The probability of a sequential Plasmodium vivax infection following asymptomatic Plasmodium falciparum and P. vivax infections in Myanmar, Vietnam, Cambodia, and Laos

Survey of malaria vectors on the Cambodia, Thailand and China-Laos Borders

BACKGROUND

Anopheles maculatus, Anopheles minimus and Anopheles dirus are the major vectors of malaria transmission in the Greater Mekong Subregion (GMS). The malaria burden in this region has decreased significantly in recent years as all GMS countries progress towards malaria elimination. It is necessary to investigate the Anopheles diversity and abundance status and assess the Plasmodium infection rates to understand the malaria transmission potential of these vector species in GMS countries to guide the development of up-to-date vector control strategies and interventions.

METHODS

A survey of mosquitoes was conducted in Stung Treng, Sainyabuli and Phongsaly Provinces on the Cambodia-Laos, Thailand-Laos and China-Laos borders, respectively. Mosquito collection was done by overnight trapping at sentinel sites in each province. After morphological identification, the 18S rRNA-based nested-PCR was performed to detect malaria parasites in the captured Anopheles mosquitoes.

RESULTS

A total of 18 965 mosquitoes comprising of 35 species of 2 subgenera (Subgenus Anopheles and Subgenus Cellia) and 4 tribes (Tribes Culicini, Aedini, Armigerini and Mansoniini) were captured. Tribe Culicini accounted for 85.66% of captures, followed by Subgenus Anopheles (8.15%). Anopheles sinensis dominated the Subgenus Anopheles by 99.81%. Plasmodium-infection was found in 25 out of the 1 683 individual or pooled samples of Anopheles. Among the 25 positive samples, 19, 5 and 1 were collected from Loum, Pangkhom and Siem Pang village, respectively. Eight Anopheles species were found infected with Plasmodium, i.e., An. sinensis, Anopheles kochi, Anopheles vagus, An. minimus, Anopheles annularis, Anopheles philippinensis, Anopheles tessellatus and An. dirus. The infection rates of Plasmodium falciparum, Plasmodium vivax and mixture of Plasmodium parasite species were 0.12% (2/1 683), 1.31% (22/1 683) and 0.06% (1/1 683), respectively.

CONCLUSIONS

Overall, this survey re-confirmed that multiple Anopheles species carry malaria parasites in the international border areas of the GMS countries. Anopheles sinensis dominated the Anopheles collections and as carriers of malaria parasites, therefore may play a significant role in malaria transmission. More extensive investigations of malaria vectors are required to reveal the detailed vector biology, ecology, behaviour, and genetics in GMS regions in order to assist with the planning and implementation of improved malaria control strategies.

Genetic Diversity and Selection of Plasmodium vivax Apical Membrane Antigen-1 in China-Myanmar Border of Yunnan Province, China, 2009-2016

Plasmodium vivax apical membrane antigen-1 (PvAMA-1) is an important vaccine candidate for vivax malaria. However, antigenic variation within PvAMA-1 is a major obstacle to the design of a global protective malaria vaccine. In this study, we analyzed the genetic polymorphism and selection of the PvAMA-1 gene from 152 P. vivax isolates from imported cases to China, collected in the China–Myanmar border (CMB) area in Yunnan Province (YP) during 2009–2011 (n = 71) and 2014–2016 (n = 81), in comparison with PvAMA-1 gene information from Myanmar (n = 73), collected from public data. The overall nucleotide diversity of the PvAMA-1 gene from the 152 YP isolates was 0.007 with 76 haplotypes identified (Hd = 0.958). Results from the population structure suggested three groups among the YP and Myanmar isolates with optimized clusters value of K = 7. In addition, YP (2014–2016) isolates generally lacked some K components that were commonly found in YP (2009–2011) and Myanmar. Meanwhile, PvAMA-1 domain I is found to be the dominant target of positive diversifying selection and most mutation loci were found in this domain. The mutation frequencies of D107N/A, R112K/T, K120R, E145A, E277K, and R438H in PvAMA-1 were more than 70% in the YP isolates. In conclusion, high genetic diversity and positive selection were found in the PvAMA-1 gene from YP isolates, which are significant findings for the design and development of PvAMA-1-based malaria vaccine.

