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Temporal changes in the prevalence of antigenic variants in Plasmodium falciparum populations have been interpreted as evidence of immune-mediated frequency-dependent selection, but evolutively neutral processes may generate similar patterns of serotype replacement. Over 4 years, we investigated the population dynamics of P. falciparum polymorphisms at the community level by using 11 putatively neutral microsatellite markers. Plasmodium falciparum populations were less diverse than sympatric P. vivax isolates, with less multiple-clone infections, lower number of alleles per locus and lower virtual heterozygosity, but both species showed significant multilocus linkage disequilibrium. Evolutively neutral P. falciparum polymorphisms showed a high turnover rate, with few lineages persisting for several months in the population. Similar results had previously been obtained, in the same community, for sympatric P. vivax isolates. In contrast, the prevalence of the 2 dimorphic types of a major antigen, MSP-2, remained remarkably stable throughout the study period. We suggest that the relatively fast turnover of parasite lineages represents the typical population dynamics of neutral polymorphisms in small populations, with clear implications for the detection of frequency-dependent selection of polymorphisms.

The extensive sequence variation in most surface antigens of Plasmodium falciparum is one of the major factors why clinical immunity to malaria develops only after repeated infections with the same species over several years. For some P. falciparum surface antigens, all observed alleles clearly fall into two allelic classes, with divergence between classes dwarfing divergence within classes. We discuss the ways in which such allelic dimorphism deviates from the expected shape of the genealogy of genes under either neutral evolution or standard balancing selection, and present a simple test, based on coalescent theory, to detect this deviation in samples of DNA sequences. We review previous hypotheses for the origin and evolution of allelic dimorphism in malarial antigens and discuss the difficulties of explaining the available data under these proposals. We conclude by offering several possible classes of explanations for allelic dimorphism, which are worthy of further theoretical and empirical exploration.

The merozoite surface protein-2 (MSP-2) is a major vaccine candidate for the asexual blood stage of Plasmodium falciparum. MSP-2 is essentially dimorphic, and allelic families are named after the representative isolates FC27 and IC1. The polymorphic central region contains immunodominant repeats, which vary in number, length, and sequence within and between allelic families. We have examined the antibody recognition of repeat regions from both MSP-2 allelic families expressed as recombinant fusion peptides. The results are summarized as follows. (1) Immunization of mice with the fusion peptides elicited IgG antibodies that cross-reacted with the native MSP-2 molecule in an allelic family-specific manner. (2) These mouse antibodies recognized the recombinant proteins in both a variant-specific and a family-specific manner, as shown in inhibition immunoassays. Antibodies raised against the peptide FC27 seemed to be essentially variant-specific, since the soluble form of the S20 antigen (a member of FC27 family) had relatively little inhibitory effect on them. (3) The overall pattern of human IgG antibody responses to MSP-2 in Karitiana Indians, a population continuously exposed to hypoendemic malaria in the Brazilian Amazon Region, differs from that described in hyperendemic areas in Africa and Papua New Guinea in two important features: there was no clear age-dependent increase in the prevalence and mean concentration of specific IgG antibodies, and there was no skewing towards the IgG3 subclass in antibody responses. (4) The relatively poor correlation between concentrations of IgG antibodies that are specific for members of the same allelic family suggests that recognition of MSP-2 peptides by naturally acquired antibodies was largely variant-specific in this population. The potential role of naturally acquired variant-specific antibodies in immune evasion, by selecting mutant parasites carrying insertions or deletions of repeat sequences, is briefly discussed.

