Impact of red blood cell polymorphisms on the antibody response to Plasmodium falciparum in Senegal

The evidence of protection afforded by red blood cell polymorphisms against either clinical malaria or Plasmodium falciparum blood levels varies with the study site and the type of malaria transmission. Nevertheless, no clear implication of an antibody-related effect has yet been established in the protection related to red blood cell polymorphisms. We performed a prospective study, where plasma IgG and IgG subclasses directed to recombinant proteins from the merozoite surface protein 2 (MSP2/3D7 and MSP2/FC27) and the ring-infected erythrocyte surface antigen (RESA) were determined in a cohort of 413 Senegalese children before the annual malaria transmission season. The antibody response was dependent on age, and to a lesser extent, on the village of residence. IgG3 responders to all proteins, IgG responders to RESA and MSP2/3D7, as well as IgG2 to RESA and IgG1 responders to MSP2/3D7, presented enhanced mean values of parasite density, as evaluated during an 18-month follow-up. The levels of IgG and IgG3 to MSP2/3D7 were negatively associated with the risk of occurrence of a malaria attack during the following transmission season. Compared to normal children, sickle cell trait carriers presented lower levels of IgG to MSP2/3D7. Similarly, G6PD A- girls had lower levels of IgG and IgG3 to MSP2/FC27 than did G6PD normal girls. The impact of these particular genetic polymorphisms on the modulation of the antibody response is discussed.

Dynamics of anti-VAR2CSA immunoglobulin G response in a cohort of senegalese pregnant women

BACKGROUND

Pregnancy-associated malaria (PAM) is precipitated by the accumulation of parasites in the placental intervillous spaces and causes maternal anemia and low birth weight. In PAM, placental parasites adhere to chondroitin sulfate A (CSA) through a unique set of variant surface antigens (VSAPAM). Several studies have shown that 1 var gene, var2csa, is transcribed at high levels and expressed in CSA-binding Plasmodium falciparum parasites.

METHODS

Plasma levels of anti-VAR2CSA immunoglobulin G (IgG) in Senegalese women were measured during pregnancy by enzyme-linked immunosorbent assay, using 3 recombinant proteins representing 3 domains of the var2csa gene product.

RESULTS

The 3 recombinant proteins were specifically recognized by plasma from pregnant women but not by control plasma. A parity-dependent recognition pattern was observed with 2 of the 3 VAR2CSA antigens. A kinetic study demonstrated that a single P. falciparum infection was able to trigger a VAR2CSA-specific antibody response. Among women with infected placentas, women with high anti-VAR2CSA IgG levels at enrollment were more likely to present with a past infection than with an acute/chronic infection.

CONCLUSIONS

Anti-VAR2CSA IgGs are involved in clinical protection against pregnancy-associated malaria and strengthens the hope for making a VAR2CSA-based vaccine.

Distribution of IgG subclass antibodies specific for Plasmodium falciparum glutamate-rich-protein molecule in sickle cell trait children with asymptomatic infections

Polymorphism in the beta-globin gene (hemoglobin S) has been associated with protection against severe forms of malaria. In a cross-sectional study, 180 young Gabonese children with and without sickle cell trait and harboring asymptomatic Plasmodium falciparum infections, were assessed for the responses to recombinant protein containing the conserved region of glutamate-rich protein (GLURP). We reported increased age-dependence of antibody prevalence and levels of total IgG (p<0.0001), IgG1 (p=0.009), and IgG3 (p<0.03) antibodies to GLURP with a cut-off at 5 years of age. Whatever the hemoglobin type, cytophilic antibodies (IgG1 and IgG3) were prevalent, but GLURP-specific IgG4 antibodies were detected at significantly (p<0.05) lower levels in HbAS children. We showed that the distribution of non-cytophilic IgG antibodies differs according to the hemoglobin type and to the malaria antigens tested. This may have possible implication for the clearance of malaria parasites and for protection against severe malaria.

