The use of PCR genotyping in the assessment of recrudescence or reinfection after antimalarial drug treatment

In the past, assessment of drug efficacy against Plasmodium falciparum malaria has been performed by microscopy, screening for parasites in blood smears. However, in areas of high endemicity, reappearing parasites might be derived from new inoculations and could be classified falsely as treatment failures. Recently, a number of studies have used polymerase chain reaction (PCR) analysis of detectable parasites after drug administration to discriminate new infections from true recrudescence. The feasibility, high sensitivity and high resolution of this technique proves that it will be practical and highly valuable in studies on both drug resistance and vaccine efficacy as well as the testing of novel antimalarial drugs. In this article, Georges Snounou and Hans-Peter Beck discuss the uncertainties in the interpretation of data inherent to the technical limitations of the PCR technique, and the constraints imposed by the biology of the parasite. They suggest that although genotyping can provide strong evidence for differentiating between true recrudescence and reinfection, it must be interpreted with caution. They also propose strategies that might help minimize these uncertainties.

Genetic polymorphisms influence Plasmodium ovale PCR detection accuracy

Detection of Plasmodium ovale by use of a nested PCR assay with a novel Plasmodium ovale primer set was superior to detection of Plasmodium ovale by real-time PCR assays. Nested PCR was also better at detecting P. malariae. The detection of P. ovale in many patients first admitted >2 months following their return to Italy indicated that P. ovale relapses are common.

Eimeria (Coccidia: Eimeridea) of hares in France: description of new taxa

The oocysts of coccidian of the genus Eimeria were sought in the caecal contents of 46 Lepus granatensis and 18 L. europaeus captured in France. Parasites were found in 34 of the hares. Parasite load was mainly very low. However, species diversity was considerable. 21 species or subspecies were identified, of which 13 species and two subspecies were not previously described. Three of the taxa, E. robertsoni, E. semisculpta and E. townsendi, previously identified on numerous occasions in western Europe and, corresponding to forms or variants created before 1960 that have been subsequently elevated to a specific level, appear to be invalid. Indeed, the parasite descriptions from the material used to effect this modification do not correspond to the original descriptions. A stable equilibrium, as generally observed in the case of many congeneric species co-infection of the same host, was not observed in the hares. This has been attributed to the solitary habits of the host and of the probable polyphyletic nature of the genus Eimeria. Paleontological data for the Leporidae indicate that rabbit parasites are derived from those of the hare.

Development of a real-time PCR assay for detection of Plasmodium falciparum, Plasmodium vivax, and Plasmodium ovale for routine clinical diagnosis

A TaqMan-based real-time PCR qualitative assay for the detection of three species of malaria parasites-Plasmodium falciparum, P. ovale, and P. vivax-was devised and evaluated using 122 whole-blood samples from patients who had traveled to areas where malaria is endemic and who presented with malaria-like symptoms and fever. The assay was compared to conventional microscopy and to an established nested-PCR assay. The specificity of the new assay was confirmed by sequencing the PCR products from all the positive samples and by the lack of cross-reactivity with Toxoplasma gondii and Leishmania infantum DNA. Real-time PCR assay showed a detection limit (analytical sensitivity) of 0.7, 4, and 1.5 parasites/ micro l for P. falciparum, P. vivax, and P. ovale, respectively. Real-time PCR, like nested PCR, brought to light errors in the species identification by microscopic examination and revealed the presence of mixed infections (P. falciparum plus P. ovale). Real-time PCR can yield results within 2 h, does not require post-PCR processing, reduces sample handling, and minimizes the risks of contamination. The assay can therefore be easily implemented in routine diagnostic malaria tests. Future studies are warranted to investigate the clinical value of this technique.

Taxonomic status and re-description of Plasmodium relictum (Grassi et Feletti, 1891), Plasmodium maior Raffaele, 1931, and description of P. bigueti n. sp. in sparrows

The first accurate re-description of Plasmodium relictum (Grassi et Feletti, 1891) in its type host was provided by Raffaele in 1931, and the name relictum should thus refer to this work. In his article, Raffaele noted the presence of an associated but distinct species, P. maior. The work of Raffaele has since remained overlooked, and the taxon relictum has been applied rather loosely to parasites found in numerous birds of diverse geographic origin. Examination of Passer domesticus specimens collected in France has confirmed the presence of the two species above, and further revealed that two other species can also be found in these birds. P. bigueti n. sp. is described here, whereas the other Plasmodium sp. was not found in sufficient numbers to allow characterisation.

[Genotyping of Plasmodium falciparum isolates by amplification of glutamate-rich protein gene using polymerase chain reaction]

AIM

To develop a genotyping method based on amplifying glutamate-rich protein (GLURP) gene for the diagnosis and identification of Plasmodium falciparum.