The merozoite surface protein-1 (MSP-1) is a major vaccine candidate for the asexual blood stage of malaria. We examined both the extent of sequence diversity in block 17, the 3' end of Msp-1 gene coding for a 19-kDa polypeptide (MSP-1(19)) putatively involved in red blood cell binding, and the patterns of linkage disequilibrium between polymorphic sites throughout the Msp-1 locus. The parasite population sample consisted of Plasmodium falciparum isolates collected between 1985 and 1998 in Rondĵnia, an area of hypoendemic malaria transmission in the southwestern Brazilian Amazon. Results were summarized as follows. (1) Seven block-17 sequence variants or haplotypes were found among 130 isolates, including two new haplotypes (novel combinations of previously reported amino acid replacements), here named Brazil-1 (E-TSR-F) and Brazil-2 (Q-TSR-F). (2) As previously shown for other Msp-1 polymorphisms, frequencies of block-17 haplotypes displayed significant temporal variation. (3) Extensive linkage disequilibrium was demonstrated between neighboring dimorphic sites within block 17, as well as between polymorphisms at the 5' and 3' ends of Msp-1 (map distance range: 3.83-4.99 kb). (4) The overall patterns of linkage disequilibrium within Msp-1 remained stable over a period of nearly one decade, and examples of possible 'epidemic' expansion of parasites carrying particular Msp-1 alleles were found in the 1980s and 1990s. These results are discussed in relation to the population biology of P. falciparum and the development of malaria vaccines based on MSP-1.

We incorporate a representation of Plasmodium falciparum recombination within a discrete-event model of malaria transmission. We simulate the introduction of a new parasite genotype into a human population in which another genotype has reached equilibrium prevalence and compare the emergence and persistence of the novel recombinant forms under differing cross-reactivity relationships between the genotypes. Cross-reactivity between the parental (initial and introduced) genotypes reduces the frequency of appearance of recombinants within three years of introduction from 100% to 14%, and delays their appearance by more than a year, on average. Cross-reactivity between parental and recombinant genotypes reduces the frequency of appearance to 36% and increases the probability of recombinant extinction following appearance from 0% to 83%. When a recombinant is cross-reactive with its parental types, its probability of extinction is influenced by cross-reactivity between the parental types in the opposite manner; that is, its probability of extinction after appearance decreases. Frequencies of P. falciparum outcrossing are mediated by frequencies of mixed-genotype infections in the host population, which are in turn mediated by the structure of cross-reactivity between parasite genotypes. The three leading hypotheses about how meiosis relates to oocyst production lead to quantitative, but no qualitative, differences in these results.

The polymorphic merozoite surface protein-2 (MSP-2) of Plasmodium falciparum is a major malaria-vaccine candidate. In the present study, PCR and hybridization with allelic-specific probes were used to type the Msp-2 gene from isolates from hypo-endemic Brazil (N = 113), meso-endemic Vietnam (N = 208) and holo-endemic Tanzania (N = 67). The typing methods were designed to group isolates into the dimorphic allelic families FC27 and IC1 and to detect possible between-family recombination events. The analysis was complemented by a comparison of 156 Msp-2 sequences from the GenBank database with 12 additional sequences obtained during the present study. Statistically significant differences were detected in pair-wise comparisons of the distribution of Msp-2 allelic types in Brazil and Vietnam, and in Brazil and Tanzania, but not in Vietnam and Tanzania. The extent of allelic diversity in the Msp-2 gene, as estimated by the total number of different alleles found in a given parasite population and the mean multiplicity of infections, clearly paralleled the levels of malaria endemicity in the study areas. However, no correlation between age and multiplicity of infections was found in the subjects. The patterns of Msp-2 diversity in Brazil appeared to be temporally stable, since no significant difference was observed in the distribution of Msp-2 allelic types among isolates collected, 10--13 years apart, in the same area of Rondônia. Despite the extensive sequence diversity found in Msp-2 alleles, especially in the central repetitive region of the molecule, several instances of identical or nearly identical alleles were found among isolates from different countries and regions, possibly as a result of extensive homoplasy. No recombinant allele was detected by molecular typing in any of the study sites, and the GenBank database included only 12 recombinant sequences (representing 7% of all reported Msp-2 sequences), all of them with an IC1-type 5' end and an FC27-type 3' end. A single, putative, crossover site was characterised for all recombinant alleles. Most of the allelic diversity observed was therefore attributable to variation in the repetitive region of the gene, instead of recombination between alleles of dimorphic families (as commonly found, for example, in the Msp-1 gene). The implications of these findings for studies on the genetic and antigenic diversity of malarial parasites are discussed.