Influence of Carriage of Hemoglobin AS and the Fcγ Receptor IIa–R131Allele on Levels of Immunoglobulin G2 Antibodies toPlasmodium falciparumMerozoite Antigens in Gabonese Children

BACKGROUND

To extend our previous findings showing an imbalanced distribution of immunoglobulin G2 (IgG2) antibodies to Plasmodium falciparum merozoite surface protein 2 (MSP2) and a higher frequency of infection with multiple P. falciparum strains in Gabonese children with sickle cell trait (hemoglobin AS), human Fc gamma receptor (Fc gamma R) IIa (CD32) polymorphism and the rate of in vitro invasion of red blood cells (RBCs) from subjects with either hemoglobin AA or AS by multiple P. falciparum strains were investigated.

METHODS

Fc gamma RIIa mutation at amino acid position 131 (arginine or histidine) was detected by polymerase chain reaction, and in vitro cultures for parasites were used to assess the invasion rate.

RESULTS

Fc gamma RIIa polymorphism is normally distributed in this population, with no preferential carriage by children with hemoglobin AS. Lower levels of IgG2 subclass antibodies to MSP2 peptides were independently associated with the Fc gamma RIIa-R131 allele and with carriage of hemoglobin AS. Our data suggest that IgG3 antibody responses to MSP2 epitopes could be exacerbated by lower IgG2 levels in children with hemoglobin AS.

CONCLUSIONS

The higher rate of invasion of RBCs in the presence of multiple strains may indicate that several invasion pathways are solicited simultaneously, and the longer persistence of ring forms in RBCs from the subjects with hemoglobin AS might reflect a slower multiplication phase, leading to a longer circulation and enhanced phagocytosis of these nonpathogenic parasite forms. This may contribute to the protection against P. falciparum malaria observed in children with hemoglobin AS.

Functional and immunological characterization of a duffy binding-like- gamma domain from Plasmodium falciparum erythrocyte membrane protein-1 expressed by a placental isolate

A recombinant Duffy binding-like (DBL)- gamma domain from a previously identified placental isolate, 732, was expressed by use of the baculovirus/insect cell system and was purified in milligram quantities. The recombinant protein binds specifically to chondroitin sulfate A (CSA) and inhibits CSA binding by placental infected erythrocytes (IEs). Polyclonal antibodies raised against the domain recognized the surfaces of live IEs from CSA-adherent clinical placental isolates. These antibodies also abrogated the in vitro binding of IEs to CSA. The 732 DBL-3 gamma domain was specifically recognized by plasma from pregnant women but not by plasma from control subjects. In addition, the protein was, comparatively, significantly more reactive with plasma from women with infected placentas, strongly suggesting that the 732 DBL-3 gamma domain carries preferentially IE-expressed immunogenic epitopes. High levels of plasma antibodies to the recombinant domain were associated with reduced placental parasite density. This is the first report of a recombinant DBL- gamma domain derived from a placental isolate that shows CSA-binding properties.

Demonstration of a high level of parasite population homology by quantification of Plasmodium falciparum alleles in matched peripheral, placental, and umbilical cord blood samples

Plasmodium falciparum msp-1 and msp-2 genes were quantified by fragment analysis in matched placental, peripheral, and cord blood samples. In the three compartments, the multiplicity of infection values were similar, and parasite populations only partially overlapped, as reported. However, identical alleles represented 80 to 95% of the overall parasite populations of each compartment, demonstrating much more homogenous parasite populations than previously thought.

High level of var2csa transcription by Plasmodium falciparum isolated from the placenta

Plasmodium falciparum parasites that bind to chondroitin sulphate A (CSA) express unique variant surface antigens that are involved in the placental sequestration that precipitates pregnancy-associated malaria (PAM). Two var gene subfamilies, var1csa and var2csa, have been associated with CSA binding. We show here that placental P. falciparum isolates highly transcribed var2csa but not var1csa. var2csa was not transcribed or was only minimally transcribed by parasites isolated from nonpregnant women. Placental parasites that effectively bound to placental chondroitin sulphate proteoglycans transcribed higher levels of var2csa. In pregnant women, levels of var2csa transcription and plasma anti-VAR2CSA immunoglobulin G were associated. These findings support the idea that VAR2CSA plays a crucial role in PAM and strengthen the rationale for the development of VAR2CSA-based vaccines.