METHODS

Two pairs of primers specific for GLURP gene of P. falciparum were designed and synthesized. R2 polymorphic domain of GLURP gene was amplified by nested PCR, which was applied to genotyping of P. falciparum isolates obtained from patients attending the malaria clinic at the village of Borai, Thailand.

RESULTS

Conspicuous polymorphism of GLURP alleles in natural populations of P. falciparum was found. 290 GLURP alleles were detected in 154 P. falciparum infections. Among the above-mentioned alleles, 12 different GLURP genotypes were distinguished according to different DNA sizes. Of them, the most frequently found allele was a variant of 770 bp, the least allele was that of 1,100 bp. More than 43% of the patients were found to be infected with mixed alleles. No apparent change for frequencies of the 12 different alleles was found in the 9-month longitudinal study.

CONCLUSION

A genotyping method is developed for the research of strain taxonomy and pathogenesis of malaria parasites.

Restricted genetic and antigenic diversity of Plasmodium falciparum under mesoendemic transmission in the Venezuelan Amazon

The study of genetic diversity in malaria populations is expected to provide new insights for the deployment of control measures. Plasmodium falciparum diversity in Africa and Asia is thought to reflect endemicity. In comprehensive epidemiological surveys reported here the genetic and antigenic structure of P. falciparum in the Venezuelan Amazon were studied over a 2-year period. DNA polymorphisms in glutamate-rich protein (GLURP), merozoite-surface protein 1 (MSP1) and MSP2 genes, in a multicopy element (PfRRM), all showed low diversity, 1 predominant genotype, and virtually no multi-clonal infections. Moreover, linkage disequilibrium was seen between GLURP, MSP1 and MSP2. Specific antibody responses against MSP1 and MSP2 recombinant antigens reflected the low genetic diversity observed in the parasite population. This is unexpected in a mesoendemic area, and suggests that the low diversity here may not only relate to endemicity but to other influences such as a bottleneck effect. Linkage disequilibrium and a predominant genotype may imply that P. falciparum frequently propagates with an epidemic or clonal population structure in the Venezuelan Amazon.

Stage-specific transcription of distinct repertoires of a multigene family during Plasmodium life cycle

Members of a multigene family in the rodent malaria parasite Plasmodium yoelii yoelii code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of py235 genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.

Association of genetic mutations in Plasmodium vivax dhfr with resistance to sulfadoxine-pyrimethamine: geographical and clinical correlates

Mutations in the Plasmodium falciparum gene (dhfr) encoding dihydrofolate reductase are associated with resistance to antifols. Plasmodium vivax, the more prevalent malaria parasite in Asia and the Americas, is considered antifol resistant. Functional polymorphisms in the dhfr gene of P. vivax (pvdhfr) were assessed by PCR-restriction fragment length polymorphism using blood samples taken from 125 patients with acute vivax malaria from three widely separated locations, Thailand (n = 100), India (n = 16), and Madagascar and the Comoros Islands (n = 9). Upon evaluation of the three important codons (encoding residues 57, 58, and 117) of P. vivax dhfr (pvdhfr), double- or triple-mutation genotypes were found in all but one case from Thailand (99%), in only three cases from India (19%) and in no cases from Madagascar or the Comoros Islands (P < 0.0001). The dhfr PCR products of P. vivax from 32 Thai patients treated with the antifolate sulfadoxine-pyrimethamine (S-P) were investigated. All samples showed either double (53%) or triple (47%) mutations. Following treatment, 34% of the patients had early treatment failures and only 10 (31%) of the patients cleared their parasitemias for 28 days. There were no significant differences in cure rates, but parasite reduction ratios at 48 h were significantly lower for patients whose samples showed triple mutations than for those whose samples showed double mutations (P = 0.01). The three mutations at the pvdhfr codons for residues 57, 58, and 117 are associated with high levels of S-P resistance in P. vivax. These mutations presumably arose from selection pressure.

Expression of the erythrocyte-binding antigen 175 in sporozoites and in liver stages of Plasmodium falciparum

Screening of a Plasmodium falciparum genomic expression library for antigens expressed at the pre-erythrocytic stages resulted in the isolation of a recombinant phage (DG249) whose insert corresponded to regions II and III of a 175-kDa erythrocyte-binding antigen (EBA-175). EBA-175 is a parasite ligand implicated in red blood cell invasion. Reverse-transcriptase polymerase chain reaction, indirect immunofluorescent antibody test, and Western blot analysis confirmed that EBA-175 is expressed not only in blood-stage parasites but also in infected hepatocytes and on the sporozoite surface. The presence of EBA-175 on pre-erythrocytic parasites enhances the vaccine potential of this antigen by adding another target to the immune responses elicited by immunization.

Meiotic recombination, cross-reactivity, and persistence in Plasmodium falciparum

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.

