Influence of infection on malaria-specific antibody dynamics in a cohort exposed to intense malaria transmission in northern Uganda

The role of submicroscopic infections in modulating malaria antibody responses is poorly understood and requires longitudinal studies. A cohort of 249 children ≤5 years of age, 126 children between 6 and 10 years and 134 adults ≥20 years was recruited in an area of intense malaria transmission in Apac, Uganda and treated with artemether/lumefantrine at enrolment. Parasite carriage was determined at enrolment and after 6 and 16 weeks using microscopy and PCR. Antibody prevalence and titres to circumsporozoite protein, apical membrane antigen-1 (AMA-1), merozoite surface protein-1 (MSP-119 ), merozoite surface protein-2 (MSP-2) and Anopheles gambiae salivary gland protein 6 (gSG6) were determined by ELISA. Plasmodium falciparum infections were detected in 38·1% (194/509) of the individuals by microscopy and in 57·1% (284/493) of the individuals by PCR at enrolment. Antibody prevalence and titre against AMA-1, MSP-119 , MSP-2 and gSG6 were related to concurrent (sub-)microscopic parasitaemia. Responses were stable in children who were continuously infected with malaria parasites but declined in children who were never parasitaemic during the study or were not re-infected after treatment. These findings indicate that continued malaria infections are required to maintain antibody titres in an area of intense malaria transmission.

Hematological and biochemical data obtained in rural northern Uganda

Reference intervals for common hematological and clinical chemistry parameters constitute an important basis for health care. Moreover, with increasing priority in drug and vaccine development for infectious diseases in Africa, the first priority is the safety evaluation and tolerability of the candidate interventions in healthy populations. To accurately assess health status and address adverse events, clinical reference intervals in the target population are necessary. We report on hematological and biochemical indices from healthy volunteers who participated in a clinical trial in Lira, northern Uganda. Median and nonparametric 95% percentiles on five hematology and 15 biochemistry analytes are shown. Although most hematological analytes conformed to reported reference intervals and trends in Africa, literature review from different African countries highlight the need for a region-specific children reference interval that can be appropriate for the population.

Improved in vitro culture of Plasmodium falciparum permits establishment of clinical isolates with preserved multiplication, invasion and rosetting phenotypes

To be able to robustly propagate P. falciparum at optimal conditions in vitro is of fundamental importance for genotypic and phenotypic studies of both established and fresh clinical isolates. Cryo-preserved P. falciparum isolates from Ugandan children with severe or uncomplicated malaria were investigated for parasite phenotypes under different in vitro growth conditions or studied directly from the peripheral blood. The parasite cultures showed a minimal loss of parasite-mass and preserved percentage of multiple infected pRBCs to that in peripheral blood, maintained adhesive phenotypes and good outgrowth and multiplication rates when grown in suspension and supplemented with gas. In contrast, abnormal and greatly fluctuating levels of multiple infections were observed during static growth conditions and outgrowth and multiplication rates were inferior. Serum, as compared to Albumax, was found necessary for optimal presentation of PfEMP1 at the pRBC surface and/or for binding of serum proteins (immunoglobulins). Optimal in vitro growth conditions of P. falciparum therefore include orbital shaking (50 rev/min), human serum (10%) and a fixed gas composition (5% O2, 5% CO2, 90% N2). We subsequently established 100% of 76 frozen patient isolates and found rosetting with schizont pRBCs in every isolate (>26% schizont rosetting rate). Rosetting during schizogony was often followed by invasion of the bound RBC as seen by regular and time-lapse microscopy as well as transmission electron microscopy. The peripheral parasitemia, the level of rosetting and the rate of multiplication correlated positively to one another for individual isolates. Rosetting was also more frequent with trophozoite and schizont pRBCs of children with severe versus uncomplicated malaria (p<0.002; p<0.004). The associations suggest that rosetting enhances the ability of the parasite to multiply within the human host.

Phase 1b randomized trial and follow-up study in Uganda of the blood-stage malaria vaccine candidate BK-SE36

Background

Up to now a malaria vaccine remains elusive. The Plasmodium falciparum serine repeat antigen-5 formulated with aluminum hydroxyl gel (BK-SE36) is a blood-stage malaria vaccine candidate that has undergone phase 1a trial in malaria-naive Japanese adults. We have now assessed the safety and immunogenicity of BK-SE36 in a malaria endemic area in Northern Uganda.

