47 mouse sera significantly reduced the number of P. vivax oocysts developing in the mosquito midgut. Limited amino acid polymorphism of these proteins was observed among 27 P. vivax isolates obtained from Thailand, Vanuatu, and Colombia; suggesting that polymorphism may not be an impediment for the utilization of Pvs48/45 and Pvs47 as TBV antigens. In one Thai isolate we found that the fourth cysteine residue in the Pvs47 cysteine-rich domain (CRD) III (amino acid position 337) is substituted to phenylalanine. However, antibodies targeting Pvs47 CRDI-III showed a significant transmission-reducing activity against this isolate, suggesting that this substitution in Pvs47 was not critical for recognition by the generated antibodies. In conclusion, our results indicate that Pvs48/45 and Pvs47 are potential transmission-blocking vaccine candidates of P. vivax.

Gradual acquisition of immunity to severe malaria with increasing exposure

Previous analyses have suggested that immunity to non-cerebral severe malaria due to Plasmodium falciparum is acquired after only a few infections, whereas longitudinal studies show that some children experience multiple episodes of severe disease, suggesting that immunity may not be acquired so quickly. We fitted a mathematical model for the acquisition and loss of immunity to severe disease to the age distribution of severe malaria cases stratified by symptoms from a range of transmission settings in Tanzania, combined with data from several African countries on the age distribution and overall incidence of severe malaria. We found that immunity to severe disease was acquired more gradually with exposure than previously thought. The model also suggests that physiological changes, rather than exposure, may alter the symptoms of disease with increasing age, suggesting that a later age at infection would be associated with a higher proportion of cases presenting with cerebral malaria regardless of exposure. This has consequences for the expected pattern of severe disease as transmission changes. Careful monitoring of the decline in immunity associated with reduced transmission will therefore be needed to ensure rebound epidemics of severe and fatal malaria are avoided.

Evaluation of non-instrumented nucleic acid amplification by loop-mediated isothermal amplification (NINA-LAMP) for the diagnosis of malaria in Northwest Ethiopia

Background

Malaria is a major public health problem in sub-Saharan African countries including Ethiopia. Early and accurate diagnosis followed by prompt and effective treatment is among the various tools available for prevention, control and elimination of malaria. This study aimed to evaluate the performance of non-instrumented nucleic acid amplification loop-mediated isothermal amplification (NINA-LAMP) compared to standard thick and thin film microscopy and nested PCR as gold standard for the sensitive diagnosis of malaria in Northwest Ethiopia.

Methods

A cross-sectional study was conducted in North Gondar, Ethiopia from March to July 2014. Eighty-two blood samples were collected from malaria suspected patients visiting Kola Diba Health Centre and analysed for Plasmodium parasites by microscopy, NINA-LAMP and nested PCR. The NINA-LAMP method was performed using the Loopamp™ Malaria Pan/Pf detection kits for detecting DNA of the genus Plasmodium and more specifically Plasmodium falciparum using an electricity-free heater. Diagnostic accuracy outcome measures (analytical sensitivity, specificity, predictive values, and Kappa scores) of NINA-LAMP and microscopy were compared to nested PCR.

Results

A total of 82 samples were tested in the primary analysis. Using nested PCR as reference, the sensitivity and specificity of the primary NINA-LAMP assay were 96.8% (95% confidence interval (CI), 83.2% - 99.5%) and 84.3% (95% CI, 71.4% - 92.9%), respectively for detection of Plasmodium genus, and 100% (95% CI, 75.1% - 100%) and 81.2% (95% CI, 69.9% - 89.6%), respectively for detection of P. falciparum parasite. Microscopy demonstrated sensitivity and specificity of 93.6% (95% CI, 78.5% - 99.0%) and 98.0% (95% CI, 89.5% - 99.7%), respectively for the detection of Plasmodium parasites. Post-hoc repeat NINA-LAMP analysis showed improvement in diagnostic accuracy, which was comparable to nested PCR performance and superior to microscopy for detection at both the Plasmodium genus level and P. falciparum parasites.

Conclusion

NINA-LAMP is highly sensitive for the diagnosis of malaria and detection of Plasmodium parasite infection at both the genus and species level when compared to nested PCR. NINA-LAMP is more sensitive than microscopy for the detection of P. falciparum and differentiation from non-falciparum species and may be a critical diagnostic modality in efforts to eradicate malaria from areas of low endemicity.

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.

Relationship between child survival and malaria transmission: an analysis of the malaria transmission intensity and mortality burden across Africa (MTIMBA) project data in Rufiji demographic surveillance system, Tanzania

BACKGROUND

The precise nature of the relationship between malaria mortality and levels of transmission is unclear. Due to methodological limitations, earlier efforts to assess the linkage have lead to inconclusive results. The malaria transmission intensity and mortality burden across Africa (MTIMBA) project initiated by the INDEPTH Network collected longitudinally entomological data within a number of sites in sub-Saharan Africa to study this relationship. This work linked the MTIMBA entomology database with the routinely collected vital events within the Rufiji Demographic Surveillance System to analyse the transmission-mortality relation in the region.

