Strain-specific humoral response to a polymorphic malaria vaccine

The 3D7 form of the merozoite surface protein 2 (MSP2) of Plasmodium falciparum was one of three subunits of the malaria vaccine Combination B that were tested in a phase I/IIb double-blind randomized placebo-controlled trial, which was undertaken with 120 Papua New Guinean children of 5 to 9 years of age. Because only one variant of the highly polymorphic MSP2 was used for vaccination, we examined whether the elicited response was directed against conserved or strain-specific epitopes. Postvaccination (week 12) titers of antibody against recombinantly expressed individual domains of MSP2 were measured by enzyme-linked immunosorbent assay and compared to baseline values. We found that vaccination with the 3D7 form of MSP2 induced a significant strain-specific humoral response directed against the repetitive and semiconserved family-specific part. The conserved N- and C-terminal domains were not immunogenic. Titers of antibody against the alternate FC27 family-specific domain showed a tendency to increase in vaccinated children, but there was no increase in antibodies against FC27-type 32-mer repeats. These results indicate that vaccination with one MSP2 variant mainly induced a strain-specific response, which can explain the selective effect of vaccination with combination B on the genotypes of breakthrough parasites. These findings support the inclusion of both family-specific domains (3D7 and FC27) in an improved vaccine formulation.

Antigenic cross-reactivity between different alleles of the Plasmodium falciparum merozoite surface protein 2

The polymorphic domain of the gene encoding Plasmodium falciparum merozoite surface protein 2 (MSP2) was PCR amplified from blood of malaria patients, genotyped, and 19 distinct fragments were cloned and expressed in E. coli. The reactivity of naturally occurring antibodies against this panel of recombinant MSP2 antigens was tested using 67 homologous or heterologous sera from a serum bank of travel clinic patients. Sera from semi-immune individuals strongly recognized almost all recombinant antigens. Sera from primary infected patients either did not react at all (9 sera), or reacted weakly against varying numbers of antigens (39 sera). The antigens that showed reactions were mostly of the allelic family corresponding to the infecting clone, but in very few cases also of the alternative allelic family. Single clone infections and repeated samples from the same individual were analysed in greater detail. Thus, we were able to quantify cross-reactivity induced by a single P. falciparum infection. Within the two allelic families of MSP2, cross-reactivity was observed between some but not all alleles of the same family, whereas antibodies cross-reactive between variants belonging to different allelic families were detected in only a few cases.

Safety and immunogenicity of a three-component blood-stage malaria vaccine (MSP1, MSP2, RESA) against Plasmodium falciparum in Papua New Guinean children

Combination B is a malaria vaccine that comprises recombinant Plasmodium falciparum (P. falciparum) blood-stage proteins MSP1, MSP2 and RESA, formulated with the adjuvant Montanide ISA 720. A phase I-IIb double-blind randomised placebo-controlled trial was undertaken in 120 children aged 5-9 years. Subjects were randomised in four groups: (i) No sulphadoxine-pyrimethamine (SP)+vaccine, (ii) No SP+placebo, (iii) SP+vaccine, (iv) SP+placebo. 15 microg of each protein were given in the thigh, at both first and second injection (4 weeks apart). The placebo was adjuvant emulsified with saline. No serious or severe AEs occurred. Moderate AEs were seen in 3% of the vaccine and 3% of the placebo recipients after first injection and in 12 and 10% after second injection. The vaccine induced significant antibody responses to all three antigens but triggered an IFN-gamma response to MSP1 only. At Week 12, the IFN-gamma response to MSP1 was substantially higher in the vaccine group where No SP had been given. Combination B proved to be safe and immunogenic in children aged 5-9 years. Vaccine immunogenicity was neither impaired by circulating parasites nor increased after pre-treatment with SP and pre-treatment is not advisable in future trials of malaria vaccines, at least for those including blood-stage antigens.

