Contribution of humoral immunity to the therapeutic response in falciparum malaria

Temperature Dependence of Plasmodium falciparum Erythrocytic Stage Development

Plasmodium falciparum infection causes febrile illness and severe disease with multiple organ failure and death when treatment is delayed. Antipyretic treatment is standard, and inducing hypothermia has been proposed to protect the brain in cerebral malaria. Here, we investigated the temperature dependence of asexual-stage parasite development and parasite multiplication in vitro. Plasmodium falciparum laboratory strain TM267 was incubated for 2 hours (short exposure) or 48 hours (continuous exposure) at different temperatures (32°C, 34°C, 35°C, 38°C, 39°C, and 40°C). The starting parasite developmental stage (ring, trophozoite, or schizont) varied between experiments. The parasite multiplication rate (PMR) was reduced under both hyper- and hypothermic conditions; after continuous exposure, the mean PMR ± SD was 9.1 ± 1.2 at 37°C compared with 2.4 ± 1.8 at 32°C, 2.3 ± 0.4 at 34°C, and 0.4 ± 0.1 at 40°C (P < 0.01). Changes in PMR were not significant after 2-hour exposure at temperatures ranging from 32°C to 40°C. Morphological changes in parasite cytoplasm and nucleus could be observed after long exposure to low or high temperature. After 48-hour incubation, rosette formation (≥ 2 uninfected red blood cells bound to infected red blood cells) was decreased at 34°C or 39°C compared with that at 37°C. In conclusion, both hyper- and hypothermia reduce PMR and delay erythrocytic stage development of P. falciparum, subsequently reducing rosette formation.

Immunity as a predictor of anti-malarial treatment failure: a systematic review

Background

Naturally acquired immunity can reduce parasitaemia and potentially influence anti-malarial treatment outcomes; however, evidence for this in the current literature provides conflicted results. The available evidence was synthesized to determine and quantify the association between host immunity and anti-malarial treatment failure.

Methods

Four databases were searched to identify studies investigating malaria antibody levels in patients receiving anti-malarial treatment for symptomatic malaria with treatment failure recorded according to the World Health Organization classification. Odds ratios or hazard ratios were extracted or calculated to quantify the association between malarial antibody levels and treatment failure, and findings from different studies were visualized using forest plots.

Results

Eight studies, including patients with falciparum malaria treated with mono- and combination therapy of artemisinin derivatives, sulfadoxine, pyrimethamine and chloroquine, were identified. Reported and calculated effect estimates varied greatly between studies, even those assessing the same antigens and treatments. An association between blood-stage IgG responses and treatment efficacy was observed. The greatest magnitudes of effect were observed for artemisinin [OR/HR (95% CI) range 0.02 (0.00, 0.45)–1.08 (0.57, 2.06)] and chloroquine [0.24 (0.04, 1.37)–0.32 (0.05, 1.96)] treatments, and larger magnitudes of effect were observed for variant surface antigen responses [0.02 (0.00, 0.45)–1.92 (0.94, 3.91)] when compared with merozoite specific responses [0.24 (0.04, 1.37)–2.83 (1.13, 7.09)].

Conclusions

Naturally acquired malarial immunity is associated with reduced anti-malarial treatment failure in malaria endemic populations. Anti-malarial IgG effects treatment outcome differently for different anti-malarial drugs and antigen targets, and had the greatest impact during treatment with the current first-line treatments, the artemisinins. This has implications for the assessment of the therapeutic efficacy of anti-malarials, particularly in the context of emerging artemisinin resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1815-y) contains supplementary material, which is available to authorized users.

Host immunity to Plasmodium falciparum and the assessment of emerging artemisinin resistance in a multinational cohort

Artemisinin-resistant falciparum malaria, defined by a slow-clearance phenotype and the presence of kelch13 mutants, has emerged in the Greater Mekong Subregion. Naturally acquired immunity to malaria clears parasites independent of antimalarial drugs. We hypothesized that between- and within-population variations in host immunity influence parasite clearance after artemisinin treatment and the interpretation of emerging artemisinin resistance. Antibodies specific to 12 Plasmodium falciparum sporozoite and blood-stage antigens were determined in 959 patients (from 11 sites in Southeast Asia) participating in a multinational cohort study assessing parasite clearance half-life (PCt_1/2) after artesunate treatment and kelch13 mutations. Linear mixed-effects modeling of pooled individual patient data assessed the association between antibody responses and PCt_1/2.P. falciparum antibodies were lowest in areas where the prevalence of kelch13 mutations and slow PCt_1/2 were highest [Spearman ρ = -0.90 (95% confidence interval, -0.97, -0.65), and Spearman ρ = -0.94 (95% confidence interval, -0.98, -0.77), respectively]. P. falciparum antibodies were associated with faster PCt_1/2 (mean difference in PCt_1/2 according to seropositivity, -0.16 to -0.65 h, depending on antigen); antibodies have a greater effect on the clearance of kelch13 mutant compared with wild-type parasites (mean difference in PCt_1/2 according to seropositivity, -0.22 to -0.61 h faster in kelch13 mutants compared with wild-type parasites). Naturally acquired immunity accelerates the clearance of artemisinin-resistant parasites in patients with falciparum malaria and may confound the current working definition of artemisinin resistance. Immunity may also play an important role in the emergence and transmission potential of artemisinin-resistant parasites.

Longitudinal analysis of antibody responses in symptomatic malaria cases do not mirror parasite transmission in peri-urban area of Cote d'Ivoire between 2010 and 2013

Background

In the agenda towards malaria eradication, assessment of both malaria exposure and efficacy of anti-vectorial and therapeutic strategies is a key component of management and the follow-up of field interventions. The simultaneous use of several antigens (Ags) as serological markers has the potential for accurate evaluation of malaria exposure. Here we aimed to measure the longitudinal evolution of the background levels of immunity in an urban setting in confirmed clinical cases of malaria.

Methods

A retrospective serological cross-sectional study on was carried out using 234 samples taken from 2010 to 2013 in peri-urban sentinel facility of Cote d’Ivoire. Antibody responses to recombinant proteins or BSA-peptides, 8 Plasmodium falciparum (PfAMA1, PfMSP4, PfMSP1, PfEMP1-DBL1α1-PF13, PfLSA1-41, PfLSA3-NR2, PfGLURP and PfCSP), one P. malariae (PmCSP) and one Anopheles gambiae salivary (gSG6-P1) antigens were measured using magnetic bead-based multiplex immunoassay (MBA). Total anti- P. falciparum IgG responses against schizont lysate from african 07/03 strain (adapted to culture) and 3D7 strain was measured by ELISA.

Results

High prevalence (7–93%) and levels of antibody responses to most of the antigens were evidenced. However, analysis showed only marginal decreasing trend of Ab responses from 2010 to 2013 that did not parallel the reduction of clinical malaria prevalence following the implementation of intervention in this area. There was a significant inverse correlation between Ab responses and parasitaemia (P<10⁻³, rho = 0.3). The particular recruitment of asymptomatic individuals in 2011 underlined a high background level of immunity almost equivalent to symptomatic patients, possibly obscuring observable yearly variations.