Methods

We performed a two-stage, randomized, single-blinded, placebo-controlled phase 1b trial (Current Controlled trials ISRCTN71619711). A computer-generated sequence randomized healthy subjects for 2 subcutaneous injections at 21-day intervals in Stage1 (21–40 year-olds) to 1-mL BK-SE36 (BKSE1.0) (n = 36) or saline (n = 20) and in Stage2 (6–20 year-olds) to BKSE1.0 (n = 33), 0.5-mL BK-SE36 (BKSE0.5) (n = 33), or saline (n = 18). Subjects and laboratory personnel were blinded. Safety and antibody responses 21-days post-second vaccination (Day42) were assessed. Post-trial, to compare the risk of malaria episodes 130–365 days post-second vaccination, Stage2 subjects were age-matched to 50 control individuals.

Results

Nearly all subjects who received BK-SE36 had induration (Stage1, n = 33, 92%; Stage2, n = 63, 96%) as a local adverse event. No serious adverse event related to BK-SE36 was reported. Pre-existing anti-SE36 antibody titers negatively correlated with vaccination-induced antibody response. At Day42, change in antibody titers was significant for seronegative adults (1.95-fold higher than baseline [95% CI, 1.56–2.43], p = 0.004) and 6–10 year-olds (5.71-fold [95% CI, 2.38–13.72], p = 0.002) vaccinated with BKSE1.0. Immunogenicity response to BKSE0.5 was low and not significant (1.55-fold [95% CI, 1.24–1.94], p = 0.75). In the ancillary analysis, cumulative incidence of first malaria episodes with ≥5000 parasites/µL was 7 cases/33 subjects in BKSE1.0 and 10 cases/33 subjects in BKSE0.5 vs. 29 cases/66 subjects in the control group. Risk ratio for BKSE1.0 was 0.48 (95% CI, 0.24–0.98; p = 0.04).

Conclusion

BK-SE36 is safe and immunogenic. The promising potential of BK-SE36, observed in the follow-up study, warrants a double-blind phase 1/2b trial in children under 5 years.

Trial Registration

Controlled-Trials.com ISRCTN71619711 ISRCTN71619711

Continuing intense malaria transmission in northern Uganda

Recent reports of reductions in malaria transmission in several African countries have resulted in optimism that malaria can be eliminated in parts of Africa where it is currently endemic. It is not known whether these trends are global or whether they are also present in areas where political instability has hindered effective malaria control. We determined malaria parasite carriage and age-dependent antibody responses to Plasmodium falciparum antigens in cross-sectional surveys in Apac, northern Uganda that was affected by political unrest. Under-five parasite prevalence was 55.8% (115/206) by microscopy and 71.9% (41/57) by polymerase chain reaction. Plasmodium ovale alone, or as a co-infection, was detected in 8.6% (12/139) and Plasmodium malariae in 4.3% (6/139) of the infections. Age seroprevalence curves gave no indication of recent changes in malaria transmission intensity. Malaria control remains a tremendous challenge in areas that have not benefited from large-scale interventions, illustrated here by the district of Apac.

var gene transcription dynamics in Plasmodium falciparum patient isolates

A major feature of Plasmodium falciparum parasitized red blood cells (pRBC) is their capacity to sequester in the microcirculation. The binding is mediated by PfEMP1 (P. falciparum erythrocyte membrane protein 1), a variable protein encoded by the var gene family. P. falciparum avoids the host antibody response generated against previously used variants by switching the expression of PfEMP1, which may affect the disease outcome. We have here studied var gene transcription over time within the life cycle of the parasite by semi-quantitative PCR and sequencing by employing three sets of degenerate primers to the 5-prime end of the var genes (corresponding to the DBL1alpha-domain). To accurately determine transcript levels, subsequent in-depth analysis was made by amplifying the 10 most frequently expressed var sequences identified in each developmental stage by quantitative PCR (Q-PCR). The maximum peak in var gene transcription seems to vary in time among parasites. In five out of seven parasites, var gene transcription was found to be higher or equal at 22-26h post-invasion compared to 4-10h post-invasion. Our data indicate that the intra-isolate var gene transcription dominance order may change between different developmental stages. The transcription of var genes in field isolates is more complex than in laboratory strains and often changes after in vitro adaption of the parasite. By using semi-quantitative PCR employing degenerate primers combined with quantitative-PCR using specific primers it is possible to monitor var gene transcription in detail during the life cycle of the parasite. The work presented here suggests that trophozoite pRBC is likely to be the optimal source of RNA for predicting the translated var gene species.