METHODS

Bayesian Bernoulli spatio-temporal Cox proportional hazards models with village clustering, adjusted for age and insecticide-treated nets (ITNs), were fitted to assess the relation between mortality and malaria transmission measured by entomology inoculation rate (EIR). EIR was predicted at household locations using transmission models and it was incorporated in the model as a covariate with measure of uncertainty. Effects of covariates estimated by the model are reported as hazard ratios (HR) with 95% Bayesian confidence interval (BCI) and spatial and temporal parameters are presented.

RESULTS

Separate analysis was carried out for neonates, infants and children 1-4 years of age. No significant relation between all-cause mortality and intensity of malaria transmission was indicated at any age in childhood. However, a strong age effect was shown. Comparing effects of ITN and EIR on mortality at different age categories, a decrease in protective efficacy of ITN was observed (i.e. neonates: HR = 0.65; 95% BCI:0.39-1.05; infants: HR = 0.72; 95% BCI:0.48-1.07; children 1-4 years: HR = 0.88; 95% BCI:0.62-1.23) and reduction on the effect of malaria transmission exposure was detected (i.e. neonates: HR = 1.15; 95% BCI:0.95-1.36; infants: HR = 1.13; 95% BCI:0.98-1.25; children 1-4 years: HR = 1.04; 95% BCI:0.89-1.18). A very strong spatial correlation was also observed.

CONCLUSION

These results imply that assessing the malaria transmission-mortality relation involves more than the knowledge on the performance of interventions and control measures. This relation depends on the levels of malaria endemicity and transmission intensity, which varies significantly between different settings. Thus, sub-regions analyses are necessary to validate and assess reproducibility of findings.

Repeated mass distributions and continuous distribution of long-lasting insecticidal nets: modelling sustainability of health benefits from mosquito nets, depending on case management

Background

Stagnating funds for malaria control have spurred interest in the question of how to sustain the gains of recent successes with long-lasting insecticidal nets (LLINs) and improved case management (CM). This simulation study examined the malaria transmission and disease dynamics in scenarios with sustained LLINs and CM interventions and tried to determine optimal LLIN distribution rates. The effects of abruptly halting LLIN distribution were also examined.

Methods

Dynamic simulations of malaria in humans and mosquitoes were run on the OpenMalaria platform, using stochastic individual-based simulation models. LLINs were distributed in a range of transmission settings, with varying CM coverage levels.

Results

In the short-term, LLINs were beneficial over the entire transmission spectrum, reducing both transmission and disease burden. In the long-term, repeated distributions sustainably reduced transmission in all settings. However, because of the resulting reduction in acquired immunity in the population, the malaria disease burden, after initially being reduced, gradually increased and eventually stabilized at a new level. This new level was higher than the pre-intervention level in previously high transmission settings, if there is a maximum disease burden in the relationship between transmission and disease burden at intermediate transmission levels. This result could lead one to conclude that sustained LLIN distribution might not be cost-effective in high transmission settings in the long term. However, improved CM rendered LLINs more cost-effective in higher transmission settings than in those without improved CM and the majority of the African population lives in areas where CM and LLINs are sustainably combined. The effects of changes in LLIN distribution rate on cost-effectiveness were relatively small compared to the effects of changes in transmission setting and CM. Abruptly halting LLIN distribution led to temporary morbidity peaks, which were particularly large in low to intermediate transmission settings.

Conclusions

This study reaffirms the importance of context specific intervention planning. Intervention planning must include combinations of malaria vector control and CM, and must consider both the pre-intervention transmission level and the intervention history to account for the loss of immunity and the potential for rebounds in disease burden.

Density-dependent blood stage Plasmodium falciparum suppresses malaria super-infection in a malaria holoendemic population

Recent studies of Plasmodium berghei malaria in mice show that high blood-stage parasitemia levels inhibit the development of subsequent liver-stage infections. Whether a similar inhibitory effect on liver-stage Plasmodium falciparum by blood-stage infection occurs in humans is unknown. We have analyzed data from a treatment-time-to-infection cohort of children < 10 years of age residing in a malaria holoendemic area of Kenya where people experience a new blood-stage infection approximately every 2 weeks. We hypothesized that if high parasitemia blocked the liver stage, then high levels of parasitemia should be followed by a "skipped" peak of parasitemia. Statistical analysis of "natural infection" field data and stochastic simulation of infection dynamics show that the data are consistent with high P. falciparum parasitemia inhibiting liver-stage parasite development in humans.

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.

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.

Anthropogenic habitat disturbance and ecological divergence between incipient species of the malaria mosquito Anopheles gambiae

BACKGROUND

Anthropogenic habitat disturbance is a prime cause in the current trend of the Earth's reduction in biodiversity. Here we show that the human footprint on the Central African rainforest, which is resulting in deforestation and growth of densely populated urban agglomerates, is associated to ecological divergence and cryptic speciation leading to adaptive radiation within the major malaria mosquito Anopheles gambiae.

METHODOLOGY/PRINCIPAL FINDINGS

In southern Cameroon, the frequency of two molecular forms--M and S--among which reproductive isolation is strong but still incomplete, was correlated to an index of urbanisation extracted from remotely sensed data, expressed as the proportion of built-up surface in each sampling unit. The two forms markedly segregated along an urbanisation gradient forming a bimodal cline of ∼6-km width: the S form was exclusive to the rural habitat, whereas only the M form was present in the core of densely urbanised settings, co-occurring at times in the same polluted larval habitats of the southern house mosquito Culex quinquefasciatus--a species association that was not historically recorded before.