The Sickle Cell Trait Is Associated with Enhanced Immunoglobulin G Antibody Responses toPlasmodium falciparumVariant Surface Antigens

The sickle cell trait (HbAS) protects against severe Plasmodium falciparum malaria in young African children. We investigated the extent of the association between HbAS and antibodies directed to parasite-derived variant surface antigens (VSAs) on the membrane of infected erythrocytes. We measured immunoglobulin G (IgG) responses with specificity for VSAs of 2 heterologous parasite isolates in 458 Gabonese children aged between 6 months and 11 years. Logistic regression analyses showed a highly significant independent association (P<.001) between carriage of HbAS and the presence of IgG anti-VSA responses; this association was related specifically to IgG1 and IgG4 subclasses in the anti-VSA profile. IgG2 and IgG3 anti-VSA responses were both independently associated with older age, consistent with the pattern observed in semi-immune adults. The results imply that enhanced levels of cross-reactive anti-VSA responses in children with HbAS may be intimately associated with the protection they have against malaria.

High prevalence of Plasmodium falciparum pfcrt K76T mutation in pregnant women taking chloroquine prophylaxis in Senegal

OBJECTIVES

The risk of malaria infection is increased during pregnancy, and many countries recommend chloroquine prophylaxis in pregnant women, despite Plasmodium falciparum chloroquine resistance. Chloroquine resistance is associated with the pfcrt gene K76T mutation. The aim of this study was to compare the prevalence rate of pfcrt T76 mutation in P. falciparum isolates from infected pregnant and non-pregnant individuals from Senegal.

METHODS

The study was conducted in the rural maternity hospital of Thiadiaye, Senegal, where malaria is seasonal. Sixty-nine P. falciparum isolates from infected women were collected at delivery. These women were part of a cohort study; they were followed from their first antenatal visit and advised to take chloroquine prophylaxis. For each woman, the earliest P. falciparum-infected blood sample was also used. A control group of 49 non-pregnant individuals with asymptomatic P. falciparum infection was enrolled.

RESULTS

During pregnancy, prevalence of T76 mutant parasites was higher than in the 49 non-pregnant controls (P<0.001). Among pregnant women, this rate was highest at delivery (P=0.06), and tended to be higher in women who had taken chloroquine prophylaxis, as assessed in urine samples (P=0.08).

CONCLUSIONS

Chloroquine prophylaxis is responsible for increased drug consumption and increased drug pressure that may lead to the selection of drug-resistant parasites. This is the first report showing that P. falciparum-infected pregnant women harbour pfcrt T76 mutant parasites more often than non-pregnant individuals, and that the prevalence of this mutation is higher at term than earlier during pregnancy.

Do African immigrants living in France have long-term malarial immunity?

Among populations living in areas endemic for malaria, repeated parasite exposure leads to a gradual increase in protective immunity to the disease. In contrast, this immunity is assumed to disappear after several years of non-exposure. This study was designed to investigate long-term immunity in subjects removed from the risk of exposure. Plasmodium falciparum malaria attacks occurring after short trips to sub-Saharan Africa were compared between 99 European patients and 252 African immigrants who had been resident in Europe for at least four years. Relative to the European patients, those originating from Africa had lower mean +/- SD parasite densities (0.8 +/- 1.5/100 red blood cells versus 1.4 +/- 2.8/100 red blood cells; P = 0.007), less frequent severe disease (4.4% versus 15.2%; P = 0.0005), accelerated parasite clearance and defervescence, and higher levels of antibodies to P. falciparum. These results suggest the persistence of acquired immunity to P. falciparum malaria after several years of non-exposure in African immigrants.

Variable Adhesion Abilities and Overlapping Antigenic Properties in PlacentalPlasmodium falciparumIsolates

BACKGROUND

Pregnancy-associated malaria is characterized by selection and multiplication, in the placenta, of a distinct population of Plasmodium falciparum expressing particular variant surface antigens (VSAs) that adhere to chondroitin sulfate A (CSA).

METHODS

The adhesion of 40 freshly collected placental parasite isolates to bovine CSA and human placental low-sulfated chondroitin proteoglycans (CSPGs) was investigated. Plasma samples from 30 pregnant women were used to test, by flow cytometry, their recognition of and their adhesion-inhibition capacity toward 6 of these isolates.

