Long-term asymptomatic carriage of Plasmodium falciparum protects from malaria attacks: a prospective study among Senegalese children.

BACKGROUND: In areas of seasonal malaria transmission, long-term asymptomatic carriage of Plasmodium falciparum throughout the dry season has been primarily studied in terms of the parasites, and the clinical consequences of persistent parasite carriage are unknown. METHODS: A prospective study was conducted in Senegal, from 2001 through 2003 among 1356 children living in areas where malaria is endemic, with seasonal transmission occurring from August through December. Cross-sectional parasitological measurements and detection of active malaria attacks were performed. A malaria attack was defined as an axillary temperature > or =37.5 degrees C, associated with a parasite density >2500 trophozoites/microL. Children harboring P. falciparum in June who did not have clinical signs were defined as asymptomatic carriers. The association of asymptomatic carriage with parasite densities and with the occurrence of malaria attacks during the rainy season were analyzed separately for the years 2002 and 2003, taking into account potential confounding covariates and use of antimalarial drugs. RESULTS: The prevalence of asymptomatic carriage was 32% (332 of 1025 persons) in June 2002 and 23% (208 of 912 persons) in June 2003. Asymptomatic P. falciparum carriers had a significantly higher mean parasite density and a significantly lower probability of developing a malaria attack during the subsequent rainy season than did noncarriers (adjusted odds ratio in 2002, 0.56; P = .01; adjusted odds ratio in 2003, 0.50; P = .01). CONCLUSIONS: These results suggest that in areas of seasonal transmission, asymptomatic carriage of P. falciparum may protect against clinical malaria. Further studies are needed to understand the immune effectors and host susceptibility that could be involved in this phenomenon.     

Frequent and persistent, asymptomatic Plasmodium falciparum infections in African infants, characterized by multilocus genotyping

To determine the duration and complexity of naturally acquired Plasmodium falciparum infections in small children, a longitudinal cohort study of 143 newborns was conducted in coastal Ghana. On average, children experienced 2 episodes of infection in their first 2 years of life, the median duration of an asymptomatic infection was <4 weeks, and estimates of the mean number of parasite genotypes per infection were 1.15-2.28. Nevertheless, 40% of the children experienced infections lasting 5 months old. The ability of very young children to clear or control malaria infections indicates the presence of effective innate or immune antiparasite mechanisms.     

Diversity of Plasmodium falciparum clones infecting children living in a holoendemic area in north-eastern Tanzania

The diversity of Plasmodium falciparum clones and their role in progression from asymptomatic to symptomatic condition in children have been investigated. Attempts to identify whether particular parasite genotypes were associated with the development of clinical symptoms have been made. A cohort of 34 initially asymptomatic parasitaemic children aged 1-5 years were followed daily for 31 days. Clinical examinations were made each day for signs and symptoms of clinical malaria, followed by parasitological investigation. Nineteen children developed symptoms suggestive of clinical malaria during this period. Daily blood parasite samples from 13 children who developed clinical malaria symptoms and 7 who remained asymptomatic were genotyped by PCR-amplification of the polymorphic regions of the merozoite surface proteins 1 and 2 (MSP1 and MSP2) and the glutamate rich protein (GLURP) genes. Infections were found to be highly complex in both groups of children. Every isolate examined from both groups had a mixture of parasite clones. Daily changes were observed in both parasite density and genotypic pattern. The mean number of genotypes per individual was estimated at 4.9 and 2.7 for asymptomatic and symptomatic groups of children, respectively. Analysis of allele frequency distributions showed that these differed significantly for the MSP1 locus only.     