CONCLUSIONS/SIGNIFICANCE

Our results indicate that when humans create novel habitats and ecological heterogeneities, they can provide evolutionary opportunities for rapid adaptive niche shifts associated with lineage divergence, whose consequences upon malaria transmission might be significant.

A new world malaria map: Plasmodium falciparum endemicity in 2010

BACKGROUND

Transmission intensity affects almost all aspects of malaria epidemiology and the impact of malaria on human populations. Maps of transmission intensity are necessary to identify populations at different levels of risk and to evaluate objectively options for disease control. To remain relevant operationally, such maps must be updated frequently. Following the first global effort to map Plasmodium falciparum malaria endemicity in 2007, this paper describes the generation of a new world map for the year 2010. This analysis is extended to provide the first global estimates of two other metrics of transmission intensity for P. falciparum that underpin contemporary questions in malaria control: the entomological inoculation rate (PfEIR) and the basic reproductive number (PfR).

METHODS

Annual parasite incidence data for 13,449 administrative units in 43 endemic countries were sourced to define the spatial limits of P. falciparum transmission in 2010 and 22,212 P. falciparum parasite rate (PfPR) surveys were used in a model-based geostatistical (MBG) prediction to create a continuous contemporary surface of malaria endemicity within these limits. A suite of transmission models were developed that link PfPR to PfEIR and PfR and these were fitted to field data. These models were combined with the PfPR map to create new global predictions of PfEIR and PfR. All output maps included measured uncertainty.

RESULTS

An estimated 1.13 and 1.44 billion people worldwide were at risk of unstable and stable P. falciparum malaria, respectively. The majority of the endemic world was predicted with a median PfEIR of less than one and a median PfRc of less than two. Values of either metric exceeding 10 were almost exclusive to Africa. The uncertainty described in both PfEIR and PfR was substantial in regions of intense transmission.

CONCLUSIONS

The year 2010 has a particular significance as an evaluation milestone for malaria global health policy. The maps presented here contribute to a rational basis for control and elimination decisions and can serve as a baseline assessment as the global health community looks ahead to the next series of milestones targeted at 2015.

Malaria morbidity and pyrethroid resistance after the introduction of insecticide-treated bednets and artemisinin-based combination therapies: a longitudinal study

BACKGROUND

Substantial reductions in malaria have been reported in several African countries after distribution of insecticide-treated bednets and the use of artemisinin-based combination therapies (ACTs). Our aim was to assess the effect of these policies on malaria morbidity, mosquito populations, and asymptomatic infections in a west African rural population.

METHODS

We did a longitudinal study of inhabitants of Dielmo village, Senegal, between January, 2007, and December, 2010. We monitored the inhabitants for fever during this period and we treated malaria attacks with artesunate plus amodiaquine. In July, 2008, we offered longlasting insecticide (deltamethrin)-treated nets (LLINs) to all villagers. We did monthly night collections of mosquitoes during the whole study period, and we assessed asymptomatic carriage from cross-sectional surveys. Our statistical analyses were by negative binomial regression, logistic regression, and binomial or Fisher exact test.

FINDINGS

There were 464 clinical malaria attacks attributable to Plasmodium falciparum during 17,858 person-months of follow-up. The incidence density of malaria attacks averaged 5·45 (95% CI 4·90-6·05) per 100 person-months between January, 2007, and July, 2008, before the distribution of LLINs. Incidence density decreased to 0·41 (0·29-0·55) between August, 2008, and August, 2010, but increased back to 4·57 (3·54-5·82) between September and December, 2010--ie, 27-30 months after the distribution of LLINs. The rebound of malaria attacks were highest in adults and children aged 10 years or older: 45 (63%) of 71 malaria attacks recorded in 2010 compared with 126 (33%) of 384 in 2007 and 2008 (p<0·0001). 37% of Anopheles gambiae mosquitoes were resistant to deltamethrin in 2010, and the prevalence of the Leu1014Phe kdr resistance mutation increased from 8% in 2007 to 48% in 2010 (p=0·0009).

INTERPRETATION

Increasing pyrethroid resistance of A gambiae and increasing susceptibility of older children and adults, probably due to decreasing immunity, caused the rebound and age shift of malaria morbidity. Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented.

FUNDING

Institut de Recherche pour le Développement and the Pasteur Institute of Dakar.

Combining indoor residual spraying and insecticide-treated nets for malaria control in Africa: a review of possible outcomes and an outline of suggestions for the future

Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are currently the preferred methods of malaria vector control. In many cases, these methods are used together in the same households, especially to suppress transmission in holoendemic and hyperendemic scenarios. Though widespread, there has been limited evidence suggesting that such co-application confers greater protective benefits than either ITNs or IRS when used alone. Since both methods are insecticide-based and intradomicilliary, this article hypothesises that outcomes of their combination would depend on effects of the candidate active ingredients on mosquitoes that enter or those that attempt to enter houses. It is suggested here that enhanced household level protection can be achieved if the ITNs and IRS have divergent yet complementary properties, e.g. highly deterrent IRS compounds coupled with highly toxic ITNs. To ensure that the problem of insecticide resistance is avoided, the ITNs and IRS products should preferably be of different insecticide classes, e.g. pyrethroid-based nets combined with organophosphate or carbamate based IRS. The overall community benefits would however depend also on other factors such as proportion of people covered by the interventions and the behaviour of vector species. This article concludes by emphasizing the need for basic and operational research, including mathematical modelling to evaluate IRS/ITN combinations in comparison to IRS alone or ITNs alone.

