Plasmodium vivax and Plasmodium falciparum infection dynamics: re-infections, recrudescences and relapses

Modeling and investigating malaria P. Falciparum and P. Vivax infections: Application to Djibouti data

Malaria is an infectious and communicable disease, caused by one or more species of Plasmodium parasites. There are five species of parasites responsible for malaria in humans, of which two, Plasmodium Falciparum and Plasmodium Vivax, are the most dangerous. In Djibouti, the two species of Plasmodium are present in different proportions in the infected population: 77% of P. Falciparum and 33% of P. Vivax. In this study we present a new mathematical model describing the temporal dynamics of Plasmodium Falciparum and Plasmodium Vivax co-infection. We focus briefly on the well posedness of this model and on the calculation of the basic reproductive numbers for the infections with each Plasmodium species that help us understand the long-term dynamics of this model (i.e., existence and stability of various eqiuilibria). Then we use computational approaches to: (a) identify model parameters using real data on malaria infections in Djibouti; (b) illustrate the influence of different estimated parameters on the basic reproduction numbers; (c) perform global sensitivity and uncertainty analysis for the impact of various model parameters on the transient dynamics of infectious mosquitoes and infected humans, for infections with each of the Plasmodium species. The originality of this research stems from employing the FAST method and the LHS method to identify the key factors influencing the progression of the disease within the population of Djibouti. In addition, sensitivity analysis identified the most influential parameter for Falciparium and Vivax reproduction rates. Finally, the uncertainty analysis enabled us to understand the variability of certain parameters on the infected compartments.

Elevated Inflammation Associated with Markers of Neutrophil Function and Gastrointestinal Disruption in Pilot Study of Plasmodium fragile Co-Infection of ART-Treated SIVmac239+ Rhesus Macaques

Human immunodeficiency virus (HIV) and malaria, caused by infection with Plasmodium spp., are endemic in similar geographical locations. As a result, there is high potential for HIV/Plasmodium co-infection, which increases the pathology of both diseases. However, the immunological mechanisms underlying the exacerbated disease pathology observed in co-infected individuals are poorly understood. Moreover, there is limited data available on the impact of Plasmodium co-infection on antiretroviral (ART)-treated HIV infection. Here, we used the rhesus macaque (RM) model to conduct a pilot study to establish a model of Plasmodium fragile co-infection during ART-treated simian immunodeficiency virus (SIV) infection, and to begin to characterize the immunopathogenic effect of co-infection in the context of ART. We observed that P. fragile co-infection resulted in parasitemia and anemia, as well as persistently detectable viral loads (VLs) and decreased absolute CD4+ T-cell counts despite daily ART treatment. Notably, P. fragile co-infection was associated with increased levels of inflammatory cytokines, including monocyte chemoattractant protein 1 (MCP-1). P. fragile co-infection was also associated with increased levels of neutrophil elastase, a plasma marker of neutrophil extracellular trap (NET) formation, but significant decreases in markers of neutrophil degranulation, potentially indicating a shift in the neutrophil functionality during co-infection. Finally, we characterized the levels of plasma markers of gastrointestinal (GI) barrier permeability and microbial translocation and observed significant correlations between indicators of GI dysfunction, clinical markers of SIV and Plasmodium infection, and neutrophil frequency and function. Taken together, these pilot data verify the utility of using the RM model to examine ART-treated SIV/P. fragile co-infection, and indicate that neutrophil-driven inflammation and GI dysfunction may underlie heightened SIV/P. fragile co-infection pathogenesis.

Dynamics of asymptomatic Plasmodium falciparum and Plasmodium vivax infections and their infectiousness to mosquitoes in a low transmission setting of Ethiopia: a longitudinal observational study

OBJECTIVE

A 15-month longitudinal study was conducted to determine the duration and infectivity of asymptomatic qPCR-detected Plasmodium falciparum and Plasmodium vivax infections in Ethiopia.

METHOD

Total parasite and gametocyte kinetics were determined by molecular methods; infectivity to Anopheles arabiensis mosquitoes by repeated membrane feeding assays. Infectivity results were contrasted with passively recruited symptomatic malaria cases.

RESULTS

For P. falciparum and P. vivax infections detected at enrolment, median durations of infection were 37 days (95% confidence interval [CI], 15-93) and 60 days (95% CI, 18-213), respectively. P. falciparum and P. vivax parasite densities declined over the course of infections. From 47 feeding assays on 22 asymptomatic P. falciparum infections, 6.4% (3/47) were infectious and these infected 1.8% (29/1579) of mosquitoes. No transmission was observed in feeding assays on asymptomatic P. vivax mono-infections (0/56); one mixed-species infection was highly infectious. Among the symptomatic cases, 4.3% (2/47) of P. falciparum and 73.3% (53/86) of P. vivax patients were infectious to mosquitoes.

CONCLUSION

The majority of asymptomatic infections were of short duration and low parasite density. Only a minority of asymptomatic individuals were infectious to mosquitoes. This contrasts with earlier findings and is plausibly due to the low parasite densities in this population.