RESULTS

Adhesion to CSA and CSPGs varied between isolates but was strongly correlated between receptors (P<.001). Adhesion of isolates to receptors strongly and negatively correlated with low birth weight (LBW) of the neonate (odds ratio [95% confidence interval], 5.2 [1.1-25.1]). In plasma samples from pregnant women, the level of specific immunoglobulin G against each placental isolate (anti-VSA(PAP)) strongly correlated with the level of anti-VSA(PAP) antibodies against all other isolates (P<.05) and increased with parity in all isolates (P<.01). Conversely, adhesion-inhibitory antibodies did not correlate with isolates or with the level of anti-VSA(PAP) antibodies.

CONCLUSION

The level of adhesion of placental parasites to chondroitin sulfate receptors is an important risk factor for LBW. Parasite heterogeneity suggests that they are composed of mixed adhesion phenotypes capable of inducing immune responses to a range of different and overlapping targets.

Malaria, from natural to supernatural: a qualitative study of mothers' reactions to fever (Dienga, Gabon)

OBJECTIVE

Decision making for health care at the household level is a crucial factor for malaria management and control among young children. This study sought to determine exactly how mothers reacted when faced with fever in a child.

DESIGN

Qualitative study based on in depth semistructured interviews of mothers and free form discussion with traditional healers (Nganga).

SETTING

Village of Dienga, a rural area of Gabon (Central Africa).

PARTICIPANTS

12 mothers and three traditional healers.

RESULTS

All mothers thought that fever and malaria were identical. Mothers home treated or went to the village treatment centre, or both, on the last episode of fever, if they judged it to be "natural" fever. However, if fever was thought to be a result of malicious intent, then a Nganga was consulted first. It was believed that strong and above all persistent fever was "supernatural". In this case, traditional treatment was thought to be best.

CONCLUSIONS

Results indicate that fever is perceived as a dual condition, with two distinct but non-mutually exclusive aetiologies (either "natural" or from witchcraft). In contrast with what is commonly believed, there seems to be no clear cut distinction between diseases suitable for management by western medicine and diseases to be managed solely by traditional health practitioners. Moreover, these data do not support the commonly held notion that the decision to seek western medicine to treat fever is considered a "last resort". Results strongly imply that some severe cases of fever, being initially considered supernatural, may partially or completely escape medical attention.

Short report: lack of prediction of amodiaquine efficacy in treating Plasmodium falciparum malaria by in vitro tests

Amodiaquine (AQ) is currently a major candidate for new antimalarial combinations, although in vivo and in vitro tests have been rarely simultaneously investigated. The efficacy of AQ was assessed at the dose of 30 mg/kg in treating Plasmodium falciparum malaria attacks in 74 children from southeast Gabon, and the in vitro activity of monodesethylamodiaquine (MdAQ), the main metabolite of AQ, was measured against P. falciparum parasites isolated from these children. Treatment failures were observed in 40.5% of the children, while 5.4% of the isolates showed in vitro resistance to MdAQ. No relationship was observed between in vivo and in vitro susceptibility. The in vitro activities of MdAQ and chloroquine were correlated. The reasons for such disparities between in vivo and in vitro AQ activities are discussed and the issue of the validity of in vitro tests to measure AQ efficacy is raised.

Analysis of IgG with specificity for variant surface antigens expressed by placental Plasmodium falciparum isolates

Background

Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes that can sequester in placental intervillous space by expressing particular variant surface antigens (VSA) that can mediate adhesion to chondroitin sulfate A (CSA) in vitro. IgG antibodies with specificity for the VSA expressed by these parasites (VSA_PAM) are associated with protection from maternal anaemia, prematurity and low birth weight, which is the greatest risk factor for death in the first month of life.

Methods

In this study, the development of anti-VSA_PAM antibodies in a group of 151 women who presented to the maternity ward of Albert Schweitzer Hospital in Lambaréné, Gabon for delivery was analysed using flow cytometry assays. Plasma samples from placenta infected primiparous women were also investigated for their capacity to inhibit parasite binding to CSA in vitro.

Results

In the study cohort, primiparous as well as secundiparous women had the greatest risk of infection at delivery as well as during pregnancy. Primiparous women with infected placentas at delivery showed higher levels of VSA_PAM-specific IgG compared to women who had no malaria infections at delivery. Placental isolates of Gabonese and Senegalese origin tested on plasma samples from Gabon showed parity dependency and gender specificity patterns. There was a significant correlation of plasma reactivity as measured by flow cytometry between different placental isolates. In the plasma of infected primiparous women, VSA_PAM-specific IgG measured by flow cytometry could be correlated with anti-adhesion antibodies measured by the inhibition of CSA binding.