Identification and differentiation of Leishmania species in clinical samples by PCR amplification of the miniexon sequence and subsequent restriction fragment length polymorphism analysis

We recently developed a new PCR-restriction fragment length polymorphism (RFLP)-based assay using the miniexon sequence from the genus Leishmania. Here we report the application of this new genotyping method to naturally infected clinical samples for the differentiation of New and Old World Leishmania species. Of the newly developed assay and four currently applied diagnostic tests (i.e., in vitro cultivation, serology, and two other molecular assays using either the small subunit-internal transcribed spacer sequence or a repetitive genomic sequence), the miniexon assay showed the highest sensitivity, 89.7%, compared to 70.6, 57.1, 51.7, and 79.3%, respectively. Species differentiation was robust and reliable compared with that by two other Leishmania genotyping techniques. The assay provides a valuable tool for the identification of Leishmania directly from clinical samples and enables determination of the infecting species by a facile technique with high discrimination power. Since Leishmania causes a broad spectrum of diseases distinguished by different parasite and host factors, detection and characterization of the infecting species is crucial for the confirmation of a diagnosis as well as the establishment of the clinical prognosis and the initiation of an adequate therapeutic approach. The miniexon PCR-RFLP assay will facilitate such determination and might improve diagnosis and treatment of leishmaniasis.

Diagnostic genotyping of Old and New World Leishmania species by PCR-RFLP

We have designed a new genotyping scheme for molecular diagnosis of the different Leishmania species pathogenic to humans. This scheme is based on PCR amplified sequences from the gene for the spliced leader RNA (mini-exon). This target was selected because it is present as tandem repeats (100 to 200 copies) in the genus Leishmania and other kinetoplastida, but is absent from the mammalian hosts and the sandfly vectors. The exon is highly conserved, whereas the intron and non-transcribed spacer region vary in size and sequence among different species. Thus, it was possible to amplify DNA from both Old and New World pathogenic Leishmania complexes using a single pair of primers deriving from the conserved region of the mini-exon tandem repeat. Species identification was performed by digesting mini-exon PCR products with one or two different restriction enzymes. Restriction fragment length polymorphism (RFLP) generated species-specific patterns of bands visualized in agarose gels, which allowed to differentiate each species unequivocally.

Molecular parasitology of malaria in Papua New Guinea

Research into the molecular biology of infectious diseases is mostly associated with well-developed countries. But in the midst of tropical Papua New Guinea, highly sophisticated molecular research has being conducted over years to understand and fight malaria and other tropical diseases. Here, we review such research carried out at the Papua New Guinea Institute of Medical Research. This Institute has considerably shaped research on molecular epidemiology through its analysis of the diversity and structure of the Plasmodium falciparum population. In addition, research has been conducted on human host factors and, more recently, the molecular analysis of drug resistance and the underlying molecular mechanisms of host-parasite interactions have been investigated.

Molecular monitoring in malaria vaccine trials

Molecular techniques offer new approaches for malaria field trials, particularly PCR techniques, which facilitate accurate diagnosis of Plasmodium infections and increase the power of estimates of vaccine effects on malaria prevalence or incidence. Molecular methods also help to assess selective effects of vaccines. Longitudinal genotyping data can be used to initiate novel analyses of parasite dynamics, including estimates of incidence of infection with individual parasite clones and duration of infections. In addition, high-throughput methods can be used to apply these techniques routinely in randomized controlled trials, as well as programme-based evaluations of malaria control.

Species-specific field testing of Entamoeba spp. in an area of high endemicity

Entamoeba histolytica has been separated in recent years into 2 morphologically identical species: the apathogenic E. dispar and the pathogenic E. histolytica, only the latter being pathogenic. Although various laboratory techniques allow discrimination between the 2 species there is a lack of field data about the suitability of available diagnostic tests for use in epidemiological studies and few epidemiological studies using species-specific diagnosis have been performed at community level in endemic areas, especially in sub-Saharan Africa. We conducted a repeated cross-sectional study of 967 schoolchildren in central Côte d'Ivoire to compare and evaluate light microscopy, 2 different antigen detection assays, and one polymerase chain reaction (PCR) assay. Microscopy and a non-specific antigen capture Entamoeba enzyme-linked immunosorbent assay (ELISA) were used for the primary screening of all children (time t0). The prevalence of the E. histolytica/E. dispar species complex at t0 was 18.8% by single microscopical examination and 31.4% using the non-specific ELISA. Approximately 2 months after the initial screening, fresh stool specimens were collected on 2 consecutive days (t1 and t2) from (i) all the children who were positive by microscopy at t0 (n = 182) and (ii) 155 randomly selected children who were negative at the primary screening. These samples were tested with a second antigen detection ELISA specific for E. histolytica (n = 238) and with a species-specific PCR assay (n = 193). The second and third examinations (t1 and t2) revealed an additional 43 infections with the species complex E. histolytica/E. dispar, so that the cumulative microscopical prevalence for t1 and t2 was 27.7%. The overall prevalence of E. histolytica by species-specific ELISA antigen detection was low (0.83%), while the prevalence of E. dispar was 15%. When analysing only microscopically positive samples by PCR (n = 129), the ratio E. histolytica: E. dispar was very low (1:46), suggesting that the vast majority of Entamoeba infections in this area were apathogenic. Both species-specific tests performed well but the ELISA was easier to use for large-scale field screening.

