Monitoring of malaria parasite resistance to chloroquine and sulphadoxine-pyrimethamine in the Solomon Islands by DNA microarray technology

Background

Little information is available on resistance to anti-malarial drugs in the Solomon Islands (SI). The analysis of single nucleotide polymorphisms (SNPs) in drug resistance associated parasite genes is a potential alternative to classical time- and resource-consuming in vivo studies to monitor drug resistance. Mutations in pfmdr1 and pfcrt were shown to indicate chloroquine (CQ) resistance, mutations in pfdhfr and pfdhps indicate sulphadoxine-pyrimethamine (SP) resistance, and mutations in pfATPase6 indicate resistance to artemisinin derivatives.

Methods

The relationship between the rate of treatment failure among 25 symptomatic Plasmodium falciparum-infected patients presenting at the clinic and the pattern of resistance-associated SNPs in P. falciparum infecting 76 asymptomatic individuals from the surrounding population was investigated. The study was conducted in the SI in 2004. Patients presenting at a local clinic with microscopically confirmed P. falciparum malaria were recruited and treated with CQ+SP. Rates of treatment failure were estimated during a 28-day follow-up period. In parallel, a DNA microarray technology was used to analyse mutations associated with CQ, SP, and artemisinin derivative resistance among samples from the asymptomatic community. Mutation and haplotype frequencies were determined, as well as the multiplicity of infection.

Results

The in vivo study showed an efficacy of 88% for CQ+SP to treat P. falciparum infections. DNA microarray analyses indicated a low diversity in the parasite population with one major haplotype present in 98.7% of the cases. It was composed of fixed mutations at position 86 in pfmdr1, positions 72, 75, 76, 220, 326 and 356 in pfcrt, and positions 59 and 108 in pfdhfr. No mutation was observed in pfdhps or in pfATPase6. The mean multiplicity of infection was 1.39.

Conclusion

This work provides the first insight into drug resistance markers of P. falciparum in the SI. The obtained results indicated the presence of a very homogenous P. falciparum population circulating in the community. Although CQ+SP could still clear most infections, seven fixed mutations associated with CQ resistance and two fixed mutations related to SP resistance were observed. Whether the absence of mutations in pfATPase6 indicates the efficacy of artemisinin derivatives remains to be proven.

Detectability of Plasmodium falciparum clones

BACKGROUND

In areas of high transmission people often harbour multiple clones of Plasmodium falciparum, but even PCR-based diagnostic methods can only detect a fraction (the detectability, q) of all clones present in a host. Accurate measurements of detectability are desirable since it affects estimates of multiplicity of infection, prevalence, and frequency of breakthrough infections in clinical drug trials. Detectability can be estimated by typing repeated samples from the same host but it has been unclear what should be the time interval between the samples and how the data should be analysed.

METHODS

A longitudinal molecular study was conducted in the Kassena-Nankana district in northern Ghana. From each of the 80 participants, four finger prick samples were collected over a period of 8 days, and tested for presence of different Merozoite Surface Protein (msp) 2 genotypes. Implications for estimating q were derived from these data by comparing the fit of statistical models of serial dependence and over-dispersion.

RESULTS

The distribution of the frequencies of detection for msp2 genotypes was close to binomial if the time span between consecutive blood samples was at least 7 days. For shorter intervals the probabilities of detection were positively correlated, i.e. the shorter the interval between two blood collections, the more likely the diagnostic results matched for a particular genotype. Estimates of q were rather insensitive to the statistical model fitted.

CONCLUSIONS

A simple algorithm based on analysing blood samples collected 7 days apart is justified for generating robust estimates of detectability. The finding of positive correlation of detection probabilities for short time intervals argues against imperfect detection being directly linked to the 48-hour periodicity of P. falciparum. The results suggest that the detectability of a given parasite clone changes over time, at an unknown rate, but fast enough to regard blood samples taken one week apart as statistically independent.