Antibody-mediated in vitro growth inhibition of field isolates of Plasmodium falciparum from asymptomatic children in Burkina Faso

Antibody-mediated inhibition of Plasmodium falciparum parasites in vitro reflects the potential parasite-neutralizing activity of the antibodies in vivo. In this study, immunoglobulins and P. falciparum isolates were collected from children with asymptomatic malaria in Burkina Faso. We demonstrate a significantly lower in vitro growth inhibitory activity against the P. falciparum field isolates by autologous host immunoglobulin compared with that of immunoglobulin from other individuals. To gain further insight to possible mechanisms for the diverse sensitivity observed, analyses of consecutive isolates taken 14 days apart were performed with regard to polymerase chain reaction-based genotyping and sensitivity to growth inhibition in vitro. All the asymptomatic infections were composed of multiple, genotypically distinct parasite clones, and at least one new parasite clone appeared in most of the day 14 isolates compared with the corresponding day 0 isolates. Apparently persisting parasite clones, present in both the day 0 and day 14 isolates from the same person, were also frequently observed. The day 14 isolates were more effectively inhibited by autologous day 14 immunoglobulin than by the corresponding day 0 immunoglobulin in 57% of the cases. However, the frequent presence of persisting parasite clones in asymptomatic children indicates that the parasite may develop a relative resistance to neutralizing immune responses.     

Serum transferrin receptor levels are increased in asymptomatic and mild Plasmodium falciparum-infection.

BACKGROUND AND OBJECTIVE: The serum transferrin receptor (sTfR) concentration in an individual reflects the extent of erythropoietic activity and is considered a useful marker of iron deficiency independent of concurrent inflammation or infection. However, data on the impact of malaria on this parameter are ambiguous. We have examined potential associations of asymptomatic and mild Plasmodium falciparum-infections and of several erythrocyte variants with sTfR values in South West Nigeria. DESIGN AND METHODS: In a cross-sectional study among 161 non-hospitalized children, sTfR concentrations and P. falciparum parasitemia were assessed. In addition, hemoglobin (Hb) and serum ferritin values, Hb-types, glucose-6-phosphate dehydrogenase (G6PD)deficiency and a-globin genotypes were determined and the effects of these factors on sTfR levels were analyzed by univariate and multivariate statistical methods. RESULTS: P. falciparum-infection was present in 77% of the children. Mean sTfR levels were higher in infected than in non-infected children (geometric mean, 3.68, 95% confidence interval [3.5-3.9] vs. 2.99 [2.7-3.3] mg/L; p = 0.0009). There was a significant trend for higher sTfR values with increasing parasite density. sTfR values decreased continuously with age. Hb-types, G6PD-, and a-globin genotypes did not correlate with sTfR levels. In the multivariate analysis, age, Hb and log ferritin values, and parasite density of P. falciparum were independently associated with log sTfR values. INTERPRETATION AND CONCLUSIONS: sTfR concentrations are increased in asymptomatic and mild P. falciparum-infections suggesting adequate bone marrow response in this condition. The diagnostic value of sTfR levels for iron deficiency may be impaired in areas where stable malaria occurs.     

Protection against clinical malaria by heterologous immunoglobulin G antibodies against malaria-infected erythrocyte variant surface antigens requires interaction with asymptomatic infections

Erythrocytes infected with mature stages of Plasmodium falciparum express variant surface antigens (VSAs) of parasite origin, including P. falciparum erythrocyte membrane protein 1. Anti-VSA antibodies protect against clinical malaria caused by parasites bearing VSAs to which they are specific (homologous), but their role in protecting against heterologous infection is unclear. Here, we report that, among 256 Kenyan children involved in a 1-year active case surveillance study, asymptomatic parasitemia was associated with an enlarged repertoire of anti-VSA immunoglobulin G (IgG) antibodies specific to apparently heterologous parasite isolates, as measured by flow cytometry. Together, asymptomatic infection and anti-VSA IgG were associated with reduced odds of experiencing an episode of clinical malaria during follow-up, whereas, independently, they were associated with increased susceptibility. These results support previous findings and underline the importance of considering the parasitological status of study participants when examining the role that immune responses to VSAs and other malaria antigens play.     