Conclusion

The use of cross-sectional clinical malaria surveys and MBA can help to identify endemic sites where control measures have unequal impact providing relevant information about population immunity and possible decrease of transmission. However, when immunity is substantially boosted despite observable clinical decline, a larger cohort including asymptomatic recruitment is needed to monitor the impact of control measures on level of immunity.

Neutralizing Antibodies against Plasmodium falciparum Associated with Successful Cure after Drug Therapy

An effective antibody response can assist drug treatment to contribute to better parasite clearance in malaria patients. To examine this, sera were obtained from two groups of adult patients with acute falciparum malaria, prior to drug treatment: patients who (1) have subsequent recrudescent infection, or (2) were cured by Day 28 following treatment. Using a Plasmodium falciparum antigen library, we examined the antibody specificities in these sera. While the antibody repertoire of both sera groups was extremely broad and varied, there was a differential antibody profile between the two groups of sera. The proportion of cured patients with antibodies against EXP1, MSP3, GLURP, RAMA, SEA and EBA181 was higher than the proportion of patients with recrudescent infection. The presence of these antibodies was associated with higher odds of treatment cure. Sera containing all six antibodies impaired the invasion of P. falciparum clinical isolates into erythrocytes. These results suggest that antibodies specific against EXP1, MSP3, GLURP, RAMA, SEA and EBA181 in P. falciparum infections could assist anti-malarial drug treatment and contribute to the resolution of the malarial infection.

Analysis of antibody profiles in symptomatic malaria in three sentinel sites of Ivory Coast by using multiplex, fluorescent, magnetic, bead-based serological assay (MAGPIX™)

BACKGROUND

Advances in malaria control have reduced the burden of disease resulting from exposure to parasite infections. The consequences on naturally acquired immunity are unclear. A magnetic bead-based immunoassay (MBA) to assess antibody levels in populations living in endemic areas was previously evaluated. In this study, the effect of clinical attacks on immunity was analysed in three sentinel sites of Ivory Coast.

METHODS

Recombinant proteins or peptides derived from liver or blood stage antigens of Plasmodium falciparum (CSP, LSA141, LSA3, SALSA, PF13-DBL1α1, GLURP, AMA1, MSP1p19, MSP4p20), the CSP of Plasmodium malariae and the salivary glands antigen of Anopheles gambiae (gSG6) were covalently linked to a colour-coded microsphere (Luminex™ beads) for the multiplex assay. ELISA was used for whole parasite extract antigen. Blood samples (n = 94) of patients consulting for symptomatic malaria attacks and living in three different malaria endemic settings (rural and periurban) were analysed.

RESULTS

Highly variable seroprevalence of antibody responses against parasite antigens was found ranging from 3 (gSG6) to 97% (MSP4p20). A marked prevalence and significantly higher level of antibodies was found in patients from the rural site (Korhogo), those harbouring the lowest level of parasitaemia. The use of whole schizont extract could not discriminate immunity level, contrary to parasite-derived recombinant proteins or peptides. Prevalence of responders to LSA141 and levels of antibodies to PF13 were significantly different between the three settings. Moreover, the post-treatment clearance of parasites was clearly associated with a significantly higher level of antibody response for almost 50% of the parasite antigens tested.

CONCLUSION

The multiplex MBA-Magpix technology assay provides an accurate high throughput monitoring of parasite-specific antibodies during symptomatic malaria. The levels of antibody responses may provide a risk criterion with respect to the degree of parasitic infection. Additionally, they can be used as an indicator in the implementation of malaria prevention and local control strategies.

A better resolution for integrating methods for monitoring Plasmodium falciparum resistance to antimalarial drugs

Effective chemotherapy is the mainstay of malaria control. However, resistance of falciparum malaria to antimalarial drugs compromised the efforts to eliminate the disease and led to the resurgence of malaria epidemics. Three main approaches are used to monitor antimalarial drug efficacy and drug resistance; namely, in vivo trials, in vitro/ex vivo assays and molecular markers of drug resistance. Each approach has its implications of use as well as its advantages and drawbacks. Therefore, there is a need to use an integrated approach that would give the utmost effect to detect resistance as early as its emergence and to track it once spread. Such integration becomes increasingly needed in the era of artemisinin-based combination therapy as a forward action to deter resistance. The existence of regional and global networks for the standardization of methodology, provision of high quality reagents for the assessment of antimalarial drug resistance and dissemination of open-access data would help in approaching an integrated resistance surveillance system on a global scale.

The roles of thepfcrt 76Tandpfmdr1 86Ymutations, immunity and the initial level of parasitaemia, in predicting the outcome of chloroquine treatment in two areas with different transmission intensities

The resistance of Plasmodium falciparum to chloroquine (CQ) is probably mediated by point mutations in two genes: pfcrt and pfmdr1. The aim of the present study was to investigate, in patients treated with CQ, the association between host factors, such as immunity and initial level of parasitaemia, and the ability to clear P. falciparum parasites carrying the key chloroquine-resistance (CQR) mutations, pfcrt 76T and pfmdr1 86Y. Identical CQ-efficacy trials were performed in 51 young children (aged <5 years) from Kibaha, in north-western Tanzania, and 44 patients (aged 3-57 years) from Darawish, in eastern Sudan. In both areas, all the CQ-treatment failures had infections with the 76T and 86Y alleles before treatment. Although the presence of these two alleles was significantly associated with treatment failure in Sudan (P=0.001), the corresponding association in Tanzania did not reach statistical significance (P=0.1). Of the 39 patients from Darawish and 44 from Kibaha who harboured parasites with the CQR mutations, 12 and 19, respectively, managed to clear their parasitaemias. The ability to clear CQR parasites was significantly associated with the initial level of parasitaemia (with P-values of 0.05 in Tanzania and 0.01 in Sudan) and with age-- the best surrogate for protective immunity in endemic areas (with P-values of 0.02 in Tanzania and 0.001 in Sudan). These results confirm previous observations that indicated that the 76T and 86Y alleles play a role in the mechanism of CQR, although other factors, such as level of parasitaemia when treated and age, are also important. The 76T and 86Y alleles could still be used as predictive markers for CQR, in non-immune individuals and low-transmission areas.

How Should Antibodies against P. falciparum Merozoite Antigens Be Measured?

Immunity against malaria develops slowly and only after repeated exposure to the parasite. Many of those that die of the disease are children under five years of age. Antibodies are an important part of immunity, but which antibodies that are protective and how these should be measured are still unclear. We discuss the pros and cons of ELISA, invasion inhibition assays/ADCI, and measurement of affinity of antibodies and what can be done to improve these assays, thereby increasing the knowledge about the immune status of an individual, and to perform better evaluation of vaccine trials.