Serodiagnosis of Echinococcus spp. infection: explorative selection of diagnostic antigens by peptide microarray

Background

Production of native antigens for serodiagnosis of helminthic infections is laborious and hampered by batch-to-batch variation. For serodiagnosis of echinococcosis, especially cystic disease, most screening tests rely on crude or purified Echinococcus granulosus hydatid cyst fluid. To resolve limitations associated with native antigens in serological tests, the use of standardized and highly pure antigens produced by chemical synthesis offers considerable advantages, provided appropriate diagnostic sensitivity and specificity is achieved.

Methodology/Principal Findings

Making use of the growing collection of genomic and proteomic data, we applied a set of bioinformatic selection criteria to a collection of protein sequences including conceptually translated nucleotide sequence data of two related tapeworms, Echinococcus multilocularis and Echinococcus granulosus. Our approach targeted alpha-helical coiled-coils and intrinsically unstructured regions of parasite proteins potentially exposed to the host immune system. From 6 proteins of E. multilocularis and 5 proteins of E. granulosus, 45 peptides between 24 and 30 amino acids in length were designed. These peptides were chemically synthesized, spotted on microarrays and screened for reactivity with sera from infected humans. Peptides reacting above the cut-off were validated in enzyme-linked immunosorbent assays (ELISA). Peptides identified failed to differentiate between E. multilocularis and E. granulosus infection. The peptide performing best reached 57% sensitivity and 94% specificity. This candidate derived from Echinococcus multilocularis antigen B8/1 and showed strong reactivity to sera from patients infected either with E. multilocularis or E. granulosus.

Conclusions/Significance

This study provides proof of principle for the discovery of diagnostically relevant peptides by bioinformatic selection complemented with screening on a high-throughput microarray platform. Our data showed that a single peptide cannot provide sufficient diagnostic sensitivity whereas pooling several peptide antigens improved sensitivity; thus combinations of several peptides may lead the way to new diagnostic tests that replace, or at least complement conventional immunodiagnosis of echinococcosis. Our strategy could prove useful for diagnostic developments in other pathogens.

Crude or purified, somatic or metabolic extracts of native antigens are routinely used for the serodiagnosis of human helminthic infections. These antigens are often cross-reactive, i.e., recognized by sera from patients infected with heterologous helminth species. To overcome limitations in antigen production, test sensitivity and specificity, chemically synthesized peptides offer a pure and standardized alternative, provided they yield acceptable operative characteristics. Ongoing genome and proteome work create new resources for the identification of antigens. Making use of the growing amount of genomic and proteomic data available in public databases, we tested a bioinformatic procedure for the selection of potentially antigenic peptides from a collection of protein sequences including conceptually translated nucleotide sequence data of Echinococcus multilocularis and E. granulosus (Plathyhelminthes, Cestoda). The in silico selection was combined with high-throughput screening of peptides on microarray and systematic validation of reactive candidates in enzyme-linked immunosorbent assay. Our study proved the applicability of this approach for selection of peptide antigens with good diagnostic characteristics. Our results suggested the pooling of several peptides to reach a high level of sensitivity required for reliable immunodiagnosis.

Treatment with coartem (artemether-lumefantrine) in Papua New Guinea

A recent drug efficacy trial reported Coartem (artemether-lumefantrine) to be highly effective against Plasmodium falciparum in children less than 5 years of age in Papua New Guinea (PNG). In contrast, we have observed high levels of treatment failures in non-trial conditions in a longitudinal cohort study in the same age group in PNG. Recrudescences were confirmed by genotyping of three different marker genes to provide optimal discrimination power between parasite clones. After excluding genetic host factors by genotyping potentially relevant cytochrome P450 loci, the high number of treatment failures in our study is best explained by poor adherence to complex dosing regimens in combination with insufficient fat supplementation, which are both crucial parameters for the outcome of Coartem treatment. In contrast to the situation in classic drug trials with ideal treatment conditions, our field survey highlights potential problems with unsupervised usage of Coartem in routine clinical practice and under program conditions.