A hybrid transmission model for Plasmodium vivax accounting for superinfection, immunity and the hypnozoite reservoir

Malaria is a vector-borne disease that exacts a grave toll in the Global South. The epidemiology of Plasmodium vivax, the most geographically expansive agent of human malaria, is characterised by the accrual of a reservoir of dormant parasites known as hypnozoites. Relapses, arising from hypnozoite activation events, comprise the majority of the blood-stage infection burden, with implications for the acquisition of immunity and the distribution of superinfection. Here, we construct a novel model for the transmission of P. vivax that concurrently accounts for the accrual of the hypnozoite reservoir, (blood-stage) superinfection and the acquisition of immunity. We begin by using an infinite-server queueing network model to characterise the within-host dynamics as a function of mosquito-to-human transmission intensity, extending our previous model to capture a discretised immunity level. To model transmission-blocking and antidisease immunity, we allow for geometric decay in the respective probabilities of successful human-to-mosquito transmission and symptomatic blood-stage infection as a function of this immunity level. Under a hybrid approximation-whereby probabilistic within-host distributions are cast as expected population-level proportions-we couple host and vector dynamics to recover a deterministic compartmental model in line with Ross-Macdonald theory. We then perform a steady-state analysis for this compartmental model, informed by the (analytic) distributions derived at the within-host level. To characterise transient dynamics, we derive a reduced system of integrodifferential equations, likewise informed by our within-host queueing network, allowing us to recover population-level distributions for various quantities of epidemiological interest. In capturing the interplay between hypnozoite accrual, superinfection and acquired immunity-and providing, to the best of our knowledge, the most complete population-level distributions for a range of epidemiological values-our model provides insights into important, but poorly understood, epidemiological features of P. vivax.

Persistent and multiclonal malaria parasite dynamics despite extended artemether-lumefantrine treatment in children

Standard diagnostics used in longitudinal antimalarial studies are unable to characterize the complexity of submicroscopic parasite dynamics, particularly in high transmission settings. We use molecular markers and amplicon sequencing to characterize post-treatment stage-specific malaria parasite dynamics during a 42 day randomized trial of 3- versus 5 day artemether-lumefantrine in 303 children with and without HIV (ClinicalTrials.gov number NCT03453840). The prevalence of parasite-derived 18S rRNA is >70% in children throughout follow-up, and the ring-stage marker SBP1 is detectable in over 15% of children on day 14 despite effective treatment. We find that the extended regimen significantly lowers the risk of recurrent ring-stage parasitemia compared to the standard 3 day regimen, and that higher day 7 lumefantrine concentrations decrease the probability of ring-stage parasites in the early post-treatment period. Longitudinal amplicon sequencing reveals remarkably dynamic patterns of multiclonal infections that include new and persistent clones in both the early post-treatment and later time periods. Our data indicate that post-treatment parasite dynamics are highly complex despite efficacious therapy, findings that will inform strategies to optimize regimens in the face of emerging partial artemisinin resistance in Africa.

Time lag effect on malaria transmission dynamics in an Amazonian Colombian municipality and importance for early warning systems

Malaria remains a significant public health problem worldwide, particularly in low-income regions with limited access to healthcare. Despite the use of antimalarial drugs, transmission remains an issue in Colombia, especially among indigenous populations in remote areas. In this study, we used an SIR Ross MacDonald model that considered land use change, temperature, and precipitation to analyze eco epidemiological parameters and the impact of time lags on malaria transmission in La Pedrera-Amazonas municipality. We found changes in land use between 2007 and 2020, with increases in forested areas, urban infrastructure and water edges resulting in a constant increase in mosquito carrying capacity. Temperature and precipitation variables exhibited a fluctuating pattern that corresponded to rainy and dry seasons, respectively and a marked influence of the El Niño climatic phenomenon. Our findings suggest that elevated precipitation and temperature increase malaria infection risk in the following 2 months. The risk is influenced by the secondary vegetation and urban infrastructure near primary forest formation or water body edges. These results may help public health officials and policymakers develop effective malaria control strategies by monitoring precipitation, temperature, and land use variables to flag high-risk areas and critical periods, considering the time lag effect.

Malaria Risk Drivers in the Brazilian Amazon: Land Use-Land Cover Interactions and Biological Diversity

Malaria is a prevalent disease in several tropical and subtropical regions, including Brazil, where it remains a significant public health concern. Even though there have been substantial efforts to decrease the number of cases, the reoccurrence of epidemics in regions that have been free of cases for many years presents a significant challenge. Due to the multifaceted factors that influence the spread of malaria, influencing malaria risk factors were analyzed through regional outbreak cluster analysis and spatio-temporal models in the Brazilian Amazon, incorporating climate, land use/cover interactions, species richness, and number of endemic birds and amphibians. Results showed that high amphibian and bird richness and endemism correlated with a reduction in malaria risk. The presence of forest had a risk-increasing effect, but it depended on its juxtaposition with anthropic land uses. Biodiversity and landscape composition, rather than forest formation presence alone, modulated malaria risk in the period. Areas with low endemic species diversity and high human activity, predominantly anthropogenic landscapes, posed high malaria risk. This study underscores the importance of considering the broader ecological context in malaria control efforts.