Alpha+ -thalassemia protects against anemia associated with asymptomatic malaria: evidence from community-based surveys in Tanzania and Kenya

BACKGROUND: In hospital-based studies, alpha(+)-thalassemia has been found to protect against severe, life-threatening falciparum malaria. alpha(+)-Thalassemia does not seem to prevent infection or high parasite densities but rather limits progression to severe disease--in particular, severe malarial anemia. We assessed to what extent alpha(+)-thalassemia influences the association between mild, asymptomatic Plasmodium falciparum infection and hemoglobin concentration. METHODS: The study was based on 2 community-based surveys conducted among afebrile children (0.5-8 years old; n=801) in Kenya and Tanzania. RESULTS: Among children without inflammation (whole-blood C-reactive protein concentration 相似文献   

Are Plasmodium falciparum parasites present in peripheral blood genetically the same as those sequestered in the tissues?

Since quinine does not inhibit the growth of Plasmodium falciparum ring stages or mature schizonts, parasites may continue to emerge from sequestration sites after starting treatment. We used polymerase chain reaction amplification of P. falciparum merozoite surface protein 1 (MSP-1) and MSP-2 alleles to distinguish genotypes infecting 58 children with severe malaria. To examine changes in parasite populations in peripheral blood over time, we compared changes in number and spectrum of genotypes in samples on admission to a hospital to those obtained up to 24 hours later. Thirty-four children lost genotypes, 21 retained genotypes, and 3 gained an extra P. falciparum genotype at one locus but not the other. The lack of novel genotypes emerging suggests that among children with severe malaria the dominant clones sequestered in deep organs are usually the same as those in peripheral circulation.     

Human genetic polymorphisms and asymptomatic Plasmodium falciparum malaria in Gabonese schoolchildren

Several studies have focused their attention on the relationship between host genetic factors and susceptibility/resistance to severe malaria. However, there is a paucity of information concerning the role of host genetic factors in asymptomatic malaria, a form of low-grade Plasmodium falciparum infection without clinical symptoms. We investigated in this study the potential relationship between the host (human) genetic polymorphisms (glucose-6-phosphate dehydrogenase [G6PD], mannose binding lectin [MBL], tumor necrosis factor alpha [TNFalpha](-308) and (-238), and nitric oxide synthase 2 [NOS2](-954)) and the prevalence and profile of asymptomatic P. falciparum infection in 158 Gabonese schoolchildren. We found that G6PD A- heterozygous females (18 of 74) have a low prevalence of asymptomatic malaria (38.9% versus 67.3%; P = 0.03, by chi-square test). Children heterozygous for TNFalpha(-238) (25 of 156) carry high number of diverse infecting parasite genotypes (2.5 versus 1.99; variance F = 3.05). No statistically significant association was found between MBL, TNFalpha(-308), or NOS2 polymorphisms and asymptomatic malaria. Upon combining our data on asymptomatic forms with those from the literature for others forms, we conclude that G6PD A- heterozygous females are protected against all forms of P. falciparum malaria, and that the TNFalpha(-238A) allele confers protection against clinical malaria.     

Cellular and humoral inhibition of Plasmodium falciparum growth in vitro and recovery from acute malaria

Both mononuclear cell cytotoxicity and serum inhibition of Plasmodium falciparum growth in vitro were found to vary according to the stage of infection in Gambian children with clinical malaria. Cytotoxicity was displayed by mononuclear cells and serum from children with acute malaria but this form of parasite killing was more effective in children with low grade P. falciparum infections of at least 10 days duration. Parasite inhibitory antibody was not evident in sera from acutely infected children but was found in sera from children recovering from malaria and reached a peak in convalescent children when P. falciparum growth was inhibited by at least 50%. The humoral response in convalescent children was strain related, being more effective against the most recent infecting parasite strain than against other 'wild' P. falciparum isolates. In contrast, mononuclear cell cytotoxicity was not strain related; when effective, multiplication of all parasite isolates tested was retarded to the same degree. The discussion considers the role of mononuclear cell cytotoxicity in the development of protective immunity and suggests that it may be a 'front line' defense mechanism during each malaria attack.     

