Bruno P. Mmbando

A progressive declining in the burden of malaria in north-eastern Tanzania

BackgroundThe planning and assessment of malaria interventions is complicated due to fluctuations in the burden of malaria over time. Recently, it has been reported that the burden of malaria in some parts of Africa has declined. However, community-based longitudinal data are sparse and the reasons for the apparent decline are not well understood.MethodsMalaria prevalence and morbidity have been monitored in two villages in north-eastern Tanzania; a lowland village and a highland village from 2003 to 2008. Trained village health workers treated presumptive malaria with the Tanzanian first-line anti-malarial drug and collected blood smears that were examined later. The prevalence of malaria parasitaemia across years was monitored through cross-sectional surveys.ResultsThe prevalence of malaria parasitaemia in the lowland village decreased from 78.4% in 2003 to 13.0% in 2008, whereas in the highland village, the prevalence of parasitaemia dropped from 24.7% to 3.1% in the same period. Similarly, the incidence of febrile malaria episodes in the two villages dropped by almost 85% during the same period and there was a marked reduction in the number of young children who suffered from anaemia in the lowland village.ConclusionThere has been a marked decline in malaria in the study villages during the past few years. This decline is likely to be due to a combination of factors that include improved access to malaria treatment provided by the trained village helpers, protection from mosquitoes by increased availability of insecticide-impregnated bed nets and a reduced vector density. If this decline in malaria morbidity is sustained, it will have a marked effect on the disease burden in this part of Tanzania.

Sequential, Ordered Acquisition of Antibodies to Plasmodium falciparum Erythrocyte Membrane Protein 1 Domains

The binding of erythrocytes infected with mature blood stage parasites to the vascular bed is key to the pathogenesis of malignant malaria. The binding is mediated by members of Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family. PfEMP1s can be divided into groups, and it has previously been suggested that parasites expressing group A or B/A PfEMP1s are most pathogenic. To test the hypothesis that the first malaria infections in infants and young children are dominated by parasites expressing A and B/A PfEMP1s, we measured the plasma Ab level against 48 recombinant PfEMP1 domains of different groupings in 1342 individuals living in five African villages characterized by markedly different malaria transmission. We show that children progressively acquire a broader repertoire of anti-PfEMP1 Abs, but that the rate of expansion is governed by transmission intensity. However, independently of transmission intensity, Abs are first acquired to particular duffy binding ligand-like domains belonging to group A or B/A PfEMP1s. The results support the view that anti-PfEMP1 Ab responses effectively structure the expenditure of the repertoire of PfEMP1 maintained by the parasite. Parasites expressing certain group A and B/A PfEMP1s are responded to first by individuals with limited previous exposure, and the resulting Abs reduce the fitness and pathogenicity of these parasites during subsequent infections. This allows parasites expressing less pathogenic PFEMP1s to dominate during later infections. The identification of PfEMP1 domains expressed by parasites causing disease in infants and young children is important for development of vaccines protecting against severe malaria.

Levels of Plasma Immunoglobulin G with Specificity against the Cysteine-Rich Interdomain Regions of a Semiconserved Plasmodium falciparum Erythrocyte Membrane Protein 1, VAR4, Predict Protection against Malarial Anemia and Febrile Episodes