Genetic diversity of Plasmodium vivax isolates from pregnant women in the Western Brazilian Amazon: a prospective cohort study

Background

Each year, 92 million pregnant women are at risk of contracting malaria during pregnancy, with the underestimation of the mortality and morbidity burden associated with Plasmodium vivax. During pregnancy, P. vivax infection is associated with low birth weight, maternal anaemia, premature delivery, and stillbirth. In the State of Acre (Brazil), high transmission leaves pregnant women at greater risk of contracting malaria and having a greater number of recurrences. The study of genetic diversity and the association of haplotypes with adverse pregnancy effects is of great importance for the control of the disease. Here we investigate the genetic diversity of P. vivax parasites infecting pregnant women across their pregnancies.

Methods

P. vivax DNA was extracted from 330 samples from 177 women followed during pregnancy, collected in the State of Acre, Brazil. All samples were negative for Plasmodium falciparum DNA. Sequence data for the Pvmsp1 gene was analysed alongside data from six microsatellite (MS) markers. Allelic frequencies, haplotype frequencies, expected heterozygosity (HE) were calculated. Whole genome sequencing (WGS) was conducted on four samples from pregnant women and phylogenetic analysis performed with other samples from South American regions.

Findings

Initially, the pregnant women were stratified into two groups-1 recurrence and 2 or more recurrences-in which no differences were observed in clinical gestational outcomes or in placental histological changes between the two groups. Then we evaluated the parasites genetically. An average of 18.5 distinct alleles were found at each of the MS loci, and the HE calculated for each marker indicates a high genetic diversity occurring within the population. There was a high percentage of polyclonal infections (61.7%, 108/175), and one haplotype (H1) occurred frequently (20%), with only 9 of the haplotypes appearing in more than one patient.

Interpretation

Most pregnant women had polyclonal infections that could be the result of relapses and/or re-infections. The high percentage of H1 parasites, along with the low frequency of many other haplotypes are suggestive of a clonal expansion. Phylogenetic analysis shows that P. vivax population within pregnant women clustered with other Brazilian samples in the region.

Funding

FAPESP and CNPq - Brazil.

Dcifer: an IBD-based method to calculate genetic distance between polyclonal infections

An essential step toward reconstructing pathogen transmission and answering epidemiologically relevant questions from genomic data is obtaining pairwise genetic distance between infections. For recombining organisms such as malaria parasites, relatedness measures quantifying recent shared ancestry would provide a meaningful distance, suggesting methods based on identity by descent (IBD). While the concept of relatedness and consequently an IBD approach is fairly straightforward for individual parasites, the distance between polyclonal infections, which are prevalent in malaria, presents specific challenges, and awaits a general solution that could be applied to infections of any clonality and accommodate multiallelic (e.g. microsatellite or microhaplotype) and biallelic [single nucleotide polymorphism (SNP)] data. Filling this methodological gap, we present Dcifer (Distance for complex infections: fast estimation of relatedness), a method for calculating genetic distance between polyclonal infections, which is designed for unphased data, explicitly accounts for population allele frequencies and complexity of infection, and provides reliable inference. Dcifer’s IBD-based framework allows us to define model parameters that represent interhost relatedness and to propose corresponding estimators with attractive statistical properties. By using combinatorics to account for unobserved phased haplotypes, Dcifer is able to quickly process large datasets and estimate pairwise relatedness along with measures of uncertainty. We show that Dcifer delivers accurate and interpretable results and detects related infections with statistical power that is 2–4 times greater than that of approaches based on identity by state. Applications to real data indicate that relatedness structure aligns with geographic locations. Dcifer is implemented in a comprehensive publicly available software package.

Prolonged fever and exaggerated hypercoagulopathy in malaria vivax relapse and COVID-19 co-infection: a case report

BACKGROUND

Coronavirus disease 2019 (COVID-19) often causes atypical clinical manifestations similar to other infectious diseases. In malaria-endemic areas, the pandemic situation will very likely result in co-infection of COVID-19 and malaria, although reports to date are still few. Meanwhile, this disease will be challenging to diagnose in areas with low malaria prevalence because the symptoms closely resemble COVID-19.

CASE PRESENTATION

A 23-year-old male patient presented to the hospital with fever, anosmia, headache, and nausea 1 week before. He was diagnosed with COVID-19 and treated for approximately 10 days, then discharged to continue self-quarantine at home. 2 weeks later, he returned to the hospital with a fever raised intermittently every 2 days and marked by a chilling-fever-sweating cycle. A laboratory test for malaria and a nasopharyngeal swab for SARS CoV-2 PCR were conducted, confirming both diagnoses. The laboratory examination showed markedly elevated D-dimer. He was treated with dihydroartemisinin-piperaquine (DHP) 4 tablets per day for 3 days and primaquine 2 tablets per day for 14 days according to Indonesian National Anti-malarial Treatment Guidelines. After 6 days of treatment, the patient had no complaints, and the results of laboratory tests had improved. This report describes the key points in considering the differential diagnosis and prompt treatment of malaria infection during the pandemic of COVID-19 in an endemic country to prevent the worse clinical outcomes. COVID-19 and malaria may also cause a hypercoagulable state, so a co-infection of those diseases may impact the prognosis of the disease.