Field isolates of Plasmodium falciparum collected from endemic areas of Southeast Asia, Solomon Islands, tropical African countries and Brazil were analyzed for the genetic diversity of the exon II of serine repeat antigen gene (SERA) by sequencing of genomic DNA. Of sixty-nine isolates, as compared to the reported FCR3, K1 and Honduras-1 types of exon II sequences, 5, 9 and 20 new allelic forms were found in 23 isolates of the FCR3 type, 36 of the K1 type and 10 of the Honduras-1 type. A group of novel non-synonymous substitutions, 4 new insertions and 3 new deletions of octamer units were found in the octamer repeat region (OR) of the exon II, and most of them clustered within a 40-residues domain. An octamer "SNPVSSEP" revealed in the OR was confirmed as a new repeat unit. Based on the sequences of the serine repeat region (SR) of the exon II, the allelic forms of the Honduras-1 type were conjectured to be the recombinant forms between the K1 type and FCR3 type. The allelic forms of K1 type with less or more repeat serine residues in the serine stretch of the SR than the reported 21 serine residues had most of the variations in the OR. Moreover, a biased geographical distribution of allelic forms was observed. Isolates from African and Southeast Asian countries accounted for most of the new allelic forms (29/33). All of the three types were detected in Southeast Asia but none of the FCR3 type in Africa. One of two groups of FCR3 new allelic forms was found solely in Brazil while another was mainly in Solomon Islands.

The polymorphic, merozoite surface protein-1 (MSP-1) of Plasmodium falciparum, an antigen of the parasite's asexual blood-stages, is a major malaria-vaccine candidate. Nucleotide sequences of each variable domain or block of this antigen may be grouped into one of three possible allelic types (K1, MAD20 and RO33), and 24 major types of the msp-1 gene may be defined, as unique combinations of allelic types in these variable blocks. Isolates collected from the Brazilian Amazon, over a period of 14 years, have now been investigated, by PCR-based typing of the msp-1 gene. Thirteen of the 24 possible gene-types were identified, and 336 P. falciparum clones were fully typed among 239 isolates. Most parasites (87%) belonged to one of the seven most frequent gene-types. Marked temporal variation in the distribution of msp-1 variants was found when comparing parasites sampled in the same sites at intervals of at least 5 years. Spatial variations were also found when comparing parasites from both neighbouring and distant sites within the Amazon Basin. The between-population variance in the frequencies of msp-1 allelic types found in Brazil, as estimated by Wright's FST statistic, is of similar magnitude to that found in previous world-wide comparisons. The potential implications of these findings for the development of an MSP-1-based, multivalent malaria vaccine are discussed.

Multilocus genotyping of microbial pathogens has revealed a range of population structures, with some bacteria showing extensive recombination and others showing almost complete clonality. The population structure of the protozoan parasite Plasmodium falciparum has been harder to evaluate, since most studies have used a limited number of antigen-encoding loci that are known to be under strong selection. We describe length variation at 12 microsatellite loci in 465 infections collected from 9 locations worldwide. These data reveal dramatic differences in parasite population structure in different locations. Strong linkage disequilibrium (LD) was observed in six of nine populations. Significant LD occurred in all locations with prevalence <1% and in only two of five of the populations from regions with higher transmission intensities. Where present, LD results largely from the presence of identical multilocus genotypes within populations, suggesting high levels of self-fertilization in populations with low levels of transmission. We also observed dramatic variation in diversity and geographical differentiation in different regions. Mean heterozygosities in South American countries (0.3-0.4) were less than half those observed in African locations (0. 76-0.8), with intermediate heterozygosities in the Southeast Asia/Pacific samples (0.51-0.65). Furthermore, variation was distributed among locations in South America (F:(ST) = 0.364) and within locations in Africa (F:(ST) = 0.007). The intraspecific patterns of diversity and genetic differentiation observed in P. falciparum are strikingly similar to those seen in interspecific comparisons of plants and animals with differing levels of outcrossing, suggesting that similar processes may be involved. The differences observed may also reflect the recent colonization of non-African populations from an African source, and the relative influences of epidemiology and population history are difficult to disentangle. These data reveal a range of population structures within a single pathogen species and suggest intimate links between patterns of epidemiology and genetic structure in this organism.