Human antibodies against Plasmodium falciparum liver-stage antigen 3 cross-react with Plasmodium yoelii preerythrocytic-stage epitopes and inhibit sporozoite invasion in vitro and in vivo

The Plasmodium falciparum liver-stage antigen 3 (LSA3), a recently identified preerythrocytic antigen, induces protection against malaria in chimpanzees. Using antibodies from individuals with hyperimmunity to malaria affinity purified on recombinant or synthetic polypeptides of LSA3, we identified four non-cross-reactive B-cell epitopes in Plasmodium yoelii preerythrocytic stages. On sporozoites the P. yoelii protein detected has a molecular mass similar to that of LSA3. T-cell epitopes cross-reacting with P. yoelii were also demonstrated using peripheral blood lymphocytes from LSA3-immunized chimpanzees. In contrast, no cross-reactive epitopes were found in Plasmodium berghei. LSA3-specific human antibodies exerted up to 100% inhibition of in vitro invasion of P. yoelii sporozoites into mouse hepatocytes. This strong in vitro activity was reproduced in vivo by passive transfer of LSA3 antibodies. These results indicate that the homologous epitopes may be biologically functional and suggest that P. yoelii could be used as a model to assess the antisporozoite activity of anti-LSA3 antibodies.

Genotyping of Plasmodium falciparum infections by PCR: a comparative multicentre study

Genetic diversity of malaria parasites represents a major issue in understanding several aspects of malaria infection and disease. Genotyping of Plasmodium falciparum infections with polymerase chain reaction (PCR)-based methods has therefore been introduced in epidemiological studies. Polymorphic regions of the msp1, msp2 and glurp genes are the most frequently used markers for genotyping, but methods may differ. A multicentre study was therefore conducted to evaluate the comparability of results from different laboratories when the same samples were analysed. Analyses of laboratory-cloned lines revealed high specificity but varying sensitivity. Detection of low-density clones was hampered in multiclonal infections. Analyses of isolates from Tanzania and Papua New Guinea revealed similar positivity rates with the same allelic types identified. The number of alleles detected per isolate, however, varied systematically between the laboratories especially at high parasite densities. When the analyses were repeated within the laboratories, high agreement was found in getting positive or negative results but with a random variation in the number of alleles detected. The msp2 locus appeared to be the most informative single marker for analyses of multiplicity of infection. Genotyping by PCR is a powerful tool for studies on genetic diversity of P. falciparum but this study has revealed limitations in comparing results on multiplicity of infection derived from different laboratories and emphasizes the need for highly standardized laboratory protocols.

The Plasmodium falciparum knob-associated PfEMP3 antigen is also expressed at pre-erythrocytic stages and induces antibodies which inhibit sporozoite invasion

The expression of the pfemp3 gene and the corresponding PfEMP3 knob-associated protein in the pre-erythrocytic stages of Plasmodium falciparum was demonstrated by RT-PCR, Western blots, IFAT and IEM. The antigen was found on the surface of the sporozoite and in the cytoplasm of mature hepatic stage parasites. Immunological cross-reactivity was observed with sporozoites from the rodent malaria parasites Plasmodium yoelii yoelii and Plasmodium berghei and was exploited to assess a potential role of this protein at the pre-erythrocytic stages. Specific antibodies from immune individuals were found to inhibit P. yoelii yoelii and P. berghei sporozoite invasion of primary hepatocyte cultures. PfEMP3 should now be added to the small list of proteins expressed at the pre-erythrocytic stages of P. falciparum, and its vaccine potential now deserves to be investigated.

Malaria multigene families: the price of chronicity

In this article, Georges Snounou, William Jarra and Peter Preiser discuss the survival strategy of malaria parasites in the light of a novel mechanism of clonal phenotypic variation recently described for a multigene family of Plasmodium yoelii yoelii. The 235 kDa rhoptry proteins (Py235) encoded by these genes may be involved in the selection of red blood cells for invasion by merozoites. The new mechanism may explain the ability of individual parasites to adapt to natural variations in red blood cell subsets, while ensuring that sufficient merozoites escape immune attack, thus maintaining a chronic infection for extended periods. This counterpoints the antigenic variation exemplified by PfEMP1 proteins (a large family of proteins derived from P. falciparum), which operates at the population level. The possibility of manipulating the expression of functionally similar genes in other Plasmodium species could lead to therapies aimed at reducing clinical severity without compromising the acquisition and maintenance of immunity.