ABSTRACT Antibodies to variant surface antigen have been implicated as mediators of malaria immunity in studies measuring immunoglobulin G (IgG) binding to infected erythrocytes. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important target for these antibodies, but no study has directly linked the presence of PfEMP1 antibodies in children to protection. We measured plasma IgG levels to the cysteine-rich interdomain region 1α (CIDR1α) of VAR4 (VAR4-CIDR1α), a member of a semiconserved PfEMP1 subfamily, by enzyme-linked immunosorbent assay in 561 Tanzanian individuals, who were monitored clinically for 7 months. The participants resided in Mkokola (a high-transmission village where malaria is holoendemic) or Kwamasimba (a moderate-transmission village). For comparison, plasma IgG levels to two merozoite surface protein 1 (MSP1) constructs, MSP1-19 and MSP1 block 2, and a control CIDR1 domain were measured. VAR4-CIDR1α antibodies were acquired at an earlier age in Mkokola than in Kwamasimba, but after the age of 10 years the levels were comparable in the two villages. After controlling for age and other covariates, the risk of having anemia at enrollment was reduced in VAR4-CIDR1α responders for Mkokola (adjusted odds ratio [AOR], 0.49; 95% confidence interval [CI], 0.29 to 0.88; P = 0.016) and Kwamasimba (AOR, 0.33; 95% CI, 0.16 to 0.68; P = 0.003) villages. The risk of developing malaria fever was reduced among individuals with a measurable VAR4-CIDR1α response from Mkokola village (AOR, 0.51; 95% CI, 0.29 to 0.89; P = 0.018) but not in Kwamasimba. Antibody levels to the MSP1 constructs and the control CIDR1α domain were not associated with morbidity protection. These data strengthen the concept of developing vaccines based on PfEMP1.

Plasmodium falciparum mutant haplotype infection during pregnancy associated with reduced birthweight, Tanzania.

Intermittent preventive treatment during pregnancy with sulfadoxine–pyrimethamine (IPTp-SP) is a key strategy in the control of pregnancy-associated malaria. However, this strategy is compromised by widespread drug resistance from single-nucleotide polymorphisms in the Plasmodium falciparum dihydrofolate reductase and dihydropteroate synthetase genes. During September 2008–October 2010, we monitored a cohort of 924 pregnant women in an area of Tanzania with declining malaria transmission. P. falciparum parasites were genotyped, and the effect of infecting haplotypes on birthweight was assessed. Of the genotyped parasites, 9.3%, 46.3%, and 44.4% had quadruple or less, quintuple, and sextuple mutated haplotypes, respectively. Mutant haplotypes were unrelated to SP doses. Compared with infections with the less-mutated haplotypes, infections with the sextuple haplotype mutation were associated with lower (359 g) birthweights. Continued use of the suboptimal IPTp-SP regimen should be reevaluated, and alternative strategies (e.g., intermittent screening and treatment or intermittent treatment with safe and effective alternative drugs) should be evaluated.

Prevalence of Single Nucleotide Polymorphisms in the Plasmodium falciparum Multidrug Resistance Gene (Pfmdr-1) in Korogwe District in Tanzania Before and After Introduction of Artemisinin-Based Combination Therapy

Tanzania implemented artemether-lumefantrine (AL) as the first-line treatment for uncomplicated malaria in November of 2006 because of resistance to sulfadoxine-pyrimethamine. AL remains highly efficacious, but widespread use may soon facilitate emergence of artemisinin tolerance/resistance, which initially may be detected at the molecular level as temporal changes in the frequency of single nucleotide polymorphisms (SNPs) in the Pfmdr-1 gene associated with AL resistance. In Tanzania, 830 Plasmodium falciparum-positive samples collected between 2003 and 2010 were examined for SNPs of Pfmdr-1 at codons 86, 184, and 1246. Both the N86 and 184F increased from 2006 to 2010 (logistic regression; N86: odds ratio [95% confidence interval] = 1.35 [1.07-1.71], P = 0.01; 184F: odds ratio = 1.42 [1.07-1.88], P = 0.02), and no change was found for D1246 (odds ratio = 1.01 [0.80-1.28], P = 0.9). The observed changes may be because of introduction of AL, and if so, this finding gives cause for concern and argues for continued surveillance of these molecular markers.

Plasma cytokine levels in Tanzanian HIV-1-infected adults and the effect of antiretroviral treatment.

