Combinatorial effects of malaria season, iron deficiency, and inflammation determine plasma hepcidin concentration in African children.

Hepcidin is the master regulatory hormone that governs iron homeostasis and has a role in innate immunity. Although hepcidin has been studied extensively in model systems, there is less information on hepcidin regulation in global health contexts where iron deficiency (ID), anemia, and high infectious burdens (including malaria) all coexist but fluctuate over time. We evaluated iron status, hepcidin levels, and determinants of hepcidin in 2 populations of rural children aged ≤8 years, in the Gambia and Kenya (total n = 848), at the start and end of a malaria season. Regression analyses and structural equation modeling demonstrated, for both populations, similar combinatorial effects of upregulating stimuli (iron stores and to a lesser extent inflammation) and downregulating stimuli (erythropoietic drive) on hepcidin levels. However, malaria season was also a significant factor and was associated with an altered balance of these opposing factors. Consistent with these changes, hepcidin levels were reduced whereas the prevalence of ID was increased at the end of the malaria season. More prevalent ID and lower hepcidin likely reflect an enhanced requirement for iron and an ability to efficiently absorb it at the end of the malaria season. These results, therefore, have implications for ID and malaria control programs.

Identifying children with excess malaria episodes after adjusting for variation in exposure: identification from a longitudinal study using statistical count models.

BACKGROUND: The distribution of Plasmodium falciparum clinical malaria episodes is over-dispersed among children in endemic areas, with more children experiencing multiple clinical episodes than would be expected based on a Poisson distribution. There is consistent evidence for micro-epidemiological variation in exposure to P. falciparum. The aim of the current study was to identify children with excess malaria episodes after controlling for malaria exposure. METHODS: We selected the model that best fit the data out of the models examined and included the following covariates: age, a weighted local prevalence of infection as an index of exposure, and calendar time to predict episodes of malaria on active surveillance malaria data from 2,463 children of under 15 years of age followed for between 5 and 15 years each. Using parameters from the zero-inflated negative binomial model which best fitted our data, we ran 100 simulations of the model based on our population to determine the variation that might be seen due to chance. RESULTS: We identified 212 out of 2,463 children who had a number of clinical episodes above the 95(th) percentile of the simulations run from the model, hereafter referred to as "excess malaria (EM)". We then identified exposure-matched controls with "average numbers of malaria" episodes, and found that the EM group had higher parasite densities when asymptomatically infected or during clinical malaria, and were less likely to be of haemoglobin AS genotype. CONCLUSIONS: Of the models tested, the negative zero-inflated negative binomial distribution with exposure, calendar year, and age acting as independent predictors, fitted the distribution of clinical malaria the best. Despite accounting for these factors, a group of children suffer excess malaria episodes beyond those predicted by the model. An epidemiological framework for identifying these children will allow us to study factors that may explain excess malaria episodes.

Negative epistasis between the malaria-protective effects of alpha+-thalassemia and the sickle cell trait.

The hemoglobinopathies, disorders of hemoglobin structure and production, protect against death from malaria. In sub-Saharan Africa, two such conditions occur at particularly high frequencies: presence of the structural variant hemoglobin S and alpha(+)-thalassemia, a condition characterized by reduced production of the normal alpha-globin component of hemoglobin. Individually, each is protective against severe Plasmodium falciparum malaria, but little is known about their malaria-protective effects when inherited in combination. We investigated this question by studying a population on the coast of Kenya and found that the protection afforded by each condition inherited alone was lost when the two conditions were inherited together, to such a degree that the incidence of both uncomplicated and severe P. falciparum malaria was close to baseline in children heterozygous with respect to the mutation underlying the hemoglobin S variant and homozygous with respect to the mutation underlying alpha(+)-thalassemia. Negative epistasis could explain the failure of alpha(+)-thalassemia to reach fixation in any population in sub-Saharan Africa.

HIV infection, malnutrition, and invasive bacterial infection among children with severe malaria.