Survival of rodent malaria merozoites in the lymphatic network: potential role in chronicity of the infection

Experiments performed during the last few years, lead us to hypothesise the existence of latent asexual forms of murine Plasmodium. In the present report we examined the organs of infected animals and describe novel structures, which we call merophores, containing merozoites which have resisted lysis seen with other asexual stage parasites. We propose that these merozoites represent a latent form of the parasite. Merophores were also found in the lymphatic circulation, and were demonstrated by subinoculation to have retained their viability. Depending on the parasite species two types of merophores were observed. For P. yoelii nigeriensis merophore sacks, with the latent merozoites found inside vesicles, were usually observed. Merophore leucocytes, where latent merozoites dispersed in the cytoplasm of macrophages or neutrophils, were solely seen with P. vinckei petteri. Both structures were seen in P. chabaudi chabaudi infections. Merophores were found in lymph nodes of rodents after the asexual parasitaemia had apparently subsided. They were formed soon after schizogony, principally in the spleen, either by pitting or by macrophage phagocytosis. Merophore numbers appeared to be proportional to the number of maturing schizonts. We propose that merophore formation and their circulation in the lymphatics play an important role in the pattern of recrudescences and chronicity of rodent malaria infections. It is further suggested that the lymphatic network, a privileged pathway for many parasites, might play a similar role in human malaria infections.

Development of irreversible lesions in the brain, heart and kidney following acute and chronic murine malaria infection

Irreversible pathological lesions were noted in the organs of mice infected with 1 of 3 rodent malaria species: Plasmodium chabaudi chabaudi, P. vinckei petteri and P. yoelii nigeriensis at different times during the course of the primary parasitaemia and long after microscopical clearance of the parasites. Moreover, similar lesions were also obtained when parasite levels were kept below 1% by subcurative drug treatment. The frequency and severity of the lesions correlated with the duration of the infection. Accumulation of tissue damage during chronic low-grade malaria infections has implications for the design of control measures.

Polymorphism at the merozoite surface protein-3alpha locus of Plasmodium vivax: global and local diversity

Allelic diversity at the Plasmodium vivax merozoite surface protein-3alpha (PvMsp-3alpha) locus was investigated using a combined polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) protocol. Symptomatic patient isolates from global geographic origins showed a high level of polymorphism at the nucleotide level. These samples were used to validate the sensitivity, specificity, and reproducibility of the PCR/RFLP method. It was then used to investigate PvMsp3alpha diversity in field samples from children living in a single village in a malaria-endemic region of Papua New Guinea, with the aim of assessing the usefulness of this locus as an epidemiologic marker of P. vivax infections. Eleven PvMsp-3alpha alleles were distinguishable in 16 samples with single infections, revealing extensive parasite polymorphism within this restricted area. Multiple infections were easily detected and accounted for 5 (23%) of 22 positive samples. Pairs of samples from individual children provided preliminary evidence for high turnover of P. vivax populations.

Biased distribution of msp1 and msp2 allelic variants in Plasmodium falciparum populations in Thailand

Plasmodium falciparum isolates were obtained from Thai patients attending a malaria clinic on the Thai-Kampuchean border over 4 cross-sectional surveys carried out at 3-monthly intervals. The genetic structure of the parasite populations was determined by nested polymerase chain reaction (PCR) amplification of polymorphic regions of 3 P. falciparum antigen genes: msp1, msp2 and glurp. Although a high degree of diversity characterized these isolates, the overall population structure of the parasites associated with patent malaria infections was observed to remain relatively stable over time. The highest degree of polymorphism was observed with msp2, and the mean number of lines per infection (multiplicity of infection) calculated with this marker was higher than that obtained using msp1 or glurp alone, or combined. Infections with > or = 2 parasite lines were seen in 76% of the samples, and were proportionally more numerous at the start and end of the rainy season. Two interesting exceptions to the random distribution were observed and involved 2 allelic variants which in one case were found dissociated (msp1 MAD20-family) and in the other were associated (msp2 FC27-family). The epidemiological significance of these types of data is discussed.

Influence of polymorphism in the genes for the interleukin (IL)-1 receptor antagonist and IL-1beta on tuberculosis

Several lines of evidence suggest that host genetic factors controlling the immune response influence infection by Mycobacterium tuberculosis. The proinflammatory cytokine interleukin (IL)-1beta and its antagonist, IL-1Ra (IL-1 receptor agonist), are strongly induced by M. tuberculosis and are encoded by polymorphic genes. The induction of both IL-1Ra mRNA and secreted protein by M. tuberculosis in IL-1Ra allele A2-positive (IL-1Ra A2(+)) healthy subjects was 1.9-fold higher than in IL-1Ra A2(-) subjects. The M. tuberculosis-induced expression of mRNA for IL-1beta was higher in subjects of the IL-1beta (+3953) A1(+) haplotype (P = 0.04). The molar ratio of IL-1Ra/IL-1beta induced by M. tuberculosis was markedly higher in IL-1Ra A2(+) individuals (P < 0.05), with minor overlap between the groups, reflecting linkage between the IL-1Ra A2 and IL-1beta (+3953) A2 alleles. In M. tuberculosis-stimulated peripheral blood mononuclear cells, the addition of IL-4 increased IL-1Ra secretion, whereas interferon gamma increased and IL-10 decreased IL-1beta production, indicative of a differential influence on the IL-1Ra/IL-1beta ratio by cytokines. In a study of 114 healthy purified protein derivative-reactive subjects and 89 patients with tuberculosis, the frequency of allelic variants at two positions (-511 and +3953) in the IL-1beta and IL-1Ra genes did not differ between the groups. However, the proinflammatory IL-1Ra A2(-)/IL-1beta (+3953) A1(+) haplotype was unevenly distributed, being more common in patients with tuberculous pleurisy (92%) in comparison with healthy M. tuberculosis-sensitized control subjects or patients with other disease forms (57%, P = 0.028 and 56%, P = 0. 024, respectively). Furthermore, the IL-1Ra A2(+) haplotype was associated with a reduced Mantoux response to purified protein derivative of M. tuberculosis: 60% of tuberculin-nonreactive patients were of this type. Thus, the polymorphism at the IL-1 locus influences the cytokine response and may be a determinant of delayed-type hypersensitivity and disease expression in human tuberculosis.