PfEMP1-DBL1alpha amino acid motifs in severe disease states of Plasmodium falciparum malaria

An infection with Plasmodium falciparum may lead to severe malaria as a result of excessive binding of infected erythrocytes in the microvasculature. Vascular adhesion is mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP1), which is encoded for by highly polymorphic members of the var-gene family. Here, we profile var gene transcription in fresh P. falciparum trophozoites from Ugandan children with malaria through var-specific DBL1alpha-PCR amplification and sequencing. A method for subsectioning region alignments into homology areas (MOTIFF) was developed to examine collected sequences. Specific PfEMP1-DBL1alpha amino acid motifs correlated with rosetting and severe malaria, with motif location corresponding to distinct regions of receptor interaction. The method is potentially applicable to other families of variant proteins and may be useful in identifying sequence-phenotype relationships. The results suggest that certain PfEMP1 sequences are predisposed to inducing severe malaria.

Genome wide gene amplifications and deletions in Plasmodium falciparum

The extent to which duplications and deletions occur in the Plasmodium falciparum genome, outside of the subtelomeres, and their contribution to the virulence of the malaria parasite is not known. Here we show the presence of multiple genome wide copy number polymorphisms (CNPs) covering 82 genes, the most extensive spanning a cumulative size of 110kilobases. CNPs were identified in both laboratory strains and fresh clinical isolates using a 70-mer oligonucleotide microarray in conjunction with fluorescent in situ hybridizations and real-time quantitative PCR. The CNPs were found on all chromosomes except on chromosomes 6 and 8 and involved a total of 50 genes with increased copy numbers and 32 genes with decreased copy numbers relative to the 3D7 parasite. The genes, amplified in up to six copies, encode molecules involved in cell cycle regulation, cell division, drug resistance, erythrocyte invasion, sexual differentiation and unknown functions. These together with previous findings, suggest that the malaria parasite employs gene duplications and deletions as general strategies to enhance its survival and spread. Further analysis of the impact of discovered genetic differences and the underlying mechanisms is likely to generate a better understanding of the biology and the virulence of the malaria parasite.

New concepts in vaccine development in malaria

PURPOSE OF REVIEW

To focus on recent novel concepts in the development of malaria vaccines.

RECENT FINDINGS

There is a renewed interest in whole attenuated sporozoite vaccines, either as irradiated or genetically modified sporozoites, because they consistently elicit solid protection against challenge infections. Enthusiasm about these vaccines is, however, tempered by technical, logistical, safety and even cultural hurdles that might need to be surmounted. Less than a score of Plasmodium falciparum proteins are currently in the development pipeline as malaria vaccines. There is an urgent need to ratchet up the process of candidate vaccine discovery, and reverse vaccinology and genome-wide surveys remain promising strategies. The development of malaria vaccines for placental malaria is an active area and chondroitin sulfate A-binding epitopes of the variant PfEMP1 have been identified. Live bacteria and viral vectors hold special promise for vaccine delivery.

SUMMARY

Attenuated sporozoite vaccines have made a resurgence to center stage in malaria vaccine development. There is an urgent need to identify more subunit vaccine candidates that can enter into the development pipeline, identify surrogate markers of immunity and design vaccines which induce long-lasting immunity.