Objective:To evaluate the role immune activation leading to the production and circulation of cytokines has in the pathogenesis of HIV infection in sub-Saharan Africa and the effect of antiretroviral treatment (ART) on these parameters. Methods:Plasma concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-8, monocyte chemotactic protein (MCP)-1, IL-10, and IL-1 receptor antagonist; plasma HIV RNA; hemoglobin concentration; and white blood cells were measured in 229 HIV-infected, 54 HIV-uninfected, and after 2 and 4 months, respectively, of ART in 35 eligible individuals in northeastern Tanzania. Results:Plasma levels of tumor necrosis factor-α, IL-6, IL-8, monocyte chemotactic protein-1, and IL-1 receptor antagonist were significantly higher in HIV-infected individuals compared with HIV-uninfected individuals and also significantly higher in HIV-infected individuals with CD4 cell counts below 200 cells per microliter than individuals with CD4 cell counts above 200 cells per microliter. HIV RNA was the strongest predictor of all cytokine expression in multivariate analysis. ART leads to a decrease in all cytokines to levels close to those of HIV-uninfected individuals. Conclusions:Our findings support the role of HIV viral replication as the most important promoter of immune activation and prove the importance of ART in reducing immune activation and viral replication even in sub-Saharan Africa where patients are exposed to an abundance of other infectious agents.

Epidemiology of malaria in an area prepared for clinical trials in Korogwe, north-eastern Tanzania.

BackgroundSite preparation is a pre-requesite in conducting malaria vaccines trials. This study was conducted in 12 villages to determine malariometric indices and associated risk factors, during long and short rainy seasons, in an area with varying malaria transmission intensities in Korogwe district, Tanzania. Four villages had passive case detection (PCD) of fever system using village health workers.MethodsFour malariometric cross-sectional surveys were conducted between November 2005 and May 2007 among individuals aged 0–19 years, living in lowland urban, lowland rural and highland strata. A total of 10,766 blood samples were collected for malaria parasite diagnosis and anaemia estimation. Blood smears were stained with Giemsa while haemoglobin level was measured by HaemoCue. Socio-economic data were collected between Jan-Apr 2006.ResultsAdjusting for the effect of age, the risk of Plasmodium falciparum parasitaemia was significantly lower in both lowland urban, (OR = 0.26; 95%CI: 0.23–0.29, p < 0.001) and highlands, (OR = 0.21; 95%CI: 0.17–0.25, p < 0.001) compared to lowland rural. Individuals aged 6–9 years in the lowland rural and 4–19 years in both lowland urban and highlands had the highest parasite prevalence, whilst children below five years in all strata had the highest parasite density. Prevalence of splenomegaly and gametocyte were also lower in both lowland urban and highlands than in lowland rural. Anaemia (Hb <11 g/dl) prevalence was lowest in the lowland urban. Availability of PCD and higher socio-economic status (SES) were associated with reduced malaria and anaemia prevalence.ConclusionHigher SES and use of bed nets in the lowland urban could be the important factors for low malaria infections in this stratum. Results obtained here were used together with those from PCD and DSS in selecting a village for Phase 1b MSP3 vaccine trial, which was conducted in the study area in year 2008.

Differences in Human Antibody Reactivity to Plasmodium falciparum Variant Surface Antigens Are Dependent on Age and Malaria Transmission Intensity in Northeastern Tanzania

ABSTRACT Plasmodium falciparum variant surface antigens (VSA) are involved in the pathogenesis of malaria. Immunoglobulin G (IgG) with specificity for VSA (anti-VSA IgG) is therefore considered important for acquired immunity. To better understand the nature and dynamics of variant-specific IgG responses at population level, we conducted an immunoepidemiological study in nearby communities in northeastern Tanzania, situated at different altitudes and therefore exposed to different levels of P. falciparum transmission intensity. Samples of plasma and infected red blood cells (IRBC) were collected from 759 individuals aged 0 to 19 years. Plasma levels of IgG with specificity for VSA expressed by a panel of different parasite isolates were measured by flow cytometry, while the ability of plasma to inhibit IRBC adhesion to CD36 was examined in cellular assays. The level and repertoire of the heterologous anti-VSA IgG response developed dramatically in individuals at 1 to 2 years of age in the high-transmission area, reaching a maximum level at around 10 years of age; only a modest further increase was observed among older children and adults. In contrast, at lower levels of malaria transmission, anti-VSA IgG levels were lower and the repertoire was more narrow, and similar age- and transmission-dependent differences were observed with regard to the ability of the plasma samples to inhibit adhesion of IRBC to CD36. These differences indicate a strong and dynamic relationship between malaria exposure and functional characteristics of the variant-specific antibody response, which is likely to be important for protection against malaria.