Global status of DDT and its alternatives for use in vector control to prevent disease

In this article I reviewed the status of dichlorodiphenyltrichloroethane (DDT), used for disease vector control, and its benefits and risks in relation to the available alternatives. Contemporary data on DDT use were obtained from questionnaires and reports as well as a Scopus search to retrieve published articles. Nearly 14 countries use DDT for disease control, and several others are reintroducing DDT. Concerns about the continued use of DDT are fueled by recent reports of high levels of human exposure associated with indoor spraying amid accumulating evidence on chronic health effects. There are signs that more malaria vectors are becoming resistant to the toxic action of DDT. Effective chemical methods are available as immediate alternatives to DDT, but the development of resistance is undermining the efficacy of insecticidal tools. Nonchemical methods are potentially important, but their effectiveness at program level needs urgent study. To reduce reliance on DDT, support is needed for integrated and multipartner strategies of vector control. Integrated vector management provides a framework for developing and implementing effective technologies and strategies as sustainable alternatives to reliance on DDT.

Protective immunity against malaria by 'natural immunization': a question of dose, parasite diversity, or both?

Plasmodium undergoes an obligate liver phase before the onset of malaria, which is caused exclusively by cyclic propagation of the parasite inside erythrocytes. The diagnostically inaccessible and clinically silent pre-erythrocytic expansion phase is a promising target for inducing sterilizing immunity against reinfections. Recent studies in rodent and human malaria models called attention to the induction of potent protective immunity by administration of anti-malarial drugs during sporozoite exposure. Here, we review the concept of drug-mediated pathogen arrest as a natural immunization strategy. This previously unrecognized immunological benefit might also open new opportunities for population-wide presumptive drug administration as an adjunct malaria control tool.

Host-mediated regulation of superinfection in malaria

In regions of high rates of malaria transmission, mosquitoes repeatedly transmit liver-tropic Plasmodium sporozoites to individuals who already have blood-stage parasitemia. This manifests itself in semi-immune children (who have been exposed since birth to Plasmodium infection and as such show low levels of peripheral parasitemia but can still be infected) older than 5 years of age by concurrent carriage of different parasite genotypes at low asymptomatic parasitemias. Superinfection presents an increased risk of hyperparasitemia and death in less immune individuals but counterintuitively is not frequently observed in the young. Here we show in a mouse model that ongoing blood-stage infections, above a minimum threshold, impair the growth of subsequently inoculated sporozoites such that they become growth arrested in liver hepatocytes and fail to develop into blood-stage parasites. Inhibition of the liver-stage infection is mediated by the host iron regulatory hormone hepcidin, whose synthesis we found to be stimulated by blood-stage parasites in a density-dependent manner. We mathematically modeled this phenomenon and show how density-dependent protection against liver-stage malaria can shape the epidemiological patterns of age-related risk and the complexity of malaria infections seen in young children. The interaction between these two Plasmodium stages and host iron metabolism has relevance for the global efforts to reduce malaria transmission and for evaluation of iron supplementation programs in malaria-endemic regions.

Daily Plasmodium yoelii infective mosquito bites do not generate protection or suppress previous immunity against the liver stage

BACKGROUND

Human populations that are naturally subjected to Plasmodium infection do not acquire complete protection against the liver stage of this parasite despite prolonged and frequent exposure. However, sterile immunity against Plasmodium liver stage can be achieved after repeated exposure to radiation attenuated sporozoites. The reasons for this different response remain largely unknown, but a suppressive effect of blood stage Plasmodium infection has been proposed as a cause for the lack of liver stage protection.

METHODS

Using Plasmodium yoelii 17XNL, the response generated in mice subjected to daily infective bites from normal or irradiated mosquitoes was compared. The effect of daily-infected mosquito bites on mice that were previously immunized against P. yoelii liver stage was also studied.

RESULTS

It was observed that while the bites of normal infected mosquitoes do not generate strong antibody responses and protection, the bites of irradiated mosquitoes result in high levels of anti-sporozoite antibodies and protection against liver stage Plasmodium infection. Exposure to daily infected mosquito bites did not eliminate the protection acquired previously with a experimental liver stage vaccine.

CONCLUSIONS

Liver stage immunity generated by irradiated versus normal P. yoelii infected mosquitoes is essentially different, probably because of the blood stage infection that follows normal mosquito bites, but not irradiated. While infective mosquito bites do not induce a protective liver stage response, they also do not interfere with previously acquired liver stage protective responses, even if they induce a complete blood stage infection. Considering that the recently generated anti-malaria vaccines induce only partial protection against infection, it is encouraging that, at least in mouse models, immunity is not negatively affected by subsequent exposure and infection with the parasite.