Molecular epidemiology of Plasmodium falciparum infections among asymptomatic inhabitants of a holoendemic malarious area in northern Ghana

Age dependence of malaria infection was assessed in an age-stratified cluster sample of 308 individuals from Kassena-Nankana District of northern Ghana during June and July 2000. Overall prevalence of Plasmodium falciparum by microscopy was 70%, with the maximum among 5-9 year olds. Parasite density was highest (geometric mean 1922/microl blood) in 1-2 year olds. Eighty-two per cent of samples were positive by polymerase chain reaction (PCR), and restriction fragment length polymorphism typing of the P. falciparum msp2 revealed a mean msp2 multiplicity of 3.4 (range: 1-8) genotypes per PCR positive sample. Multiplicity increased with age until 5-9 years and then started to reduce again into adulthood. About 49.3% of infections belonged to the msp2 FC27 allelic family and 50.7% to the 3D7 family. On the day of the survey, only 3.6% of the participants had fever (axillary temperature >or= 37.5 degrees C) and 2.3% had fever associated with parasitaemia. The correlation between parasite density and msp2 multiplicity was 0.42; highest among infants, and decreased with age to a minimum among 5-9 year olds. Contrasting with results from Tanzania, this correlation increased with age in adolescents and adults. Parasite multiplicity is very high in this community, and the patterns of age dependence are similar to those in other holoendemic sites in Africa, validating the use of the age-multiplicity relationship as an indicator of malaria endemicity.

A point mutation in codon 76 of pfcrt of P. falciparum is positively selected for by Chloroquine treatment in Tanzania

This study was undertaken to validate the relevance of Chloroquine (CQ) resistance markers pfcrt(76) and pfmdr1(86) in an endemic area in Tanzania. After treatment with CQ, recrudescence was distinguished from new infection by msp2 genotyping, and the number of concurrent infections was also determined. The rate of children with recrudescent parasites at day 7 and/or day 14 amounted to a parasitological failure rate of 22.4% using PCR. The mean multiplicity of infection at day 0 was 3.2 (n=71). The allelic frequencies of the mutated pfcrt(76) and pfmdr1(86) were estimated to be 92 and 77%, respectively. Both values exceeded by far the observed frequency of 14% of recrudescent parasites as calculated on the whole analysed parasite population taking multiple infections into account. Although neither mutant allele is of predictive value for parasitological resistance, there is evidence for a role of pfcrt(76) in CQ resistance in the natural parasite population. All wild-type pfcrt(76) alleles were eliminated before day 3, after the onset of CQ treatment and no recrudescent parasite with the wild-type allele was observed at later time points. The discrepancy between the rate of resistant parasites (14%) and the frequency of the mutant pfcrt(76) allele (92%), however, indicates that other polymorphisms and other factors must be involved in CQ resistance. No selective elimination of the pfmdr1(86) wild-type allele was observed.

A recombinant blood-stage malaria vaccine reduces Plasmodium falciparum density and exerts selective pressure on parasite populations in a phase 1-2b trial in Papua New Guinea

The malaria vaccine Combination B comprises recombinant Plasmodium falciparum ring-infected erythrocyte surface antigen and 2 merozoite surface proteins (MSP1 and MSP2) formulated in oil-based adjuvant. A phase 1-2b double-blind, randomized, placebo-controlled trial in 120 children (5-9 years old) in Papua New Guinea demonstrated a 62% (95% confidence limits: 13%, 84%) reduction in parasite density in children not pretreated with sulfadoxine-pyrimethamine. Vaccinees had a lower prevalence of parasites carrying the MSP2-3D7 allelic form (corresponding to that in the vaccine) and a higher incidence of morbid episodes associated with FC27-type parasites. These results demonstrate functional activity of Combination B against P. falciparum in individuals with previous malaria exposure. The specific effects on parasites with particular msp2 genotypes suggest that the MSP2 component, at least in part, accounted for the activity. The vaccine-induced selection pressure exerted on the parasites and its consequences for morbidity strongly argue for developing vaccines comprising conserved antigens and/or multiple components covering all important allelic types.