Differential patterns of infection and disease with P. falciparum and P. vivax in young Papua New Guinean children

Background

Where P. vivax and P. falciparum occur in the same population, the peak burden of P. vivax infection and illness is often concentrated in younger age groups. Experiences from malaria therapy patients indicate that immunity is acquired faster to P. vivax than to P. falciparum challenge. There is however little prospective data on the comparative risk of infection and disease from both species in young children living in co-endemic areas.

Methodology/Principal Findings

A cohort of 264 Papua New Guinean children aged 1-3 years (at enrolment) were actively followed-up for Plasmodium infection and febrile illness for 16 months. Infection status was determined by light microscopy and PCR every 8 weeks and at each febrile episode. A generalised estimating equation (GEE) approach was used to analyse both prevalence of infection and incidence of clinical episodes. A more pronounced rise in prevalence of P. falciparum compared to P. vivax infection was evident with increasing age. Although the overall incidence of clinical episodes was comparable (P. falciparum: 2.56, P. vivax 2.46 episodes / child / yr), P. falciparum and P. vivax infectious episodes showed strong but opposing age trends: P. falciparum incidence increased until the age of 30 months with little change thereafter, but incidence of P. vivax decreased significantly with age throughout the entire age range. For P. falciparum, both prevalence and incidence of P. falciparum showed marked seasonality, whereas only P. vivax incidence but not prevalence decreased in the dry season.

Conclusions/Significance

Under high, perennial exposure, children in PNG begin acquiring significant clinical immunity, characterized by an increasing ability to control parasite densities below the pyrogenic threshold to P. vivax, but not to P. falciparum, in the 2^nd and 3^rd year of life. The ability to relapse from long-lasting liver-stages restricts the seasonal variation in prevalence of P. vivax infections.

Comparison of Plasmodium falciparum allelic frequency distribution in different endemic settings by high-resolution genotyping

Background

The diversity of genotyping markers of Plasmodium falciparum depends on transmission intensity. It has been reported that the diversity of the merozoite surface protein 2 (msp2) is greater in areas of high compared to low endemicity, however, results for msp1 were inconsistent. These previous reports relied on low resolution genotyping techniques.

Methods

In the present study, a high-resolution capillary electrophoresis-based technique was applied to genotype samples from areas of different endemicity in Papua New Guinea and Tanzania. For both endemic settings, the diversity of msp1 and msp2 was investigated; the mean multiplicity of infection (MOI) and the F_ST values were determined to investigate whether more accurate sizing generates different results.

Results and Conclusion

The results of the present study confirmed previous reports of a higher mean MOI for both marker genes and increased genetic diversity in areas of higher endemicity as estimated by the total number of distinct alleles for msp2. For msp1 a minor increase in diversity was observed. Measures of between population variance in allele frequencies (F_ST) indicated little genetic differentiation for both marker genes between the two populations from different endemic settings. MOI adjusted for the probability of multiple infections sharing the same allele was estimated by using the msp2 allele frequency distribution and the distribution of observed numbers of concurrent infections. For the high-resolution typing technique applied in this study, this adjustment made little difference to the estimated mean MOI compared to the observed mean MOI.

Sequence conservation in Plasmodium falciparum alpha-helical coiled coil domains proposed for vaccine development

Background

The availability of the P. falciparum genome has led to novel ways to identify potential vaccine candidates. A new approach for antigen discovery based on the bioinformatic selection of heptad repeat motifs corresponding to α-helical coiled coil structures yielded promising results. To elucidate the question about the relationship between the coiled coil motifs and their sequence conservation, we have assessed the extent of polymorphism in putative α-helical coiled coil domains in culture strains, in natural populations and in the single nucleotide polymorphism data available at PlasmoDB.