A rhoptry-protein-associated mechanism of clonal phenotypic variation in rodent malaria

The recognition and invasion of host cells are mediated by components of the apical complex of the ookinete, sporozoite and merozoite stages of Plasmodium parasites. The paired rhoptries (organelles involved in host-cell recognition) in the apical complex contain many proteins of as-yet unknown function. In the rodent malaria agent P. yoelii yoelii, a multigene family codes for merozoite rhoptry proteins of relative molecular mass 235,000 (p235 proteins); these proteins are thought to determine the subset of erythrocytes that the parasites invade. Further support for this idea came from the identification of a region in p235 with weak but significant homology to reticulocyte-binding protein-2 of P. vivax and the demonstration that at least one p235 member binds to the erythrocyte surface membrane. Here, using single, micromanipulated P.y.yoelii parasites, we describe a new mechanism of gene expression by which the merozoites originating from a single schizont each express a distinct member of this multigene family. We propose that this new type of clonal phenotypic variation provides the parasite with a survival strategy in the mammalian host; this strategy contributes to the observed chronicity of malarial infections. This phenomenon is genetically and functionally distinct from classical antigenic variation, which is mediated by the var multigene family of P. falciparum.

A genus- and species-specific nested polymerase chain reaction malaria detection assay for epidemiologic studies

A nested polymerase chain reaction (PCR) assay that uses Plasmodium genus-specific primers for the initial PCR (nest 1) amplification and either genus- or species-specific primers for the nest 2 amplifications was tested on laboratory and field samples. With in vitro cultured Plasmodium falciparum-infected blood samples, it was capable of detecting six parasites/microl of blood using DNA prepared from 25-microl blood spots on filter paper. The assay was evaluated on fingerprick blood samples collected on filter paper from 129 individuals living in a malaria-endemic area in Malaysia. Malaria prevalence by genus-specific nested PCR was 35.6% (46 of 129) compared with 28.7% (37 of 129) by microscopy. The nested PCR detected seven more malaria samples than microscopy in the first round of microscopic examination, malaria in three microscopically negative samples, six double infections identified as single infections by microscopy and one triple infection identified as a double infection by microscopy. The nested PCR assay described is a sensitive technique for collecting accurate malaria epidemiologic data. When coupled with simple blood spot sampling, it is particularly useful for screening communities in remote regions of the world.

Complexity of Plasmodium falciparum infections is consistent over time and protects against clinical disease in Tanzanian children

The complexity of Plasmodium falciparum populations in 21 children was studied in repetitive samples over 4 years in an area of Tanzania where the organism is holoendemic. Genotyping was done by a polymerase chain reaction method that targets three highly polymorphic regions of the merozoite surface protein (MSP) 1 block 2, MSP 2, and the glutamine-rich protein. Eight children were repeatedly parasitemic, 5 had scanty parasitemias, and 8 were consistently nonparasitemic. Varying numbers of genotypes were detected in the parasitemic children, but the multiplicity of infection was significantly constant within each child. The children with frequent parasitemias experienced fewer clinical episodes during the study period than those without parasitemias. There was also a tendency for children with more complex infections to experience fewer episodes. The children had consistent parasitologic profiles over the 4 years. Although few subjects were studied and the results will require confirmation, the results suggest that asymptomatic (especially polyclonal) P. falciparum infection protects against clinical disease from new infections.

A clonal Plasmodium falciparum population in an isolated outbreak of malaria in the Republic of Cabo Verde

We present the first parasitological, molecular and longitudinal analysis of an isolated outbreak of malaria. This outbreak occurred on Santiago Island (Republic of Cabo Verde), a region where malaria is hypoendemic and controlled, and thus the population is considered non-immune. Blood samples were collected from the inhabitants over 1 month and during cross-sectional surveys in the following year. The presence and nature of the parasites was determined by PCR. Plasmodium falciparum was the only species detected. Genetic analysis revealed that the circulating parasites were genetically homogeneous, and probably clonal. Gametocytes were found throughout this period. Our data suggest that this represented a focal outbreak, resulting in the infection of at least 40% of the villagers with a clonal parasite line. Thus, P. falciparum infections can persist for at least 1 year in a substantial proportion (10%) of the hosts. Implications for malaria control and the interpretation of epidemiological data are discussed.