Prospective risk of morbidity in relation to multiplicity of infection with Plasmodium falciparum in São Tomé

The prospective risk of acute morbidity was analysed in relation to multiplicity of Plasmodium falciparum infection in 491 individuals in a peri-urban community in São Tomé. In an initial cross-sectional survey, 40.5% of individuals were recorded by microscopy as infected with P. falciparum, and by PCR 60.5%, with the maximum prevalence in children aged 5-10 years. PCR-RFLP typing of the msp-2 gene of P. falciparum found a mean of 2.4 parasite genotypes per infected person, with little age dependence in this multiplicity and a total of 43 different msp-2 alleles identified. None of these were unique for São Tomé. Study participants were encouraged to report to a project worker whenever they suffered a febrile illness. During the 3 months following the parasitological survey the recorded incidence rates decreased with increasing baseline msp-2 multiplicity, both for P. falciparum-positive episodes and for fever without parasitaemia. While this is consistent with suggestions that multiple P. falciparum infections may protect against super-infecting parasites, confounding by patterns of health service usage is an alternative explanation. The incidence of clinical malaria episodes was only a little higher in children than in adults. This weak age-dependence in clinical immunity might be a consequence of a cohort effect resulting from resurgence of the disease after the breakdown of malaria control programs in the 1980s.

Limited polymorphism in Plasmodium falciparum sexual-stage antigens

In areas highly endemic for malaria, individuals are frequently found to be infected simultaneously with multiple Plasmodium falciparum clones. This raises the question of whether all parasite clones produce gametocytes equally or whether gametocytogenesis is suppressed in some clones. In order to assess this in epidemiological studies, polymorphic genes specifically expressed in gametocytes could be analyzed by both amplification of genomic DNA from blood samples and by reverse transcribed polymerase chain reaction amplifying expressed gametocyte-specific genes only. Here we report the analysis of diversity in the three gametocyte-specific genes Pfs16, Pfs48/45, and Pfs230. In addition to the previously published data, limited polymorphism was found in the coding sequences of Pfs16 and Pfs48/45. Larger polymorphism was identified in Pfs230, which might allow the development of a discriminating PCR-based genotyping scheme for transmission studies. However, the limited polymorphism in Pfs16 and Pfs48/45 renders these molecules poorly useful for such studies.

Plasmodium falciparum: expression of gametocyte-specific genes in monolayer cultures and malaria-positive blood samples

In order to facilitate molecular epidemiological studies on the transmission of the malaria parasite Plasmodium falciparum a sensitive assay for gametocyte detection based on RT-PCR was developed. The transcription of the sexual stage-specific genes Pfs16, Pfs48/45, Pfs230, and Pfs25, as well as the sexual stage- and sporozoite-specific S 18S rRNA, was detected by RT-PCR. S 18S rRNA was present in seven of nine P. falciparum-positive blood samples, despite the lack of microscopic detection of gametocytes and a parasitemia below 0.1%. Expression of the other four gametocyte-specific genes was detected less frequently in malaria-positive blood samples. These findings indicate that RT-PCR of S 18S rRNA is a highly sensitive method for gametocyte detection and, furthermore, that gametocytes are present in the peripheral blood of most malaria carriers, even if the parasitemia is below 0.1%. To determinate the expression pattern of sexual stage-specific genes in more detail, RT-PCR was performed at consecutive time points of highly synchronized monolayer cell cultures. Transcripts of all examined genes except Pfs25 were detected directly after invasion of merozoites of the strain NF54 in red blood cells.

Genomic distribution and functional characterisation of two distinct and conserved Plasmodium falciparum var gene 5' flanking sequences

Approximately 50 highly diverse var genes distributed throughout the haploid genome of the malaria parasite Plasmodium falciparum code for PfEMP1 variants located on the surface of infected erythrocytes. PfEMP1 is involved in cytoadherence of parasitised red blood cells and undergoes antigenic variation through differential expression of var genes. Members of the var gene family are located in chromosome-internal positions on chromosomes 4, 7, 8 and 12, and in subtelomeric regions of all chromosomes. Here we show that there are two distinct and conserved types of 5' upstream regions (var17-type and 5B1-type) of var genes, and suggest that most subtelomeric var genes are flanked by a var17-type 5' upstream sequence. In contrast, 5B1-type 5' upstream are localised to chromosomes that have been shown to contain var genes within chromosome-internal regions. Transcriptional analysis using RT-PCR revealed that var genes flanked by either type of 5' upstream sequence are transcribed in in vitro cultured trophozoite stage parasites. In addition, we have shown that the 5' flanking sequences of four different var genes are able to drive transient expression of the cat reporter gene. Our results suggest that at least the minimal regulatory sequences required for transcription of var genes are conserved among both subgroups of the var gene family. Furthermore, these sequences provide new markers for the investigation of the chromosomal organisation of var genes.

