The role of asymptomatic P. falciparum parasitaemia in the evolution of antimalarial drug resistance in areas of seasonal transmission

Plasmodium falciparum genetic diversity and multiplicity of infection based on msp-1, msp-2, glurp and microsatellite genetic markers in sub-Saharan Africa: a systematic review and meta-analysis

BACKGROUND

In sub-Saharan Africa (SSA), Plasmodium falciparum causes most of the malaria cases. Despite its crucial roles in disease severity and drug resistance, comprehensive data on Plasmodium falciparum genetic diversity and multiplicity of infection (MOI) are sparse in SSA. This study summarizes available information on genetic diversity and MOI, focusing on key markers (msp-1, msp-2, glurp, and microsatellites). The systematic review aimed to evaluate their influence on malaria transmission dynamics and offer insights for enhancing malaria control measures in SSA.

METHODS

The review was conducted following the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines. Two reviewers conducted article screening, assessed the risk of bias (RoB), and performed data abstraction. Meta-analysis was performed using the random-effects model in STATA version 17.

RESULTS

The review included 52 articles: 39 cross-sectional studies and 13 Randomized Controlled Trial (RCT)/cohort studies, involving 11,640 genotyped parasite isolates from 23 SSA countries. The overall pooled mean expected heterozygosity was 0.65 (95% CI: 0.51-0.78). Regionally, values varied: East (0.58), Central (0.84), Southern (0.74), and West Africa (0.69). Overall pooled allele frequencies of msp-1 alleles K1, MAD20, and RO33 were 61%, 44%, and 40%, respectively, while msp-2 I/C 3D7 and FC27 alleles were 61% and 55%. Central Africa reported higher frequencies (K1: 74%, MAD20: 51%, RO33: 48%) than East Africa (K1: 46%, MAD20: 42%, RO33: 31%). For msp-2, East Africa had 60% and 55% for I/C 3D7 and FC27 alleles, while West Africa had 62% and 50%, respectively. The pooled allele frequency for glurp was 66%. The overall pooled mean MOI was 2.09 (95% CI: 1.88-2.30), with regional variations: East (2.05), Central (2.37), Southern (2.16), and West Africa (1.96). The overall prevalence of polyclonal Plasmodium falciparum infections was 63% (95% CI: 56-70), with regional prevalences as follows: East (62%), West (61%), Central (65%), and South Africa (71%).

CONCLUSION

The study shows substantial regional variation in Plasmodium falciparum parasite genetic diversity and MOI in SSA. These findings suggest a need for malaria control strategies and surveillance efforts considering regional-specific factors underlying Plasmodium falciparum infection.

Long-term effects of increased adoption of artemisinin combination therapies in Burkina Faso

Artemisinin combination therapies (ACTs) are the WHO-recommended first-line therapies for uncomplicated Plasmodium falciparum malaria. The emergence and spread of artemisinin-resistant genotypes is a major global public health concern due to the increased rate of treatment failures that result. This is particularly germane for WHO designated 'high burden to high impact' (HBHI) countries, such as Burkina Faso, where there is increased emphasis on improving guidance, strategy, and coordination of local malaria response in an effort to reduce the prevalence of P. falciparum malaria. To explore how the increased adoption of ACTs may affect the HBHI malaria setting of Burkina Faso, we added spatial structure to a validated individual-based stochastic model of P. falciparum transmission and evaluated the long-term effects of increased ACT use. We explored how de novo emergence of artemisinin-resistant genotypes, such as pfkelch13 580Y, may occur under scenarios in which private-market drugs are eliminated or multiple first-line therapies (MFT) are deployed. We found that elimination of private market drugs would result in lower treatment failures rates (between 11.98% and 12.90%) when compared to the status quo (13.11%). However, scenarios incorporating MFT with equal deployment of artemether-lumefantrine (AL) and dihydroartemisinin-piperaquine (DHA-PPQ) may accelerate near-term drug resistance (580Y frequency ranging between 0.62 to 0.84 in model year 2038) and treatment failure rates (26.69% to 34.00% in 2038), due to early failure and substantially reduced treatment efficacy resulting from piperaquine-resistant genotypes. A rebalanced MFT approach (90% AL, 10% DHA-PPQ) results in approximately equal long-term outcomes to using AL alone but may be difficult to implement in practice.

