Development of vaccines for Plasmodium vivax malaria

Plasmodium vivax continues to cause significant morbidity outside Africa with more than 50% of malaria cases in many parts of South and South-east Asia, Pacific islands, Central and South America being attributed to P. vivax infections. The unique biology of P. vivax, including its ability to form latent hypnozoites that emerge months to years later to cause blood stage infections, early appearance of gametocytes before clinical symptoms are apparent and a shorter development cycle in the vector makes elimination of P. vivax using standard control tools difficult. The availability of an effective vaccine that provides protection and prevents transmission would be a valuable tool in efforts to eliminate P. vivax. Here, we review the latest developments related to P. vivax malaria vaccines and discuss the challenges as well as directions toward the goal of developing highly efficacious vaccines against P. vivax malaria.     

Azadirachta indica extract–artesunic acid combination produces an increased cure rate of Plasmodium berghei-infected mice

Context: Available artemisinin-combination therapies (ACTs) are expensive. Various traditional herbal remedies for malaria involve plants, proven scientifically to have antiplasmodial effects, e.g., Azadirachta indica A. Juss. (Meliaceae).

Objective: Combination of an artemisinin-based compound and a medicinal herb extract will provide an indigenous alternative/herb-based ACT.

Materials and methods: The in vivo schizontocidal activity of the crude aqueous extract of 100, 500, and 1000?mg/kg of A. indica fresh leaves (NCE) and 6, 15, and 20?mg/kg of artesunic acid were determined, alone and in combination, while keeping the dose of artesunic acid constant at 15?mg/kg, using the Peter’s 4-day suppressive test and Swiss albino mice. The ED₅₀ was calculated from the dose-response relationships. Percentage survival and cure were also determined.

Results: The average yield of two extractions of NCE was 8.33?±?1.67%. Combination of 1000?mg/kg of NCE and 15?mg/kg of artesunic acid, produced a significant reduction of parasitemia (96.87%), compared to 20?mg/kg of artesunic acid alone (68.14%). The combination had an ED₅₀ of 0.58?mg/kg while that of artesunic acid alone was 8.814?mg/kg. The combinations of NCE with artesunic acid produced a cure, although the artesunic acid did not produce a cure in 30?d.

Discussion: NCE increased the activity of artesunic acid in terms of reduction in parasitemia, and increased survival time and cure rate.

Conclusion: The combination of an artemisinin and aqueous extract of neem leaf is possible, providing a potentiated reduction of parasitemia, and increased cure rate.     

Novel therapies for the prevention of malaria

Background: Malaria continues to exact a huge toll on the health of residents of endemic countries. Thus, new approaches to prevention and treatment are needed. Objective: To provide an update on novel therapies for the prevention of malaria. Methods: Systematic MEDLINE search from 1956 to 2008 using the search term ‘malaria’ (with the subheadings ‘intermittent preventive treatment’, ‘mass drug administration’, ‘chemotherapy’, ‘artemisinin-based combination therapy’ and ‘home-based management of malaria’). Conclusions: Chemoprophylaxis is used as a short-term protective measure for non-immune visitors to malaria-endemic countries. However, in malaria-endemic areas, chemoprophylaxis has not been implemented widely because of concerns related to sustainability, cost-effectiveness, appropriate delivery systems and development of drug resistance. Intermittent preventive treatment, a novel approach to malaria control, has the potential to provide some of the benefits of sustained chemoprophylaxis without some of its drawbacks.     

Genetic diversity of VAR2CSA ID1-DBL2Xb in worldwide Plasmodium falciparum populations: Impact on vaccine design for placental malaria

In placental malaria (PM), sequestration of infected erythrocytes in the placenta is mediated by an interaction between VAR2CSA, a Plasmodium falciparum protein expressed on erythrocytes, and chondroitin sulfate A (CSA) on syncytiotrophoblasts. Recent works have identified ID1-DBL2Xb as the minimal CSA-binding region within VAR2CSA able to induce strong protective immunity, making it the leading candidate for the development of a vaccine against PM. Assessing the existence of population differences in the distribution of ID1-DBL2Xb polymorphisms is of paramount importance to determine whether geographic diversity must be considered when designing a candidate vaccine based on this fragment. In this study, we examined patterns of sequence variation of ID1-DBL2Xb in a large collection of P. falciparum field isolates (n = 247) from different malaria-endemic areas, including Africa (Benin, Senegal, Cameroon and Madagascar), Asia (Cambodia), Oceania (Papua New Guinea), and Latin America (Peru). Detection of variants and estimation of their allele frequencies were performed using next-generation sequencing of DNA pools. A considerable amount of variation was detected along the whole gene segment, suggesting that several allelic variants may need to be included in a candidate vaccine to achieve broad population coverage. However, most sequence variants were common and extensively shared among worldwide parasite populations, demonstrating long term persistence of those polymorphisms, probably maintained through balancing selection. Therefore, a vaccine mixture including such stable antigen variants will be putatively applicable and efficacious in all world regions where malaria occurs. Despite similarity in ID1-DBL2Xb allele repertoire across geographic areas, several peaks of strong population differentiation were observed at specific polymorphic loci, pointing out putative targets of humoral immunity subject to positive immune selection.     