Release of sequestered malaria parasites upon injection of a glycosaminoglycan

Severe human malaria is attributable to an excessive sequestration of Plasmodium falciparum–infected and uninfected erythrocytes in vital organs. Strains of P. falciparum that form rosettes and employ heparan sulfate as a host receptor are associated with development of severe forms of malaria. Heparin, which is similar to heparan sulfate in that it is composed of the same building blocks, was previously used in the treatment of severe malaria, but it was discontinued due to the occurrence of serious side effects such as intracranial bleedings. Here we report to have depolymerized heparin by periodate treatment to generate novel glycans (dGAG) that lack anticoagulant-activity. The dGAGs disrupt rosettes, inhibit merozoite invasion of erythrocytes and endothelial binding of P. falciparum–infected erythrocytes in vitro, and reduce sequestration in in vivo models of severe malaria. An intravenous injection of dGAGs blocks up to 80% of infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestered parasites into circulation. P. falciparum–infected human erythrocytes that sequester in the non-human primate Macaca fascicularis were similarly found to be released in to the circulation upon a single injection of 500 μg of dGAG. We suggest dGAGs to be promising candidates for adjunct therapy in severe malaria.

Severe Plasmodium falciparum malaria is common and in part the result of an excessive binding of infected erythrocytes in the microvasculature. The parasite employs heparan sulfate during the adherence to the vascular endothelium and to erythrocytes. Heparin, which is related to heparan sulfate in that it is composed of the same building blocks, was here periodate-treated to generate depolymerized glycosaminoglycans (dGAGs) that possess no anticoagulant activity. The dGAGs disrupt erythrocyte and endothelial binding of P. falciparum–infected erythrocytes in vitro. An intravenous injection of dGAGs blocks infected erythrocytes from binding in the micro-vasculature of the rat and releases already sequestrated parasites into circulation both in the rat and in a non-human primate. If this approach is successfully translated to the clinical setting, it may offer help to patients whereby the injection of a dGAG releases already sequestered parasite-infected erythrocytes and re-establishes the micro-vascular blood flow. The authors suggest dGAGs to be promising candidates of adjunct therapy that may have an important impact on malaria mortality.

Pigmented monocytes are negative correlates of protection against severe and complicated malaria in Ugandan children

Pigmented leukocytes are reported to be associated with severe malaria (SM). Blood smears from a case-control study of SM conducted in Apac Hospital in Northern Uganda were examined for pigmented leukocytes to investigate their association with measures of disease and clinical immunity in children less than 5 years old. Pigmented leukocytes, predominated by monocytes, were significantly greater in number in SM by comparison with uncomplicated malaria (UM). SM children with no pigmented leukocytes had significantly elevated hemoglobin, packed cell volumes, and titers of IgG anti-SERA5 by comparison with SM children with pigmented leukocytes. These differences were not observed in UM. A Spearman rank correlation analysis showed, in addition, a negative but weak correlation between pigmented monocytes and titers of IgG anti-Plasmodium falciparum lysate and IgG anti-EBA-175 in both SM and UM children. Thus, numbers of pigmented monocytes might be negative correlates of clinical immunity in a region of holoendemic malaria.

High titers of IgG antibodies against Plasmodium falciparum serine repeat antigen 5 (SERA5) are associated with protection against severe malaria in Ugandan children

Plasmodium falciparum serine repeat antigen (SERA5) is a promising asexual blood stage malaria candidate vaccine. However, there is a paucity of information about natural immune responses to SERA5 in children from malaria-endemic regions. We undertook a hospital-based case-control study of severe malaria in Apac District, Northern Uganda, in children 6-59 months of age. The commonest symptoms observed in children with severe malaria (SM) were respiratory distress (53.4%) and prostration (40.4%) followed by circulatory collapse (7.4%), severe anemia (Hb < 5 g/dL, 7.0%), and seizures (2.6%). None of the SM children had impaired consciousness, coma, or cerebral malaria. We measured serum IgG antibodies using a recombinant construct of SERA5 (SE36) in enzyme-linked immunosorbent assays. High titers of IgG anti-SE36 were associated with protection against severe malaria in children under 5 years old.

Two mutations in dihydrofolate reductase combined with one in the dihydropteroate synthase gene predict sulphadoxine–pyrimethamine parasitological failure in Ugandan children with uncomplicated falciparum malaria