Reduction in the mean number of Plasmodium falciparum genotypes in Gambian children immunized with the malaria vaccine SPf66

SPf66, a synthetic peptide Plasmodium falciparum vaccine, did not protect young Gambian children against clinical attacks of malaria. Nevertheless, Gambian children who had been vaccinated with SPf66 and who were parasitaemic at the end of the first malaria transmission season after vaccination had significantly fewer detectable P. falciparum genotypes than control children, as determined by polymerase chain reaction analysis of 3 polymorphic loci--the msp1 block 2 repeat region, the msp2 repeat region, and the R11 region of the glutamate-rich protein gene (glurp). Geometric mean numbers of genotypes were 1.66 vs. 1.87, 1.95 vs. 2.43, and 1.21 vs. 1.50 for msp1, msp2 and glurp, respectively (P = 0.31, P = 0.04 and P < 0.01). Differences between groups became a little more marked for msp1 and msp2 when children with symptomatic malaria were excluded. No significant difference was found between parasites obtained from SPf66-vaccinated or control children in the prevalences of amino acid alleles at positions 44 and 47 in the 11 amino acid sequence of the merozoite surface protein 1 molecule, which is present in SPf66. The reduction in the number of genotypes observed could not be explained by a difference in parasite densities between SPf66-vaccinated and control children, as geometric mean parasite densities were almost identical in the 2 groups. These observations suggest that SPf66 vaccine may have induced an immune response which reduced the incidence of new infections in immunized children or accelerated the rate of clearance of parasites of individual genotypes. However, no reduction in the prevalence or density of parasitaemia was recorded in SPf66-vaccinated children, suggesting the existence of some kind of density-dependent mechanism for controlling low levels of malaria parasitaemia.

Sampling and storage of blood and the detection of malaria parasites by polymerase chain reaction

Polymerase chain reaction (PCR) is now widely used in malaria research for analysis of field samples. However, little has been reported regarding loss of sensitivity due to field methodology. Therefore, studies were carried out in relation to blood sampling (anticoagulants, culture medium, filter paper), storage (temperature, time and immediate lysis) and handling (repeated thawing and freezing). The PCR was unaffected by citrate and EDTA but partly inhibited by heparin (inhibition was reversed by heparinase at optimal concentrations). Samples collected on filter paper showed a significant 100-fold lower sensitivity (compared to control samples frozen immediately after collection) when stored at 30 degrees C and 60% humidity; and the paper quality appeared to be critical. Storage of unprocessed whole blood at 4 degrees C, 20 degrees C or 30 degrees C rarely resulted in any loss of sensitivity. Repeated thawing generally resulted in 10-fold loss of sensitivity compared to blood kept frozen until DNA extraction. The presence of antimalarial drug did not apparently affect sensitivity. We conclude that the mode of collection and storage of blood samples may influence the sensitivity of detection of malaria parasites by PCR. This may be critical in studies including individuals with low parasitaemia, mixed infections and comparison of data from different settings.

Limited genetic diversity of Plasmodium falciparum in field isolates from Honduras

The genetic diversity displayed by Plasmodiumfalciparum field isolates, the occurrence of variant forms of the parasite at different frequencies in different geographic areas, and the complexity of the infections represent major obstacles for the development of effective malaria control measures. However, since most of the existing studies have been performed in regions where P. falciparum transmission is high, little is known about the diversity and complexity of parasite populations circulating in areas of low malaria endemicity. We investigated the extent of genetic polymorphism in P. falciparum field isolates from Honduras, a region where its transmission is low and seasonal. Allelic diversity was analyzed in the highly polymorphic parasite genes encoding the merozoite surface proteins- (MSP-1) and -2 (MSP-2) and the glutamate-rich protein (GLURP) by the polymerase chain reaction. Gene polymorphism was also assessed in the EB200 region derived from the highly size polymorphic Pf332 gene. Limited size polymorphism was detected in all genes analyzed, with four and three variants for the MSP-1 and MSP-2 alleles, respectively, and two size variants for the GLURP and Pf332 genes. Moreover, based on the studied genetic markers, most infections consisted of only a few genetically distinct parasite clones. These results suggest that the P. falciparum parasite populations circulating in this region are genetically homogeneous and point to an association between the extent of parasite genetic diversity and the intensity of malaria transmission.