Most studies of genetic variability of Plasmodium falciparum have focused on protein antigens and the genes that encode them. The consensus is that populations exhibit high levels of genetic polymorphism, most notably the genes encoding surface proteins of the merozoite (Msp1, Msp2) and the sporozoite (Csp). The age and derivation of this variation is a subject that warrants further careful consideration, as discussed here by Stephen Rich, Marcelo Ferreira and Francisco Ayala.

Intragenic recombination in the merozoite surface protein-1 gene (Msp-1) of Plasmodium falciparum is a major mechanism for allelic variation among natural parasite populations. The frequency of recombination depends on the intensity of transmission in the vector mosquito. In the present study, linkage disequilibrium between polymorphic 'loci' in the 5'- and 3'-regions of Msp-1 was examined in parasite populations from Brazilian Amazon and southern Vietnam and compared with that in a Thai population previously reported. The R2 test identified clusters of linkage disequilibria between the 5'- and 3'-regions, which are different among the three populations. However, the overall strength of linkage disequilibria was stronger in Brazil, a hypoendemic area, than in Vietnam and Thailand, mesoendemic areas, suggesting that linkage disequilibrium in Msp-1 inversely correlates with the intensity of transmission. To investigate possible mechanisms for linkage disequilibrium in Msp-1, we applied the Fst index, which measures the inter-population variance in allele frequency, to 'loci' in Msp-1 among the three populations. The Fst test identified two distinct regions with respect to inter-population allele frequency in Msp-1: one for highly divergent 'loci' in the 5'-region and the other for non-divergent 'loci' in the 3'-region. These results suggest that genetic drift is not the sole mechanism for linkage disequilibrium, but selection operates on 'loci' in the 3'-region in hypo- and mesoendemic areas of malaria.

The polymorphic merozoite surface protein (MSP-1) of Plasmodium falciparum is a major asexual blood-stage malaria vaccine candidate. The impact of allelic diversity on recognition of MSP-1 during the immune response remains to be investigated in areas of hypoendemicity such as the Brazilian Amazon region. In this study, PCR was used to type variable regions, blocks 2, 4, and 10, of the msp-1 gene and to characterize major gene types (unique combinations of allelic types in variable blocks) in P. falciparum isolates collected across the Amazon basin over a period of 12 years. Twelve of the 24 possible gene types were found among 181 isolates, and 68 (38%) of them had more than one gene type. Temporal, but not spatial, variation was found in the distribution of MSP-1 gene types in the Amazon. Interestingly, some gene types occurred more frequently than expected from random assortment of allelic types in different blocks, as previously found in other areas of endemicity. We also compared the antibody recognition of polymorphic (block 2), dimorphic (block 6), and conserved (block 3) regions of MSP-1 in Amazonian malaria patients and clinically immune Africans, using a panel of recombinant peptides. Results were summarized as follows. (i) All blocks were targeted by naturally acquired cytophilic antibodies of the subclasses IgG1 and IgG3, but the balance between IgG1 and IgG3 depended on the subjects' cumulative exposure to malaria. (ii) The balance between IgG1 and IgG3 subclasses and the duration of antibody responses differed in relation to distinct MSP-1 peptides. (iii) Antibody responses to variable blocks 2 and 6 were predominantly type specific, but variant-specific antibodies that target isolate-specific repetitive motifs within block 2 were more frequent in Amazonian patients than in previously studied African populations.