The point mutations in the Plasmodium falciparum dihydrofolate reductase (dhfr) and the dihydropteroate synthase (dhps) genes that are linked to sulphadoxine-pyrimethamine (SP) resistance in vitro have been well characterised. To determine whether a few of these mutations could predict SP treatment failure in vivo, two mutations (Asn-108 and Arg-59) in the dhfr gene and one (Glu-540) in the dhps gene were analysed according to the risk of SP parasitological failure (RI-RIII) at day 28 in pre-treatment isolates in 79 Ugandan children aged 6-59 (mean = 18.4, S.D. = 8.8) months with uncomplicated falciparum malaria. Neither the dhfr-108 (P = 0.3) nor the dhps-540 (P = 0.6) or dhfr-108 + dhps-540 (P = 0.04) mutations were significantly associated with SP parasitological failure. However, the dhfr-108 + dhfr-59 (P = 0.04), the dhfr-59 + dhps-540 (P = 0.04) and the dhfr-108 + dhfr-59 + dhps-540 (P = 0.02) mutations significantly increased the risk for SP parasitological failure. Our findings confirm an earlier report that the dhfr-59 and the dhps-540 mutations could be useful genetic markers for rapid screening of populations at high risk of SP resistance.

Population-based validation of dihydrofolate reductase gene mutations for the prediction of sulfadoxine-pyrimethamine resistance in Uganda

Mutations in the dihydrofolate reductase gene (dhfr) of Plasmodium falciparum have been proposed as molecular markers for the surveillance of sulfadoxine-pyrimethamine (SP)-resistant malaria, but such proposals have not been validated. At 7 Ugandan sites in 1999, we determined the population-based prevalence of infections with mutations and the mutant allele frequency of dhfr codons 108, 51, and 59 using a random sample of infected individuals aged 1-45 years. Sulfadoxine-pyrimethamine treatment failure was independently estimated by in vivo tests in 327 children aged 6-59 months with clinical malaria. The prevalence of infections with the single point mutations and the dhfr codons 108 and 51 mutant allele frequency were not correlated to SP treatment failure. However, the dhfr codon 59 mutant allele frequency was positively correlated to SP treatment failure (r = 0.72, P = 0.06). The ratio of the infections with the mutant to wild genotype (M/W) and that of the mutant to wild allele (MA/WA) had the same values. Both dhfr codon 59 M/W and MA/WA ratio were significantly and positively correlated to SP treatment failure (r = 0.73, P = 0.05). Moreover, the prevalence of infections with only 2 mutations (Asn-108 plus Ile-51) was significantly and inversely correlated to the prevalence of infections with 3 mutations (Asn-108 plus Ile-51 plus Arg-59) (r = 0.92, P = 0.004), suggesting the stepwise accumulation of the dhfr mutations is Asn-108 Ile-51 Arg-59 and further supporting the idea of using the dhfr codon 59 M/W ratio as a molecular index for the prediction of SP treatment failure. Atthe population level, the dhfr codon 59 M/W ratio is a simple and stable index for the estimation of SP treatment failure.

Fine specificity of serum antibodies to Plasmodium falciparum merozoite surface protein, PfMSP-1(19), predicts protection from malaria infection and high-density parasitemia

Antibodies to the C terminus of the Plasmodium falciparum merozoite surface protein, PfMSP-1(19), may inhibit merozoite invasion or block the effects of inhibitory antibodies. Here, using a competition enzyme-linked immunosorbent assay and antibody binding to wild-type and mutated recombinant proteins, we show that there are marked variations between individuals in the fine specificity of naturally acquired anti-MSP-1(19) antibodies. Furthermore, although neither the prevalence nor the concentration of total anti-MSP-1(19) antibodies was associated with resistance to malaria in African children, significant associations were observed between antibody fine specificity and subsequent risk of infection and high-density parasitemia during a follow-up period. Thus, the fine specificity of naturally acquired human anti-MSP-1(19) antibodies is crucial in determining their function. Future field studies, including the evaluation of PfMSP-1 vaccine trials, should include assays that explore antibody fine specificity as well as titer.

Efficacy of sulphadoxine-pyrimethamine alone or combined with amodiaquine or chloroquine for the treatment of uncomplicated falciparum malaria in Ugandan children