Analysis of Plasmodium falciparum infections in a village community in Northern Nigeria: determination of msp2 genotypes and parasite-specific IgG responses

The genetic diversity of P. falciparum and multiplicity of infection has been studied in a village in Northern Nigeria at the end of the rainy season, when transmission is high. We analysed blood samples from 104 individuals aged 5-70 years by polymerase chain reaction (PCR) amplifying the gene for the merozoite surface protein MSP2 followed by genotyping based on restriction fragment length polymorphism (RFLP). 94.2% of all samples were parasite positive by PCR and over 80% of those had multiple infections. The age distribution of the average number of parasite clones present in P. falciparum infections showed an initial increase, then reached a peak multiplicity in children 8-10 years of age, and afterwards decreased significantly with age. Mean multiplicity in those 8-10-year-old children was 5.4 clones per carrier. Peak multiplicity and parasite diversity in Nigerian individuals is compared to findings from other study sites in Africa and PNG. The prevalence of IgG antibodies against the circumsporozoite protein (CSP), an indicator for malaria exposure, was over 85% in all age groups showing a high exposure of villagers to P. falciparum. OD values in ELISA were positively correlated with age. There was no correlation between the level of IgG against CSP and the multiplicity of P. falciparum infections determined by PCR of msp2. These results imply that in highly endemic areas multiplicity of infection is not directly correlated with exposure to P. falciparum.

Consequences of multiple infection with Plasmodium falciparum in an area of high endemicity

Most Plasmodium falciparum infections occur in partially immune hosts in highly endemic areas. In such situations, many hosts are simultaneously infected with multiple parasite genotypes, which must lead to intense competition between different parasite populations. We here summarise a series of studies of multiple infection, mostly using polymerase chain reaction-restriction fragment polymorphism (PCR-RFLP) genotyping of the highly polymorphic msp-2 gene. These indicate that chronic infections, characteristic of the partially immune host, appear to protect against super-infecting parasites. This protection is not seen in infants. A consequence is that selection for fast-growing (virulent) parasites, occurs mainly in the youngest, immunologically naïve, hosts. The normal situation for P. falciparum is one in which the host is partially immune, and competition between parasite genotypes in this situation is not expected to result in selection for virulence.

Rapid DNA extraction for molecular epidemiological studies of malaria

DNA isolation from blood samples collected in molecular epidemiological studies is crucial for the quality and reproducibility of data. Blood samples from two malaria endemic sites have been prepared by four different DNA isolation methods with subsequent PCR amplification of the msp2 locus of Plasmodium falciparum. We tested a rapid boiling method; the guanadine isothiocyanate DNA extraction; QIAmp blood kit; and the ISOCODE STIX PCR template preparation dipstick, and analysed the numbers of concurrent infections/sample. The rapid boiling method and the ISOCODE STIX provided overall the best sensitivity combined with ease of handling. The possibility to store and ship the ISOCODE STIX at ambient temperature adds further advantage to this method.

Malaria infection and morbidity in infants in relation to genetic polymorphisms in Tanzania

To investigate the effects of host polymorphisms on malaria morbidity and infection, the frequency distributions of TNF alpha promotor gene and sickle cell trait were studied in infants in an area highly endemic for Plasmodium falciparum transmission in Tanzania. Differences in parasite prevalence, density, febrile episodes with and without parasitaemia, PCV levels and the frequencies of different MSP-2 parasite infections were assessed by genotype. The frequency of the TNF alpha promotor allele 2 was 0.09, and the trait was in Hardy-Weinberg equilibrium. There were no differences in malariametric indices between infants with the normal TNF alpha promoter gene and those who were heterozygous for this trait. Infants heterozygous for the TNF alpha promotor gene appeared to have fewer febrile episodes when they were free of parasites. The two infants homozygous for the TNF alpha promoter allele 2 had both a much higher incidence of fever, independently of parasitaemia, than the average for the other genotypes. The frequency of the sickle cell allele S was 0.06 and the trait was also in Hardy-Weinberg equilibrium. Infants heterozygous for the sickle cell trait had significantly lower parasite densities, but similar prevalence, and MSP-2 infections compared to infants with normal haemoglobin. PCV levels, and the incidence of febrile episodes both with and without parasitaemia were also similar. In contrast to the sickle cell trait, the TNF alpha promotor polymorphism appeared not to have any protective effect on malaria in this study, and its importance in other unspecified fever-causing diseases in this population needs further investigation.