CONCLUSION

This case report shows that considering the possibility of a co-infection in a COVID-19 patient who presents with fever can prevent delayed treatment that can worsen the disease outcome. Paying more attention to a history of travel to malaria-endemic areas, a history of previous malaria infection, and exploring anamnesis regarding the fever patterns in patients are important points in making a differential diagnosis of malaria infection during the COVID-19 pandemic.

Effects of CYP2D6 genotypes on Plasmodium vivax recurrence after primaquine treatment: A meta-analysis

OBJECTIVES

To elucidate the relationship between CYP2D6 polymorphisms and Plasmodium vivax recurrence in patients receiving primaquine-based treatment through systematic review and meta-analysis.

METHODS

We searched the PubMed, EMBASE, Cochrane Library, and Web of Science databases for eligible studies published up to August of 2021. We included studies investigating the associations between CYP2D6 polymorphisms and P. vivax recurrence. We evaluated the pooled odds ratio (OR) and 95% confidence interval (CI).

RESULTS

Data from nine studies, including 970 patients, were analyzed. We found that CYP2D6 poor metabolizers (PMs), intermediate metabolizers (IMs), or normal metabolizers slow (NM-Ss) were associated with a 1.8-fold (95% CI, 1.34-2.45; P = 0.0001) higher recurrence of P. vivax than normal metabolizers fast (NM-Fs), extensive metabolizers (EMs), or ultrarapid metabolizer (UMs). Subgroup analysis showed that studies on both Brazilian and Southeast or East Asian individuals had similar results to the main results. Sensitivity analysis by sequentially excluding individual studies also showed robust results (OR range: 1.63-2.01).

CONCLUSIONS

This meta-analysis confirmed that CYP2D6 PMs, IMs, or NM-Ss increased the risk of P. vivax recurrence compared to NM-Fs, EMs, or UMs. The results of this study could be used to predict P. vivax recurrence and suggest CYP2D6 genotype-based primaquine dosing.

qPCR in a suitcase for rapid Plasmodium falciparum and Plasmodium vivax surveillance in Ethiopia

Many Plasmodium spp. infections, both in clinical and asymptomatic patients, are below the limit of detection of light microscopy or rapid diagnostic test (RDT). Molecular diagnosis by qPCR can be valuable for surveillance, but is often hampered by absence of laboratory capacity in endemic countries. To overcome this limitation, we optimized and tested a mobile qPCR laboratory for molecular diagnosis in Ziway, Ethiopia, where transmission intensity is low. Protocols were optimized to achieve high throughput and minimize costs and weight for easy transport. 899 samples from febrile patients and 1021 samples from asymptomatic individuals were screened by local microscopy, RDT, and qPCR within a period of six weeks. 34/52 clinical Plasmodium falciparum infections were missed by microscopy and RDT. Only 4 asymptomatic infections were detected. No hrp2 deletions were observed among 25 samples typed, but 19/24 samples carried hrp3 deletions. The majority (25/41) of Plasmodium vivax infections (1371 samples screened) were found among asymptomatic individuals. All asymptomatic P. vivax infections were negative by microscopy and RDT. In conclusion, the mobile laboratory described here can identify hidden parasite reservoirs within a short period of time, and thus inform malaria control activities.

Bayesian spatio-temporal distributed lag modeling for delayed climatic effects on sparse malaria incidence data

Background

In many areas of the Greater Mekong Subregion (GMS), malaria endemic regions have shrunk to patches of predominantly low-transmission. With a regional goal of elimination by 2030, it is important to use appropriate methods to analyze and predict trends in incidence in these remaining transmission foci to inform planning efforts. Climatic variables have been associated with malaria incidence to varying degrees across the globe but the relationship is less clear in the GMS and standard methodologies may not be appropriate to account for the lag between climate and incidence and for locations with low numbers of cases.

Methods

In this study, a methodology was developed to estimate the spatio-temporal lag effect of climatic factors on malaria incidence in Thailand within a Bayesian framework. A simulation was conducted based on ground truth of lagged effect curves representing the delayed relation with sparse malaria cases as seen in our study population. A case study to estimate the delayed effect of environmental variables was used with malaria incidence at a fine geographic scale of sub-districts in a western province of Thailand.

Results

From the simulation study, the model assumptions which accommodated both delayed effects and excessive zeros appeared to have the best overall performance across evaluation metrics and scenarios. The case study demonstrated lagged climatic effect estimation of the proposed modeling with real data. The models appeared to be useful to estimate the shape of association with malaria incidence.

Conclusions

A new method to estimate the spatiotemporal effect of climate on malaria trends in low transmission settings is presented. The developed methodology has potential to improve understanding and estimation of past and future trends in malaria incidence. With further development, this could assist policy makers with decisions on how to more effectively distribute resources and plan strategies for malaria elimination.