Pharmacokinetic disposition of sulfadoxine in children with acute uncomplicated falciparum malaria treated with sulfadoxine-pyrimethamine in South West Nigeria

Sulfadoxine (SDX)-pyrimethamine is currently recommended as a partner drug with artesunate in the chemotherapy of malaria. However, information on pharmacokinetic disposition of SDX-pyrimethamine in children is limited. Efforts in this study were thus devoted to evaluation of pharmacokinetic disposition of SDX using high-pressure liquid chromatographic techniques and effects of pharmacokinetic variability on treatment outcome in Nigerian children with falciparum malaria. The blood concentration profile of SDX was similar in patients whose infection responded to treatment and those who failed treatment; mean SDX concentration values were similar for day 3 (179 vs 157 μg/mL, P = 0.734), day 7 (84 vs 51 μg/mL, P = 0.365), and day 14 (50 vs 14 μg/mL, P = 0.151). Extent of exposure (area under the curve) to SDX was also similar in the patients (1196 vs 1013 μg d/mL, P = 0.561). Pearson's correlation, showed significant correlation between area under the curve and D3 or D7 concentration of SDX (P = 0.001, r = 0.702 or P = 0.001, r = 0.835, respectively). Age-stratified analysis showed that SDX concentrations were significantly higher in older children (older than 5 years); the mean maximum concentration (125 vs 295 μg/mL, P = 0.001), extent of exposure (812 vs 1562 μg d/mL, P = 0.001), day 3 concentration (98 vs 250 μg/mL, P = 0.001), and day 7 concentration (54 vs 128 μg/mL, P = 0.007) were higher. The study revealed no differences in posttreatment blood SDX concentrations in patients who responded to treatment and those who failed to respond to treatment. Furthermore, there was an age-related pharmacokinetic variability of SDX in the group of children studied with potential impact on treatment outcome.

Associations between antibodies to a panel of Plasmodium falciparum specific antigens and response to sub-optimal antimalarial therapy in Kampala, Uganda

Background

Antibodies are important in the control of blood stage Plasmodium falciparum infection. It is unclear which antibody responses are responsible for, or even associated with protection, partly due to confounding by heterogeneous exposure. Assessment of response to partially effective antimalarial therapy, which requires the host to assist in clearing parasites, offers an opportunity to measure protection independent of exposure.

Methods

A cohort of children aged 1–10 years in Kampala, Uganda were treated with amodiaquine+sulfadoxine-pyrimethamine for uncomplicated malaria. Serum samples from the time of malaria diagnosis and 14 days later were analyzed for total IgG to 8 P. falciparum antigens using a quantitative indirect ELISA. Associations between antibody levels and risk of treatment failure were estimated using Cox proportional hazard regression.

Results

Higher levels of antibodies to apical membrane antigen 1 (AMA-1), but to none of the other 7 antigens were significantly associated with protection against treatment failure (HR 0.57 per 10-fold increase in antibody level, CI 0.41–0.79, p = 0.001). Protection increased consistently across the entire range of antibody levels.

Conclusions

Measurement of antibody levels to AMA-1 at the time of malaria may offer a quantitative biomarker of blood stage immunity to P. falciparum, a tool which is currently lacking.

Efficacy of artemether-lumefantrine, the nationally-recommended artemisinin combination for the treatment of uncomplicated falciparum malaria, in southern Laos

Background

The Lao Government changed the national policy for uncomplicated Plasmodium falciparum malaria from chloroquine to artemether-lumefantrine (AL) in 2005. Since then, no information on AL efficacy has been reported. With evidence of resistance to artemisinin derivatives in adjacent Cambodia, there has been a concern as to AL efficacy. Monitoring of AL efficacy would help the Lao Government to make decisions on appropriate malaria treatment.

Methods

The efficacy of a three-day, twice daily oral artemether-lumefantrine for the treatment of uncomplicated falciparum malaria in Xepon District, Savannakhet Province, southern Laos was studied over 42 days follow-up. This was part of a trial of thiamin supplementation in falciparum malaria.

Results

Of 630 patients with P. falciparum enrolled in the trial of thiamin treatment, 549 (87%, 357 children ≤15 years and 192 adults) were included in this study. The per protocol 42-day cure rates were 97% (524/541) [96% (337/352) for children and 99% (187/189) for adults, p = 0.042]. By conventional intention-to-treat analysis, the 42-day cure rates adjusted for re-infection, were 97% (532/549) [96% (342/357) in children and 99% (190/192) in adults, p = 0.042]. The proportion of patients who remained parasitaemic at day 1 after treatment was significantly higher in children [33% (116/356)] compared to adults [15% (28/192)] (p < 0.001) and only one adult patient had detectable parasitaemia on day 2. There were no serious adverse events. Potential side effects after treatment were reported more commonly in adults (32%) compared to children (15%) (p < 0.001). Patients with recrudescent infections were significantly younger, had longer mean time to fever clearance, and had longer median time to parasite clearance compared to those who were cured.

Conclusions

The current nationally-recommended anti-malarial treatment (artemether-lumefantrine) remains highly efficacious for the treatment of uncomplicated falciparum malaria five years after introduction in Laos. Regular monitoring is required in case artemisinin-resistant P. falciparum parasites should appear.

Trial registration

ISRCTN85411059.

Antibodies to malaria vaccine candidates are associated with chloroquine or sulphadoxine/pyrimethamine treatment efficacy in children in an endemic area of Burkina Faso

BACKGROUND

Patient immune status is thought to affect the efficacy of anti-malarial chemotherapy. This is a subject of some importance, since evidence of immunity-related interactions may influence our use of chemotherapy in populations with drug resistance, as well as assessment of the value of suboptimal vaccines. The study aim was to investigate relationship between antibodies and anti-malarial drug treatment outcomes.

METHODS

Some 248 children aged 0.5 and 15 years were recruited prior to the high malaria transmission season. Venous blood (5 ml) was obtained from each child to measure antibody levels to selected malaria antigens, using ELISA. Blood smears were also performed to assess drug efficacy and malaria infection prevalence. Children were actively followed up to record clinical malaria cases.

RESULTS

IgG levels to MSP3 were always higher in the successfully treated group than in the group with treatment failure. The same observation was made for GLURP but the reverse observation was noticed for MSP1-19. Cytophilic and non-cytophilic antibodies were significantly associated with protection against all three antigens, except for IgG4 to MSP1-19 and GLURP.

CONCLUSION

Acquired anti-malarial antibodies may play an important role in the efficacy of anti-malarial drugs in younger children more susceptible to the disease.

High affinity antibodies to Plasmodium falciparum merozoite antigens are associated with protection from malaria

Background

Malaria kills almost 1 million people every year, but the mechanisms behind protective immunity against the disease are still largely unknown.

Methodology/Principal Findings

In this study, surface plasmon resonance technology was used to evaluate the affinity (measured as k^d) of naturally acquired antibodies to the Plasmodium falciparum antigens MSP2 and AMA1. Antibodies in serum samples from residents in endemic areas bound with higher affinities to AMA1 than to MSP2, and with higher affinities to the 3D7 allele of MSP2-3D7 than to the FC27 allele. The affinities against AMA1 and MSP2-3D7 increased with age, and were usually within similar range as the affinities for the monoclonal antibodies also examined in this study. The finding of MSP2-3D7 type parasites in the blood was associated with a tendency for higher affinity antibodies to both forms of MSP2 and AMA1, but this was significant only when analyzing antibodies against MSP2-FC27, and individuals infected with both allelic forms of MSP2 at the same time showed the highest affinities. Individuals with the highest antibody affinities for MSP2-3D7 at baseline had a prolonged time to clinical malaria during 40 weeks of follow-up, and among individuals who were parasite positive at baseline higher antibody affinities to all antigens were seen in the individuals that did not experience febrile malaria during follow up.

Conclusions/Significance

This study contributes important information for understanding how immunity against malaria arises. The findings suggest that antibody affinity plays an important role in protection against disease, and differs between antigens. In light of this information, antibody affinity measurements would be a key assessment in future evaluation of malaria vaccine formulations.