Mapping partner drug resistance to guide antimalarial combination therapy policies in sub-Saharan Africa

Resistance to artemisinin-based combination therapies (ACTs) threatens the global control of Plasmodium falciparum malaria. ACTs combine artemisinin-derived compounds with partner drugs to enable multiple mechanisms of clearance. Although ACTs remain widely effective in sub-Saharan Africa, long-standing circulation of parasite alleles associated with reduced partner drug susceptibility may contribute to the development of clinical resistance. We fitted a hierarchical Bayesian spatial model to data from over 500 molecular surveys to predict the prevalence and frequency of four key markers in transporter genes (pfcrt 76T and pfmdr1 86Y, 184F, and 1246Y) in first-level administrative divisions in sub-Saharan Africa from the uptake of ACTs (2004 to 2009) to their widespread usage (2010 to 2018). Our models estimated that the pfcrt 76T mutation decreased in prevalence in 90% of regions; the pfmdr1 N86 and D1246 wild-type genotypes increased in prevalence in 96% and 82% of regions, respectively; and there was no significant directional selection at the pfmdr1 Y184F locus. Rainfall seasonality was the strongest predictor of the prevalence of wild-type genotypes, with other covariates, including first-line drug policy and transmission intensity more weakly associated. We lastly identified regions of high priority for enhanced surveillance that could signify decreased susceptibility to the local first-line ACT. Our results can be used to infer the degree of molecular resistance and magnitude of wild-type reversion in regions without survey data to inform therapeutic policy decisions.

Asymptomatic malaria infection prevailing risks for human health and malaria elimination

There has been a consistent rise in malaria cases in the last few years. The existing malaria control measures are challenged by insecticide resistance in the mosquito vector, drug résistance in parasite populations, and asymptomatic malaria (ASM) in healthy individuals. The absence of apparent malaria symptoms and the presence of low parasitemia makes ASM a hidden reservoir for malaria transmission and an impediment in malaria elimination efforts. This review focuses on ASM in malaria-endemic countries and the past and present research trends from those geographical locations. The harmful impacts of asymptomatic malaria on human health and its contribution to disease transmission are highlighted. We discuss certain crucial genetic changes in the parasite and host immune response necessary for maintaining low parasitemia leading to long-term parasite survival in the host. Since the chronic health effects and the potential roles for disease transmission of ASM remain mostly unknown to significant populations, we offer proposals for developing general awareness. We also suggest advanced technology-based diagnostic methods, and treatment strategies to eliminate ASM.

Defining symptoms of malaria in India in an era of asymptomatic infections

Background

Malaria is a major public health problem in India. Data from surveys totaling 3031 participants at three sites revealed a high proportion of asymptomatic infections, complicating diagnosis. The aim of this study was to identify differences in complaints and symptoms between sites, and factors associated with asymptomatic Plasmodium infections.

Methods

Published data from community-based cross-sectional studies conducted between 2012 and 2015 in Nadiad (Gujarat), Chennai (Tamil Nadu), and Rourkela (Odisha) as part of the Center for the Study of Complex Malaria in India were analysed. Complaints and symptoms were systematically recorded, and Plasmodium infections confirmed using microscopy, rapid diagnostic tests (RDTs), and polymerase chain reaction (PCR). Multivariate analyses were conducted to determine the association between general symptoms and age, season, or gender, and factors associated with asymptomatic Plasmodium infections were assessed.