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.     

Falciparum malaria and beta-thalassaemia trait in northern Liberia

In a study in northern Liberia of the malaria and beta-thalassaemia hypothesis, the frequencies of beta-thalassaemia and HbS traits were 9.1 and 3.4% in the Mano and 9.5 and 1.7% in the Gio tribal samples. HbC and HbN were present at low frequency. G6PD deficiency was found in 16% of males. An observed increase with age of beta-thalassaemia trait frequencies was consistent with the selection hypothesis. However, we could not entirely exclude that associated iron deficiency influenced the results in the six to 11 month age group. Malaria was holoendemic; Plasmodium falciparum predominated, P. malariae and P. ovale were also identified. Plasmodium falciparum prevalence rates were similar in normal and beta-thalassaemia trait children but parasite densities were consistently lower in the latter. Using the criterion of a falciparum parasite density of 1 x 10(9) 1(-1) or greater to indicate a potentially important infection, the relative risk in beta-thalassaemia traits one to four years old from the cross-sectional study was 0.45 (upper 95% confidence interval 0.79) and 0.41 (0.61) in two to nine year trait carriers from a longitudinal study. Plasmodium falciparum gametocyte rates were lower in beta-thalassaemia trait children (P less than 0.005). The geometric mean titre of P. falciparum antibodies was lower in beta-thalassaemia trait children from the one to four year group (P less than 0.05). Otherwise immunological studies showed little difference between the different Hb types. Parasitological findings were consistent with relative resistance of HbS trait carriers towards P. falciparum infection. We found no evidence for relative resistance of beta-thalassaemia traits towards P. malariae infection nor that G6PD deficient males were more resistant to P. falciparum than those with normal activity. We conclude that the results are consistent with relative resistance of beta-thalassaemia trait carriers to P. falciparum malaria.     

Revisiting host/parasite interactions: molecular analysis of parasites collected during longitudinal and cross-sectional surveys in humans

This paper summarizes the first conclusions arising from an analysis of parasite diversity in blood samples collected during longitudinal surveys conducted in Senegal. Parasite typing was carried out using a PCR-based molecular analysis of allelic polymorphism. The parasite populations circulating in the village of Dielmo during periods of intense transmission (when the inoculation rate was 0.5–1 infective bite/night) are characterized by a considerable allelic diversity of the MSP-1, MSP-2 and TRAP loci. A large proportion of blood samples contained several MSP-1 or MSP-2 alleles. In asymptomatic carriers, the complexity of the infections (number of alleles and genetic diversity of these alleles) was age-dependent. In children, the trend was for a reduced complexity during clinical episodes. Molecular typing showed that successive clinical episodes experienced by children were caused by genetically distinct parasites. Longitudinal analysis of asymptomatic carriers indicated that in the absence of transmission, the same parasite types were carried for long periods, while rapidly changing profiles were observed during intense transmission season. The consequences of these findings on our understanding of acquired anti-parasite immunity in humans living in endemic are discussed.     

Association of Schistosoma haematobium infection with protection against acute Plasmodium falciparum malaria in Malian children

Plasmodium falciparum and Schistosoma haematobium are co-endemic parasitic diseases with worldwide distribution. Evidence suggests interactions occur between helminthic and malaria infections, although it is unclear whether this effect is beneficial or harmful to the host. Malian children 4-14 years of age with asymptomatic S. haematobium infection (SP) (n = 338) were prospectively matched by age, sex, and residence to children without schistosomiasis (SN) (n = 338) who were cleared of occult intestinal parasites, and followed-up for one malaria transmission season (25 weeks). The time to the first clinical malaria infection, incidence of malaria episodes, and parasitemia were recorded. Age associated protection from malaria in children with schistosomiasis was observed. SP children (4-8 years of age) compared with SN children demonstrated delayed time to first clinical malaria infection (74 versus 59 days; P = 0.04), fewer numbers of malaria episodes (1.55 versus 1.81 infections; P = 0.03) and lower geometric mean parasite densities (6,359 versus 9,874 asexual forms/mm(3); P = 0.07) at first infection. No association between schistosomiasis and P. falciparum malaria was observed in children 9-14 years of age. We conclude that underlying schistosomiasis is associated with protection against clinical falciparum malaria in an age-dependent manner.     

