Genetic Diversity of Circumsporozoite Surface Protein of Plasmodium vivax from the Central Highlands, Vietnam.

The circumsporozoite surface protein of Plasmodium vivax (PvCSP) plays a critical role in parasite biology. It has been extensively studied as a leading vivax-malaria-vaccine candidate. In this study, the genetic polymorphism and natural selection of pvcsp in P. vivax isolates collected from the Central Highlands, Vietnam were analyzed to understand the genetic structure of the parasite circulating in the endemic area and to provide baseline information for effective vaccine development based on the protein. Only two major alleles, VK210 and VK247, were detected in Vietnamese pvcsp, with VK247 being the predominant one. The N-terminal and C-terminal regions of Vietnamese VK210 and VK247 variants showed a low genetic diversity. Amino acid substitutions, insertions of a single amino acid or octapeptide (ANKKAEDA in VK210 and ANKKAGDA in VK247), and tetrapeptide repeat motifs (GGNA) were the main factors generating genetic diversity in the two regions of the Vietnamese VK210 and VK247 variants. Interestingly, these two regions of Vietnamese pvcsp displayed a unique natural selection pressure distinct from global pvcsp, particularly with the neighboring Southeast Asian pvcsp population. Meanwhile, the central repeat region (CRR) in both the VK210 and VK247 variants showed a high degree of polymorphic characters, caused by varying numbers, types, and combinations of peptide repeat motifs (PRMs) in Vietnamese pvcsp. Highly complicated polymorphic patterns of the CRR were also detected in global pvcsp. These results expand our understanding of the genetic structure of Vietnamese pvcsp and the population dynamics of P. vivax in the Central Highlands, Vietnam.

Genetic Polymorphism and Natural Selection of Apical Membrane Antigen-1 in Plasmodium falciparum Isolates from Vietnam.

Apical membrane antigen-1 of Plasmodium falciparum (PfAMA-1) is a leading malaria vaccine candidate antigen. However, the genetic diversity of pfama-1 and associated antigenic variation in global P. falciparum field isolates are major hurdles to the design of an efficacious vaccine formulated with this antigen. Here, we analyzed the genetic structure and the natural selection of pfama-1 in the P. falciparum population of Vietnam. A total of 37 distinct haplotypes were found in 131 P. falciparum Vietnamese isolates. Most amino acid changes detected in Vietnamese pfama-1 were localized in the ectodomain, domains I, II, and III. Overall patterns of major amino acid changes in Vietnamese pfama-1 were similar to those of global pfama-1, but the frequencies of the amino acid changes slightly differed by country. Novel amino acid changes were also identified in Vietnamese pfama-1. Vietnamese pfama-1 revealed relatively lower genetic diversity than currently analyzed pfama-1 in other geographical regions, and suggested a distinct genetic differentiation pattern. Evidence for natural selection was detected in Vietnamese pfama-1, but it showed purifying selection unlike the global pfama-1 analyzed so far. Recombination events were also found in Vietnamese pfama-1. Major amino acid changes that were commonly identified in global pfama-1 were mainly localized to predicted B-cell epitopes, RBC-binding sites, and IUR regions. These results provide important information for understanding the genetic nature of the Vietnamese pfama-1 population, and have significant implications for the design of a vaccine based on PfAMA-1.

Cross-sectional survey of asymptomatic malaria in Dak Nong province in the Central Highlands of Vietnam for the malaria elimination roadmap.

Asymptomatic parasite carriers represent a "silent" infective reservoir for malaria transmission and contributes to malaria persistence. However, limited data are available on asymptomatic malaria in Vietnam. Between November 2018 and March 2019, we conducted a malaria epidemiological survey of asymptomatic people (children ≥ 10 years old and adults ≥18 years old, n = 2,809) residing in three communes in Tuy Duc district, Dak Nong province in the Central Highlands of Vietnam. Based on the national stratification of malaria risk, Dak Buk So, Dak Ngo and Quang Truc communes were classified by the National Malaria Control Programme as low, moderate and high malaria endemic areas, respectively. Using participants' finger prick blood samples, malaria parasites were detected by one-step reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The median age (Interquartile Range) for adults and children were 35 years (26-50) and 12 years (11-14), respectively. The prevalence of asymptomatic malaria was 1.7% (22/1,328), 3.5% (31/890) and 12.2% (72/591) for participants from Dak Buk So, Dak Ngo and Quang Truc, respectively. The prevalence of asymptomatic malaria was lower in children compared to adults: 2.6% (9/352) versus 4.7% (116/2,457) (Odds Ratio 0.53, 95% Confidence Interval 0.28 to1.02). Ownership of long-lasting insecticide-treated bed nets and hammocks was 97.1%, 99.0% and 94.7% for participants in Dak Buk So, Dak Ngo and Quang Truc, respectively, however, only 66.0%, 57.3% and 42.8% of the participants reported using bed nets every night. Of the several risk factors examined, going to the forest two weeks prior to enrolment into the study and sleeping in the forest had a significant association with participants being infected with asymptomatic malaria in Quang Truc, but not in the other two communes. Knowledge of the prevalence and distribution of asymptomatic malaria will help design and evaluate future intervention strategies for malaria elimination in Vietnam.

Molecular surveillance of malaria in the Central Highlands, Vietnam.

Vietnam achieved outstanding success against malaria in the last few decades. The mortality and morbidity of malaria in Vietnam have decreased remarkably in recent years, but malaria is still a major public health concern in the country, particularly in the Central Highlands region. In this study, molecular analyses of malaria parasites in the Central Highlands were performed to understand the population structure and genetic diversity of the parasites circulating in the region. Plasmodium falciparum (68.7%) and P. vivax (27.4%) along with mixed infections with P. falciparum/P. vivax (3.9%) were detected in 230 blood samples from patients with malaria. Allele-specific nested-polymerase chain reaction (PCR) or PCR-restriction fragment length polymorphism (PCR-RFLP) analyses of pfmsp-1, pfama-1, pvcsp, and pvmsp-1 revealed complex genetic makeup in P. falciparum and P. vivax populations of Vietnam. Substantial multiplicity of infection (MOI) was also identified, suggesting significant genetic diversity and polymorphism of P. falciparum and P. vivax populations in the Central Highlands of Vietnam. These results provide fundamental insight into the current patterns of dispersion and genetic nature of malaria parasites as well as for the development of malaria elimination strategies in the endemic region.

Multidrug-Resistant Plasmodium falciparum Parasites in the Central Highlands of Vietnam Jeopardize Malaria Control and Elimination Strategies.

Plasmodium falciparum resistance to dihydroartemisinin-piperaquine has spread through the Greater Mekong Subregion to southwestern Vietnam. In 2018 to 2019, we collected 127 P. falciparum isolates from Dak Nong (36), Dak Lak (55), Gia Lai (13), and Kon Tum (23) provinces in Vietnam's Central Highlands and found parasites bearing the Pfkelch13 C580Y mutation and multiple plasmepsin 2/3 genes (mean prevalence, 17.9%; range, 4.3% to 27.8%), conferring resistance to dihydroartemisinin-piperaquine. This information is important for drug policy decisions in Vietnam.