Declining responsiveness of Plasmodium falciparum infections to artemisinin-based combination treatments on the Kenyan coast

Background

The emergence of artemisinin-resistant P. falciparum malaria in South-East Asia highlights the need for continued global surveillance of the efficacy of artemisinin-based combination therapies.

Methods

On the Kenyan coast we studied the treatment responses in 474 children 6–59 months old with uncomplicated P. falciparum malaria in a randomized controlled trial of dihydroartemisinin-piperaquine vs. artemether-lumefantrine from 2005 to 2008. (ISRCTN88705995)

Results

The proportion of patients with residual parasitemia on day 1 rose from 55% in 2005–2006 to 87% in 2007–2008 (odds ratio, 5.4, 95%CI, 2.7–11.1; P<0.001) and from 81% to 95% (OR, 4.1, 95%CI, 1.7–9.9; P = 0.002) in the DHA-PPQ and AM-LM groups, respectively. In parallel, Kaplan-Meier estimated risks of apparent recrudescent infection by day 84 increased from 7% to 14% (P = 0.1) and from 6% to 15% (P = 0.05) with DHA-PPQ and AM-LM, respectively. Coinciding with decreasing transmission in the study area, clinical tolerance to parasitemia (defined as absence of fever) declined between 2005–2006 and 2007–2008 (OR body temperature >37.5°C, 2.8, 1.9–4.1; P<0.001). Neither in vitro sensitivity of parasites to DHA nor levels of antibodies against parasite extract accounted for parasite clearance rates or changes thereof.

Conclusions

The significant, albeit small, decline through time of parasitological response rates to treatment with ACTs may be due to the emergence of parasites with reduced drug sensitivity, to the coincident reduction in population-level clinical immunity, or both. Maintaining the efficacy of artemisinin-based therapy in Africa would benefit from a better understanding of the mechanisms underlying reduced parasite clearance rates.

Trial Registration

Controlled-Trials.com ISRCTN88705995

The parasite clearance curve

Parasite clearance rates are important measures of anti-malarial drug efficacy. They are particularly important in the assessment of artemisinin resistance. The slope of the log-linear segment in the middle of the parasite clearance curve has the least inter-individual variance and is the focus of therapeutic assessment. The factors affecting parasite clearance are reviewed. Methods of presentation and the approaches to analysis are discussed.

Host candidate gene polymorphisms and clearance of drug-resistant Plasmodium falciparum parasites

Background

Resistance to anti-malarial drugs is a widespread problem for control programmes for this devastating disease. Molecular tests are available for many anti-malarial drugs and are useful tools for the surveillance of drug resistance. However, the correlation of treatment outcome and molecular tests with particular parasite markers is not perfect, due in part to individuals who are able to clear genotypically drug-resistant parasites. This study aimed to identify molecular markers in the human genome that correlate with the clearance of malaria parasites after drug treatment, despite the drug resistance profile of the protozoan as predicted by molecular approaches.

Methods

3721 samples from five African countries, which were known to contain genotypically drug resistant parasites, were analysed. These parasites were collected from patients who subsequently failed to clear their infection following drug treatment, as expected, but also from patients who successfully cleared their infections with drug-resistant parasites. 67 human polymorphisms (SNPs) on 17 chromosomes were analysed using Sequenom's mass spectrometry iPLEX gold platform, to identify regions of the human genome, which contribute to enhanced clearance of drug resistant parasites.

Results

An analysis of all data from the five countries revealed significant associations between the phenotype of ability to clear drug-resistant Plasmodium falciparum infection and human immune response loci common to all populations. Overall, three SNPs showed a significant association with clearance of drug-resistant parasites with odds ratios of 0.76 for SNP rs2706384 (95% CI 0.71-0.92, P = 0.005), 0.66 for SNP rs1805015 (95% CI 0.45-0.97, P = 0.03), and 0.67 for SNP rs1128127 (95% CI 0.45-0.99, P = 0.05), after adjustment for possible confounding factors. The first two SNPs (rs2706384 and rs1805015) are within loci involved in pro-inflammatory (interferon-gamma) and anti-inflammatory (IL-4) cytokine responses. The third locus encodes a protein involved in the degradation of misfolded proteins within the endoplasmic reticulum, and its role, if any, in the clearance phenotype is unclear.

Conclusions

The study showed significant association of three loci in the human genome with the ability of parasite to clear drug-resistant P. falciparum in samples taken from five countries distributed across sub-Saharan Africa. Both SNP rs2706384 and SNP1805015 have previously been reported to be associated with risk of malaria infection in African populations. The loci are involved in the Th1/Th2 balance, and the association of SNPs within these genes suggests a key role for antibody in the clearance of drug-resistant parasites. It is possible that patients able to clear drug-resistant infections have an enhanced ability to control parasite growth.

Characterization of peripheral blood T lymphocyte subsets in Chinese rhesus macaques with repeated or long-term infection with Plasmodium cynomolgi

T lymphocytes play a vital role in antimalaria immunity, but there is little information about the role of T cells in malaria infection. In order to explore the profile of T cells in malaria immunity, we infected Chinese rhesus macaques with the malaria parasite (Plasmodium cynomolgi) and examined the dynamics of T cell subsets. Both repeated and long-term infections were involved. Our results showed that the monkeys in the repeated infection group acquired protective immunity through primary infection, which was evidenced by a much lower parasitemia, milder anemia, and milder fever during reinfection; the monkeys in the long-term infection group also developed protective immunity, but this was not sufficient to eliminate the parasite. The total counts of leukocytes, neutrophils, CD3+ T cells, CD4+ or CD8+ T cells, and naïve and memory CD4+ and CD8+ T cells declined during the acute phase of malaria but increased after the parasite was controlled. The total number of activated CD4+ T cells significantly increased during malaria in animals with a long-term infection, which remained at least 3 months after the termination of malaria. However, the activated CD4+ T cells decreased during the acute phase of infection in the repeated infection group and converted to preinfection levels after malaria was cured. Regulatory CD4+ T cells continued to increase during the malaria infections and quickly reverted to preinfection levels after the parasite was controlled. Our study provides a systematic analysis of the kinetic profiles of T lymphocyte subsets during malaria infections and provides some experimental insight into malaria immunology.

Plasmodium falciparum clearance with artemisinin-based combination therapy (ACT) in patients with glucose-6-phosphate dehydrogenase deficiency in Mali

Background

Artemisinin-based combination therapy (ACT) is currently the most effective medicine for the treatment of uncomplicated malaria. Artemisinin has previously been shown to increase the clearance of Plasmodium falciparum in malaria patients with haemoglobin E trait, but it did not increase parasite inhibition in an in vitro study using haemoglobin AS erythrocytes. The current study describes the efficacy of artemisinin derivatives on P. falciparum clearance in patients with glucose-6-phosphate dehydrogenase deficiency (G6PD), a haemoglobin enzyme deficiency, not yet studied in the same context, but nonetheless is a common in malaria endemic areas, associated with host protection against uncomplicated and severe malaria. The impact of G6PD deficiency on parasite clearance with ACT treatment was compared between G6PD-deficient patients and G6PD-normal group.