Dynamics of multiple Plasmodium falciparum infections in infants in a highly endemic area of Tanzania

The force of infection and recovery rate for malaria in infants in a highly endemic area of Tanzania were analysed using polymerase chain reaction-restriction fragment length polymorphism genotyping of the Plasmodium falciparum msp2 locus in 99 paired blood samples. Overall, new genotypes were acquired at a rate of 0.064 per day, and the average duration of infections was estimated to be 23 d. The highest recovery rates were in children under 4 months of age. The higher susceptibility of infants to clinical malaria in comparison with older children, in areas of very high transmission, may be largely a consequence of the short duration of infections which precludes the establishment of concomitant immunity. The high turnover of infections also implies that infection prevalence and multiplicity approach an equilibrium even in very young children, and calls into question the use of infant conversion rates as a measure of transmission intensity.

Effect of insecticide-treated bed nets on the dynamics of multiple Plasmodium falciparum infections

The rates of acquisition and loss of individual genotypes belonging to the FC27 family of the Plasmodium falciparum merozoite surface protein 2 (msp2) gene were studied in 120 children aged 5 months to 2.5 years, in a randomized controlled trial of insecticide-treated bed nets (ITNs) in Kiberege village, Tanzania. Analysis of longitudinal changes in positivity for individual alleles in samples collected at intervals of one month indicated that the average duration of infections, allowing for undetected parasite genotypes, was 73 d in those aged < 18 months and 160 d in children aged > or = 18 months, consistent with a shift from acute to chronic infection with age. Overall, 51% of genotypes infecting the host were estimated to be detected by polymerase chain reaction-restriction fragment length polymorphism analysis in any one sample of 0.5 microL of packed peripheral blood cells. In children less than 18 months old this sensitivity was 61% (SE = 6%) compared with 41% (SE = 6%) in older children. Conversely, the rate of appearance of new parasite genotypes was higher in children < 18 months of age than in older children, but this partly reflected the difference in sensitivity. The overall incidence of new infections was estimated to be reduced by 17% in ITN users. There was no statistically significant difference between users and non-users in observed infection multiplicity, sensitivity, recovery rate, or estimated infection rates for individual alleles. This suggests that, in areas of high P. falciparum endemicity, ITNs have little effect on the establishment of chronic malaria infection.

Premunition in Plasmodium falciparum infection: insights from the epidemiology of multiple infections

Epidemiological studies of multiple clone infections by Plasmodium falciparum in highly endemic areas have demonstrated age dependence in both the multiplicity of infection and the relationships between this multiplicity and the risk of acute illness. We hypothesize that, in infants, host defence against blood-stage infections with P. falciparum relies mainly on fever and cytokine activities, and the infections are of short duration. In older children, a high multiplicity of infection is characteristic of low-level chronic parasitaemia. This appears to confer cross-protection against newly inoculated parasites, via partially genotype-specific responses which are short-term, lasting little longer than the infections themselves. This has important implications for our understanding of immunity against P. falciparum, its ecological niche, and the epidemiological impact of interventions against it.

Multiple Plasmodium falciparum infections in Tanzanian infants

Paired blood samples from 99 Tanzanian infants were analysed to examine the infection dynamics of Plasmodium falciparum during the first year of life. Infecting parasites were genotyped by polymerase chain reaction amplification of the polymorphic gene for the merozoite surface protein 2 and subsequent analysis according to the resulting restriction fragment length polymorphism pattern. The same samples served as controls in a parallel case-control study for which an additional blood sample was taken from each child during a fever episode. The relationship of the number of concurrent infections (multiplicity) with age and morbidity was analysed and results were compared to those of a similar study on older children between 2 and 7 years of age, carried out in the same village at the same time. The mean of 2 infecting genotypes per positive blood sample from community surveys was low compared to that in older children, and there was no significant age-dependency of multiplicity within the first year of life. Multiplicity of infection in fever cases was also independent of age. In infants, multiplicity was positively associated with parasite density and risk of clinical malaria, in contrast to the situation in older children (> 2 years). The findings help in the understanding of infection dynamics, premunition, and development of semi-immunity in malaria.