Results

Complaints of any illness were lowest in Chennai (17.7%), 30.6% in Rourkela and 42.7% in Nadiad. Complaints were more often reported for children; gender differences were noted in Rourkela only. In Nadiad, 7.0% of 796 participants were positive for malaria by PCR (32% Plasmodium falciparum); 78.6% had a history of fever or documented fever, 14.3% had other symptoms, and 7.1% were “truly asymptomatic”. For Chennai this was 29.2%, 4.2% and 66.7% respectively, with a malaria prevalence of 2.6% by PCR of 928 participants (29% P. falciparum). In Rourkela, with 7.7% of 1307 participants positive for malaria by PCR (82% P. falciparum), the percentages were 35.6%, 24.8% and 39.6%, respectively. In Rourkela, asymptomatic infections were associated with young age and male gender (microscopy or RDT), and with rainy season (PCR). In the same site, participants with Plasmodium vivax were more likely to be asymptomatic (11/18 or 61.1%) than persons with P. falciparum mono-infections (27/78 or 34.6%); gametocytes for P. falciparum were evenly distributed between symptomatic and asymptomatic infections (2/53 vs. 2/49, respectively). The addition of the symptoms “headache”, “aches” and “chills” to fever improved the case-definition of symptomatic malaria.

Conclusion

There were considerable differences in complaints at the three sites in India. Malaria and asymptomatic infections differ by region, indicating that malaria elimination will require localized approaches.

Early transmission of sensitive strain slows down emergence of drug resistance in Plasmodium vivax

The spread of drug resistance of Plasmodium falciparum and Plasmodium vivax parasites is a challenge towards malaria elimination. P. falciparum has shown an early and severe drug resistance in comparison to P. vivax in various countries. In fact, P. vivax differs in its life cycle and treatment in various factors: development and duration of sexual parasite forms differ, symptoms severity are unequal, relapses present only in P. vivax cases and the Artemisinin-based combination therapy (ACT) is only mandatory in P. falciparum cases. We compared the spread of drug resistance for both species through two compartmental models using ordinary differential equations. The model structure describes how sensitive and resistant parasite strains infect a human population treated with antimalarials. We found that an early transmission,i.e., before treatment and low effectiveness of drug coverage, supports the prevalence of sensitive parasites delaying the emergence of resistant P. vivax. These results imply that earlier attention of both symptomatic cases and reservoirs of P. vivax are essential in controlling transmission but also accelerate the spread of drug resistance.

The genomic architecture of antimalarial drug resistance

Plasmodium falciparum and Plasmodium vivax, the two protozoan parasite species that cause the majority of cases of human malaria, have developed resistance to nearly all known antimalarials. The ability of malaria parasites to develop resistance is primarily due to the high numbers of parasites in the infected person's bloodstream during the asexual blood stage of infection in conjunction with the mutability of their genomes. Identifying the genetic mutations that mediate antimalarial resistance has deepened our understanding of how the parasites evade our treatments and reveals molecular markers that can be used to track the emergence of resistance in clinical samples. In this review, we examine known genetic mutations that lead to resistance to the major classes of antimalarial medications: the 4-aminoquinolines (chloroquine, amodiaquine and piperaquine), antifolate drugs, aryl amino-alcohols (quinine, lumefantrine and mefloquine), artemisinin compounds, antibiotics (clindamycin and doxycycline) and a napthoquinone (atovaquone). We discuss how the evolution of antimalarial resistance informs strategies to design the next generation of antimalarial therapies.