Methods

Blood samples from children and adults participants (1 to 70 years old) with uncomplicated P. falciparum malaria residing in Kambila, Mali were analysed. Study participants were randomly assigned to receive either artemether-lumefantrine (Coartem^®) or artesunate plus mefloquine (Artequin™). A restriction-fragment length polymorphism analysis of PCR-amplified DNA samples was used to identify the (A-) allele of the gene mutation responsible for G6PD deficiency (G6PD*A-). 470 blood samples were thus analysed and of these, DNA was extracted from 315 samples using the QIAamp kit for PCR to identify the G6PD*A- gene.

Results

The DNA amplified from 315 samples using PCR showed that G6PD*A- deficiency was present in 56 participants (17.8%). The distribution of the specific deficiency was 1%, 7% and, 9.8% respectively for homozygous, hemizygous, and heterozygous genotypes. Before treatment, the median parasitaemia and other baseline characteristics (mean haemoglobin, sex and age groups) between G6PD deficiency (hemizygous, heterozygous, and homozygous) and G6PD-normal participants were comparable (p > 0.05). After treatment, parasite clearance did not change significantly whether the participants were G6PD deficient or G6PD normal on day 1 (OR = 1.3; CI = 0.70-2.47; p > 0.05) and on day 2 (OR = 0.859; CI = 0.097-7.61; p > 0.05).

Conclusions

The presence of G6PD deficiency does not appear to significantly influence the clearance of P. falciparum in the treatment of uncomplicated malaria using ACT.

Modulating effects of plasma containing anti-malarial antibodies on in vitro anti-malarial drug susceptibility in Plasmodium falciparum

Background

The efficacy of anti-malarial drugs is determined by the level of parasite susceptibility, anti-malarial drug bioavailability and pharmacokinetics, and host factors including immunity. Host immunity improves the in vivo therapeutic efficacy of anti-malarial drugs, but the mechanism and magnitude of this effect has not been characterized. This study characterized the effects of 'immune' plasma to Plasmodium falciparumon the in vitro susceptibility of P. falciparum to anti-malarial drugs.

Methods

Titres of antibodies against blood stage antigens (mainly the ring-infected erythrocyte surface antigen [RESA]) were measured in plasma samples obtained from Thai patients with acute falciparum malaria. 'Immune' plasma was selected and its effects on in vitro parasite growth and multiplication of the Thai P. falciparum laboratory strain TM267 were assessed by light microscopy. The in vitro susceptibility to quinine and artesunate was then determined in the presence and absence of 'immune' plasma using the ³H-hypoxanthine uptake inhibition method. Drug susceptibility was expressed as the concentrations causing 50% and 90% inhibition (IC₅₀ and IC₉₀), of ³H-hypoxanthine uptake.

Results

Incubation with 'immune' plasma reduced parasite maturation and decreased parasite multiplication in a dose dependent manner. ³H-hypoxanthine incorporation after incubation with 'immune' plasma was decreased significantly compared to controls (median [range]; 181.5 [0 to 3,269] cpm versus 1,222.5 [388 to 5,932] cpm) (p= 0.001). As a result 'immune' plasma reduced apparent susceptibility to quinine substantially; median (range) IC₅₀ 6.4 (0.5 to 23.8) ng/ml versus 221.5 (174.4 to 250.4) ng/ml (p = 0.02), and also had a borderline effect on artesunate susceptibility; IC₅₀ 0.2 (0.02 to 0.3) ng/ml versus 0.8 (0.2 to 2.3) ng/ml (p = 0.08). Effects were greatest at low concentrations, changing the shape of the concentration-effect relationship. IC₉₀ values were not significantly affected; median (range) IC₉₀ 448.0 (65 to > 500) ng/ml versus 368.8 (261 to 501) ng/ml for quinine (p > 0.05) and 17.0 (0.1 to 29.5) ng/ml versus 7.6 (2.3 to 19.5) ng/ml for artesunate (p = 0.4).

Conclusions

'Immune' plasma containing anti-malarial antibodies inhibits parasite development and multiplication and increases apparent in vitro anti-malarial drug susceptibility of P. falciparum. The IC₉₀ was much less affected than the IC₅₀ measurement.

Host immunity as a determinant of treatment outcome in Plasmodium falciparum malaria

Host immunity is an important but poorly understood determinant of antimalarial efficacy, influencing the outcome of prevention and treatment trials. Variations in host immunity might explain why factors such as host genetics, age, pregnancy, infection with HIV, parasite density, and malaria transmission intensity, can raise or lower apparent cure rates. Recently, attempts have been made to characterise immunological correlates of treatment outcome in Plasmodium falciparum malaria, but not yet for Plasmodium vivax. A better understanding of such correlates might improve trials of antimalarial drugs and provide leads for vaccine development. Greater understanding of the relation between host immunity and treatment outcome is crucial to making informed choices about the use of safe but partly effective drugs for malaria prevention in children and pregnant women. With increasing malaria control efforts worldwide, declining population immunity might alter drug response profiles. Improved methods for assessing antimalarial immunity will strengthen malaria control efforts.

The relationship of Plasmodium falciparum humeral immunity with HIV-1 immunosuppression and treatment efficacy in Zambia

Background

HIV-1 infection affects malaria humeral immunity during pregnancy, but data for non-pregnant adults are lacking. This study reports the impact of HIV-1 infection and other variables on the level of malaria humeral immunity in adults with clinical malaria and whether humeral immune suppression was a risk factor for treatment failure.

Methods

Sera of 224 HIV-1 infected and 115 uninfected adults were compared for IgG to merozoite antigens AMA-1 and MSP2 (3D7 and FC27 types) determined by ELISA, and for IgG to the Variant Surface Antigens (VSA) of three different parasite line E8B, A4 and HCD6 determined by flow cytometry.

Results

Compared to HIV-1 uninfected adults, AMA-1 IgG was lower in HIV-1 infected (P = 0.02) and associated with low CD4 count AMA-1 IgG (P = 0.003). Low IgG to all three merozoite antigens was associated with less anemia (P = 0.03). High parasite load was associated with low MSP2 IgG 3D7 and FC27 types (P = 0.02 and P = 0.08). Antibody levels to VSA did not differ between HIV-1 infected and uninfected adults. However, low VSA IgGs were associated with high parasite load (P ≤ 0.002 for each parasite line) and with treatment failure (P ≤ 0.04 for each parasite line).

Conclusion

HIV-1 affects humeral responses to AMA-1, but seems to marginally or not affect humeral responses to other merozoite antigens and VSAs. The latter were important for controlling parasite density and predict treatment outcome.

Antibodies to variant surface antigens of Plasmodium falciparum-infected erythrocytes are associated with protection from treatment failure and the development of anemia in pregnancy

BACKGROUND

In pregnancy-associated malaria (PAM), Plasmodium falciparum-infected erythrocytes (IEs) express variant surface antigens (VSA-PAM) that evade existing immunity and mediate placental sequestration. Antibodies to VSA-PAM develop with gravidity and block placental adhesion or opsonize IEs for phagocytic clearance, helping to prevent maternal anemia and low birth weight in infants.

METHODS

Using serum samples from 141 pregnant Malawian women with parasitemia enrolled in a randomized trial of antimalarials and VSA-PAM-expressing CS2 IEs, we quantified levels of immunoglobulin (Ig) G to VSA-PAM by flow cytometry and levels of opsonizing antibodies by measuring uptake of IEs by THP1 promonocytes.