Parasitic carriage of the causative agents of tropical malaria

In 1993 to 1999, a total of 1020 Russians temporarily living in the Republic of Guinea were examined; of them 26 (2.5%) persons were found to be asymptomatic carriers of P. falciparum. Single studies of cellular and humoral immunity were first conducted in 15 asymptomatic parasitic carriers of P. falciparum. Asymptomatic parasitic carriage of P. falciparum was established in nonimmune persons who developed immunopathological reactions. The nature of immune disorders gives grounds to regard parasitic carriage as a subclinical form of tropical malaria for the first time. By taking into account asymptomatic carriage of P. falciparum, a parasitological blood study should be made in all non-immune persons when they are leaving an endemic focus for Russia.     

Limited advantage of multiple consecutive samples for genotyping Plasmodium falciparum populations during the first days of treatment

The informative value of genotyping Plasmodium falciparum populations in single blood samples was studied before and during treatment in 13 patients with P. falciparum malaria. Genotyping of the two merozoite surface proteins (msp1 [block 2] and msp2) and the glutamate-rich protein showed multiple genotypes in seven patients, and single genotypes in the remaining six patients. The same genotype profiles were detected in consecutive samples obtained every 12 hours during treatment from the respective patients, although some genotypes were cleared earlier than others. These patterns are in contrast to the extensive daily dynamics previously described in asymptomatic infections. The genotypes detected in one pre-treatment sample thus appear to reflect the parasite subpopulations of the clinical malaria infection during the following days, and additional sampling does not provide any additional information.     

Plasmodium falciparum merozoite surface protein 1 (MSP1): genotyping and humoral responses to allele-specific variants.

The present study is the first to investigate Plasmodium falciparum merozoite surface protein 1 (MSP1) allele-specific humoral responses in residents of central Africa. In endemic areas, acquired immune responses to malaria are assumed to reflect the need to be infected with a large number of antigenically diverse parasite populations. In the work presented here, the relationship between antibody specificity and the infecting parasite genotype was investigated in asymptomatic subjects and patients with uncomplicated malaria in order to possibly clarify the relationship between anti-MSP1 block2 antibodies and clinical malaria. Overall isolates were typed by nested PCR using allele-specific primers of the P. falciparum MSP1 gene to identify the infecting parasite genotype. The K1 type was the predominant allelic family in both clinical groups. Polyinfection (number of isolates with more than one parasite genotype) and the complexity of infections (mean number of parasite genotype per infected subject) were higher in isolates from asymptomatic individuals. Total immunoglobulins G (IgG) responses to schizont crude extract antigens and to MSP1 variant-specific peptides were assessed by ELISA test. More than 90% of the sera reacted against schizont extract, whatever the clinical group and the K1 seroprevalence was the highest in both clinical groups. Our results showed an age-dependence in the number of different variants of MSP1 block2 recognised by serum. Indeed, isolates from older (>14 years) subjects showed lower multiplicity of infection and higher was the mean number of different MSP1 variants recognised by their serum. This corresponded to the age reported for the acquisition of anti-parasite immunity under high malaria endemicity. The contribution of variant-specific immunity in asymptomatic malaria infections is discussed.     