The risk of Plasmodium vivax parasitaemia after P. falciparum malaria: An individual patient data meta-analysis from the WorldWide Antimalarial Resistance Network

BACKGROUND

There is a high risk of Plasmodium vivax parasitaemia following treatment of falciparum malaria. Our study aimed to quantify this risk and the associated determinants using an individual patient data meta-analysis in order to identify populations in which a policy of universal radical cure, combining artemisinin-based combination therapy (ACT) with a hypnozoitocidal antimalarial drug, would be beneficial.

METHODS AND FINDINGS

A systematic review of Medline, Embase, Web of Science, and the Cochrane Database of Systematic Reviews identified efficacy studies of uncomplicated falciparum malaria treated with ACT that were undertaken in regions coendemic for P. vivax between 1 January 1960 and 5 January 2018. Data from eligible studies were pooled using standardised methodology. The risk of P. vivax parasitaemia at days 42 and 63 and associated risk factors were investigated by multivariable Cox regression analyses. Study quality was assessed using a tool developed by the Joanna Briggs Institute. The study was registered in the International Prospective Register of Systematic Reviews (PROSPERO: CRD42018097400). In total, 42 studies enrolling 15,341 patients were included in the analysis, including 30 randomised controlled trials and 12 cohort studies. Overall, 14,146 (92.2%) patients had P. falciparum monoinfection and 1,195 (7.8%) mixed infection with P. falciparum and P. vivax. The median age was 17.0 years (interquartile range [IQR] = 9.0-29.0 years; range = 0-80 years), with 1,584 (10.3%) patients younger than 5 years. 2,711 (17.7%) patients were treated with artemether-lumefantrine (AL, 13 studies), 651 (4.2%) with artesunate-amodiaquine (AA, 6 studies), 7,340 (47.8%) with artesunate-mefloquine (AM, 25 studies), and 4,639 (30.2%) with dihydroartemisinin-piperaquine (DP, 16 studies). 14,537 patients (94.8%) were enrolled from the Asia-Pacific region, 684 (4.5%) from the Americas, and 120 (0.8%) from Africa. At day 42, the cumulative risk of vivax parasitaemia following treatment of P. falciparum was 31.1% (95% CI 28.9-33.4) after AL, 14.1% (95% CI 10.8-18.3) after AA, 7.4% (95% CI 6.7-8.1) after AM, and 4.5% (95% CI 3.9-5.3) after DP. By day 63, the risks had risen to 39.9% (95% CI 36.6-43.3), 42.4% (95% CI 34.7-51.2), 22.8% (95% CI 21.2-24.4), and 12.8% (95% CI 11.4-14.5), respectively. In multivariable analyses, the highest rate of P. vivax parasitaemia over 42 days of follow-up was in patients residing in areas of short relapse periodicity (adjusted hazard ratio [AHR] = 6.2, 95% CI 2.0-19.5; p = 0.002); patients treated with AL (AHR = 6.2, 95% CI 4.6-8.5; p < 0.001), AA (AHR = 2.3, 95% CI 1.4-3.7; p = 0.001), or AM (AHR = 1.4, 95% CI 1.0-1.9; p = 0.028) compared with DP; and patients who did not clear their initial parasitaemia within 2 days (AHR = 1.8, 95% CI 1.4-2.3; p < 0.001). The analysis was limited by heterogeneity between study populations and lack of data from very low transmission settings. Study quality was high.

CONCLUSIONS

In this meta-analysis, we found a high risk of P. vivax parasitaemia after treatment of P. falciparum malaria that varied significantly between studies. These P. vivax infections are likely attributable to relapses that could be prevented with radical cure including a hypnozoitocidal agent; however, the benefits of such a novel strategy will vary considerably between geographical areas.

Plasmodium vivax in the Era of the Shrinking P. falciparum Map

Plasmodium vivax is an important cause of malaria, associated with a significant public health burden. Whilst enhanced malaria-control activities have successfully reduced the incidence of Plasmodium falciparum malaria in many areas, there has been a consistent increase in the proportion of malaria due to P. vivax in regions where both parasites coexist. This article reviews the epidemiology and biology of P. vivax, how the parasite differs from P. falciparum, and the key features that render it more difficult to control and eliminate. Since transmission of the parasite is driven largely by relapses from dormant liver stages, its timely elimination will require widespread access to safe and effective radical cure.