RESULTS

After controlling for gravidity and antimalarial treatment, higher levels of IgG to VSA-PAM were associated with decreased anemia at delivery (odds ratio [OR], 0.66 [95% confidence interval {CI}, 0.46-0.93]; P = .018) and were weakly associated with decreased parasitological failure (OR, 0.78 [95% CI, 0.60-1.03]; P = .075), especially reinfection (OR, 0.73 [95% CI, 0.53-1.01]; P = .057). Higher levels of opsonizing antibodies to CS2 IEs were associated with less maternal anemia (OR, 0.31 [95% CI, 0.13-0.74]; P = .008) and treatment failure (OR, 0.48 [95% CI, 0.25-0.90]; P = .023), primarily because of recrudescent infection (OR, 0.49 [95% CI, 0.21-1.12]; P = .089).

CONCLUSION

Higher levels of both IgG antibodies to VSA-PAM and opsonizing antibodies, a functional measure of immunity, correlate with parasite clearance and less anemia in pregnancy malaria.

Spread of anti-malarial drug resistance: mathematical model with implications for ACT drug policies

BACKGROUND

Most malaria-endemic countries are implementing a change in anti-malarial drug policy to artemisinin-based combination therapy (ACT). The impact of different drug choices and implementation strategies is uncertain. Data from many epidemiological studies in different levels of malaria endemicity and in areas with the highest prevalence of drug resistance like borders of Thailand are certainly valuable. Formulating an appropriate dynamic data-driven model is a powerful predictive tool for exploring the impact of these strategies quantitatively.

METHODS

A comprehensive model was constructed incorporating important epidemiological and biological factors of human, mosquito, parasite and treatment. The iterative process of developing the model, identifying data needed, and parameterization has been taken to strongly link the model to the empirical evidence. The model provides quantitative measures of outcomes, such as malaria prevalence/incidence and treatment failure, and illustrates the spread of resistance in low and high transmission settings. The model was used to evaluate different anti-malarial policy options focusing on ACT deployment.

RESULTS

The model predicts robustly that in low transmission settings drug resistance spreads faster than in high transmission settings, and treatment failure is the main force driving the spread of drug resistance. In low transmission settings, ACT slows the spread of drug resistance to a partner drug, especially at high coverage rates. This effect decreases exponentially with increasing delay in deploying the ACT and decreasing rates of coverage. In the high transmission settings, however, drug resistance is driven by the proportion of the human population with a residual drug level, which gives resistant parasites some survival advantage. The spread of drug resistance could be slowed down by controlling presumptive drug use and avoiding the use of combination therapies containing drugs with mismatched half-lives, together with reducing malaria transmission through vector control measures.

CONCLUSION

This paper has demonstrated the use of a comprehensive mathematical model to describe malaria transmission and the spread of drug resistance. The model is strongly linked to the empirical evidence obtained from extensive data available from various sources. This model can be a useful tool to inform the design of treatment policies, particularly at a time when ACT has been endorsed by WHO as first-line treatment for falciparum malaria worldwide.

Impact of intermittent preventive treatment with sulfadoxine-pyrimethamine on antibody responses to erythrocytic-stage Plasmodium falciparum antigens in infants in Mozambique

We evaluated the impact of intermittent preventive treatment in infants (IPTi) with sulfadoxine-pyrimethamine (SP), which was given at ages 3, 4, and 9 months through the Expanded Program on Immunization (EPI), on the development of antibody responses to Plasmodium falciparum in Mozambique. Immunoglobulin M (IgM) and IgG subclass antibodies specific to whole asexual parasites and to recombinant MSP-1(19), AMA-1, and EBA-175 were measured at ages 5, 9, 12, and 24 months for 302 children by immunofluorescence antibody tests and by enzyme-linked immunosorbent assays. Antibody responses did not significantly differ between children receiving IPTi with SP and those receiving a placebo at any time point measured, with the exception of the responses of IgG and IgG1 to AMA-1 and/or MSP-1(19), which were significantly higher in the SP-treated group than in the placebo group at ages 5, 9, and/or 24 months. IPTi with SP given through the EPI reduces the frequency of malarial illness while allowing the development of naturally acquired antibody responses to P. falciparum antigens.

Effects of Plasmodium falciparum parasite population size and patient age on early and late parasitological outcomes of antimalarial treatment in children

The design and interpretation of trials assessing the chemotherapeutic effects of antimalarial drugs depend on our understanding of how different selection criteria affect treatment outcomes. In this study, we analyzed the effects of baseline parameters on the initial parasite elimination rate and the risk of subsequent recrudescence as a marker for incompletely eliminated asexual blood-stage parasites in pediatric patients with uncomplicated Plasmodium falciparum infection treated with amodiaquine in a high-transmission area. We found that (i) parasite population size and patient age independently determine early and late parasitological treatment outcome measurements; (ii) the rate of recrudescence is higher in patients 1 to 3 years of age than in patients aged <1 or >3 years; (iii) patients aged >5 years with parasite densities between 2,000 and 10,000/microl have a lower recrudescence rate (13%; 95% confidence interval [CI], 8% to 21%) than patients aged <5 years with parasite densities of >10,000/microl (40%; 95% CI, 30% to 50%); and (iv) the sensitivity of detecting recrudescences outside this high-risk group, i.e., in patients of >5 years of age or with parasite densities of <10,000/microl, is as low as 27% or 22%, respectively. In conclusion, these findings highlight the need to use adequate selection criteria and to report parasitological outcome results adjusted for the readily available determinants of chemotherapeutic failure, i.e., patient age and baseline parasitemia. The thresholds may vary by transmission intensity and drug regimen. A better understanding of the limitations of antimalarial regimens in high-risk subgroups of patients has important implications for setting policy recommendations.

Potential impact of host immunity on malaria treatment outcome in Tanzanian children infected with Plasmodium falciparum

Background

In malaria endemic areas children may recover from malaria after chemotherapy in spite of harbouring genotypically drug-resistant Plasmodium falciparum. This phenomenon suggests that there is a synergy between drug treatment and acquired immunity. This hypothesis was examined in an area of moderately intense transmission of P. falciparum in Tanzania during a drug trail with sulphadoxine-pyrimethamine (SP) or amodiaquine (AQ).

Methods

One hundred children with uncomplicated malaria were treated with either SP or AQ and followed for 28 days. Mutations in parasite genes related to SP and AQ-resistance as well as human sickle cell trait and alpha-thalassaemia were determined using PCR and sequence-specific oligonucleotide probes and enzyme-linked immunosorbent assay (SSOP-ELISA), and IgG antibody responses to a panel of P. falciparum antigens were assessed and related to treatment outcome.

Results

Parasitological or clinical treatment failure (TF) was observed in 68% and 38% of children receiving SP or AQ, respectively. In those with adequate clinical and parasitological response (ACPR) compared to children with TF, and for both treatment regimens, prevalence and levels of anti-Glutamate-rich Protein (GLURP)-specific IgG antibodies were significantly higher (P < 0.001), while prevalence of parasite haplotypes associated with SP and AQ resistance was lower (P = 0.02 and P = 0.07, respectively). Interestingly, anti-GLURP-IgG antibodies were more strongly associated with treatment outcome than parasite resistant haplotypes, while the IgG responses to none of the other 11 malaria antigens were not significantly associated with ACPR.