Conclusion

Recognition of placental parasites shows a parity- and sex- dependent pattern, like that previously observed in laboratory strains selected to bind to CSA. Placental infections at delivery in primiparous women appear to be sufficient to induce functional antibodies which can both recognize the surface of the infected erythrocytes as well as block their binding to CSA. The correlation between serum reactivities of placental field isolates from different geographic locations and collected at different times is indicative of the conserved nature of the antigen(s) mediating PAM.

Monocyte Activation and T Cell Inhibition inPlasmodium falciparum–Infected Placenta

BACKGROUND

During healthy pregnancy, T helper (Th) 1-type and inflammatory-type responses are down-regulated, and Th2-type and proinflammatory-type responses predominate. In Plasmodium falciparum-infected females, these responses induce enhanced production of tumor necrosis factor- alpha and interferon- gamma.

METHODS

To assess the respective implication of monocytes and T cells in this placental immunomodulation, we cocultured cells from delivering females living in an area where malaria is endemic. Monocytes and T cells from both peripheral and intervillous blood were crossed in in vitro cultures, to compare the proliferative response to several antigens. Moreover, monocyte cell-surface molecules were quantified by flow cytometry.

RESULTS

Coculture results confirmed placental immunomodulation and suggested that the most affected cells are not the intervillous monocytes, which are as able to present the antigen as the peripheral monocytes, but the intervillous T cells. Monocyte staining showed significant increases in human leukocyte antigen D-related, CD54, CD80, and CD86 surface markers in intervillous blood, compared with peripheral blood, which suggests a relative activation of monocytes in the placenta.

CONCLUSION

A state of T cell deactivation and monocyte activation is present at delivery. The T cell deactivation in reaction to purified protein derivative could be explained by the presence of local T cell immunoregulatory factors.

Temporal variations in immune responses to conserved regions of Plasmodium falciparum merozoite surface proteins related to the severity of a prior malaria episode in Gabonese children

We measured cellular and humoral responses to conserved regions of Plasmodium falciparum merozoite surface proteins 1 and 2 (MSP-1 and MSP-2) at different times during and after acute infection in matched groups of Gabonese children who presented with either mild or severe malaria. We used an MSP-1(19) recombinant protein and peptides corresponding to conserved epitopes in MSP-1 and MSP-2 N- and C-terminal regions. The prevalences of peptide-induced cell-mediated responses were maximal in both groups when they were healthy, but were consistently higher in the mild malaria group, whereas peptide-specific antibody responses were consistently highest in the severe malaria group. Recombinant MSP-1(19) protein-specific antibody levels in the 2 groups were similar both prior to and 1 month post-treatment but declined later when the children were healthy and parasite-free, to a significantly lower level in those admitted with severe malaria, reflecting the profile of the predominant MSP-1(19)-specific immunoglobulin G1 isotype. This finding implies a defect in the ability of children with a history of severe malaria to maintain an antibody response putatively associated with immunity to P. falciparum malaria.

DHFR and DHPS genotypes of Plasmodium falciparum isolates from Gabon correlate with in vitro activity of pyrimethamine and cycloguanil, but not with sulfadoxine-pyrimethamine treatment efficacy

OBJECTIVES

To assess the relationship between the presence of DHFR and DHPS mutations in Plasmodium falciparum, parasite in vitro resistance, and in vivo efficacy of sulfadoxine-pyrimethamine (SP) treatment.

PATIENTS AND METHODS

Measurement of SP treatment efficacy in malaria-infected children in Gabon was combined with in vitro tests of susceptibility to pyrimethamine and cycloguanil, and molecular genotyping at several DHFR and DHPS loci of parasites isolated before treatment. DHFR was studied at codons 108, 51, and 59, whereas DHPS gene was typed at positions 436, 437, 540 and 581.