2005-2011年青神县输入性疟疾个案调查

目的掌握青神县输入性疟疾流行现状,为制定疟疾防治策略提供科学依据。方法对2005-2011年青神县实验室确诊输入性疟疾病例的个案调查进行分析。结果 2005-2011年共有实验室确诊输入性疟疾病例65例,其中间日疟37例,恶性疟27例,混合感染1例。病例分布于青神8个乡镇,占全县乡镇的80%;除12月外其余各月均有发病;男女性别比为15.25∶1;年龄在17～46岁间;职业全为民工。感染地主要为云南、缅甸等疟疾高发区,病例为62例,占输入病例95.39%。结论青神县输入性疟疾均为外出打工的青壮年,以男性为主,病例感染地主要为云南、缅甸疟疾高发区。     

Comparison of functional assays used in the clinical development of a placental malaria vaccine

BackgroundMalaria in pregnancy is associated with significant morbidity in pregnant women and their offspring. Plasmodium falciparum infected erythrocytes (IE) express VAR2CSA that mediates binding to chondroitin sulphate A (CSA) in the placenta. Two VAR2CSA-based vaccines for placental malaria are in clinical development. The purpose of this study was to evaluate the robustness and comparability of binding inhibition assays used in the clinical development of placental malaria vaccines.MethodsThe ability of sera from animals immunised with different VAR2CSA constructs to inhibit IE binding to CSA was investigated in three in vitro assays using 96-well plates, petri dishes, capillary flow and an ex vivo placental perfusion assay.ResultsThe inter-assay variation was not uniform between assays and ranged from above ten-fold in the flow assay to two-fold in the perfusion assay. The intra-assay variation was highest in the petri dish assay. A positive correlation between IE binding avidity and the level of binding after antibody inhibition in the petri dish assay indicate that high avidity IE binding is more difficult to inhibit. The highest binding inhibition sensitivity was found in the 96-well and petri dish assays compared to the flow and perfusion assays where binding inhibition required higher antibody titers.ConclusionsThe inhibitory capacity of antibodies is not easily translated between assays and the high sensitivity of the 96-well and petri dish assays stresses the need for comparing serial dilutions of serum. Furthermore, IE binding avidity must be in the same range when comparing data from different days. There was an overall concordance in the capacity of antibody-mediated inhibition, when comparing the in vitro assays with the perfusion assay, which more closely represents in vivo conditions. Importantly the ID1-ID2a protein in a liposomal formulation, currently in a phase I trial, effectively induced antibodies that inhibited IE adhesion in placental tissue.     

When is a malaria immune complex not an immune complex?

Immune complexes (ICs) are believed to play an important role in malaria pathology, and an interesting article by Mibei et al. recently published by Parasite Immunology suggests that IgG4 and IgE are particularly important. However, researchers should be aware of potential pitfalls to current assays aimed at measuring plasma ICs and correlating these to deposition in tissues.     

Efficacy of oral single dose therapy with artemisinin-naphthoquine phosphate in uncomplicated falciparum malaria

All artemisinin-based combination therapies (ACTs), recommended by the World Health Organization, are 3-day regimens. A considerable level of non-compliance on ACTs has been reported from some countries. The study aimed to assess the therapeutic efficacy of single dose treatment with new generation ACT containing artemisinin plus naphthoquine. An oral single dose of eight tablets (400 mg of naphthoquine + 1000 mg artemisinin) of the combination drug was administered to adult uncomplicated falciparum malaria patients. Observations of fever, parasite clearance and reappearance, and other clinical manifestations were made on Days 0, 1, 2, 3, 7, 14, 21 and 28. Fifty-three adult falciparum positive cases, with fever or history of fever within the previous 24 h, were included in the final evaluation of the study. Mean fever clearance time, parasite clearance time were 18.2 ± 8.6 h and 34.6 ± 14.3 h, respectively. Adequate clinical and parasitological response was achieved in 52 cases, the rate being 98.1% (95% CI, 91.1-99.9). One patient was classified as late parasitological failure because of the reappearance of falciparum parasite on Day 14. The drug was well tolerated and no adverse reactions were detected in the patients. Since it is a single dose therapy, health workers can administer the drug as directly observed treatment.