Estimating the global clinical burden of Plasmodium falciparum malaria in 2007

BACKGROUND

The epidemiology of malaria makes surveillance-based methods of estimating its disease burden problematic. Cartographic approaches have provided alternative malaria burden estimates, but there remains widespread misunderstanding about their derivation and fidelity. The aims of this study are to present a new cartographic technique and its application for deriving global clinical burden estimates of Plasmodium falciparum malaria for 2007, and to compare these estimates and their likely precision with those derived under existing surveillance-based approaches.

METHODS AND FINDINGS

In seven of the 87 countries endemic for P. falciparum malaria, the health reporting infrastructure was deemed sufficiently rigorous for case reports to be used verbatim. In the remaining countries, the mapped extent of unstable and stable P. falciparum malaria transmission was first determined. Estimates of the plausible incidence range of clinical cases were then calculated within the spatial limits of unstable transmission. A modelled relationship between clinical incidence and prevalence was used, together with new maps of P. falciparum malaria endemicity, to estimate incidence in areas of stable transmission, and geostatistical joint simulation was used to quantify uncertainty in these estimates at national, regional, and global scales. Combining these estimates for all areas of transmission risk resulted in 451 million (95% credible interval 349-552 million) clinical cases of P. falciparum malaria in 2007. Almost all of this burden of morbidity occurred in areas of stable transmission. More than half of all estimated P. falciparum clinical cases and associated uncertainty occurred in India, Nigeria, the Democratic Republic of the Congo (DRC), and Myanmar (Burma), where 1.405 billion people are at risk. Recent surveillance-based methods of burden estimation were then reviewed and discrepancies in national estimates explored. When these cartographically derived national estimates were ranked according to their relative uncertainty and replaced by surveillance-based estimates in the least certain half, 98% of the global clinical burden continued to be estimated by cartographic techniques.

CONCLUSIONS AND SIGNIFICANCE

Cartographic approaches to burden estimation provide a globally consistent measure of malaria morbidity of known fidelity, and they represent the only plausible method in those malaria-endemic countries with nonfunctional national surveillance. Unacceptable uncertainty in the clinical burden of malaria in only four countries confounds our ability to evaluate needs and monitor progress toward international targets for malaria control at the global scale. National prevalence surveys in each nation would reduce this uncertainty profoundly. Opportunities for further reducing uncertainty in clinical burden estimates by hybridizing alternative burden estimation procedures are also evaluated.

Climate change and highland malaria: fresh air for a hot debate

In recent decades, malaria has become established in zones at the margin of its previous distribution, especially in the highlands of East Africa. Studies in this region have sparked a heated debate over the importance of climate change in the territorial expansion of malaria, where positions range from its neglect to the reification of correlations as causes. Here, we review studies supporting and rebutting the role of climatic change as a driving force for highland invasion by malaria. We assessed the conclusions from both sides of the argument and found that evidence for the role of climate in these dynamics is robust. However, we also argue that over-emphasizing the importance of climate is misleading for setting a research agenda, even one which attempts to understand climate change impacts on emerging malaria patterns. We review alternative drivers for the emergence of this disease and highlight the problems still calling for research if the multidimensional nature of malaria is to be adequately tackled. We also contextualize highland malaria as an ongoing evolutionary process. Finally, we present Schmalhausen's law, which explains the lack of resilience in stressed systems, as a biological principle that unifies the importance of climatic and other environmental factors in driving malaria patterns across different spatio-temporal scales.

Age-patterns of malaria vary with severity, transmission intensity and seasonality in sub-Saharan Africa: a systematic review and pooled analysis

Background

There is evidence that the age-pattern of Plasmodium falciparum malaria varies with transmission intensity. A better understanding of how this varies with the severity of outcome and across a range of transmission settings could enable locally appropriate targeting of interventions to those most at risk. We have, therefore, undertaken a pooled analysis of existing data from multiple sites to enable a comprehensive overview of the age-patterns of malaria outcomes under different epidemiological conditions in sub-Saharan Africa.

Methodology/Principal Findings

A systematic review using PubMed and CAB Abstracts (1980–2005), contacts with experts and searching bibliographies identified epidemiological studies with data on the age distribution of children with P. falciparum clinical malaria, hospital admissions with malaria and malaria-diagnosed mortality. Studies were allocated to a 3×2 matrix of intensity and seasonality of malaria transmission. Maximum likelihood methods were used to fit five continuous probability distributions to the percentage of each outcome by age for each of the six transmission scenarios. The best-fitting distributions are presented graphically, together with the estimated median age for each outcome. Clinical malaria incidence was relatively evenly distributed across the first 10 years of life for all transmission scenarios. Hospital admissions with malaria were more concentrated in younger children, with this effect being even more pronounced for malaria-diagnosed deaths. For all outcomes, the burden of malaria shifted towards younger ages with increasing transmission intensity, although marked seasonality moderated this effect.

Conclusions

The most severe consequences of P. falciparum malaria were concentrated in the youngest age groups across all settings. Despite recently observed declines in malaria transmission in several countries, which will shift the burden of malaria cases towards older children, it is still appropriate to target strategies for preventing malaria mortality and severe morbidity at very young children who will continue to bear the brunt of malaria deaths in Sub-Saharan Africa.