Methodology/Principal Findings

14 α-helical coiled coil domains were selected based on preclinical experimental evaluation. They were tested by PCR amplification and sequencing of different P. falciparum culture strains and field isolates. We found that only 3 out of 14 α-helical coiled coils showed point mutations and/or length polymorphisms. Based on promising immunological results 5 of these peptides were selected for further analysis. Direct sequencing of field samples from Papua New Guinea and Tanzania showed that 3 out of these 5 peptides were completely conserved. An in silico analysis of polymorphism was performed for all 166 putative α-helical coiled coil domains originally identified in the P. falciparum genome. We found that 82% (137/166) of these peptides were conserved, and for one peptide only the detected SNPs decreased substantially the probability score for α-helical coiled coil formation. More SNPs were found in arrays of almost perfect tandem repeats. In summary, the coiled coil structure prediction was rarely modified by SNPs. The analysis revealed a number of peptides with strictly conserved α-helical coiled coil motifs.

Conclusion/Significance

We conclude that the selection of α-helical coiled coil structural motifs is a valuable approach to identify potential vaccine targets showing a high degree of conservation.

Evaluation of Plasmodium vivax genotyping markers for molecular monitoring in clinical trials

BACKGROUND

Many antimalarial interventions are accompanied by molecular monitoring of parasite infections, and a number of molecular typing techniques based on different polymorphic marker genes are used. Here, we describe a genotyping technique that provides a fast and precise approach to study Plasmodium vivax infection dynamics during circumstances in which individual clones must be followed over time. The method was tested with samples from an in vivo drug efficacy study.

METHODS

The sizes of polymerase chain reaction fragments were evaluated by capillary electrophoresis to determine the extent of size polymorphism for 9 potential genetic markers (5 genes of merozoite surface proteins [msp] and 4 microsatellites) in 93-108 P. vivax-positive blood samples from 3 villages in Papua New Guinea.

RESULTS

The microsatellites MS16 and Pv3.27 showed the greatest diversity in the study area, with 66 and 31 different alleles, respectively, followed by 2 fragments of msp1 and 2 other microsatellites. msp3alpha, msp4, and msp5 revealed limited polymorphism.

CONCLUSIONS

Even for the most diverse markers, the highest allelic frequencies reached 6% (MS16) or 13% (Pv3.27). To reduce the theoretical probability of superinfection with parasites that have the same haplotype as that detected at baseline, we propose to combine at least 2 markers for genotyping individual P. vivax infections.

Optimization and validation of multi-coloured capillary electrophoresis for genotyping of Plasmodium falciparum merozoite surface proteins (msp1 and 2)

Background

Genotyping of Plasmodium falciparum based on PCR amplification of the polymorphic genes encoding the merozoite surface proteins 1 and 2 (msp1 and msp2) is well established in the field of malaria research to determine the number and types of concurrent clones in an infection. Genotyping is regarded essential in anti-malarial drug trials to define treatment outcome, by distinguishing recrudescent parasites from new infections. Because of the limitations in specificity and resolution of gel electrophoresis used for fragment analysis in most genotyping assays it became necessary to improve the methodology. An alternative technique for fragment analysis is capillary electrophoresis (CE) performed using automated DNA sequencers. Here, one of the most widely-used protocols for genotyping of P. falciparum msp1 and msp2 has been adapted to the CE technique. The protocol and optimization process as well as the potentials and limitations of the technique in molecular epidemiology studies and anti-malarial drug trials are reported.

Methods

The original genotyping assay was adapted by fluorescent labeling of the msp1 and msp2 allelic type specific primers in the nested PCR and analysis of the final PCR products in a DNA sequencer. A substantial optimization of the fluorescent assay was performed. The CE method was validated using known mixtures of laboratory lines and field samples from Ghana and Tanzania, and compared to the original PCR assay with gel electrophoresis.