BACKGROUND: Human immunodeficiency virus (HIV) infection, malnutrition, and invasive bacterial infection (IBI) are reported among children with severe malaria. However, it is unclear whether their cooccurrence with falciparum parasitization and severe disease happens by chance or by association among children in areas where malaria is endemic. METHODS: We examined 3068 consecutive children admitted to a Kenyan district hospital with clinical features of severe malaria and 592 control subjects from the community. We performed multivariable regression analysis, with each case weighted for its probability of being due to falciparum malaria, using estimates of the fraction of severe disease attributable to malaria at different parasite densities derived from cross-sectional parasitological surveys of healthy children from the same community. RESULTS: HIV infection was present in 133 (12%) of 1071 consecutive parasitemic admitted children (95% confidence interval [CI], 11%-15%). Parasite densities were higher in HIV-infected children. The odds ratio for admission associated with HIV infection for admission with true severe falciparum malaria was 9.6 (95% CI, 4.9-19); however, this effect was restricted to children aged 1 year. Malnutrition was present in 507 (25%) of 2048 consecutive parasitemic admitted children (95% CI, 23%-27%). The odd ratio associated with malnutrition for admission with true severe falciparum malaria was 4.0 (95% CI, 2.9-5.5). IBI was detected in 127 (6%) of 2048 consecutive parasitemic admitted children (95% CI, 5.2%-7.3%). All 3 comorbidities were associated with increased case fatality. CONCLUSIONS: HIV, malnutrition and IBI are biologically associated with severe disease due to falciparum malaria rather than being simply alternative diagnoses in co-incidentally parasitized children in an endemic area.

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.

Breadth and magnitude of antibody responses to multiple Plasmodium falciparum merozoite antigens are associated with protection from clinical malaria.

Individuals living in areas where malaria is endemic are repeatedly exposed to many different malaria parasite antigens. Studies on naturally acquired antibody-mediated immunity to clinical malaria have largely focused on the presence of responses to individual antigens and their associations with decreased morbidity. We hypothesized that the breadth (number of important targets to which antibodies were made) and magnitude (antibody level measured in a random serum sample) of the antibody response were important predictors of protection from clinical malaria. We analyzed naturally acquired antibodies to five leading Plasmodium falciparum merozoite-stage vaccine candidate antigens, and schizont extract, in Kenyan children monitored for uncomplicated malaria for 6 months (n = 119). Serum antibody levels to apical membrane antigen 1 (AMA1) and merozoite surface protein antigens (MSP-1 block 2, MSP-2, and MSP-3) were inversely related to the probability of developing malaria, but levels to MSP-1(19) and erythrocyte binding antigen (EBA-175) were not. The risk of malaria was also inversely associated with increasing breadth of antibody specificities, with none of the children who simultaneously had high antibody levels to five or more antigens experiencing a clinical episode (17/119; 15%; P = 0.0006). Particular combinations of antibodies (AMA1, MSP-2, and MSP-3) were more strongly predictive of protection than others. The results were validated in a larger, separate case-control study whose end point was malaria severe enough to warrant hospital admission (n = 387). These findings suggest that under natural exposure, immunity to malaria may result from high titers antibodies to multiple antigenic targets and support the idea of testing combination blood-stage vaccines optimized to induce similar antibody profiles.

Acquisition of naturally occurring antibody responses to recombinant protein domains of Plasmodium falciparum erythrocyte membrane protein 1.

BACKGROUND: Antibodies targeting variant antigens expressed on the surface of Plasmodium falciparum infected erythrocytes have been associated with protection from clinical malaria. The precise target for these antibodies is unknown. The best characterized and most likely target is the erythrocyte surface-expressed variant protein family Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). METHODS: Using recombinant proteins corresponding to five domains of the expressed A4 var gene, A4 PfEMP1, the naturally occurring antibody response was assessed, by ELISA, to each domain in serum samples obtained from individuals resident in two communities of differing malaria transmission intensity on the Kenyan coast. Using flow cytometry, the correlation in individual responses to each domain with responses to intact A4-infected erythrocytes expressing A4 PfEMP1 on their surface as well as responses to two alternative parasite clones and one clinical isolate was assessed. RESULTS: Marked variability in the prevalence of responses between each domain and between each transmission area was observed, as wasa strong correlation between age and reactivity with some but not all domains. Individual responses to each domain varied strikingly, with some individuals showing reactivity to all domains and others with no reactivity to any, this was apparent at all age groups. Evidence for possible cross-reactivity in responses to the domain DBL4gamma was found. CONCLUSION: Individuals acquire antibodies to surface expressed domains of a highly variant protein. The finding of potential cross-reactivity in responses to one of these domains is an important initial finding in the consideration of potential vaccine targets.