A mathematical model for malaria involving differential susceptibility, exposedness and infectivity of human host

The main purpose of this article is to formulate a deterministic mathematical model for the transmission of malaria that considers two host types in the human population. The first type is called "non-immune" comprising all humans who have never acquired immunity against malaria and the second type is called "semi-immune". Non-immune are divided into susceptible, exposed and infectious and semi-immune are divided into susceptible, exposed, infectious and immune. We obtain an explicit formula for the reproductive number, R(0) which is a function of the weight of the transmission semi-immune-mosquito-semi-immune, R(0a), and the weight of the transmission non-immune-mosquito-non-immune, R(0e). Then, we study the existence of endemic equilibria by using bifurcation analysis. We give a simple criterion when R(0) crosses one for forward and backward bifurcation. We explore the possibility of a control for malaria through a specific sub-group such as non-immune or semi-immune or mosquitoes.

Global status of DDT and its alternatives for use in vector control to prevent disease

OBJECTIVE

I review the status of dichlorodiphenyltrichloroethane (DDT), used for disease vector control, along with current evidence on its benefits and risks in relation to the available alternatives.

DATA SOURCES AND EXTRACTION

Contemporary data on DDT use were largely obtained from questionnaires and reports. I also conducted a Scopus search to retrieve published articles.

DATA SYNTHESIS

DDT has been recommended as part of the arsenal of insecticides available for indoor residual spraying until suitable alternatives are available. Approximately 14 countries use DDT for disease control, and several countries are preparing to reintroduce DDT. The effectiveness of DDT depends on local settings and merits close consideration in relation to the alternatives. Concerns about the continued use of DDT are fueled by recent reports of high levels of human exposure associated with indoor spraying amid accumulating evidence on chronic health effects. There are signs that more malaria vectors are becoming resistant to the toxic action of DDT, and that resistance is spreading to new countries. A comprehensive cost assessment of DDT versus its alternatives that takes side effects into account is missing. Effective chemical methods are available as immediate alternatives to DDT, but the choice of insecticide class is limited, and in certain areas the development of resistance is undermining the efficacy of insecticidal tools. New insecticides are not expected in the short term. Nonchemical methods are potentially important, but their effectiveness at program level needs urgent study.

CONCLUSIONS

To reduce reliance on DDT, support is needed for integrated and multipartner strategies of vector control and for the continued development of new technologies. Integrated vector management provides a framework for developing and implementing effective technologies and strategies as sustainable alternatives to reliance on DDT.

Effect of agricultural activities on prevalence rates, and clinical and presumptive malaria episodes in central Côte d'Ivoire

Agricultural activities, among other factors, can influence the transmission of malaria. In two villages of central Côte d'Ivoire (Tiémélékro and Zatta) with distinctively different agro-ecological characteristics, we assessed Plasmodium prevalence rates, fever and clinically confirmed malaria episodes among children aged 15 years and below by means of repeated cross-sectional surveys. Additionally, presumptive malaria cases were monitored in dispensaries for a 4-year period. In Tiémélékro, we observed a decrease in malaria prevalence rates from 2002 to 2005, which might be partially explained by changes in agricultural activities from subsistence farming to cash crop production. In Zatta, where an irrigated rice perimeter is located in close proximity to human habitations, malaria prevalence rates in 2003 were significantly lower than in 2002 and 2005, which coincided with the interruption of irrigated rice farming in 2003/2004. Although malaria transmission differed by an order of magnitude in the two villages in 2003, there was no statistically significant difference between the proportions of severe malaria episodes (i.e. axillary temperature>37.5 degrees C plus parasitaemia>5000 parasites/microl blood). Our study underscores the complex relationship between malaria transmission, prevalence rate and the dynamics of malaria episodes. A better understanding of local contextual determinants, including the effect of agricultural activities, will help to improve the local epidemiology and control of malaria.

Microbial larvicide application by a large-scale, community-based program reduces malaria infection prevalence in urban Dar es Salaam, Tanzania

Background

Malaria control in Africa is most tractable in urban settlements yet most research has focused on rural settings. Elimination of malaria transmission from urban areas may require larval control strategies that complement adult mosquito control using insecticide-treated nets or houses, particularly where vectors feed outdoors.

Methods and Findings

Microbial larvicide (Bacillus thuringiensis var. israelensis (Bti)) was applied weekly through programmatic, non-randomized community-based, but vertically managed, delivery systems in urban Dar es Salaam, Tanzania. Continuous, randomized cluster sampling of malaria infection prevalence and non-random programmatic surveillance of entomological inoculation rate (EIR) respectively constituted the primary and secondary outcomes surveyed within a population of approximately 612,000 residents in 15 fully urban wards covering 55 km². Bti application for one year in 3 of those wards (17 km² with 128,000 residents) reduced crude annual transmission estimates (Relative EIR [95% Confidence Interval] = 0.683 [0.491–0.952], P = 0.024) but program effectiveness peaked between July and September (Relative EIR [CI] = 0.354 [0.193 to 0.650], P = 0.001) when 45% (9/20) of directly observed transmission events occurred. Larviciding reduced malaria infection risk among children ≤5 years of age (OR [CI] = 0.284 [0.101 to 0.801], P = 0.017) and provided protection at least as good as personal use of an insecticide treated net (OR [CI] = 0.764 [0.614–0.951], P = 0.016).