Results

The CE-based method showed high precision and reproducibility in determining fragment size (< 1 bp). More genotypes were detected in mixtures of laboratory lines and blood samples from malaria infected children, compared to gel electrophoresis. The capacity to distinguish recrudescent parasites from new infections in an anti-malarial drug trial was similar by both methods, resulting in the same outcome classification, however with more precise determination by CE.

Conclusion

The improved resolution and reproducibility of CE in fragment sizing allows for comparison of alleles between separate runs and determination of allele frequencies in a population. The more detailed characterization of individual msp1 and msp2 genotypes may contribute to improved assessments in anti-malarial drug trials and to a further understanding of the molecular epidemiology of these polymorphic P. falciparum antigens.

Monkey malaria in a European traveler returning from Malaysia

In 2007, a Finnish traveler was infected in Peninsular Malaysia with Plasmodium knowlesi, a parasite that usually causes malaria in monkeys. P. knowlesi has established itself as the fifth Plasmodium species that can cause human malaria. The disease is potentially life-threatening in humans; clinicians and laboratory personnel should become more aware of this pathogen in travelers.

The N-terminal domain of Plasmodium falciparum circumsporozoite protein represents a target of protective immunity

The N-terminal domain of the circumsporozoite protein (CSP) has been largely neglected in the search for a malaria vaccine in spite of being a target of inhibitory antibodies and protective T cell responses in mice. Thus, in order to develop this region as a vaccine candidate to be eventually associated with other candidates and, in particular, with the very advanced C-terminal counterpart, synthetic constructs representing N- and C-terminal regions of Plasmodium falciparum and Plasmodium berghei CSP were administered as single or combined formulations in mice. We show that the antisera generated against the combinations inhibit sporozoite invasion of hepatocytes in vitro better than antisera against single peptides. Furthermore, two different P. falciparum CSP N-terminal constructs (PfCS22-110 and PfCS65-110) were recognized by serum samples from people living in malaria-endemic regions. Importantly, recognition of the short N-terminal peptide (PfCS65-110) by sera from children living in a malaria-endemic region was associated with protection from disease. Taken together, these results underline the potential of using such fragments as malaria vaccine candidates.

Heterogeneous distribution of Plasmodium falciparum drug resistance haplotypes in subsets of the host population

Background

The emergence of drug resistance is a major problem in malaria control. For mathematical modelling of the transmission and spread of drug resistance the determinant parameters need to be identified and measured. The underlying hypothesis is that mutations associated with drug resistance incur fitness costs to the parasite in absence of drug pressure. The distribution of drug resistance haplotypes in different subsets of the host population was investigated. In particular newly acquired haplotypes after radical cure were characterized and compared to haplotypes from persistent infections.

Methods

Mutations associated with antimalarial drug resistance were analysed in parasites from children, adults, and new infections occurring after treatment. Twenty-five known single nucleotide polymorphisms from four Plasmodium falciparum genes associated with drug resistance were genotyped by DNA chip technology.

Results

Haplotypes were found to differ between subsets of the host population. A seven-fold mutated haplotype was significantly reduced in adults compared to children and new infections, whereas parasites harbouring fewer mutations were more frequent in adults.

Conclusion

The reduced frequency of highly mutated parasites in chronic infections in adults is likely a result of fitness costs of drug resistance that increases with number of mutations and is responsible for reduced survival of mutant parasites.

Identification of differentially expressed genes in a Giardia lamblia WB C6 clone resistant to nitazoxanide and metronidazole

OBJECTIVES

The characterization of differential gene expression in Giardia lamblia WB C6 strain C4 resistant to metronidazole and nitazoxanide using microarray technology and quantitative real-time PCR.

METHODS

In a previous study, we created and characterized the G. lamblia WB C6 clone C4 resistant to nitazoxanide and metronidazole. In this study, using a microarray-based approach, we have identified open-reading frames (ORFs) that were differentially expressed in C4 when compared with its wild-type WB C6. Using quantitative real-time PCR, we have validated the expression patterns of some of those ORFs, focusing on chaperones such as heat-shock proteins in wild-type and C4 trophozoites. In order to induce an antigenic shift, trophozoites of both strains were subjected to a cycle of en- and excystation. Expression of selected genes and resistance to nitazoxanide and metronidazole were investigated after this cycle.