Prevalence and correlates of home delivery amongst HIV-infected women attending care at a rural public health facility in Coastal Kenya.

BACKGROUND: Home delivery, referring to pregnant women giving birth in the absence of a skilled birth attendant, is a significant contributor to maternal mortality, and is encouragingly reported to be on a decline in the general population in resource limited settings. However, much less is known about home delivery amongst HIV-infected women in sub-Saharan Africa (sSA). We described the prevalence and correlates of home delivery among HIV-infected women attending care at a rural public health facility in Kilifi, Coastal Kenya. METHODS: A cross-sectional design using mixed methods was used. Quantitative data were collected using interviewer-administered questionnaires from HIV-infected women with a recent pregnancy (within 5 years, n = 425), whilst qualitative data were collected using focused group discussions (FGD, n = 5). Data were analysed using logistic regression and a thematic framework approach respectively. RESULTS: Overall, 108 (25.4%, [95% CI: 21.3-29.8]) participants delivered at home. Correlates of home delivery included lack of formal education (aOR 12.4 [95% CI: 3.4-46.0], p<0.001), history of a previous home delivery (2.7 [95% CI:1.2-6.0], p = 0.019) and being on highly active antiretroviral therapy (HAART, 0.4 [95% CI:0.2-0.8], p = 0.006).Despite a strong endorsement against home delivery, major thematic challenges included consumer-associated barriers, health care provider associated barriers and structural barriers. CONCLUSION: A quarter of HIV-infected women delivered at home, which is comparable to estimates reported from the general population in this rural setting, and much lower than estimates from other sSA settings. A tailored package of care targeting women with no formal education and with a history of a previous home delivery, coupled with interventions towards scaling up HAART and improving the quality of maternal care in HIV-infected women may positively contribute to a decline in home delivery and subsequent maternal mortality in this setting.

The haptoglobin 2-2 genotype is associated with a reduced incidence of Plasmodium falciparum malaria in children on the coast of Kenya.

BACKGROUND: Haptoglobin (Hp) genotype determines the efficiency of hemoglobin clearance after malaria-induced hemolysis and alters antioxidant and immune functions. The Hp2 allele is thought to have spread under strong selection pressure, but it is unclear whether this is due to protection from malaria or other diseases. METHODS: We monitored the incidence of febrile malaria and other childhood illnesses with regard to Hp genotype in a prospective cohort of 312 Kenyan children during 558.3 child-years of follow-up. We also conducted 7 cross-sectional surveys to determine the prevalence of Plasmodium falciparum parasitemia. RESULTS: The Hp2/2 genotype was associated with a 30% reduction in clinical malarial episodes (adjusted incidence rate ratio, 0.67; P=.008 for Hp2/2 vs. Hp1/1 and Hp2/1 combined). Protection increased with age; there was no protection in the first 2 years of life, 30% protection at > or = 2 years of age, and 50% protection from 4-10 years of age. Children with the Hp1/1 genotype had a significantly lower rate of nonmalarial fever (P=.001). CONCLUSIONS: Balancing selection pressures may have influenced the spread of the Hp gene. Our observations suggest that the Hp2 allele may have spread as a result of protection from malaria, and the Hp1 allele may be sustained by protection from other infections.

Defining childhood severe falciparum malaria for intervention studies.

BACKGROUND: Clinical trials of interventions designed to prevent severe falciparum malaria in children require a clear endpoint. The internationally accepted definition of severe malaria is sensitive, and appropriate for clinical purposes. However, this definition includes individuals with severe nonmalarial disease and coincident parasitaemia, so may lack specificity in vaccine trials. Although there is no "gold standard" individual test for severe malaria, malaria-attributable fractions (MAFs) can be estimated among groups of children using a logistic model, which we use to test the suitability of various case definitions as trial endpoints. METHODS AND FINDINGS: A total of 4,583 blood samples were taken from well children in cross-sectional surveys and from 1,361 children admitted to a Kenyan District hospital with severe disease. Among children under 2 y old with severe disease and over 2,500 parasites per microliter of blood, the MAFs were above 85% in moderate- and low-transmission areas, but only 61% in a high-transmission area. HIV and malnutrition were not associated with reduced MAFs, but gastroenteritis with severe dehydration (defined by reduced skin turgor), lower respiratory tract infection (clinician's final diagnosis), meningitis (on cerebrospinal fluid [CSF] examination), and bacteraemia were associated with reduced MAFs. The overall MAF was 85% (95% confidence interval [CI] 83.8%-86.1%) without excluding these conditions, 89% (95% CI 88.4%-90.2%) after exclusions, and 95% (95% CI 94.0%-95.5%) when a threshold of 2,500 parasites/mul was also applied. Applying a threshold and exclusion criteria reduced sensitivity to 80% (95% CI 77%-83%). CONCLUSIONS: The specificity of a case definition for severe malaria is improved by applying a parasite density threshold and by excluding children with meningitis, lower respiratory tract infection (clinician's diagnosis), bacteraemia, and gastroenteritis with severe dehydration, but not by excluding children with HIV or malnutrition.