Malaria in Nigeria: a revisit

The frequency of asymptomatic malaria parasitaemia was investigated in rural and urban school-children aged six to 12 years in southwestern Nigeria between January 1987 and October 1988. Asymptomatic parasitaemia was detected in the rural school-children all year round with the lowest parasite rate in January and the highest in July, corresponding to the mid-dry and wet seasons respectively. Asymptomatic parasitaemia was also common amongst urban school-children, but the frequency was lower than in the rural children. Parasite density was less than or equal to 1000 microliters-1 in 42% of parasite-positive asymptomatic children and was greater than 10,000 microliters-1 in only 20% of them. Mass treatment with chloroquine, to which the parasites were fully sensitive, was followed by the same rate of re-infection in the parasite-positive and parasite-negative groups. Of 7713 patients clinically diagnosed as having malaria 4425 were found to have parasitologically-proven malaria, and of these 4239 had pure Plasmodium falciparum malaria. Of the patients with falciparum malaria only 4.6% were below the age of one year. In 47% the parasite count was less than or equal to 1000 microliters-1, and it was over 10,000 microliters-1 in 37% and over 250,000 microliters-1 in 16%. There was no significant difference between the asymptomatic children and the acutely ill patients in the percentage with parasite densities less than or equal to 1000 microliters-1, but the percentage with parasite densities greater than 10,000 microliters-1 was significantly greater in the acute malaria patients than in those with asymptomatic parasitaemia.     

Distinguishing Plasmodium falciparum treatment failures from re-infections by using polymerase chain reaction genotyping in a holoendemic area in northeastern Tanzania.

An in vivo drug sensitivity study was conducted in Magoda village in northeastern Tanzania to evaluate the usefulness of polymerase chain reaction (PCR)-based genotyping of Plasmodium falciparum parasites to distinguish between re-infection and treatment failure. The study tested P. falciparum susceptibility to a combination of sulfadoxine/pyrimethamine (Fansidar; F. Hoffmann La Roche, Basel, Switzerland). Blood samples were collected before treatment and on days 7, 14, or 28 post-treatment in 51 asymptomatic children, of which 26 could not clear parasitemia within seven days post-treatment. Among the remaining 25 children who had no detectable parasites on day 7, only five remained parasite negative up to day 28. Primary and recrudescent P. falciparum parasites were analyzed by PCR using family specific primers for merozoite surface protein-1 (MSP-1), MSP-2, and glutamate-rich protein (GLURP). All samples contained multiple P. falciparum infections. For all children with recrudescent P. falciparum, common alleles were detected in both the primary and recrudescent samples. However, in no child were the exact same alleles detected in both samples, indicating that probably at least some of the recrudescing parasites originated from new infections. The study demonstrates the general usefulness of PCR genotyping technique in distinguishing re-infections from true recrudescences following therapeutic drug treatment.     

Epidemiology of Plasmodium falciparum in a rice field and a savanna area in Burkina Faso: seasonal fluctuations of gametocytaemia and malarial infectivity.

For a better understanding of the epidemiology of Plasmodium falciparum in an African savanna area, the authors have: (a) defined the real gametocyte reservoir in the native population; (b) followed the fluctuations of gametocytaemia throughout the transmission period; and (c) measured the infectiousness of malarious individuals to mosquitoes. The transversal surveys, in different villages of this endemic area, have shown that gametocyte carrier rates decreased with age and malaria experience; 10.9% of the whole population were potentially infectious to mosquitoes, and of these 73% were children and only 27% were adults. The longitudinal studies have shown that the P. falciparum gametocyte rate depends on the equilibrium between the gametocyte conversion rates and the density of the asexual forms. When there are large numbers of children who become carriers of the sexual stage of the parasite and at the same time a small number who lose their gametocyte infection, the gametocyte rate increases in the population; and vice versa. The circumstances under which gametocytes are produced are not well-known. Two factors seem to be important: the level of the parasite density and immune mechanisms. The infectiousness of malarious individuals was estimated by the 'mosquito infection probability'. The percentage of mosquitoes infected after feeding on gametocyte carriers (which may partly reflect the infectiousness of a human population to mosquitoes) was multiplied by the percentage of gametocyte carriers in the population. This indicated that, in this endemic area, 4% of biting mosquitoes would become infected; but this theoretical mosquito infection probability is over-estimated.(ABSTRACT TRUNCATED AT 250 WORDS)