RESULTS

Forty of a total of 9115 ORFs were found to be up-regulated and 46 to be down-regulated in C4 when compared with wild-type. After a cycle of en- and excystation, resistance of C4 to nitazoxanide and metronidazole was lost. Resistance formation and en-/excystation were correlated with changes in expression of ORFs encoding for major surface antigens such as the variant surface protein TSA417 or AS7 ('antigenic shift'). Moreover, expression patterns of the cytosolic heat-shock protein HSP70 B2, HSP40, and of the previously identified nitazoxanide-binding proteins nitroreductase and protein disulphide isomerase PDI4 were correlated with resistance and loss of resistance after en-/excystation. C4 trophozoites had a higher thermotolerance level than wild-type trophozoites. After en-/excystation, this tolerance was lost.

CONCLUSIONS

These results suggest that resistance formation in Giardia to nitazoxanide and metronidazole is correlated with altered expression of genes involved in stress response such as heat-shock proteins.

Rapid microarray-based method for monitoring of all currently known single-nucleotide polymorphisms associated with parasite resistance to antimalaria drugs

Parasite drug resistance is partly conferred by single-nucleotide polymorphisms (SNPs), and monitoring them has been proposed as an alternative to monitoring drug resistance. Therefore, techniques are required to facilitate analyses of multiple SNPs on an epidemiological scale. We report a rapid and affordable microarray technique for application in epidemiological studies of malaria drug resistance. We have designed a multiwell microarray that is used in conjunction with PCR-amplified target genes implicated in the drug resistance of malaria with subsequent one-tube minisequencing using two fluorochromes. The drug-resistance-associated genes pfdhfr, pfdhps, pfcrt, pfmdr1, and pfATPase were amplified and analyzed for cultured Plasmodium falciparum strains and from field samples. We obtained a specificity of 94%, and comparison of field sample results to those of restriction fragment length polymorphism (RFLP) typing resulted in an overall consistency of >90%, except for pfdhfr51, for which most discrepancies were due to false determinations by RFLP of mixed infections. The system is sufficiently sensitive to assay parasites in clinical malaria cases and in most asymptomatic cases, and it allows high throughput with minimal hands-on time. The cost for the assay has been calculated as 0.27 euros/SNP (US $0.33), which is below the cost incurred with other systems. Due to the simplicity of the approach, newly identified SNPs can be incorporated rapidly. Such a monitoring system also makes it possible to identify the reemergence of drug-susceptible parasites once a drug has been withdrawn.

Rapid identification of malaria vaccine candidates based on alpha-helical coiled coil protein motif

To identify malaria antigens for vaccine development, we selected alpha-helical coiled coil domains of proteins predicted to be present in the parasite erythrocytic stage. The corresponding synthetic peptides are expected to mimic structurally "native" epitopes. Indeed the 95 chemically synthesized peptides were all specifically recognized by human immune sera, though at various prevalence. Peptide specific antibodies were obtained both by affinity-purification from malaria immune sera and by immunization of mice. These antibodies did not show significant cross reactions, i.e., they were specific for the original peptide, reacted with native parasite proteins in infected erythrocytes and several were active in inhibiting in vitro parasite growth. Circular dichroism studies indicated that the selected peptides assumed partial or high alpha-helical content. Thus, we demonstrate that the bioinformatics/chemical synthesis approach described here can lead to the rapid identification of molecules which target biologically active antibodies, thus identifying suitable vaccine candidates. This strategy can be, in principle, extended to vaccine discovery in a wide range of other pathogens.