An outbreak of cholera in western Kenya, 2015: a case control study.

INTRODUCTION: in February 2015, an outbreak of acute watery diarrhea was reported in two sub counties in western Kenya. Vibrio cholerae 01 serotype Ogawa was isolated from 26 cases and from water samples collected from a river mainly used by residents of the two sub-counties for domestic purposes. We carried out an investigation to determine factors associated with the outbreak. METHODS: we conducted a frequency matched case control study in the community. We defined cases as episodes of watery diarrhea (at least three motions in 24 hours) in persons ≥ 2 years who were residents of Rongo or Ndhiwa sub-counties from January 23-February 25, 2015. Cases were systematically recruited from a cholera line list and matched to two controls (persons without diarrhea since January 23, 2015) by age category and residence. A structured questionnaire was administered to evaluate exposures in cases and controls and multivariable logistic regression done to determine independent factors associated with the outbreak. RESULTS: we recruited 52 cases and 104 controls. Females constituted 61% (95/156) of all participants. Overall latrine coverage was 58% (90/156). Latrine coverage was 44% (23/52) for cases and 64% (67/104) for controls. Having no latrine at home (aOR = 10.9; 95% CI: 3.02-39.21), practicing communal hand washing in a basin (aOR = 6.5; 95% CI: 2.30-18.11) and vending of food as an occupation (aOR = 3.4; 95% CI: 1.06-10.74) were independently associated with the outbreak. CONCLUSION: poor latrine coverage and personal hygiene practices were identified as the main drivers of the outbreak. We recommended improved public health education on latrine usage and promotion of hand washing with soap and water in the community.

The effect of alpha+-thalassaemia on the incidence of malaria and other diseases in children living on the coast of Kenya.

BACKGROUND: The alpha-thalassaemias are the commonest genetic disorders of humans. It is generally believed that this high frequency reflects selection through a survival advantage against death from malaria; nevertheless, the epidemiological description of the relationships between alpha-thalassaemia, malaria, and other common causes of child mortality remains incomplete. METHODS AND FINDINGS: We studied the alpha+-thalassaemia-specific incidence of malaria and other common childhood diseases in two cohorts of children living on the coast of Kenya. We found no associations between alpha+-thalassaemia and the prevalence of symptomless Plasmodium falciparum parasitaemia, the incidence of uncomplicated P. falciparum disease, or parasite densities during mild or severe malaria episodes. However, we found significant negative associations between alpha+-thalassaemia and the incidence rates of severe malaria and severe anaemia (haemoglobin concentration < 50 g/l). The strongest associations were for severe malaria anaemia (> 10,000 P. falciparum parasites/mul) and severe nonmalaria anaemia; the incidence rate ratios and 95% confidence intervals (CIs) for alpha+-thalassaemia heterozygotes and homozygotes combined compared to normal children were, for severe malaria anaemia, 0.33 (95% CI, 0.15,0.73; p = 0.006), and for severe nonmalaria anaemia, 0.26 (95% CI, 0.09,0.77; p = 0.015). CONCLUSIONS: Our observations suggest, first that selection for alpha+-thalassaemia might be mediated by a specific effect against severe anaemia, an observation that may lead to fresh insights into the aetiology of this important condition. Second, although alpha+-thalassaemia is strongly protective against severe and fatal malaria, its effects are not detectable at the level of any other malaria outcome; this result provides a cautionary example for studies aimed at testing malaria interventions or identifying new malaria-protective genes.

Contrasting signatures of selection on the Plasmodium falciparum erythrocyte binding antigen gene family.