Conclusions

In this context, larviciding reduced malaria prevalence and complemented existing protection provided by insecticide-treated nets. Larviciding may represent a useful option for integrated vector management in Africa, particularly in its rapidly growing urban centres.

Evaluation of Bacillus sphaericus bioinsecticide produced with white soybean meal as culture medium for the control of Culex (Culex) quinquefasciatus

Bioinsecticides are shown to be useful in control programs to prevent several diseases, based on their specificity and efficiency against insect vectors. In the current study a bioinsecticide based on Bacillus sphaericus was produced using a white soybean culture medium and applied to larvae of Culex quinquefasciatus, the susceptible species, and Aedes aegypti, the refractory species used as the negative control. Efficacy was compared with that of the product fermented with the Luria Bertani (LB) reference medium. The experiments showed that C. quinquefasciatus was highly susceptible to the product prepared with white soybean meal, reaching 100% larval mortality even at 10mg/L, while A. aegypti failed to reach 70% mortality at a concentration of 1g/L. By comparison with the reference medium, the proposed culture medium showed high larvicidal power, reaching a LD90 of 2.26 mg/L, while 4.37 mg/L was needed for the LB medium to achieve the same mortality rate. Cost comparison between the formulations favored the use of the bioinsecticide produced with white soybean meal. After factoring in the LD90 value, the cost ratio favored the new raw material by nearly 1:220.

Loss of population levels of immunity to malaria as a result of exposure-reducing interventions: consequences for interpretation of disease trends

BACKGROUND

The persistence of malaria as an endemic infection and one of the major causes of childhood death in most parts of Africa has lead to a radical new call for a global effort towards eradication. With the deployment of a highly effective vaccine still some years away, there has been an increased focus on interventions which reduce exposure to infection in the individual and -by reducing onward transmission-at the population level. The development of appropriate monitoring of these interventions requires an understanding of the timescales of their effect.

METHODS & FINDINGS

Using a mathematical model for malaria transmission which incorporates the acquisition and loss of both clinical and parasite immunity, we explore the impact of the trade-off between reduction in exposure and decreased development of immunity on the dynamics of disease following a transmission-reducing intervention such as insecticide-treated nets. Our model predicts that initially rapid reductions in clinical disease incidence will be observed as transmission is reduced in a highly immune population. However, these benefits in the first 5-10 years after the intervention may be offset by a greater burden of disease decades later as immunity at the population level is gradually lost. The negative impact of having fewer immune individuals in the population can be counterbalanced either by the implementation of highly-effective transmission-reducing interventions (such as the combined use of insecticide-treated nets and insecticide residual sprays) for an indefinite period or the concurrent use of a pre-erythrocytic stage vaccine or prophylactic therapy in children to protect those at risk from disease as immunity is lost in the population.

CONCLUSIONS

Effective interventions will result in rapid decreases in clinical disease across all transmission settings while population-level immunity is maintained but may subsequently result in increases in clinical disease many years later as population-level immunity is lost. A dynamic, evolving intervention programme will therefore be necessary to secure substantial, stable reductions in malaria transmission.

Age patterns of severe paediatric malaria and their relationship to Plasmodium falciparum transmission intensity

Background

The understanding of the epidemiology of severe malaria in African children remains incomplete across the spectrum of Plasmodium falciparum transmission intensities through which communities might expect to transition, as intervention coverage expands.

Methods

Paediatric admission data were assembled from 13 hospitals serving 17 communities between 1990 and 2007. Estimates of Plasmodium falciparum transmission intensity in these communities were assembled to be spatially and temporally congruent to the clinical admission data. The analysis focused on the relationships between community derived parasite prevalence and the age and clinical presentation of paediatric malaria in children aged 0–9 years admitted to hospital.

Results

As transmission intensity declined a greater proportion of malaria admissions were in older children. There was a strong linear relationship between increasing transmission intensity and the proportion of paediatric malaria admissions that were infants (R² = 0.73, p < 0.001). Cerebral malaria was reported among 4% and severe malaria anaemia among 17% of all malaria admissions. At higher transmission intensity cerebral malaria was a less common presentation compared to lower transmission sites. There was no obvious relationship between the proportions of children with severe malaria anaemia and transmission intensity.

Conclusion

As the intensity of malaria transmission declines in Africa through the scaling up of insecticide-treated nets and other vector control measures a focus of disease prevention among very young children becomes less appropriate. The understanding of the relationship between parasite exposure and patterns of disease risk should be used to adapt malaria control strategies in different epidemiological settings.

Effect of a fall in malaria transmission on morbidity and mortality in Kilifi, Kenya

BACKGROUND

As efforts to control malaria are expanded across the world, understanding the role of transmission intensity in determining the burden of clinical malaria is crucial to the prediction and measurement of the effectiveness of interventions to reduce transmission. Furthermore, studies comparing several endemic sites led to speculation that as transmission decreases morbidity and mortality caused by severe malaria might increase. We aimed to assess the epidemiological characteristics of malaria in Kilifi, Kenya, during a period of decreasing transmission intensity.