Genetic diversity and transmissibility of imported Plasmodium vivax in Qatar and three countries of origin

Malaria control program in the Arabian Peninsula, backed by adequate logistical support, has interrupted transmission with exception of limited sites in Saudi Arabia and sporadic outbreaks in Oman. However, sustained influx of imported malaria represents a direct threat to the above success. Here we examined the extent of genetic diversity among imported P. vivax in Qatar, and its ability to produce gametocytes, compared to parasites in main sites of imported cases, the Indian subcontinent (india) and East Africa (Sudan and Ethiopia). High diversity was seen among imported P. vivax in Qatar, comparable to parasites in the Indian subcontinent and East Africa. Limited genetic differentiation was seen among imported P. vivax, which overlapped with parasites in India, but differentiated from that in Sudan and Ethiopia. Parasite density among imported cases, ranged widely between 26.25–7985934.1 Pv18S rRNA copies/µl blood, with a high prevalence of infections carried gametocytes detectable by qRT-PCR. Parasitaemia was a stronger predictor for P. vivax gametocytes density (r = 0.211, P = 0.04). The extensive diversity of imported P. vivax and its ability to produce gametocytes represent a major threat for re-introduction of malaria in Qatar. The genetic relatedness between P. vivax reported in Qatar and those in India suggest that elimination strategy should target flow and dispersal of imported malaria into the region.

A mathematical model for malaria transmission with asymptomatic carriers and two age groups in the human population

In this paper a conceptual mathematical model of malaria transmission proposed in a previous paper has been analyzed in a deeper detail. Among its key epidemiological features of this model, two-age-classes (child and adult) and asymptomatic carriers have been included. The extra mortality of mosquitoes due to the use of long-lasting treated mosquito nets (LLINs) and Indoor Residual Spraying (IRS) has been included too. By taking advantage of the natural double time scale of the parasite and the human populations, it has been possible to provide interesting threshold results. In particular it has been shown that key parameters can be identified such that below a threshold level, built on these parameters, the epidemic tends to extinction, while above another threshold level it tends to a nontrivial endemic state, for which an interval estimate has been provided. Numerical simulations confirm the analytical results.

Malaria Pathogenesis

In the mosquito-human life cycle, the six species of malaria parasites infecting humans (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale wallickeri, Plasmodium ovale curtisi, Plasmodium malariae, and Plasmodium knowlesi) undergo 10 or more morphological states, replicate from single to 10,000+ cells, and vary in total population from one to many more than 10⁶ organisms. In the human host, only a small number of these morphological stages lead to clinical disease and the vast majority of all malaria-infected patients in the world produce few (if any) symptoms in the human. Human clinical disease (e.g., fever, anemia, coma) is the result of the parasite preprogrammed biology in concert with the human pathophysiological response. Caveats and corollaries that add variation to this host-parasite interaction include parasite genetic diversity of key proteins, coinfections, comorbidities, delays in treatment, human polymorphisms, and environmental determinants.

Residual malaria transmission dynamics varies across The Gambia despite high coverage of control interventions

Over the last decades, malaria has declined substantially in The Gambia but its transmission has not been interrupted. In order to better target control interventions, it is essential to understand the dynamics of residual transmission. This prospective cohort study was conducted between June 2013 and April 2014 in six pairs of villages across The Gambia. Blood samples were collected monthly during the transmission season (June-December) from all residents aged ≥6 months (4,194 individuals) and then in April (dry season). Entomological data were collected monthly throughout the malaria transmission season. Ownership of Long-Lasting Insecticidal Nets was 71.5% (2766/3869). Incidence of malaria infection and clinical disease varied significantly across the country, with the highest values in eastern (1.7/PYAR) than in central (0.2 /PYAR) and western (0.1/PYAR) Gambia. Malaria infection at the beginning of the transmission season was significantly higher in individuals who slept outdoors (HR = 1.51, 95% CI: 1.02-2.23, p = 0.04) and in those who had travelled outside the village (HR = 2.47, 95% CI: 1.83-3.34, p <0.01). Sub-patent infections were more common in older children (HR = 1.35, 95% CI: 1.04-1.6, p <0.01) and adults (HR = 1.53, 95% CI: 1.23-1.89, p<0.01) than in younger children. The risk of clinical malaria was significantly higher in households with at least one infected individual at the beginning of the transmission season (HR = 1.76, p<0.01). Vector parity was significantly higher in the eastern part of the country, both in the south (90.7%, 117/129, p<0.01) and the north bank (81.1%, 227/280, p<0.01), than in the western region (41.2%, 341/826), indicating higher vector survival. There is still significant residual malaria transmission across The Gambia, particularly in the eastern region. Additional interventions able to target vectors escaping Long-Lasting Insecticidal Nets and indoor residual spraying are needed to achieve malaria elimination.