Erythrocyte binding antigens of Plasmodium falciparum are involved in erythrocyte invasion, and may be targets of acquired immunity. Of the five eba genes, protein products have been detected for eba-175, eba-181 and eba-140, but not for psieba-165 or ebl-1, providing opportunity for comparative analysis of genetic variation to identify selection. Region II of each of these genes was sequenced from a cross-sectional sample of parasites in an endemic Kenyan population, and the frequency distributions of polymorphisms analysed. A positive value of Tajima's D was observed for eba-175 (D=1.13) indicating an excess of intermediate frequency polymorphisms, while all other genes had negative values, the most negative being ebl-1 (D=-2.35) followed by psieba-165 (D=-1.79). The eba-175 and ebl-1 genes were then studied in a sample of parasites from Thailand, for which a positive Tajima's D value was again observed for eba-175 (D=1.79), and a negative value for ebl-1 (D=-1.85). This indicates that eba-175 is under balancing selection in each population, in strong contrast to the other members of the gene family, particularly ebl-1 and psieba-165 that may have been under recent directional selection. Population expansion simulations were performed under a neutral model, further supporting the departures from neutrality of these genes.

Cellular reactivity to the p. Falciparum protein trap in adult kenyans: novel epitopes, complex cytokine patterns, and the impact of natural antigenic variation.

Malaria vaccines based on thrombospondin-related adhesive protein of Plasmodium falciparum (Pf TRAP) are currently undergoing clinical trials in humans. This study was designed to investigate naturally acquired cellular immunity to Pf TRAP in adults from a target population for future trials of TRAP-based vaccines in Kilifi, Kenya. We first tested reactivity to a panel of 53 peptides spanning Pf TRAP and identified 26 novel T-cell epitopes. A panel of naturally occurring polymorphic variant epitope peptides were made to the most commonly recognized epitope regions and tested for ability to elicit IFN-gamma, IL-4, and IL-10 production. These data provide for the first time a complex cytokine matrix mapping naturally induced T-cell responses to TRAP and suggest that T-cell responses boosted by vaccination with Pf TRAP could stimulate the release of competing pro- and anti-inflammatory cytokines. They further define polymorphic variants able to boost specific Th1, Th2, and possibly Tr1 reactivity.

A polymorphism of intercellular adhesion molecule-1 is associated with a reduced incidence of nonmalarial febrile illness in Kenyan children.

An intercellular adhesion molecule-1 polymorphism (ICAM-1(Kilifi)) is present at a high frequency across sub-Saharan Africa, and its presence may increase susceptibility to cerebral malaria. Here, we report that, compared with children in whom wild-type intercellular adhesion molecule-1 is present, the incidence of nonmalarial fever is significantly lower among those homozygous for ICAM-1(Kilifi). We propose that ICAM-1(Kilifi) may be associated with reduced rates of tissue damage and of death due to sepsis.

Heritability of malaria in Africa.

BACKGROUND: While many individual genes have been identified that confer protection against malaria, the overall impact of host genetics on malarial risk remains unknown. METHODS AND FINDINGS: We have used pedigree-based genetic variance component analysis to determine the relative contributions of genetic and other factors to the variability in incidence of malaria and other infectious diseases in two cohorts of children living on the coast of Kenya. In the first, we monitored the incidence of mild clinical malaria and other febrile diseases through active surveillance of 640 children 10 y old or younger, living in 77 different households for an average of 2.7 y. In the second, we recorded hospital admissions with malaria and other infectious diseases in a birth cohort of 2,914 children for an average of 4.1 y. Mean annual incidence rates for mild and hospital-admitted malaria were 1.6 and 0.054 episodes per person per year, respectively. Twenty-four percent and 25% of the total variation in these outcomes was explained by additively acting host genes, and household explained a further 29% and 14%, respectively. The haemoglobin S gene explained only 2% of the total variation. For nonmalarial infections, additive genetics explained 39% and 13% of the variability in fevers and hospital-admitted infections, while household explained a further 9% and 30%, respectively. CONCLUSION: Genetic and unidentified household factors each accounted for around one quarter of the total variability in malaria incidence in our study population. The genetic effect was well beyond that explained by the anticipated effects of the haemoglobinopathies alone, suggesting the existence of many protective genes, each individually resulting in small population effects. While studying these genes may well provide insights into pathogenesis and resistance in human malaria, identifying and tackling the household effects must be the more efficient route to reducing the burden of disease in malaria-endemic areas.