METHODS

We analyse 18 years (1990-2007) of surveillance data from a paediatric ward in a malaria-endemic region of Kenya. The hospital has a catchment area of 250 000 people. Clinical data and blood-film results for more than 61 000 admissions are reported.

FINDINGS

Hospital admissions for malaria decreased from 18.43 per 1000 children in 2003 to 3.42 in 2007. Over 18 years of surveillance, the incidence of cerebral malaria initially increased; however, malaria mortality decreased overall because of a decrease in incidence of severe malarial anaemia since 1997 (4.75 to 0.37 per 1000 children) and improved survival among children admitted with non-severe malaria. Parasite prevalence, the mean age of children admitted with malaria, and the proportion of children with cerebral malaria began to change 10 years before hospitalisation for malaria started to fall.

INTERPRETATION

Sustained reduction in exposure to infection leads to changes in mean age and presentation of disease similar to those described in multisite studies. Changes in transmission might not lead to immediate reductions in incidence of clinical disease. However, longitudinal data do not indicate that reductions in transmission intensity lead to transient increases in morbidity and mortality.

Protective immunity induced by daily bites from irradiated mosquitoes infected with Plasmodium yoelii

Individuals in malaria endemic regions do not develop fully protective immune responses against Plasmodium liver stage infections. In high transmission areas, individuals can be exposed to more than two infective mosquito bites daily. Their exposure to Plasmodium sporozoites, therefore, is in the form of small and frequent doses. This is very different from individuals studied in controlled immunization trials where the delivery of large numbers of radiation-attenuated sporozoites in a limited number of doses can induce sterile protective immunity. Using irradiated mosquitoes infected with Plasmodium yoelii 17XNL, we tested whether daily bites from a few mosquitoes can induce a protective immune response in mice. This immunization strategy successfully induced a protective response, preventing the development of liver stages when mice were challenged with nonirradiated sporozoites. These results provide further support for the development of liver stage vaccines. They are also a call for further study into why fully protective responses against the liver stage are not seen in individuals from endemic regions.

Various carrier system(s)- mediated genetic vaccination strategies against malaria

The introduction of vaccine technology has facilitated an unprecedented multiantigen approach to develop an effective vaccine against complex pathogens, such as Plasmodium spp., that cause severe malaria. The capacity of multisubunit DNA vaccines encoding different stage Plasmodium antigens to induce CD8(+) cytotoxic T lymphocytes and IFN-gamma responses in mice, monkeys and humans has been observed. Moreover, genetic vaccination may be multi-immune (i.e., capable of eliciting more than one type of immune response, including cell-mediated and humoral). In the case of malaria parasites, a cytotoxic T-lymphocyte response is categorically needed against the intracellular hepatocyte stage while a humoral response, with antibodies targeted against antigens from all stages of the life cycle, is also needed. Therefore, the key to success for any DNA-based therapy is to design a vector able to serve as a safe and efficient delivery system. This has encouraged the development of nonviral DNA-mediated gene-transfer techniques, such as liposomes, virosomes, microspheres and nanoparticles. Efficient and relatively safe DNA transfection using lipoplexes makes them an appealing alternative to be explored for gene delivery. In addition, liposome-entrapped DNA has been shown to enhance the potency of DNA vaccines, possibly by facilitating uptake of the plasmid by antigen-presenting cells. Another recent technology using cationic lipids has been deployed and has generated substantial interest in this approach to gene transfer. This review comprises various aspects that could be decisive in the formulation of efficient and stable carrier system(s) for the development of malaria vaccines.

Comparison of all-cause and malaria-specific mortality from two West African countries with different malaria transmission patterns

Background

Malaria is a leading cause of death in children below five years of age in sub-Saharan Africa. All-cause and malaria-specific mortality rates for children under-five years old in a mesoendemic malaria area (The Gambia) were compared with those from a hyper/holoendemic area (Burkina Faso).

Methods

Information on observed person-years (PY), deaths and cause of death was extracted from online search, using key words: "Africa, The Gambia, Burkina Faso, malaria, Plasmodium falciparum, mortality, child survival, morbidity". Missing person-years were estimated and all-cause and malaria-specific mortality were calculated as rates per 1,000 PY. Studies were classified as longitudinal/clinical studies or surveys/censuses. Linear regression was used to investigate mortality trends.

Results

Overall, 39 and 18 longitudinal/clinical studies plus 10 and 15 surveys and censuses were identified for The Gambia and Burkina Faso respectively (1960–2004). Model-based estimates for under-five all-cause mortality rates show a decline from 1960 to 2000 in both countries (Burkina Faso: from 71.8 to 39.0), but more markedly in The Gambia (from 104.5 to 28.4). The weighted-average malaria-specific mortality rate per 1000 person-years for Burkina Faso (15.4, 95% CI: 13.0–18.3) was higher than that in The Gambia (9.5, 95% CI: 9.1–10.1). Malaria mortality rates did not decline over time in either country.

Conclusion

Child mortality in both countries declined significantly in the period 1960 to 2004, possibly due to socio-economic development, improved health services and specific intervention projects. However, there was little decline in malaria mortality suggesting that there had been no major impact of malaria control programmes during this period. The difference in malaria mortality rates across countries points to significant differences in national disease control policies and/or disease transmission patterns.