Application of 23 Novel Serological Markers for Identifying Recent Exposure to Plasmodium vivax Parasites in an Endemic Population of Western Thailand

Thailand is aiming for malaria elimination by the year 2030. However, the high proportion of asymptomatic infections and the presence of the hidden hypnozoite stage of Plasmodium vivax are impeding these efforts. We hypothesized that a validated surveillance tool utilizing serological markers of recent exposure to P. vivax infection could help to identify areas of ongoing transmission. The objective of this exploratory study was to assess the ability of P. vivax serological exposure markers to detect residual transmission "hot-spots" in Western Thailand. Total IgG levels were measured against a panel of 23 candidate P. vivax serological exposure markers using a multiplexed bead-based assay. A total of 4,255 plasma samples from a cross-sectional survey conducted in 2012 of endemic areas in the Kanchanaburi and Ratchaburi provinces were assayed. We compared IgG levels with multiple epidemiological factors that are associated with an increased risk of P. vivax infection in Thailand, including age, gender, and spatial location, as well as Plasmodium infection status itself. IgG levels to all proteins were significantly higher in the presence of a P. vivax infection (n = 144) (T-test, p < 0.0001). Overall seropositivity rates varied from 2.5% (PVX_097625, merozoite surface protein 8) to 16.8% (PVX_082670, merozoite surface protein 7), with 43% of individuals seropositive to at least 1 protein. Higher IgG levels were associated with older age (>18 years, p < 0.05) and males (17/23 proteins, p < 0.05), supporting the paradigm that men have a higher risk of infection than females in this setting. We used a Random Forests algorithm to predict which individuals had exposure to P. vivax parasites in the last 9-months, based on their IgG antibody levels to a panel of eight previously validated P. vivax proteins. Spatial clustering was observed at the village and regional level, with a moderate correlation between PCR prevalence and sero-prevalence as predicted by the algorithm. Our data provides proof-of-concept for application of such surrogate markers as evidence of recent exposure in low transmission areas. These data can be used to better identify geographical areas with asymptomatic infection burdens that can be targeted in elimination campaigns.

Prospective Clinical and Molecular Evaluation of Potential Plasmodium ovale curtisi and wallikeri Relapses in a High-transmission Setting

Background

Plasmodium ovale curtisi and wallikeri are perceived as relapsing malarial parasites. Contrary to Plasmodium vivax, direct evidence for this hypothesis is scarce. The aim of this prospective study was to characterize the reappearance patterns of ovale parasites.

Methods

P. ovale spp. infected patients were treated with artemether-lumefantrine and followed biweekly for up to 1 year for the detection of reappearing parasitemia. Molecular analysis of reappearing isolates was performed to identify homologous isolates by genotyping and to define cases of relapse following predefined criteria.

Results

At inclusion, 26 participants were positive for P. ovale curtisi and/or P. ovale wallikeri. The median duration of follow-up was 35 weeks. Reappearance of the same P. ovale species was observed in 46% of participants; 61% of P. ovale curtisi and 19% of P. ovale wallikeri infection-free intervals were estimated to end with reappearance by week 32. Based on the predefined criteria, 23% of participants were identified with 1 or 2 relapses, all induced by P. ovale curtisi.

Conclusion

These findings are in line with the currently accepted relapse theory inasmuch as the reappearance of P. ovale curtisi strains following initial blood clearance was conclusively demonstrated. Interestingly, no relapse of P. ovale wallikeri was observed.

Plasmodium malariae and Plasmodium ovale infections and their association with common red blood cell polymorphisms in a highly endemic area of Uganda

BACKGROUND

Plasmodium ovale and Plasmodium malariae infections are scarcely studied in sub-Saharan Africa, where the Plasmodium falciparum species predominates. The objective of this study is to investigate the prevalence of P. ovale and P. malariae infections and their relationship with common red blood cell polymorphisms in a cohort of 509 individuals from Uganda.

METHODS

Three cross-sectional surveys were conducted in individuals of 1-10 and >20 y of age from the Apac district at baseline and 6 and 16 weeks after drug treatment. Malaria infections were assessed by polymerase chain reaction and genotyping was performed for the sickle-cell allele, α-thalassaemia and glucose-6-phosphate dehydrogenase.

RESULTS

At baseline, the prevalence of infection was 7.5%, 12.6% and 57.4% for P. ovale, P. malariae and P. falciparum species, respectively. Co-infections were present in 14.1% of individuals, all including P. falciparum parasites. In children 1-5 y of age, the prevalence of P. ovale mono-infections increased significantly from 1.7% to 7.3% over time (p=0.004) while the prevalence of P. malariae and P. falciparum infections declined significantly during this study. After adjusting for confounding and multiple testing, only α-thalassaemia had a statistically significant increase in the odds of P. falciparum infections (odds ratio 1.93 [95% confidence interval 1.26 to 2.94]).

CONCLUSIONS

Common red blood cell polymorphisms do not show strong effects on mild Plasmodium infections in this Ugandan population. To understand the extent of this result, similar studies should be carried out in other populations using larger cohorts.

Malaria cross-sectional surveys identified asymptomatic infections of Plasmodium falciparum, Plasmodium vivax and Plasmodium knowlesi in Surat Thani, a southern province of Thailand

OBJECTIVES

Malaria cross-sectional surveys are rarely conducted in very low transmission settings. This study aimed to determine the prevalence and risk factors of Plasmodium infection in a near-elimination setting in southern Thailand.