An immune basis for malaria protection by the sickle cell trait.

BACKGROUND: Malaria resistance by the sickle cell trait (genotype HbAS) has served as the prime example of genetic selection for over half a century. Nevertheless, the mechanism of this resistance remains the subject of considerable debate. While it probably involves innate factors such as the reduced ability of Plasmodium falciparum parasites to grow and multiply in HbAS erythrocytes, recent observations suggest that it might also involve the accelerated acquisition of malaria-specific immunity. METHODS AND FINDINGS: We studied the age-specific protection afforded by HbAS against clinical malaria in children living on the coast of Kenya. We found that protection increased with age from only 20% in the first 2 y of life to a maximum of 56% by the age of 10 y, returning thereafter to 30% in participants greater than 10 y old. CONCLUSIONS: Our observations suggest that malaria protection by HbAS involves the enhancement of not only innate but also of acquired immunity to the parasite. A better understanding of the underlying mechanisms might yield important insights into both these processes.

Ex vivo interferon-gamma immune response to thrombospondin-related adhesive protein in coastal Kenyans: longevity and risk of Plasmodium falciparum infection.

Thrombospondin-related adhesive protein (TRAP) of Plasmodium falciparum is currently being tested in human vaccine studies. However, its natural reactivity in the field remains poorly characterized. More than 40% of 217 Kenyan donors responded in an ex vivo interferon-gamma (IFN-gamma) enzyme-linked immunospot (ELISPOT) assay to at least one of 14 20mer peptides spanning 42% of the antigen. Reactivity was comparable from early childhood (>1 year of age) to old age, and the maximal precursor frequency of TRAP-specific cells to all 14 peptides was 1 in 4,000. Prospective follow-up for one year indicated that these low-level ex vivo responses to TRAP did not protect against the subsequent development of malaria. Retesting of selected donors after one year showed a complete change in the reactivity pattern, suggesting that malaria-specific ex vivo IFN-gamma ELISPOT assay responses are short lived in naturally exposed donors, even to conserved epitopes. This study provides important information regarding natural reactivity to a key malaria antigen.

The effects of untreated bednets on malaria infection and morbidity on the Kenyan coast.

A study was conducted in order to determine whether children that slept under untreated bednets were protected against both malaria infection and clinical disease compared with children not sleeping under bednets. The study was conducted in Kilifi District, Kenya, during the malaria season (June-August, 2000) and involved 416 children aged < or = 10 years. Data collected from a cross-sectional survey showed evidence of protection against malaria infection among children sleeping under untreated bednets in good condition compared with those not using nets (adjusted odds ratio [AOR] = 0.4, 95% CI 0.22-0.72, P = 0.002). There was no evidence of a protective effect against infection when comparing those that used untreated bednets that were worn and those not using nets (AOR = 0.75, 95% CI 0.34-1.63, P = 0.47). When these same children were followed-up during the malaria season, there was evidence of a lower rate of clinical malaria among those that used untreated nets in good condition (adjusted incidence rate ratio = 0.65, 95% CI 0.45-0.94, P = 0.022), while the rate of clinical malaria among those that used untreated bednets that were worn was similar to that of those that did not use bednets. In the face of persistent failure of communities to take up net retreatment, there is hope that untreated nets will offer some protection against malaria infection and disease compared with not using nets at all.

Transmission-dependent tolerance to multiclonal Plasmodium falciparum infection.

Whether the number of concurrent clones in asymptomatic Plasmodium falciparum infections reflects the degree of host protection was investigated in children living in areas with different levels of transmission on the coast of Kenya. The number of concurrent clones was determined on the basis of polymorphism in msp2, which encodes the vaccine candidate antigen merozoite surface protein 2. In a low-transmission area, most children had monoclonal infections, and diversity did not predict a risk of clinical malaria. In an area of moderate transmission, asymptomatic infections with 2 clones were, compared with 1 clone, associated with an increased risk of subsequent malaria. In a comparative assessment in a high-transmission area in Tanzania, multiclonal infections conferred a reduced risk. The different nonlinear associations between the number of clones and malaria morbidity suggest that levels of tolerance to multiclonal infections are transmission dependent as a result of cumulative exposure to antigenically diverse P. falciparum infections.