Epidemiological consequences of immune sensitisation by pre-exposure to vector saliva

Blood-feeding arthropods—like mosquitoes, sand flies, and ticks—transmit many diseases that impose serious public health and economic burdens. When a blood-feeding arthropod bites a mammal, it injects saliva containing immunogenic compounds that facilitate feeding. Evidence from Leishmania, Plasmodium and arboviral infections suggests that the immune responses elicited by pre-exposure to arthropod saliva can alter disease progression if the host later becomes infected. Such pre-sensitisation of host immunity has been reported to both exacerbate and limit infection symptoms, depending on the system in question, with potential implications for recovery. To explore if and how immune pre-sensitisation alters the effects of vector control, we develop a general model of vector-borne disease. We show that the abundance of pre-sensitised infected hosts should increase when control efforts moderately increase vector mortality rates. If immune pre-sensitisation leads to more rapid clearance of infection, increasing vector mortality rates may achieve greater than expected disease control. However, when immune pre-sensitisation prolongs the duration of infection, e.g., through mildly symptomatic cases for which treatment is unlikely to be sought, vector control can actually increase the total number of infected hosts. The rising infections may go unnoticed unless active surveillance methods are used to detect such sub-clinical individuals, who could provide long-lasting reservoirs for transmission and suffer long-term health consequences of those sub-clinical infections. Sensitivity analysis suggests that these negative consequences could be mitigated through integrated vector management. While the effect of saliva pre-exposure on acute symptoms is well-studied for leishmaniasis, the immunological and clinical consequences are largely uncharted for other vector-parasite-host combinations. We find a large range of plausible epidemiological outcomes, positive and negative for public health, underscoring the need to quantify how immune pre-sensitisation modulates recovery and transmission rates in vector-borne diseases.

Many diseases of health and economic importance are transmitted by arthropod vectors, like mosquitoes, sand flies, and ticks. When a blood-feeding arthropod bites a mammal, it injects saliva containing compounds that facilitate feeding. The immune responses elicited by previous exposure to vector saliva can alter disease severity if the host later becomes infected. Such pre-sensitisation of host immunity has been linked to either exacerbation or mitigation of symptoms in a number of disease systems. We develop a general model of vector-borne disease to examine how vector control efforts alter the frequency of immune pre-sensitisation and thus change the epidemiological impact of control. We show that the abundance of pre-sensitised infected hosts should increase when control efforts moderately increase vector mortality rates. When immune pre-sensitisation leads to longer infections—by generating sub-clinical cases for which treatment is not rapidly sought—killing vectors can lead to unexpected increases in the number of infected hosts. The rising case burden may go unnoticed unless sub-clinical individuals are tested for infection. Conversely, if immune pre-sensitisation leads to more rapid clearance of infection, increasing vector mortality rates may achieve greater than expected disease control. Our findings highlight the need to quantify how immune pre-sensitisation modulates clinical outcomes and parasite transmission in humans.

Biolayer interferometry predicts ELISA performance of monoclonal antibody pairs for Plasmodium falciparum histidine-rich protein 2

Predicting antibody pair performance in a sandwich format streamlines development of antibody-based diagnostics and laboratory research tools, such as enzyme-linked immunosorbent assays (ELISAs) and lateral flow immunoassays (LFAs). We have evaluated panels of monoclonal antibodies against the malarial parasite biomarker Plasmodium falciparum histidine rich protein 2 (HRP2), including 9 new monoclonal antibodies, using biolayer interferometry (BLI) and screened antibody pairs in a checkerboard ELISA. This study showed BLI predicts antibody pair ELISA performance for HRP2. Pairs that included capture antibodies with low off-rate constants and detection antibodies with high on-rate constants performed best in an ELISA format.