Allelic diversity in the merozoite surface protein-1 (MSP-1) of Plasmodium falciparum, a major malaria vaccine candidate, was examined in clinical isolates from holoendemic northern Tanzania. The variable blocks 2, 4a, 4b, 6, and 10 of the MSP-1 gene were typed by allelic type-specific polymerase chain reaction. Twenty-four possible MSP-1 gene types were defined as unique combinations of allelic types detected in each variable block. Thirteen gene types were identified, and 187 P. falciparum populations were fully typed among 79 isolates. In contrast with recent findings in Vietnam, we were unable to detect nonrandom associations between allelic types in the typed variable blocks. Most patients (60%) harbored more than 1 genetically distinct parasite population (average: 2.37 populations per isolate) and, in 1 patient, 6 different versions of this single-copy gene were found. Statistical analysis suggests that parasites carrying different MSP-1 gene types are not independently distributed in the host population. The epidemiological consequences of these findings are discussed.

Nucleotide sequences of each variable block in the Plasmodium falciparum merozoite surface protein-1 gene (PfMSP-1) may be grouped into one of two or three possible allelic types, named after the reference isolates MAD20, K1, and RO33. Allelic diversity at this locus basically results from different combinations of allelic types in variable blocks. We used a polymerase chain reaction (PCR)-based strategy to type the variable blocks 2, 4a, 4b, and 10 of the PfMSP-1 gene of P. falciparum isolates from 54 symptomatic malaria patients living in Rondonia, a hypoendemic area in the southwestern Brazilian Amazon. Ten different PfMSP-1 gene types, defined as unique combinations of allelic types in variable blocks, were identified among the 54 isolates. Twenty-one isolates (39%) harbored more than one gene type and two had at least three genetically distinct clones. Hybrid sequences, with a MAD20-type sequence in the 5' segment (4a) and a K1-type sequence in the 3' segment (4b), were quite common in block 4. Direct sequencing of block 4 PCR products revealed a new putative recombination site in four isolates. In contrast with previous studies, the observed distribution of gene types does not deviate significantly from that expected under the null hypothesis of random association between allelic types detected in each variable block. These contradictory data are discussed with reference to the immunoepidemiologic features prevailing in distinct malaria-endemic areas.

The merozoite surface protein-1 (MSP-1) locus of Plasmodium falciparum codes for a major asexual blood-stage antigen currently proposed as a major malaria vaccine candidate. The protein, however, shows extensive polymorphism, which may compromise its use in sub-unit vaccines. Here we compare the patterns of allelic diversity at the MSP-1 locus in wild isolates from three epidemiologically distinct malaria-endemic areas: the hypoendemic southwestern Brazilian Amazon (n = 54), the mesoendemic southern Vietnam (n = 238) and the holoendemic northern Tanzania (n = 79). Fragments of the variable blocks 2, 4a, 4b and 6 or 10 of this single-copy gene were amplified by the polymerase chain reaction, and 24 MSP-1 gene types were defined as unique combinations of allelic types in each variable block. Ten different MSP-1 types were identified in Brazil, 23 in Vietnam and 13 in Tanzania. The proportion of genetically mixed infections (isolates with parasites carrying more than one MSP-1 version) ranged from 39% in Brazil to 44% in Vietnam and 60% in Tanzania. The vast majority (90%) of the typed parasite populations from Brazil and Tanzania belonged to the same seven most frequent MSP-1 gene types. In contrast, these seven gene types corresponded to only 61% of the typed parasite populations from Vietnam. Non-random associations were found between allelic types in blocks 4a and 6 among Vietnamese isolates, the same pattern being observed in independent studies performed in 1994, 1995 and 1996. These results suggest that MSP-1 is under selective pressure in the local parasite population. Nevertheless, the finding that similar MSP-1 type frequencies were found in 1994 and 1996 argues against the prominence of short-term frequency-dependent immune selection of MSP-1 polymorphisms. Non-random associations between MSP-1 allelic types, however, were not detected among isolates from Brazil and Tanzania. A preliminary analysis of the distribution of MSP-1 gene types per host among isolates from Tanzania, but not among those from Brazil and Vietnam, shows significant deviation from that expected under the null hypothesis of independent distribution of parasites carrying different gene types in the human hosts. Some epidemiological consequences of these findings are discussed.