No influence of age on infection complexity and allelic distribution in Plasmodium falciparum infections in Ndiop, a Senegalese village with seasonal, mesoendemic malaria

We have shown previously that in Dielmo, a Senegalese village with intense perennial Plasmodium falciparum transmission, the infection complexity and the distribution of some allelic types harbored by asymptomatic carriers was age-dependent. We report here an investigation of these parameters in Ndiop, a village located 5 km from Dielmo, where malaria is mesoendemic and seasonal, and where immunity is acquired at a very low rate, as indicated by the lifelong distribution of P. falciparum clinical attacks. Blood was collected from 143 and 125 inhabitants, including 122 individuals sampled in both surveys, during two cross-sectional surveys at one-month intervals during the 1994 transmission season. Plasmodium falciparum parasites were genotyped for three polymorphic single copy genes. Genetic diversity was very large, with 17, 43, and nine distinct alleles detected for the merozoite surface protein-1 (MSP-1), MSP-2, and glutamate-rich protein loci, respectively. These figures, similar to those previously observed in Dielmo, indicate that the parasite genetic diversity is not directly related to the inoculation rate, at least in the range of transmission intensity studied here. The complexity of the asymptomatic infections (average number of distinct genotypes per isolate) was more than two-fold lower in Ndiop than in Dielmo and importantly, did not decrease with age. Likewise, the allele distribution was not influenced by age, contrasting with the observations made in Dielmo. This indicates that the number of parasite types per isolate and the influence of age on complexity and allele distribution depend on the level of endemicity, consistent with the interpretation that they reflect acquired anti-parasite immunity.

Only viable parasites are detected by PCR following clearance of rodent malarial infections by drug treatment or immune responses

Detection and analysis of pathogens by PCR plays an important role in infectious disease research. The value of these studies would be diminished if nuclear material from dead parasites were found to remain in circulation for extended periods and thus result in positive amplification. This possibility was tested in experimental rodent malaria infections. Blood samples were obtained from infected mice during and following drug or immune clearance of Plasmodium chabaudi chabaudi parasitemias. Detection of parasite DNA by a sensitive Plasmodium-specific PCR amplification assay was associated with the presence of viable parasites, as detected by subinoculation. No parasite DNA could be detected by PCR 48 h after the injection of killed parasites into mice. Nuclear material from parasites removed by drug or immune responses is rapidly cleared from the circulation and does not contribute significantly to amplification. Thus, results from PCR analysis of malaria-infected blood accurately reflect the presence of live parasites.

Non-immune patients in the Democratic Republic of São Tomé e Principe reveal a high level of transmission of P. ovale and P. vivax despite low frequency in immune patients

Malaria parasite species circulating in immune inhabitants of the Democratic Republic of São Tomé e Principe (DRSTP) and those acquired by non-immune travellers returning from this country have been compared. Using sensitive detection and species identification by PCR, Plasmodium parasites were found in the blood of 16 of the 43 travellers, who reported during the first 8 months of 1995 to a clinical diagnosis laboratory in Lisbon. Plasmodium vivax was found in four (25%) and Plasmodium ovale in ten (63%) of these patients. The observed prevalence of these two species in infected patients of DRSTP during 1995/1996 was <2%.

A randomized controlled trial of artemether/benflumetol, a new antimalarial and pyrimethamine/sulfadoxine in the treatment of uncomplicated falciparum malaria in African children

We report here the results of a randomized double blind trial comparing coartemether (CGP56697), a combination of artemether and benflumetol, with pyrimethamine/sulfadoxine (P/S). Two hundred eighty-seven children 1-5 years of age with uncomplicated falciparum malaria were enrolled at two centers in The Gambia between July 1996 and December 1996. Following treatment, children were visited at home every 24 hr until a blood film free of asexual parasites was obtained. Genotyping of parasites was used to distinguish recrudescence from new infections. Three days after the start of treatment, 133 (100%) of the CGP56697-treated children compared with 128 (93.4%) of children treated with P/S had cleared their parasites (P = 0.003). The day 15 cure rate was 93.3% for CGP56697 and 97.7% for P/S (P = 0.13). Within the third and fourth week after initiation of therapy, 20 children treated with CGP56697 and one of the P/S-treated children returned with second malaria episodes (P < 0.0001). Genotyping suggested that the majority (19 of 23 [82.6%]) of these second episodes were due to new infections, supporting the World Health Organization recommendation that longer follow-up is not relevant for the assessment of drug efficacy. At the two-week follow-up, 28.9% of the P/S treated children but none of the CGP56697-treated children carried gametocytes (P < 0.0001). This study showed that CGP56697 is safe in African children with acute uncomplicated falciparum malaria, clears parasites more rapidly than P/S, and results in fewer gametocyte carriers. More frequent new infections within the third and fourth week following treatment with CGP56697 than treatment with P/S are likely to be due to the short prophylactic effect of CGP56697.