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Kinetic parameters of 15 anti-HRP2 antibodies are measured by biolayer interferometry.

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Kinetic constants are compared to a checkerboard ELISA of 225 antibody pairs.

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Biolayer interferometry predicts antibody pair performance for HRP2 ELISA.

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Capture mAbs with low k_off and detection mAbs with high k_on are best in HRP2 ELISA.

New adenovirus-based vaccine vectors targeting Pfs25 elicit antibodies that inhibit Plasmodium falciparum transmission

Background

An effective malaria transmission-blocking vaccine (TBV) would be a major advance in the current efforts to eliminate and, ultimately, eradicate malaria. Antibodies against Plasmodium falciparum surface protein, Pfs25, are known to block parasite development in the mosquito vector. However, in initial clinical trials the limited immunogenicity of recombinant Pfs25 protein-in-adjuvant vaccines has been a challenge.

Methods

Novel human adenovirus type 5 (Ad5) vectors were used in heterologous prime boost vaccination strategies to augment the immune response against Pfs25. Specifically, an Ad5 vector that directs expression of full-length, membrane-bound Pfs25 was used as a priming immunization followed by a boost with Ad5 viral particles displaying only the Pfs25 epitope targeted by transmission-blocking antibodies 4B7 and 1D2 (Pfs25 aa 122–134) in hypervariable region 5 of the hexon capsid protein.

Results

This heterologous prime-boost vaccine strategy induced antibodies that significantly inhibit P. falciparum transmission to mosquitoes in a standard membrane-feeding assay. Further, immunized mice generated a robust anti-Pfs25 antibody response characterized by higher titer, higher relative avidity and a broader IgG subclass profile than observed with a homologous prime-boost with recombinant Pfs25/alum.

Conclusion

The data suggest that focusing the immune response against defined epitopes displayed on the viral capsid is an effective strategy for transmission-blocking vaccine development.

High Plasmodium falciparum longitudinal prevalence is associated with high multiclonality and reduced clinical malaria risk in a seasonal transmission area of Mali

The effects of persistent Plasmodium falciparum (Pf) infection and multiclonality on subsequent risk of clinical malaria have been reported, but the relationship between these 2 parameters and their relative impacts on the clinical outcome of infection are not understood. A longitudinal cohort study was conducted in a seasonal and high-transmission area of Mali, in which 500 subjects aged 1-65 years were followed for 1 year. Blood samples were collected every 2 weeks, and incident malaria cases were diagnosed and treated. Pf infection in each individual at each time point was assessed by species-specific nested-PCR, and Pf longitudinal prevalence per person (PfLP, proportion of Pf-positive samples over 1 year) was calculated. Multiclonality of Pf infection was measured using a 24-SNP DNA barcoding assay at 4 time-points (two in wet season, and two in dry season) over one year. PfLP was positively correlated with multiclonality at each time point (all r≥0.36; all P≤0.011). When host factors (e.g., age, gender), PfLP, and multiclonality (at the beginning of the transmission season) were analyzed together, only increasing age and high PfLP were associated with reduced clinical malaria occurrence or reduced number of malaria episodes (for both outcomes, P<0.001 for age, and P = 0.005 for PfLP). When age, PfLP and baseline Pf positivity were analyzed together, the effect of high PfLP remained significant even after adjusting for the other two factors (P = 0.001 for malaria occurrence and P<0.001 for number of episodes). In addition to host age and baseline Pf positivity, both of which have been reported as important modifiers of clinical malaria risk, our results demonstrate that persistent parasite carriage, but not baseline multiclonality, is associated with reduced risk of clinical disease in this population. Our study emphasizes the importance of considering repeated parasite exposure in future studies that evaluate clinical malaria risk.