METHODS

Two cross-sectional surveys were conducted in areas of active transmission in the Surat Thani province of Thailand in January and May 2019. PCR was used to detect Plasmodium infection.

RESULTS

The prevalence of Plasmodium blood infection was 0.45% and 0.61% in January and May 2019, respectively. The major parasite species was Plasmodium falciparum in January and Plasmodium vivax in May. Unexpectedly, Plasmodium knowlesi infections were also detected. Most infections, including those of Plasmodium knowlesi, were asymptomatic. Being male and staying outdoors at night-time were the only significant identified risk factors. Of people infected in January 28.0% were positive in May for the same parasite species, suggesting persistent asymptomatic infections.

CONCLUSIONS

Despite the very low incidence rate in Surat Thani, most malaria infections were asymptomatic. Outdoor mosquito biting at night-time is likely an important mode of malaria transmission. Unexpectedly, asymptomatic Plasmodium knowlesi infection was found, confirming previous reports of such infection in mainland Southeast Asia.

Differential impact of malaria control interventions on P. falciparum and P. vivax infections in young Papua New Guinean children

INTRODUCTION

As malaria transmission declines, understanding the differential impact of intensified control on Plasmodium falciparum relative to Plasmodium vivax and identifying key drivers of ongoing transmission is essential to guide future interventions.

METHODS

Three longitudinal child cohorts were conducted in Papua New Guinea before (2006/2007), during (2008) and after scale-up of control interventions (2013). In each cohort, children aged 1-5 years were actively monitored for infection and illness. Incidence of malaria episodes, molecular force of blood-stage infections (_molFOB) and population-averaged prevalence of infections were compared across the cohorts to investigate the impact of intensified control in young children and the key risk factors for malaria infection and illness in 2013.

RESULTS

Between 2006 and 2008, P. falciparum infection prevalence, _molFOB, and clinical malaria episodes reduced by 47%, 59% and 69%, respectively, and a further 49%, 29% and 75% from 2008 to 2013 (prevalence 41.6% to 22.1% to 11.2%; _molFOB: 3.4 to 1.4 to 1.0 clones/child/year; clinical episodes incidence rate (IR) 2.6 to 0.8 to IR 0.2 episodes/child/year). P. vivax clinical episodes declined at rates comparable to P. falciparum between 2006, 2008 and 2013 (IR 2.5 to 1.1 to 0.2), while P. vivax _molFOB (2006, 9.8; 2008, 12.1) and prevalence (2006, 59.6%; 2008, 65.0%) remained high in 2008. However, in 2013, P. vivax _molFOB (1.2) and prevalence (19.7%) had also substantially declined. In 2013, 89% of P. falciparum and 93% of P. vivax infections were asymptomatic, 62% and 47%, respectively, were sub-microscopic. Area of residence was the major determinant of malaria infection and illness.

CONCLUSION

Intensified vector control and routine case management had a differential impact on rates of P. falciparum and P. vivax infections but not clinical malaria episodes in young children. This suggests comparable reductions in new mosquito-derived infections but a delayed impact on P. vivax relapsing infections due to a previously acquired reservoir of hypnozoites. This demonstrates the need to strengthen implementation of P. vivax radical cure to maximise impact of control in co-endemic areas. The high heterogeneity of malaria in 2013 highlights the importance of surveillance and targeted interventions to accelerate towards elimination.

Resolving the cause of recurrent Plasmodium vivax malaria probabilistically

Relapses arising from dormant liver-stage Plasmodium vivax parasites (hypnozoites) are a major cause of vivax malaria. However, in endemic areas, a recurrent blood-stage infection following treatment can be hypnozoite-derived (relapse), a blood-stage treatment failure (recrudescence), or a newly acquired infection (reinfection). Each of these requires a different prevention strategy, but it was not previously possible to distinguish between them reliably. We show that individual vivax malaria recurrences can be characterised probabilistically by combined modelling of time-to-event and genetic data within a framework incorporating identity-by-descent. Analysis of pooled patient data on 1441 recurrent P. vivax infections in 1299 patients on the Thailand-Myanmar border observed over 1000 patient follow-up years shows that, without primaquine radical curative treatment, 3 in 4 patients relapse. In contrast, after supervised high-dose primaquine only 1 in 40 relapse. In this region of frequent relapsing P. vivax, failure rates after supervised high-dose primaquine are significantly lower (∼3%) than estimated previously.

Pharmacokinetics/pharmacodynamics of chloroquine and artemisinin-based combination therapy with primaquine

Background

Activation of hypnozoites of vivax malaria causes multiple clinical relapses, which contribute to the Plasmodium vivax burden and continuing transmission. Artemisinin-based combination therapy (ACT) is effective against blood-stage P. vivax but requires co-administration with primaquine to achieve radical cure. The therapeutic efficacy of primaquine depends on the generation of a therapeutically active metabolite via cytochrome P450 2D6 (CYP2D6). Impaired CYP2D6 metabolism has been associated with primaquine treatment failure. This study investigated the association between impaired CYP2D6 genotypes, drug-exposure to the long-acting ACT component (schizonticidal drugs) and tolerance and efficacy.