Conclusion

These findings suggest that GLURP-specific IgG antibodies in this setting contribute to clearance of drug-resistant infections and support the hypothesis that acquired immunity enhances the clinical efficacy of drug therapy. The results should be confirmed in larger scale with greater sample size and with variation in transmission intensity.

Dynamics of immune response and drug resistance in malaria infection

Background

Malaria parasites that concurrently infect a host compete on the basis of their intrinsic growth rates and by stimulating cross-reactive immune responses that inhibit each others' growth. If the phenotypes also show different drug sensitivities ('sensitive' vs. 'resistant' strains), drug treatment can change their joint dynamics and the long-term outcome of the infection: most obviously, persistent drug pressure can permit the more resistant, but otherwise competitively-inferior, strains to dominate.

Methods

Here a mathematical model is developed to analyse how these and more subtle effects of antimalarial drug use are modulated by immune response, repeated re-inoculation of parasites, drug pharmacokinetic parameters, dose and treatment frequency.

Results

The model quantifies possible effects of single and multiple (periodic) treatment on the outcome of parasite competition. In the absence of further inoculation, the dosage and/or treatment frequency required for complete clearance can be estimated. With persistent superinfection, time-average parasite densities can be derived in terms of the basic immune-regulating parameters, the drug efficacy and treatment regimen.

Conclusion

The functional relations in the model are applicable to a wide range of conditions and transmission environments, allowing predictions to be made on both the individual and the community levels, and, in particular, transitions from drug-sensitive to drug-resistant parasite dominance to be projected on both levels.

Antimalarial drug resistance in Africa: strategies for monitoring and deterrence

Despite the initiation in 1998 by the World Health Organization of a campaign to 'Roll Back Malaria', the rates of disease and death caused by Plasmodium falciparum malaria in sub-Saharan Africa are growing. Drug resistance has been implicated as one of the main factors in this disturbing trend. The efforts of international agencies, governments, public health officials, advocacy groups and researchers to devise effective strategies to deter the spread of drug resistant malaria and to ameliorate its heavy burden on the people of Africa have not succeeded. This review will not attempt to describe the regional distribution of drug resistant malaria in Africa in detail, mainly because information on resistance is limited and has been collected using different methods, making it difficult to interpret. Instead, the problems of defining and monitoring resistance and antimalarial drug treatment outcomes will be discussed in hopes of clarifying the issues and identifying ways to move forward in a more coordinated fashion. Strategies to improve measurement of resistance and treatment outcomes, collection and use of information on resistance, and potential approaches to deter and reduce the impact of resistance, will all be considered. The epidemiological setting and the goals of monitoring determine how antimalarial treatment responses should be measured. Longitudinal studies, with incidence of uncomplicated malaria episodes as the primary endpoint, provide the best information on which to base treatment policy changes, while simpler standard in vivo efficacy studies are better suited for ongoing efficacy monitoring. In the absence of an ideal antimalarial combination regimen, different treatment alternatives are appropriate in different settings. But where chloroquine has failed, policy changes are long overdue and action must be taken now.

Immunoglobulin G antibodies to merozoite surface antigens are associated with recovery from chloroquine-resistant Plasmodium falciparum in Gambian children

We examined the hypothesis that recovery from uncomplicated malaria in patients carrying drug-resistant Plasmodium falciparum is a measure of acquired functional immunity and may therefore be associated with humoral responses to candidate vaccine antigens. Gambian children with malaria were treated with chloroquine in 28-day trials, and recovery was defined primarily as the absence of severe clinical malaria at any time and absence of parasitemia with fever after 3 days. Plasma samples from these children were assayed by enzyme-linked immunosorbent assay for immunoglobulin G (IgG) to recombinant merozoite antigens: apical membrane antigen 1 (AMA-1) and the 19-kDa C-terminal region of merozoite surface protein 1 (MSP-1(19)), including antigenic variants of MSP-1(19) with double and triple substitutions. Antigen-specific IgG was more frequent in children who recovered, particularly that for MSP-1(19) (age-adjusted odds ratios: 0.32 [95% confidence interval, 0.05, 1.87; P = 0.168] for AMA-1, 0.19 [0.03, 1.11; P = 0.019] for recombinant MSP-1(19), 0.24 [0.04, 1.31; P = 0.032] for the recombinant MSP-1(19) double variant, and 0.18 [0.03, 0.97; P = 0.013] for the triple variant). IgG titers to MSP-1(19) and to the triple variant were higher in plasma samples taken 7 days after chloroquine treatment from children who carried resistant parasites but recovered and remained parasite free. Moreover, in children who were parasitemic on day 14 or day 28, there was an age-independent relationship between parasite density and IgG to both MSP-1(19) and the triple variant (coefficients of -0.550 and -0.590 and P values of 0.002 and 0.001, respectively). The results validate the use of this approach to identify antigens that are associated with protection from malaria.

Association between the pharmacokinetics and in vivo therapeutic efficacy of sulfadoxine-pyrimethamine in Malawian children

Sulfadoxine-pyrimethamine (SP) has been widely used in recent years to treat acute uncomplicated Plasmodium falciparum malaria. Risk factors for SP therapeutic failure include young age, subtherapeutic SP concentrations, and resistance-conferring genetic mutations in parasite target enzymes. A substantial proportion of patients are able to clear genetically highly resistant P. falciparum genotypes. To determine whether blood SP concentrations independently affect the patient's ability to clear resistant genotypes, we compared SP pharmacokinetics of cases of adequate clinical and parasitological response (ACPR) with cases of treatment failure (TF). When patients with ACPR and TF were compared, mean values were similar for the day 3 blood pyrimethamine (205 ng/ml versus 172 ng/ml; P = 0.25) and estimated maximum sulfadoxine (79 +/- 6.52 versus 69 +/- 6.27 mug/ml; P = 0.60) concentrations, for sulfadoxine terminal-phase elimination half-lives (7.15 versus 6.41 days; P = 0.42), and for the extents of sulfadoxine absorption (areas under the concentration-time curve of 932 +/- 100 versus 888 +/- 78.9 mug day ml(-1); P = 0.72). Among patients infected with the quintuple resistant parasites, day 3 blood pyrimethamine concentrations were higher in those who cleared the infection than in those who did not (305 +/- 35.4 versus 228 +/- 21.7 ng/ml; P = 0.037). Within this subgroup, this finding remained significant after adjusting for endogenous folate levels, age, site, and resistance-conferring mutations (odds ratio: 1.011 [1.003 to 1.024]; P = 0.018). However, as a subgroup analysis, our biologically plausible observation that higher blood pyrimethamine concentrations enhance the ability of patients to clear resistant P. falciparum should be interpreted with caution and needs further validation.