Age and seasonal variation in the transition rates and detectability of Plasmodium falciparum malaria

The effect of acquired immunity on the duration of Plasmodium falciparum infections is unclear, although this is an important term in models of malaria transmission. It is problematical to determine the duration of infections because of the difficulty of distinguishing persisting infections from new ones, and because parasite densities are often transiently below the limit of detection. We recently developed a dynamic model for infection incidence, clearance and detection of multiple genotype P. falciparum infections and fitted it to a panel dataset from a longitudinal study in Northern Ghana. We now extend this model to allow for seasonal and age variation in infection rates and also age dependence in clearance and in detectability of infections. These models indicate that there is seasonal variation in the infection rate, and age dependence in detectability. The best fitting models had no age dependence in infection or clearance rates, suggesting that acquired immunity mainly affects detectability.

Comparison of PCR-RFLP and Genescan-based genotyping for analyzing infection dynamics of Plasmodium falciparum

Parameters describing the infection dynamics of Plasmodium falciparum are important determinants of the potential impact of interventions and are potential outcome measurements for malaria intervention trials. Low parasite densities, periodic sequestration of parasites, and the presence of multiple concurrent infections make it essential to use molecular techniques to estimate the force of infection and duration of infections in endemic areas. We now compare two approaches for tracking individual genotypes of the highly polymorphic merozoite surface protein 2: 1) fluorescence-labeled polymerase chain reaction (PCR) and GeneScan-sizing and 2) restriction fragment length polymorphism (RFLP). We analyze samples from a longitudinal field study in Ghana and use statistical approaches that allow for imperfect detectability. The two methods gave broadly similar estimates of parasite dynamics, but GeneScan is more precise and can achieve a higher throughput. The analysis of parasite dynamics indicated an average duration of infection of 210 days by GeneScan versus 152 days by PCR-RFLP in the study population in Kassena-Nankana, Northern Ghana. This reflects the good performance of GeneScan-based genotyping for studies of parasite infection dynamics.

Effect of the malaria vaccine Combination B on merozoite surface antigen 2 diversity

Extensive genetic polymorphism is generally found in Plasmodium falciparum surface antigens. This poses a considerable obstacle to the development of a malaria vaccine. In order to assess possible effects of a polymorphic vaccine, we have analyzed the genetic diversity of parasites collected in the course of a phase 2b field trial of the blood stage vaccine Combination B in Papua New Guinea. The full-length 3D7 allele of the merozoite surface protein 2 (MSP2) was included in Combination B as one of three subunits. Vaccinees had a lower prevalence of parasites carrying a 3D7-type allele (corresponding to that in the vaccine) and selection appeared to favour the alternative FC27-type alleles resulting in a higher incidence of morbid episodes associated with FC27-type parasites. We sequenced MSP2 alleles detected in study participants after vaccination to identify breakthrough genotypes. Extensive genetic diversity of MSP2 was observed in both the repetitive and family-specific domains, but alleles occurring in vaccine recipients were no different from those found in placebo recipients. A phylogenetic analysis showed no clustering of 3D7-type breakthrough infections from vaccine recipients. The repeat unit present in the vaccine molecule occurred in a number of alleles from the trial area and was also observed in vaccinated individuals. Thus the anti-repeat immune response did not lead to elimination of parasites carrying the same repeat unit. We conclude that the conserved epitopes in the family-specific domain were the most important determinants of the vaccine effect against new 3D7-type infections and that the hypervariable domains were not subject to selective effects of the vaccine.

[Prevalence, genetic diversity and multiplicity of Plasmodium falciparum infection in school children in central Cote d'Ivoire]