Application of magnetic cytosmear for the estimation of Plasmodium falciparum gametocyte density and detection of asexual stages in asymptomatic children

Background

Conventional malaria parasite detection methods, such as rapid diagnostic tests (RDT) and light microscopy (LM), are not sensitive enough to detect low level parasites and identification of gametocytes in the peripheral blood. A modified and sensitive laboratory prototype, Magnetic Deposition Microscopy (MDM) was developed to increase the detection of sub-microscopic parasitaemia and estimation of gametocytes density in asymptomatic school children.

Methods

Blood samples were collected from 303 asymptomatic school children from seven villages in Bagamoyo district in Tanzania. Participants were screened for presence of malaria parasites in the field using RDT and MDM whereas further examination of malaria parasites was done in the laboratory by LM. LM and MDM readings were used to calculate densities and estimate prevalence of asexual and sexual stages of the parasite.

Results

Plasmodium falciparum parasites (asexual and sexual stages) were detected in 23 (7.6 %), 52 (17.2 %), and 59 (19.5 %) out of 303 samples by LM, RDT and MDM respectively. Gametocytes were detected in 4 (1.3 %) and 12 (4.0 %) out of the same numbers of samples by LM, and MDM, respectively. Likewise, in vitro results conducted on two laboratory strains of P. falciparum, 3D7 and NF54 to assess MDM sensitivity on gametocytes detection and its application on concentrating gametocytes indicated that gametocytes were enriched by MDM by 10-fold higher than LM. Late stages of the parasite strains, 3D7 and NF54 were enriched by MDM by a factor of 20.5 and 35.6, respectively. MDM was more specific than LM and RDT by 87.5 % (95 %, CI 71.2–89.6 %) and 89.0 % (95 % CI 82.9–91.4) respectively. It was also found that MDM sensitivity was 62.5 % (95 % CI 49.5–71.8) when compared with RDT while with LM was 36.5 % (95 % CI 32.2–60.5).

Conclusions

These findings provide strong evidence that MDM enhanced detection of sub-microscopic P. falciparum infections and estimation of gametocyte density compared to current malaria diagnostic tools. In addition, MDM is superior to LM in detecting sub-microscopic gametocytaemia. Therefore, MDM is a potential tool for low-level parasitaemia identification and quantification with possible application in malaria transmission research.

Malaria Parasites in the Asymptomatic: Looking for the Hay in the Haystack

With malaria elimination back on the international agenda, programs face the challenge of targeting all Plasmodium infections, not only symptomatic cases. As asymptomatic individuals are unlikely to seek treatment, they are missed by passive surveillance while remaining infectious to mosquitoes, thus acting as silent reservoirs of transmission. To estimate the risk of asymptomatic infections in various phases of malaria elimination, we need a deeper understanding of the underlying mechanisms favoring carriage over disease, which may involve both pathogen and host factors. Here we review our current knowledge on the determinants leading to Plasmodium falciparum symptomless infections. Understanding the host-pathogen interactions that are most likely to affect transitions between malaria disease states could guide the development of tools to tackle asymptomatic carriers in elimination settings.

Numerical Distributions of Parasite Densities During Asymptomatic Malaria

BACKGROUND

Asymptomatic parasitemia is common even in areas of low seasonal malaria transmission, but the true proportion of the population infected has not been estimated previously because of the limited sensitivity of available detection methods.

METHODS

Cross-sectional malaria surveys were conducted in areas of low seasonal transmission along the border between eastern Myanmar and northwestern Thailand and in western Cambodia. DNA was quantitated by an ultrasensitive polymerase chain reaction (uPCR) assay (limit of accurate detection, 22 parasites/mL) to characterize parasite density distributions for Plasmodium falciparum and Plasmodium vivax, and the proportions of undetected infections were imputed.