RESULTS

SP treatment was effective in 86% of children by day 28. Seventy-five percent of isolates were in vitro resistant to pyrimethamine and 65.5% to cycloguanil. No mutation was detected at codons 540 and 581 of the DHPS gene. Most isolates (71.8%) presented with the triple mutant DHFR genotype, whereas 64.3% combined at least three DHFR and one DHPS mutations. The increase in the number of DHFR mutations was associated with an increase in in vitro resistance to pyrimethamine and cycloguanil; three DHFR mutations conferred pyrimethamine and to a lesser extent cycloguanil resistance. Treatment failures only occurred with isolates presenting at least two DHFR mutations (S108N and C59R) and one DHPS mutation (S436A or A437G), but SP treatment of infections with such parasites gave treatment success in 82.0% of children.

CONCLUSIONS

DHFR mutations that lead to high-level in vitro resistance to pyrimethamine plus 1-2 DHPS mutations are not sufficient to induce in vivo failure of SP treatment in young children from Gabon.

Polymorphism in two merozoite surface proteins of Plasmodium falciparum isolates from Gabon

BACKGROUND

Plasmodium falciparum antigenic diversity and polymorphism confuses the issue of antimalarial vaccine development. Merozoite surface protein (MSP)-1 and -2 are two highly polymorphic vaccine candidates. Characterisation of their precise polymorphism in endemic regions may facilitate the design of an effective vaccine.

METHODS

Isolates obtained in 52 Gabonese children presenting with uncomplicated malaria were genotyped by nested-PCR of msp-1 block 2, and msp-2 block 3, to analyze both parasite population polymorphism and clone fluctuations.

RESULTS

Twenty-five and 19 different alleles were respectively obtained for msp-1 and msp-2 loci, the RO33 family of msp-1 being poorly polymorphic. Four cases of non-random distribution of alleles were reported of the FC27, and/or 3D7 families of msp-2. All but two isolates were composed of more than one genotype, and the multiplicity of infection (MOI) was 4.0. Neither parasite density nor age was related to MOI. Clone fluctuations were studied for ten subjects who were sampled again at reappearance of parasites in blood. Disappearance and reappearance of alleles were observed following treatment, suggesting difficulties in assessing polymorphism and in distinguishing reinfection from recrudescence.

CONCLUSION

P. falciparum polymorphism is extensive in Southeast Gabon, and most of infections are composed of multiple clones. The fluctuation of clones contributes to parasite diversity.

In vitro susceptibility to a new antimalarial organometallic analogue, ferroquine, of Plasmodium falciparum isolates from the Haut-Ogooué region of Gabon

OBJECTIVES

To assess the activity of a new organometallic chloroquine analogue, ferroquine, against numerous Plasmodium falciparum isolates from Gabon.

METHODS

The in vitro susceptibility of 116 P. falciparum isolates to chloroquine and ferroquine was assessed using the isotopic microtest. All isolates were from outpatients in the Franceville and Bakoumba medical centres in the province of Haut-Ogooué, south-east Gabon.

RESULTS

The in vitro resistance to chloroquine was 51.8% in Franceville and 96.7% in Bakoumba. The IC50 geometric mean (95% CI) of ferroquine against isolates in Franceville was 16.0 (14.4-17.8) nM, with individual values ranging from 1.0 to 47.0 nM; in Bakoumba it was 27.9 (23.4-33.2) nM, with individual values ranging from 1.0 to 62.0 nM. Compared with chloroquine, ferroquine was 5.3 times more active on isolates susceptible to chloroquine, and 13.3 times more active on isolates resistant to chloroquine. A weak positive correlation was observed between responses of these two drugs, but too low to demonstrate cross-resistance.

CONCLUSIONS

Ferroquine may be useful as an alternative drug for treating chloroquine-resistant malaria.

Relationship between entomological inoculation rate, Plasmodium falciparum prevalence rate, and incidence of malaria attack in rural Gabon

To assess the relationships between variations of Plasmodium falciparum transmission and those of peripheral parasitaemia prevalence or malaria attack incidence rates in regions with limited fluctuations of transmission, we conducted a follow-up in two Gabonese populations. Entomological surveys were carried out from May 1995 to April 1996 in Dienga, and from May 1998 to April 1999 in Benguia. In Dienga, malaria transmission was seasonal, being not detected during two 3-month periods. Mean entomological inoculation rate (EIR) was 0.28 infective bite/person/night. In Benguia, malaria transmission was perennial with seasonal fluctuations, mean EIR being 0.76 infective bite/person/night. In Dienga, 301 schoolchildren were followed from October 1995 to March 1996. Clinical malaria attack was defined as fever associated with >5000 parasites/microl of blood. P. falciparum prevalence varied from 28 to 42%, and monthly malaria attack incidence from 30 to 169 per thousand. In Benguia, the entire population (122 persons) was followed from November 1998 to April 1999. Prevalence varied from 22 to 50%, and monthly malaria attack incidence from 52 to 179 per thousand. In each area, entomological variations were not related to parasite prevalence, but preceded malaria attack incidence with 1- or 2-month time lag, corresponding to the pre-patency period that differs in the two populations, possibly according to differences in immunity related to parasite transmission.