The rapid development of falciparum resistance to sulphadoxine-pyrimethamine (SP) in East and Central Africa has raised concerns as to the efficacy of combining it with another drug. In 2002, we assessed the efficacy of SP alone and combined with amodiaquine (AQ/SP) or chloroquine (CQ/SP) in Ugandan children with uncomplicated falciparum malaria. At day 14, adequate clinical response was 100% (84/84) for AQ/SP, 93% (92/101) for CQ/SP and 91% (73/80) for SP. At day 28, parasitological failure (RI-RIII) occurred in 16% (13/80) of children treated with AQ/SP, in 48% (48/100) of those treated with CQ/SP and in 61% (48/79) of those treated with SP alone. Compared with the AQ/SP arm, the odds for parasitological failure at day 28 were five times higher (95% CI, 2-10) in the CQ/SP group and sevenfold higher (95% CI, 3-17) in that of SP alone. CQ/SP does not offer any significant added benefit over SP alone while AQ/SP is an efficacious low-cost combination. These findings have important policy implications for Uganda and other resource-constrained African countries faced with the problematic choice of a new first-line antimalarial treatment in a context of high CQ resistance.

Intensity of transmission and spread of gene mutations linked to chloroquine and sulphadoxine-pyrimethamine resistance in falciparum malaria

The number of malaria parasite clones per infection-multiplicity of parasite clones-is affected by the transmission intensity, multiplicity increases with increasing transmission. This affects the frequency of parasites' sexual recombination and, if several mutations in different genes are involved, can break down drug resistant genotypes. Therefore, the effects of malaria transmission intensity on the spread of drug resistance could vary depending on the number of genes involved. Here we show that, compared to low transmission, intermediate-high transmission is associated with a 20-100-fold lower risk for the mutations linked to chloroquine resistance and a 6-17 times higher risk for those linked to sulphadoxine-pyrimethamine resistance. This is consistent with the hypothesis of a multigenic basis for chloroquine resistance and a monogenic basis for that of sulphadoxine-pyrimethamine. Reducing transmission intensity could slow the spread of resistance. However, a reduction below a critical threshold (e.g. when parasite prevalence in children 2-9 years old is around 60-80%) could, paradoxically, accelerate the spread of resistance to chloroquine and possibly to other drug combinations whose basis is multigenic. Our findings have important implications for malaria control because increasing drug resistance has a substantial impact on mortality.

Serine Repeat Antigen (SERA5) Is Predominantly Expressed among the SERA Multigene Family of Plasmodium falciparum, and the Acquired Antibody Titers Correlate with Serum Inhibition of the Parasite Growth

The Plasmodium falciparum serine repeat antigen (SERA) is one of the blood stage malaria vaccine candidates. The malaria genome project has revealed that SERA is a member of the SERA multigene family consisting of eight SERA homologues clustered on chromosome 2 and one SERA homologue on chromosome 9. Northern blotting and real time quantitative reverse transcription-PCR with five independent parasite strains, including three allelic representative forms of the SERA gene, have shown that all of the SERA homologues are transcribed most actively at trophozoite and schizont stages and that SERA5 (SERA/SERP) is transcribed predominantly among the family. Polyclonal antibodies were raised against recombinant proteins representing the N-terminal portions of four significantly transcribed SERA homologues (SERA3 to -6) in the center of the cluster on chromosome 2. Using these antibodies, indirect immunofluorescence microscopy detected the expression of SERA3 to -6, with similar localization, in all trophozoite- and schizont-infected erythrocytes. We have examined 40 sera from Ugandan adults for their antibody reactivity and found that enzyme-linked immunosorbent assay titer against SERA5 N-terminal domain, but not against other SERA proteins, is positively correlated with the inhibition of in vitro parasite growth by individual sera. Our data confirm the usefulness of the N-terminal domain of SERA5 as a promising malaria candidate vaccine.

The Lys-76-Thr mutation in PfCRT and chloroquine resistance in Plasmodium falciparum isolates from Uganda

Recent molecular studies of chloroquine (CQ) resistance of Plasmodium falciparum have demonstrated an association between a mutation in the PfCRT gene and CQ resistance. We identified wild type and mutant alleles of the PfCRT codon 76 in baseline pre-CQ treatment P. falciparum isolates collected during 1999 and investigated their relationship to CQ efficacy in 3 different sites with different levels of CQ parasite resistance in Uganda. Of 32 isolates from Mulago Hospital, all were mutant (100%), while of 45 isolates from Tororo, 5 (11%) were mixed wild type and mutant and 40 (89%) were mutants only. Of 41 isolates from Apac, 13 (32%) were mixed wild type and mutant whereas 28 (68%) were mutants only. The finding of 100% prevalence of the Thr-76 mutant allele in all isolates at the 3 sites was remarkable. We found no association between the presence of Thr-76 mutation and treatment outcome at all the sites. However, the prevalence of the wild-type Lys-76 allele was higher in Apac, an area with lower CQ parasite resistance, compared to Tororo and Mulago which have relatively higher CQ parasite resistance. The Thr-76 allele as a marker of CQ resistance is probably useful in regions where the allele frequency has not yet plateaued.