Comparison of chloroquine, sulfadoxine/pyrimethamine, mefloquine and mefloquine-artesunate for the treatment of falciparum malaria in Kachin State, North Myanmar

Multi-drug resistant falciparum malaria is widespread in Asia. In Thailand, Cambodia and Vietnam the national protocols have changed largely to artesunate combined treatment regimens but elsewhere in East and South Asia chloroquine (CQ) and sulfadoxine-pyrimethamine (SP) are still widely recommended by national malaria control programmes. In Kachin State, northern Myanmar, an area of low seasonal malaria transmission, the efficacy of CQ (25 mg base/kg) and SP (1.25/25 mg/kg), the nationally recommended treatments at the time, were compared with mefloquine alone (M; 15 mg base/kg) and mefloquine combined with artesunate (MA; 15:4 mg/kg). An open randomized controlled trial enrolled 316 patients with uncomplicated Plasmodium falciparum malaria, stratified prospectively into three age-groups. Early treatment failures (ETF) occurred in 41% (32/78) of CQ treated patients and in 24% of patients treated with SP (18/75). In young children the ETF rates were 87% after CQ and 35% after SP. Four children (two CQ, two SP) developed symptoms of cerebral malaria within 3 days after treatment. By day 42, failure rates (uncorrected for reinfections) had increased to 79% for CQ and 81% for SP. ETF rates were 2.5% after treatment with M and 3.9% after treatment with MA (P > 0.2). Overall uncorrected treatment failure rates at day 42 following M and MA were 23% and 21%, respectively. Chloroquine and SP are completely ineffective for the treatment of falciparum malaria in northern Myanmar. Mefloquine treatment is much more effective, but three day combination regimens with artesunate will be needed for optimum efficacy and protection against resistance.

In vivo assessment of drug efficacy against Plasmodium falciparum malaria: duration of follow-up

To determine the optimum duration of follow-up for the assessment of drug efficacy against Plasmodium falciparum malaria, 96 trial arms from randomized controlled trials (RCTs) with follow-up of 28 days or longer that were conducted between 1990 and 2003 were analyzed. These trials enrolled 13,772 patients, and participating patients comprised 23% of all patients enrolled in RCTs over the past 40 years; 61 (64%) trial arms were conducted in areas where the rate of malaria transmission was low, and 58 (50%) trial arms were supported by parasite genotyping to distinguish true recrudescences from reinfections. The median overall failure rate reported was 10% (range, 0 to 47%). The widely used day 14 assessment had a sensitivity of between 0 and 37% in identifying treatment failures and had no predictive value. Assessment at day 28 had a sensitivity of 66% overall (28 to 100% in individual trials) but could be used to predict the true failure rate if either parasite genotyping was performed (r(2) = 0.94) or if the entomological inoculation rate was known. In the assessment of drug efficacy against falciparum malaria, 28 days should be the minimum period of follow-up.

Randomized Comparison of Chloroquine plus Sulfadoxine-Pyrimethamine versus Artesunate plus Mefloquine versus Artemether-Lumefantrine in the Treatment of Uncomplicated Falciparum Malaria in the Lao People's Democratic Republic

BACKGROUND

Recent clinical trials in the Lao People's Democratic Republic have demonstrated that chloroquine and sulfadoxine-pyrimethamine, which are national malaria treatment policy, are no longer effective in the treatment of uncomplicated Plasmodium falciparum malaria.

METHODS

A randomized comparison of 3 oral antimalarial combinations--chloroquine plus sulfadoxine-pyrimethamine versus artesunate plus mefloquine versus artemether-lumefantrine--with 42-day follow-up period, was conducted among 330 patients with acute uncomplicated falciparum malaria in southern Laos.

RESULTS

The 42-day cure rates, as determined by intention-to-treat analysis and adjusted for reinfection, were 100%, 97%, and 93% for the groups receiving artesunate plus mefloquine, artemether-lumefantrine, and chloroquine plus sulfadoxine-pyrimethamine, respectively. Of 8 patients receiving chloroquine plus sulfadoxine-pyrimethamine who experienced treatment failure, 6 had early treatment failure. The mean parasite clearance time was significantly longer in patients treated with chloroquine plus sulfadoxine-pyrimethamine (2.9 days; 95% confidence interval [CI], 2.8-3.0 days) than in those treated with artesunate plus mefloquine (2.07 days; 95% CI, 2.0-2.1 days; P<.001) and artemether-lumefantrine (2.08 days; 95% CI, 2.0-2.1 days; P<.001). Cure rates with artemether-lumefantrine were high despite low mean daily dietary fat intake (13.8 g; 95% CI, 12.5-15.1 g) and day 7 plasma lumefantrine concentrations (0.47 mu g/mL; 95% CI, 0.38-0.56 mu g/mL).

CONCLUSION

Oral artesunate plus mefloquine and artemether-lumefantrine are highly effective for the treatment of uncomplicated falciparum malaria in Laos.

Proteomics approach reveals novel proteins on the surface of malaria-infected erythrocytes

Proteins on the surface of parasite-infected erythrocytes (PIESPs) have been one of the major focuses of malaria research due to their role in pathogenesis and their potential as targets for immunity and drug intervention. Despite intense scrutiny, only a few surface proteins have been identified and characterized. We report the identification of two novel surface proteins from Plasmodium falciparum-infected erythrocytes. Surface proteins were fractionated through biotin-streptavidin interaction and analyzed by shotgun proteomics. From a list of 36 candidates, two were selected for further characterization. The surface location of both proteins was confirmed by confocal microscopy using specific antibodies. PIESP1 and PIESP2 are unlikely to be associated with knobs, the protrusions on the parasite-infected erythrocyte (PIE) surface. In contrast to other known PIESPs, such as PfEMP1 and Rifin, these novel proteins are encoded by single copy genes, highly conserved across Plasmodium ssp., making them good targets for interventions with a broad specificity to various P. falciparum isolates.

A systematic overview of published antimalarial drug trials

Systematic database searches identified 435 antimalarial drug treatment trials, involving 82,616 patients, conducted and published between 1966 and December 2002. Of these trials 72% were randomised; 64 (15%) trials involved severe malaria, 47 (11%) studied Plasmodium vivax, 3 Plasmodium malariae or Plasmodium ovale, and the remainder (74%) assessed treatment responses in uncomplicated falciparum malaria. Twelve trials (2.7%) specifically evaluated antimalarial treatments in pregnant women. Overall 49% of trials were conducted in Asia (29% from Thailand alone) and 42% in Africa. Half of all the patients studied had been in trials published in the past 7 years. There has been a recent rise in the proportion of trial enrolling children, and a tripling in the average number of patients recruited per trial (from approximately 100 in the 1970s to 300 currently). Chloroquine was given to over half the patients in antimalarial drug trials (n = 53552) compared with artemisinin derivatives (n = 12463), mefloquine-sulphadoxine-pyrimethamine (n = 9153), mefloquine (n = 5546) and sulphadoxine-pyrimethamine (n = 5909). The quality of safety and efficacy data for recently evaluated drugs contrasts with a relative paucity of data for older 'established' compounds.

The assessment of antimalarial drug efficacy

Antimalarial drug efficacy in uncomplicated malaria should be assessed parasitologically in large, community-based trials, enrolling the age groups most affected by clinical disease. For rapidly eliminated drugs, a 28-day follow-up is needed, but, for slowly eliminated drugs, up to nine weeks could be required to document all recrudescences, and, when possible, the drug levels should also be measured. The WHO 14-day assessments are neither sensitive nor specific. In tropical Plasmodium vivax and Plasmodium ovale infections treated with chloroquine, the first relapse is usually suppressed by residual drug levels. A relapse cannot be distinguished confidently from a recrudescence. Host immunity is a major contributor to the therapeutic response, and can make failing drugs appear effective.