Methods

Adult patients with acute vivax malaria were enrolled in a recently completed trial and treated with artesunate–mefloquine, chloroquine or artemether–lumefantrine. All received concomitant primaquine (0.5 mg/kg/day for 7–9 days). The association between efficacy and safety and drug exposure was explored using area-under-the-curve (AUC) and half-life (t_1/2) estimates obtained by non-compartmental analysis of the long half-life drugs. Parasite recurrences by day 63 were categorized as related relapses or re-infections/unrelated hypnozoite activation by genotyping three microsatellite loci and two polymorphic loci of merozoite surface antigen-1. The CYP2D6 genotype was identified with Taqman assays by real-time PCR to 9 polymorphisms (8 SNPs and one deletion). Impaired CYP2D6 activity was inferred using the Activity Score System.

Results

Most recurrences in the ASMQ (67%), CQ (80%) and AL (85%) groups were considered related relapses. Eight of nine (88.9%) of the patients with impaired CYP2D6 activity relapsed with related parasite compared to 18/25 (72%) with normal activity (RR = 1.23, 0.88; 1.72, p = 0.40). There were no associations between the measured PK parameters and recurrence. Patients with longer chloroquine half-lives had more pruritus (RR = 1.09, 1.03; 1.14, p = 0.001). Higher CQ AUCs were associated with reduced falls in haemoglobin by day 14 (Coef − 0.02, − 0.005; − 0.03, p = 0.01). All regimens were well tolerated.

Conclusion

Genotyping of P. vivax showed that activation of related (homologous) hypnozoites was the most frequent cause of recurrence. The high proportion of the impaired CYP2D6 activity among patients with recurrent infections suggests that slow primaquine metabolism might influence related relapse rates in Brazil among patients receiving primaquine for radical cure, although confirmatory studies are needed. There was no association between drug exposure of the long-acting ACT component (schizonticidal drugs) and risk of related relapse. ACT was well tolerated. These results provide further re-assurance about the safety and efficacy of ACT when combined with short course primaquine to treat uncomplicated malaria vivax in Brazil.

Trial registration RBR-79s56s (http://www.ensaiosclinicos.gov.br/rg/RBR-79s56s/)

Malaria elimination transmission and costing in the Asia-Pacific: a multi-species dynamic transmission model

Background: The Asia-Pacific region has made significant progress in combatting malaria since 2000 and a regional goal for a malaria-free Asia Pacific by 2030 has been recognised at the highest levels. External financing has recently plateaued and with competing health risks, countries face the risk of withdrawal of funding as malaria is perceived as less of a threat. An investment case was developed to provide economic evidence to inform policy and increase sustainable financing. Methods: A dynamic epidemiological-economic model was developed to project rates of decline to elimination by 2030 and determine the costs for elimination in the Asia-Pacific region. The compartmental model was used to capture the dynamics of Plasmodium falciparum and Plasmodium vivax malaria for the 22 countries in the region in a metapopulation framework. This paper presents the model development and epidemiological results of the simulation exercise. Results: The model predicted that all 22 countries could achieve Plasmodium falciparum and Plasmodium vivax elimination by 2030, with the People’s Democratic Republic of China, Sri Lanka and the Republic of Korea predicted to do so without scaling up current interventions. Elimination was predicted to be possible in Bangladesh, Bhutan, Malaysia, Nepal, Philippines, Timor-Leste and Vietnam through an increase in long-lasting insecticidal nets (and/or indoor residual spraying) and health system strengthening, and in the Democratic People’s Republic of Korea, India and Thailand with the addition of innovations in drug therapy and vector control. Elimination was predicted to occur by 2030 in all other countries only through the addition of mass drug administration to scale-up and/or innovative activities. Conclusions: This study predicts that it is possible to have a malaria-free region by 2030. When computed into benefits and costs, the investment case can be used to advocate for sustained financing to realise the goal of malaria elimination in Asia-Pacific by 2030.

Highly heterogeneous residual malaria risk in western Thailand

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There is a highly heterogenous risk of malaria infection among villagers in western Thailand.

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The molecular force of infection was determined in a low endemic setting.

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There is a strong correlation between malaria prevalence and the force of infection.

Over the past decades, the malaria burden in Thailand has substantially declined. Most infections now originate from the national border regions. In these areas, the prevalence of asymptomatic infections is still substantial and poses a challenge for the national malaria elimination program. To determine epidemiological parameters as well as risk factors for malaria infection in western Thailand, we carried out a cohort study in Kanchanaburi and Ratchaburi provinces on the Thailand-Myanmar border. Blood samples from 999 local participants were examined for malaria infection every 4 weeks between May 2013 and Jun 2014. Prevalence of Plasmodium falciparum and Plasmodium vivax was determined by quantitative PCR (qPCR) and showed a seasonal variation with values fluctuating from 1.7% to 4.2% for P. vivax and 0% to 1.3% for P. falciparum. Ninety percent of infections were asymptomatic. The annual molecular force of blood-stage infection (_molFOB) was estimated by microsatellite genotyping to be 0.24 new infections per person-year for P. vivax and 0.02 new infections per person-year for P. falciparum. The distribution of infections was heterogenous, that is, the vast majority of infections (>80%) were found in a small number of individuals (<8% of the study population) who tested positive at multiple timepoints. Significant risk factors were detected for P. vivax infections, including previous clinical malaria, occupation in agriculture and travel to Myanmar. In contrast, indoor residual spraying was associated with a protection from infection. These findings provide a recent landscape of malaria epidemiology and emphasize the importance of novel strategies to target asymptomatic and imported infections.