IgG Antibodies to Endothelial Protein C Receptor-Binding Cysteine-Rich Interdomain Region Domains of Plasmodium falciparum Erythrocyte Membrane Protein 1 Are Acquired Early in Life in Individuals Exposed to Malaria

ABSTRACT Severe malaria syndromes are precipitated by Plasmodium falciparum parasites binding to endothelial receptors on the vascular lining. This binding is mediated by members of the highly variant P. falciparum erythrocyte membrane protein 1 (PfEMP1) family. We have previously identified a subset of PfEMP1 proteins associated with severe malaria and found that the receptor for these PfEMP1 variants is endothelial protein C receptor (EPCR). The binding is mediated through the amino-terminal cysteine-rich interdomain region (CIDR) of the subtypes α1.1 and α1.4 to α1.8. In this study, we investigated the acquisition of anti-CIDR antibodies using plasma samples collected in four study villages with different malaria transmission intensities in northeastern Tanzania during a period with a decline in malaria transmission. We show that individuals exposed to high levels of malaria transmission acquire antibodies to EPCR-binding CIDR domains early in life and that these antibodies are acquired more rapidly than antibodies to other CIDR domains. The rate by which antibodies to EPCR-binding CIDR domains are acquired in populations in areas where malaria is endemic is determined by the malaria transmission intensity, and on a population level, the antibodies are rapidly lost if transmission is interrupted. This indicates that sustained exposure is required to maintain the production of the antibodies.

Parasite threshold associated with clinical malaria in areas of different transmission intensities in north eastern Tanzania.

BackgroundIn Sub-Sahara Africa, malaria due to Plasmodium falciparum is the main cause of ill health. Evaluation of malaria interventions, such as drugs and vaccines depends on clinical definition of the disease, which is still a challenge due to lack of distinct malaria specific clinical features. Parasite threshold is used in definition of clinical malaria in evaluation of interventions. This however, is likely to be influenced by other factors such as transmission intensity as well as individual level of immunity against malaria.MethodsThis paper describes step function and dose response model with threshold parameter as a tool for estimation of parasite threshold for onset of malaria fever in highlands (low transmission) and lowlands (high transmission intensity) strata. These models were fitted using logistic regression stratified by strata and age groups (0-1, 2-3, 4-5, 6-9, and 10-19 years). Dose response model was further extended to fit all age groups combined in each stratum. Sub-sampling bootstrap was used to compute confidence intervals. Cross-sectional and passive case detection data from Korogwe district, north eastern Tanzania were used.ResultsDose response model was better in the estimation of parasite thresholds. Parasite thresholds (scale = log parasite/μL) were high in lowlands than in highlands. In the lowlands, children in age group 4-5 years had the highest parasite threshold (8.73) while individuals aged 10-19 years had the lowest (6.81). In the highlands, children aged 0-1 years had the highest threshold (7.12) and those aged 10-19 years had the lowest (4.62). Regression analysis with all ages combined showed similar pattern of thresholds in both strata, whereby, in the lowlands the threshold was highest in age group 2-5 years and lowest in older individuals, while in the highlands was highest in age group 0-1 and decreased with increased age. The sensitivity of parasite threshold by age group ranged from 64%-74% in the lowlands and 67%-97% in the highlands; while specificity ranged between 67%-90% in the lowlands and 37%-73% in the highlands.ConclusionDose response model with threshold parameter can be used to estimate parasite threshold associated with malaria fever onset. Parasite threshold were lower in older individuals and in low malaria transmission area.