A critical role has been proposed for the switch from non-cytophilic IgG2 to cytophilic antibodies of IgG1 and IgG3 subclasses observed in the humoral immune responses to Plasmodium falciparum of some Africans. These Africans have acquired clinically immunity naturally, after several years of exposure to holo-endemic malaria. In the present study, the possibility that life-long exposure to low levels of malarial endemicity may be associated with changes in the IgG-subclass composition of antibodies to P. falciparum was investigated in a native Amazonian community. The subjects were 138 malaria-exposed but non-infected Karitiana Indians. In a separate investigation, the concentrations of IgG-subclass antibodies in acutely ill patients with severe malaria (N = 22) were compared with those in age- and sex-matched controls who had uncomplicated malaria (N = 44). Plasma concentrations of IgG against a detergent-soluble extract of P. falciparum schizonts were measured by quantitative ELISA, using indirect standardization. Among the Karitiana, the concentrations of anti-parasite antibodies of all subclasses increased with age, and there was no correlation between age and the proportion of such antibodies which was cytophilic. The predominance of cytophilic IgG1 and non-cytophilic IgG2 antibodies in all age-groups of the Karitiana provides an example of an intermediate pattern of immune responses to P. falciparum which contrasts with those previously described in both clinically immune and non-immune populations. Although mean concentrations of cytophilic IgG1 against P. falciparum were significantly higher in the controls than in the patients with severe malaria, there were no significant differences in other IgG subclasses. Lack of exposure to malaria in the past was associated with disease severity (odds ratio = 4.75; 95% confidence interval = 1.31-17.42), and may explain, at least partially, the occurrence of defective, low-IgG1 antibody responses to P. falciparum in those subjects who had severe malaria.

The extent of allelic diversity at the Merozoite Surface Protein-1 locus of Plasmodium falciparum (PfMSP-1) was examined in isolates collected from symptomatic patients living in a mesoendemic area in southern Vietnam. The variable blocks 2, 4 and 10 were typed by polymerase chain reaction and 24 PfMSP-1 gene types were defined as unique combinations of allelic types detected in each variable block. Nineteen PfMSP-1 gene types were identified and 182 parasite populations were fully typed among 102 isolates. Forty-eight (47%) patients harbored more than one typed parasite population, and one patient had at least eight genetically distinct subpopulations. As previously shown in the same endemic area, recombination between blocks 4 and 10 was significantly less frequent than expected from random assortment of allelic types. The distribution of PfMSP-1 gene types, however, did not differ significantly from that observed in isolates collected in the same area 17-24 mo before the present study. Furthermore, the prevalence of the most common gene types and the average number of different gene types harbored by the same host did not decrease with age. This argues against the prominence of frequency-dependent immune selection of PfMSP-1 polymorphisms in this parasite population.

Allelic variation in the Plasmodium falciparum merozoite surface protein 1 (MSP1) gene is expressed as an association of allelic types in variable blocks. In this study, a PCR strategy that can detect 24 different MSP1 association types was used to investigate allelic variation in the MSP1 gene. We identified 236 distinct association type clones in 136 wild isolates collected from southern Vietnam, analysis of which revealed that (1) recombination between two representative allelic types in the central part of the MSP1 gene did not exist, (2) frequency distribution of MSP1 association types did not differ in different population groups, and (3) particular MSP1 association types were predominant. Statistical analysis for the association of allelic types indicated significant, nonrandom associations between blocks 4 and 6 but not between blocks 2 and 4, and 2 and 6. These results suggest that selection operates in favor of particular MSP1 association types. In addition, direct sequencing of 31 isolates confirmed reported sequence substitutions in the C-terminal 19-kDa Cys-rich region of MSP1, supporting a notion of limited variations in this region, a strong vaccine candidate molecule.