A study was carried out in the village of Taabo, located in the vicinity of a large man-made lake in central Côte d'Ivoire. The objectives were (i) to determine the level of prevalence, genetic diversity and multiplicity of Plasmodiumfakiparum infection in schoolchildren and (ii) to compare the diagnostic performance of light microscopy and polymerase chain reaction (PCR). A total of 424 schoolchildren ranging in age from 5 to 15 years underwent diagnostic testing using both light microscopy of blood smears and PCR. Multiplicity of P. falciparum infection was investigated in 196 children (46.2%). The prevalence of malaria was 54.7% based on light microscopy and 83.9% based on PCR. Genotyping based on polymorphism in the length of the restriction fragment of the gene encoding the merozoite surface protein-2 (msp2) showed that 86.5% of cases involved multiple infection with a geometric mean of 3.87 genotypes per positive child. There was a strong positive correlation between multipcity of infection and parasite density in the 56-year old age group. A total of 50 genotypes including six observed for the first time were identified and classified into families with similar-sized sequence groups: 26 x FC27 (52%) and 24 x 3D7 (48%). In comparison with PCR, the sensitivity and specificity of light microscopy for diagnosis of P. falciparum was 81.3% and 88.2% respectively. Data are discussed in the light of similar studies carried out in sub-Saharan Africa and elsewhere. These findings can serve as a basis for monitoring the longterm effect of major water resource management projects on the prevalence, genetic diversity and multiplicity of P. falciparum infection.

Efficacy of sulfadoxine-pyrimethamine in Tanzania after two years as first-line drug for uncomplicated malaria: assessment protocol and implication for treatment policy strategies

Background

Systematic surveillance for resistant malaria shows high level of resistance of Plasmodium falciparum to sulfadoxine-pyrimethamine (SP) across eastern and southern parts of Africa. This study assessed in vivo SP efficacy after two years of use as an interim first-line drug in Tanzania, and determined the rates of treatment failures obtained after 14 and 28 days of follow-up.

Methods

The study was conducted in the Ipinda, Mlimba and Mkuranga health facilities in Tanzania. Children aged 6–59 months presenting with raised temperature associated exclusively with P. falciparum (1,000–100,000 parasites per μl) were treated with standard dose of SP. Treatment responses were classified according to the World Health Organization (WHO) definition as Adequate Clinical and Parasitological Response (ACPR), Early Treatment Failure (ETF), Late Clinical Failure (LCF) and Late Parasitological Failure (LPF) on day 14 and day 28.

Results

Overall 196 (85.2%) of 230 patients had ACPR on day 14 but only 116 (50.9%) on day 28 (57.7% after excluding new infections by parasite genotyping). Altogether 21 (9.1%) and 13 (5.7%) of the 230 patients assessed up to day 14 and 39 (17.1%) and 55 (24.1%) of the 228 followed up to day 28 had clinical and parasitological failure, respectively.

Conclusion

These findings indicate that SP has low therapeutic value in Tanzania. The recommendation of changing first line treatment to artemether + lumefantrine combination therapy from early next year is, therefore, highly justified. These findings further stress that, for long half-life drugs such as SP, establishment of cut-off points for policy change in high transmission areas should consider both clinical and parasitological responses beyond day 14.

A prospective study of Plasmodium falciparum multiplicity of infection and morbidity in Tanzanian children

Several studies suggest that in individuals with substantial previous exposure to malaria, co-infection with multiple clones of Plasmodium falciparum can protect against subsequent clinical malaria attacks. Other studies, mainly of individuals with little previous exposure, found the converse relationship. To test whether acquisition of such cross-protection tracks the acquisition of clinical immunity in general, 610 Tanzanian children aged 0-6 years were enrolled in a nine-month prospective study of the risk of morbidity in relation to parasitological status and merozoite surface protein 2 genotypes on enrolment. Prevalence of parasitaemia and multiplicity of infection increased with age. In the first year of life, the incidence of clinical malaria was almost three times higher in children with parasites at baseline than in those without. In older children, baseline P. falciparum infections appeared to protect against both parasitaemic and non-parasitaemic fever episodes. In children aged less than three years, baseline multiple infection tended to be associated with higher prospective risk of clinical malaria than single infection while in children aged more than three years the converse was found, but these effects were not statistically significant. These results provide further evidence that relationships between asymptomatic malaria infections and clinical malaria change with cumulative exposure.