Natural human immunoglobulin G subclass responses to Plasmodium falciparum serine repeat antigen in Uganda

Serum samples from Ugandan residents of a malaria-hyperendemic region were tested by enzyme-linked immunosorbent assay for reactivity against recombinant constructs of the 47 (SE47')- and 50 (SE50A)-kDa fragments of Plasmodium falciparum serine repeat antigen (SERA). Immunoglobulin (Ig) G3 and IgG1 were the predominant subclass responses to SE47' and SE50A, respectively. The geometric mean optical density (OD) for IgG3 anti-SE47' was significantly lower in children < 15 years compared with adults > or = 15 years (P < 0.0001). By contrast, the geometric mean IgG1 anti-SE50A was slightly higher in children compared with adults (P < 0.01). The proportion of high responders (ODs > 0.5) to SE47' was significantly lower in children compared with adults (P < 0.001), whereas the proportion of high responders to SE50A was comparable in children and adults (P = 0.07). This first detailed study of SERA in a malaria-hyperendemic region suggests that natural human IgG3 anti-SE47' might be associated with immunity to malaria.

Plasmodium falciparum malariometric indices in Apac district, northern Uganda

OBJECTIVE

To establish Plasmodium falciparum malariometric indices in a field study site in Apac district, northern Uganda.

DESIGN

A community-based cross sectional survey.

SETTINGS

Atopi Parish, Apac district, Uganda, 1995.

SUBJECTS

One thousand two hundred and thirty four volunteers aged below one and ninety years.

MAIN OUTCOME MEASURES

P. falciparum parasitaemia rates and parasite density, splenomegaly, bednet use and chloroquine consumption.

INTERVENTIONS

All subjects with P. falciparum positive smears were treated with chloroquine.

RESULTS

The population prevalence of parasitaemia was 62.1% with the predominant species being P. falciparum (100%) and P. malariae in the minority (3.5%); P. ovale was not seen. The prevalence of parasitaemia in subjects older than 20 years and in those under ten years was 36% and 85%, respectively. The geometric mean parasite density started to decline by the age of six years. The splenomegaly rate in subjects over the age of 12 years and in those under nine years was 19.8% and 63.1%, respectively. Bednet use and chloroquine consumption was low. Interestingly, the reported use of chloroquine in the week immediately preceding the study was more frequent in children under two years old than in the rest of the population.

CONCLUSION

Malaria transmission in Atopi Parish in northern Uganda is hyperendemic and age-related acquired anti-parasite immunity seems to appear by seven years of age.

[Relationship between the intensity of Loa loa filariasis transmission and prevalence of infections]

Filarial loiasis differs from other filariases in that most infected subjects are amicrofilaremic. This difference raises the notion of occult infection. The aim of this study was to evaluate the relationship between the intensity of transmission and incidence of infection. For this purpose we determined the incidence of loiasis both microscopically and by PCR in 201 subjects from three villages in the province of Haut Ogooue in Gabon. Intensity of transmission, expressed in ATP (annual transmission potential) in these villages was estimated to be 250 infecting larvae per individual per year (L3/man/yr) in Moyabi, 180 L3/man/yr in N'dokaye, and 43,000 L3/man/yr in Okoumbi. Although there was no significant difference between the three villages with regard to the incidence of microfilaremia (21 p. 100 and 22 p. 100), the incidence of occult infection, i.e., positive PCR in amicrofilaremic subjects, was 45 p. 100 in Moyabi, 79 p. 100 in N'dokaye and 80 p. 100 in Okoumbi. The overall incidence of loiasis was 57 p. 100 in Moyabi and 85 p. 100 in both N'dokaye and Okoumbi. These findings demonstrate that the incidence of loiasis is correlated with the intensity of transmission (p < 0.001), especially in children. Taking this information into account will improve control of Loa loa in endemic areas.