Malaria elimination transmission and costing in the Asia-Pacific: a multi-species dynamic transmission model

Background: The Asia-Pacific region has made significant progress in combatting malaria since 2000 and a regional goal for a malaria-free Asia Pacific by 2030 has been recognised at the highest levels. External financing has recently plateaued and with competing health risks, countries face the risk of withdrawal of funding as malaria is perceived as less of a threat. An investment case was developed to provide economic evidence to inform policy and increase sustainable financing. Methods: A dynamic epidemiological-economic model was developed to project rates of decline to elimination by 2030 and determine the costs for elimination in the Asia-Pacific region. The compartmental model was used to capture the dynamics of Plasmodium falciparum and Plasmodium vivax malaria for the 22 countries in the region in a metapopulation framework. This paper presents the model development and epidemiological results of the simulation exercise. Results: The model predicted that all 22 countries could achieve Plasmodium falciparum and Plasmodium vivax elimination by 2030, with the People’s Democratic Republic of China, Sri Lanka and the Republic of Korea predicted to do so without scaling up current interventions. Elimination was predicted to be possible in Bangladesh, Bhutan, Malaysia, Nepal, Philippines, Timor-Leste and Vietnam through an increase in long-lasting insecticidal nets (and/or indoor residual spraying) and health system strengthening, and in the Democratic People’s Republic of Korea, India and Thailand with the addition of innovations in drug therapy and vector control. Elimination was predicted to occur by 2030 in all other countries only through the addition of mass drug administration to scale-up and/or innovative activities. Conclusions: This study predicts that it is possible to have a malaria-free region by 2030. When computed into benefits and costs, the investment case can be used to advocate for sustained financing to realise the goal of malaria elimination in Asia-Pacific by 2030.

The temporal dynamics and infectiousness of subpatent Plasmodium falciparum infections in relation to parasite density

Malaria infections occurring below the limit of detection of standard diagnostics are common in all endemic settings. However, key questions remain surrounding their contribution to sustaining transmission and whether they need to be detected and targeted to achieve malaria elimination. In this study we analyse a range of malaria datasets to quantify the density, detectability, course of infection and infectiousness of subpatent infections. Asymptomatically infected individuals have lower parasite densities on average in low transmission settings compared to individuals in higher transmission settings. In cohort studies, subpatent infections are found to be predictive of future periods of patent infection and in membrane feeding studies, individuals infected with subpatent asexual parasite densities are found to be approximately a third as infectious to mosquitoes as individuals with patent (asexual parasite) infection. These results indicate that subpatent infections contribute to the infectious reservoir, may be long lasting, and require more sensitive diagnostics to detect them in lower transmission settings.

The role of subpatent infections for malaria transmission and elimination is unclear. Here, Slater et al. analyse several malaria datasets to quantify the density, detectability, course of infection and infectiousness of subpatent infections.

Deciphering genetic regulation of CD14 by SP1 through characterization of peripheral blood mononuclear transcriptome of P. faiciparum and P. vivax infected malaria patients

BACKGROUND

Plasmodium falciparum and Plasmodium vivax are two major parasites responsible for malaria which remains a threat to almost 50% of world's population despite decade-long eradication program. One possible reason behind this conundrum is that the bases of clinical variability in malaria caused by either species are complex and poorly understood.

METHODS

Whole-genome transcriptome was analyzed to identify the active and predominant pathways in the PBMC of P. falciparum and P. vivax infected malaria patients. Deregulated genes were identified and annotated using R Bioconductor and DAVID/KEGG respectively. Genetic and functional regulation of CD14, a prioritized candidate, were established by quantitative RT-PCR, genotyping using RFLP and resequencing, mapping of transcription factor binding using CONSITE and TFBIND, dual luciferase assay, western blot analysis, RNAi- mediated gene knockdown and chromatin-immunoprecipation.

FINDINGS

The study highlighted that deregulation of host immune and inflammatory genes particularly CD14 as a key event in P. falciparum malaria. An abundance of allele-C of rs5744454, located in CD14 promoter, in severe malaria motivated us to establish an allele-specific regulation of CD14 by SP1. An enhancement of SP1 and CD14 expression was observed in artemisinin treated human monocyte cell line.

INTERPRETATION

Our data not only reinstates that CD14 of TLR pathway plays a predominant role in P. falciparum malaria, it establishes a functional basis for genetic association of rs5744454 with P. falciparum severe malaria by demonstrating a cis-regulatory role of this promoter polymorphism. Moreover, the study points towards a novel pharmacogenetic aspect of artemisinin-based anti-malarial therapy. FUND: DST-SERB, Govt. of India, SR/SO/HS-0056/2013.