Efficacy of permethrin-impregnated bed nets on malaria control in a hyperendemic area in Irian Jaya, Indonesia III. Antibodies to circumsporozoite protein and ring-infected erythrocyte surface antigen

A two years intervention study was carried out using permethrin impregnated bed nets in a hyperendemic area, in Irian Jaya, Indonesia. To assess the influence of this intervention on natural immunity, concurrent immunological studies to determine levels of antibodies to the circumsporozoite (CS) and ring-infected erythrocyte surface antigen (RESA) proteins were conducted. Prevalence and titers of immunoglobulins (Ig)G and IgG subclasses were periodically measured in 138 individuals (30 children under the age of ten and 108 villagers ten years old and older). In the younger group, seropositivity of total IgG against CS fluctuated according to the parasite infection rates; however, IgG seropositive reaction against RESA gradually increased. In the older age group, seropositivity of both kinds of antibodies was stable during the whole study period. Nevertheless, the geometric mean titers of total IgG against CS and RESA were significantly reduced in this latter group in individuals who contained these antibodies before and after intervention. The geometric mean titer of IgG3 subclass against RESA was decreased at a highly significant level (p = 0.0005), and that of IgG4 against the same antigen was also decreased although to a lesser extent (p = 0.02).

Maternally transmitted antibodies to pregnancy-associated variant antigens on the surface of erythrocytes infected with Plasmodium falciparum: relation to child susceptibility to malaria

The consequences of pregnancy-associated malaria on a child's health have been poorly investigated. Malarial infection of the placenta seems to result in a higher susceptibility of children to the parasite during their first year of life. In 1993-1995, the authors investigated the role of antibodies in variable surface antigens (VSA) specific to chondroitin sulfate A (CSA)-binding parasites to assess the parasitologic status of the child. Flow cytometry was used to measure levels of antibodies to VSA from CSA-selected and -unselected parasite lines in the cord blood of 79 newborns in Ebolowa, Cameroon. These newborns were subsequently followed up for 2 years to determine the date of first occurrence of blood parasites and mean parasite density during follow-up. Maternally transmitted antibodies to VSA expressed by CSA-binding parasites, but not antibodies to any other specificity, were negatively related to time of first appearance of Plasmodium falciparum in a child's blood and were positively related to mean parasite density during the first 2 years of life. If maternal infection is thought to be the main mechanism influencing susceptibility of the newborn to malaria, antibodies to VSA may better denote maternal malaria infection during almost the entire pregnancy as opposed to placental blood smear reflecting infection during the last few months of pregnancy.

Combination of drug level measurement and parasite genotyping data for improved assessment of amodiaquine and sulfadoxine-pyrimethamine efficacies in treating Plasmodium falciparum malaria in Gabonese children