The heat-shock protein Pf72/Hsp70-1 from the human malaria parasite Plasmodium falciparum has been suggested as a potential candidate antigen for a multivalent vaccine. We have investigated the prevalence and levels of IgG antibodies to the recombinant protein PfR44, derived from Pf72/Hsp70-1, in individuals from different age groups living in Candeias do Jamari, an Amazonian town characterized by unstable and hypoendemic malaria transmission. Blood were collected from a household-based random sample comprising 241 people and the sera were comparatively tested against recombinant antigen PfR44 and a detergent-soluble extract of P. falciparum (PfAg-T). The prevalence and levels of IgG antibodies to both recombinant and total P. falciparum antigens were positively correlated with cumulative exposure to malaria, as estimated by the age of the individuals and the duration of their stay in the study area. Nevertheless, correlations between antibody responses to Pf72/Hsp70-1 and the acquisition of protective anti-malarial immunity could not be derived from our data.

A critical role has been proposed for cytophilic IgG1 and IgG3 subclass antibodies and monocytes and macrophages in antimalarial immunity. Here we compared the isotype composition and avidity of naturally acquired antibodies, as measured by enzyme immunoassay against a detergent-soluble extract of Plasmodium falciparum schizonts, in clinically immune Senegalese adults (n = 33) and semi-immune, adult Amazonian patients (n = 25). Plasma were collected during an acute symptomatic P. falciparum attack and two months later, and in the absence of recrudescence or reinfection. Specific IgG, IgM, IgA, and IgG subclass antibodies were assessed. The results are summarized as follows: 1) high-avidity cytophilic antibodies predominated in clinically immune Senegalese subjects; 2) acutely ill Amazonian patients produced high levels of low-avidity cytophilic antibody; 3) such a response was shortlived, since two months later, the concentrations of cytophilic antibodies were significantly lower; 4) however, affinity maturation of IgG antibodies was observed in Amazonian patients two months after the acute malaria attack. A considerable proportion (35-46%) of anti-P. falciparum IgG1 antibodies produced by African and Amazonian patients was shown to recognize periodate-sensitive carbohydrate epitopes. The potential impact of these findings on the design and evaluation of antimalarial vaccines is discussed.

Antibodies of IgG and IgM isotypes reacting with Plasmodium falciparum and P. vivax thicksmear antigens were searched for by the indirect fluorescent antibody test (IFAT) in a random sample of 230 blood donors at the transfusion centre of Porto Velho (HEMERON), Rondônia State, western Brazilian Amazon. A high prevalence of IgG seropositivity (32% against P. falciparum, 24% against P. vivax and 37% against either P. falciparum or P. vivax antigens) was detected among them, despite the fact that candidates reporting recent (< 12 months) malaria attacks were not eligible. Only a small proportion of them had also detectable IgM antibodies to these antigens. These data suggest an intense, relatively recent exposure to malaria in such an urban population sample. However, parasitaemia (as detected by microscopical examination of Giemsa-stained thick smears) was patent in only one prospective donor. The antibody profile of blood donors was compared with that of healthy subjects of all age groups, living in a close endemic area (Candeias village, 21 km east of Porto Velho). The villagers were classified into two groups according to their history of a recent (< 12 months) or a remote (> 12 months) past malaria attack due to either P. falciparum or P. vivax. Extensive overlapping was observed when the distribution of antibody titres of healthy subjects from Candeias village with a recent and remote malaria history was compared. In conclusion, subjects with a recent or a remote malaria history could not be distinguished by serological criteria alone.