Treatment of African children with uncomplicated falciparum malaria with a new antimalarial drug, CGP 56697

New antimalarial drugs are urgently needed. The use of short courses of the new antimalarial drug artemether as monotherapy has been limited by secondary malaria episodes following parasite clearance. Therefore, a new antimalarial drug, CGP 56697, has been developed, which combines artemether with a longer-acting antimalarial agent, benflumetol. A safety trial was undertaken in 60 Gambian children 1-6 years old with uncomplicated Plasmodium falciparum malaria. All children treated with CGP 56697 cleared their parasites 72 h after the start of treatment. No neurologic, cardiac, or other adverse reactions were observed. Second episodes of falciparum malaria were recorded in 16 (27%) of the children. Second infections were more frequent during the rainy season than during the dry season. Molecular epidemiologic studies suggested that 12 of the 14 second episodes of malaria in children treated with CGP 56697 were due to new infections. CGP 56697 proved to be a safe and effective antimalarial drug in African children.

Relapsing Plasmodium vivax malaria with atypical parasite forms and phagocytosis by peripheral neutrophils

A case of atypical Plasmodium vivax malaria is presented. The clinical follow-up has allowed to characterize three consecutive malaria clinical episodes within one year. At the first attack, 39% of the infected red blood cells were parasitized by gametocytes. Furthermore, rare crisis forms, exceptional "pseudoparthenogenesis" forms, a few equatorial trophozoites, malaria pigment-containing leucocytes and phagocytized parasites were also found in the thin blood smears. At the second malaria episode, morphological aspects were quite similar, but the gametocyte percentage decreased and that of the equatorial trophozoite forms increased. Only at the third attack, was the morphology typical of P. vivax. The Plasmodium species and the absence of mixed infection were unequivocally confirmed using polymerase chain reaction. Atypical strains of P. vivax are relatively frequent. Nevertheless, to our knowledge, neither so high a gametocyte percentage, nor extensive P. vivax peripheral phagocytosis were previously reported.

Daily dynamics of Plasmodium falciparum subpopulations in asymptomatic children in a holoendemic area

Plasmodium falciparum is the major cause of malaria morbidity and mortality in the world. Biologic and antigenic diversity is a characteristic of this parasite and infections can consist of several genetically diverse parasites. The daily dynamics of these parasite subpopulations were investigated in asymptomatic children in rural Tanzania. Fingerprick blood samples were collected on 14 consecutive days from 20 children. Parasite densities were detected by light microscopy and genotyping of P. falciparum was done using a nested polymerase chain reaction (PCR) assay targeting polymorphic regions on the merozoite surface protein-1 (MSP-1), MSP-2, and glutamine-rich protein (GLURP) genes. In the eight children harboring P. falciparum throughout the study period, infections were found to be highly complex with daily changes in both parasite density and genotypic pattern. A nonrandom. 48-hr periodicity in these fluctuations suggests that P. falciparum infections consist of inherently synchronous subpopulations of parasites. These findings have important biologic and epidemiologic implications since one blood sample may only partly reflect the whole parasite population in an infected individual.

Transmission of mixed malaria species and strains by mosquitoes, as detected by PCR, in a study area in Guinea-Bissau

Parasites present in blood samples of asymptomatic carriers and in the midgut of mosquitoes collected within a few days from the same households, have been analysed by PCR. A high prevalence (32%) of infected mosquitoes was observed and, in half of these, two parasite species were found simultaneously. The distribution of parasite species in the mosquito correlated with that found in the infected persons. Genotype patterns of Plasmodium falciparum populations were however found to be different in the two sets of samples. These results and the potential of PCR are discussed with reference to investigations of the dynamics of malaria transmission.

Evidence for selection for the tyrosine-86 allele of the pfmdr 1 gene of Plasmodium falciparum by chloroquine and amodiaquine

The 4-aminoquinolines chloroquine (CQ) and amodiaquine (AM) were used to treat Gambian children with uncomplicated falciparum malaria in a randomized drug trial. Blood samples were taken immediately before treatment (day 0), and at day 7 and day 28 after treatment. Samples from those parasitologically positive at day 7 following treatment ('early positives') and those positive at day 28 but negative at day 7 ('late positives') have been studied by PCR followed by restriction enzyme digestion to determine the allelic status of the pfmdr 1 locus at the codon-86 position (asparagine or tyrosine), previously associated with resistance to CQ. A significantly higher prevalence of the tyr-86 allele was observed in samples taken immediately before treatment (day 0) in the early positives group when compared with the late positives group. This suggests the tyr-86 allele contributes to drug resistance in the early positives group. This association remained significant for both CQ and AM groups, implying a common genetic basis of resistance. Predominance of the allele at day 7 is consistent with a strong selection in the first week following treatment. In the late positives group, a significantly higher prevalence of the tyr-86 allele was observed in the samples at day 28 when compared with those at day 0, suggestive of selection during the period day 7 to day 28. Differences were observed in the extent of this selection in the CQ and AM groups. The samples were genotyped at 3 unlinked polymorphic loci. These analyses suggested that most parasites observed at day 7 were probably recrudescences whereas most of those at day 28 were reinfections.