Many African countries currently use a sulfadoxine-pyrimethamine combination (SP) or amodiaquine (AQ) to treat uncomplicated Plasmodium falciparum malaria. Both drugs represent the last inexpensive alternatives to chloroquine. However, resistant P. falciparum populations are largely reported in Africa, and it is compulsory to know the present situation of resistance. The in vivo World Health Organization standard 28-day test was used to assess the efficacy of AQ and SP to treat uncomplicated falciparum malaria in Gabonese children under 10 years of age. To document treatment failures, molecular genotyping to distinguish therapeutic failures from reinfections and drug dosages were undertaken. A total of 118 and 114 children were given AQ or SP, respectively, and were monitored. SP was more effective than AQ, with 14.0 and 34.7% of therapeutic failures, respectively. Three days after initiation of treatment, the mean level of monodesethylamodiaquine (MdAQ) in plasma was 149 ng/ml in children treated with amodiaquine. In those treated with SP, mean levels of sulfadoxine and pyrimethamine in plasma were 100 microg/ml and 212 ng/ml, respectively. Levels of the three drugs were higher in patients successfully treated with AQ (MdAQ plasma levels) or SP (sulfadoxine and pyrimethamine plasma levels). Blood concentration higher than breakpoints of 135 ng/ml for MdAQ, 100 micro g/ml for sulfadoxine, and 175 ng/ml for pyrimethamine were associated with treatment success (odds ratio: 4.5, 9.8, and 11.8, respectively; all P values were <0.009). Genotyping of merozoite surface proteins 1 and 2 demonstrated a mean of 4.0 genotypes per person before treatment. At reappearance of parasitemia, both recrudescent parasites (represented by common bands in both samples) and newly inoculated parasites (represented by bands that were absent before treatment) were present in the blood of most (51.1%) children. Only 3 (6.4%) therapeutic failures were the result not of treatment inefficacy but of new infection. In areas where levels of drug resistance and complexity of infections are high, drug dosage and parasite genotyping may be of limited interest in improving the precision of drug efficacy measurement. Their use should be weighted according to logistical constraints.

[Malaria during pregnancy: consequences and interventional perspectives]

The impact of malaria during pregnancy varies greatly according to the intensity of transmission. Severe acute complications including cerebral malaria or materno-fetal death seem to be confined to areas of unstable transmission where malaria is uncommon except during epidemics. In areas of stable endemicity, the main consequences are maternal anemia and intra-uterine growth retardation resulting in low birthweight (LBW) particularly after first pregnancies. Recent studies have demonstrated that frequency and severity of placental malaria are greater in pregnant women with concurrent HIV infection. Since 1964 several controlled trials have been conducted to evaluate chemoprophylaxis in pregnant women mainly in tropical Africa where malaria transmission is stable. Findings have usually demonstrated an increase in mean birthweight after prophylaxis especially among primigravidae. Prophylaxis also had beneficial effects on anemia. Another finding of these trials was that prevention is less effective for women with HIV co-infection and that higher doses may therefore be required in such cases. In our opinion prophylaxis should be actively promoted as a routine public health measure for pregnant women in endemic areas. Current recommendations call for the use of a sulfadoxine-pyrimethamine twice or three times during pregnancy in antenatal clinics. This combination is more effective as a result of strong resistance of parasites to chloroquine. High cost and possible adverse effects in pregnant women prohibit routine use of mefloquine in developing countries. Integration of malaria prophylaxis into antenatal care services with nutrition and immunization measures should enhance the overall efficacy of prevention in outlying clinical facilities. Recent identification of molecular receptors involved in the cytoadherence of parasitized red blood cells to the placenta may lead to the development of new therapeutic or vaccinal approaches for pregnant women.

In vitro activity of chloroquine, quinine, mefloquine and halofantrine against Gabonese isolates of Plasmodium falciparum

OBJECTIVES

To determine the in vitro activity of antimalarial drugs against isolates of Plasmodium falciparum in Gabon.

METHODS

Plasmodium falciparum isolates were collected from symptomatic infections in the hospitals of Bakoumba and Franceville, south-east Gabon and in 2000. In vitro activity of chloroquine, quinine, mefloquine, halofantrine was measured by the isotopic microtest.

RESULTS

A total of 60 and 62 isolates gave interpretable data in Franceville and Bakoumba, respectively. In Franceville, 50.0% (mean IC50 = 111.7 nm), 0% (mean IC50 = 156.7 nm), and 21.2% (mean IC50 = 12.4 nm) of isolates, respectively, showed in vitro resistance to chloroquine, quinine and mefloquine. In Bakoumba, we saw resistance to chloroquine, quinine, mefloquine and halofantrine in 95.0% (mean IC50 = 325.8 nm), 10.2% (mean IC50 = 385.5 nm), 47.5% (mean IC50 = 24.5 nm) and 18.2% (mean IC50 = 1.9 nm) of isolates, respectively. Activities of chloroquine and mefloquine, chloroquine and quinine, and mefloquine and quinine were positively correlated.

CONCLUSIONS

Antimalarial drug resistance is high in this area of Gabon. The extent of resistance is disparate, as all tested drugs were less efficacious in Bakoumba than in Franceville.