RESULTS

The prevalence of asymptomatic malaria as determined by uPCR was 27.5% (1303 of 4740 people tested). Both P. vivax and P. falciparum density distributions were unimodal and log normal, with modal values well within the quantifiable range. The estimated proportions of all parasitemic individuals identified by uPCR were >70% among individuals infected with P. falciparum and >85% among those infected with P. vivax. Overall, 83% of infections were predicted to be P. vivax infections, 13% were predicted to be P. falciparum infections, and 4% were predicted to be mixed infections. Geometric mean parasite densities were similar; 5601 P. vivax parasites/mL and 5158 P. falciparum parasites/mL.

CONCLUSIONS

This uPCR method identified most infected individuals in malaria-endemic areas. Malaria parasitemia persists in humans at levels that optimize the probability of generating transmissible gametocyte densities without causing illness.

Asymptomatic Multiclonal Plasmodium falciparum Infections Carried Through the Dry Season Predict Protection Against Subsequent Clinical Malaria

BACKGROUND

Immunity to the antigenically diverse parasite Plasmodium falciparum is acquired gradually after repeated exposure. Studies in areas of high malaria transmission have shown that asymptomatic individuals infected with multiclonal infections are at reduced risk of febrile malaria during follow-up.

METHODS

We assessed the relationship between the genetic diversity of clones in P. falciparum infections that persist through the dry season and the subsequent risk of febrile malaria in 225 individuals aged 2-25 years in Mali, where the 6-month malaria and dry seasons are sharply demarcated. Polymerase chain reaction-based genotyping of the highly polymorphic merozoite surface protein 2 gene was performed on blood samples collected at 5 cross-sectional surveys.

RESULTS

In an age-adjusted analysis, individuals with multiclonal P. falciparum infections before the rainy season were at reduced risk of febrile malaria, compared with individuals who were uninfected (hazard ratio [HR], 0.28; 95% confidence interval [CI], .11-.69). In contrast, there was no significant association between risk of malaria and having 1 clone at baseline (HR, 0.71; 95% CI, .36-1.40).

CONCLUSIONS

The results suggest that persistent multiclonal infections carried through the dry season contribute to protection against subsequent febrile malaria, possibly by maintaining protective immune responses that depend on ongoing parasite infection.

Optimal control problems of epidemic systems with parameter uncertainties: application to a malaria two-age-classes transmission model with asymptomatic carriers

The main scope of this paper is to study the optimal control practices of malaria, by discussing the implementation of a catalog of optimal control strategies in presence of parameter uncertainties, which is typical of infectious diseases data. In this study we focus on a deterministic mathematical model for the transmission of malaria, including in particular asymptomatic carriers and two age classes in the human population. A partial qualitative analysis of the relevant ODE system has been carried out, leading to a realistic threshold parameter. For the deterministic model under consideration, four possible control strategies have been analyzed: the use of Long-lasting treated mosquito nets, indoor residual spraying, screening and treatment of symptomatic and asymptomatic individuals. The numerical results show that using optimal control the disease can be brought to a stable disease free equilibrium when all four controls are used. The Incremental Cost-Effectiveness Ratio (ICER) for all possible combinations of the disease-control measures is determined. The numerical simulations of the optimal control in the presence of parameter uncertainty demonstrate the robustness of the optimal control: the main conclusions of the optimal control remain unchanged, even if inevitable variability remains in the control profiles. The results provide a promising framework for the designing of cost-effective strategies for disease controls with multiple interventions, even under considerable uncertainty of model parameters.

A male and female gametocyte functional viability assay to identify biologically relevant malaria transmission-blocking drugs

Malaria elimination will require interventions that prevent parasite transmission from the human host to the mosquito. Experimentally, this is usually determined by the expensive and laborious Plasmodium falciparum standard membrane feeding assay (PfSMFA), which has limited utility for high-throughput drug screening. In response, we developed the P. falciparum dual gamete formation assay (PfDGFA), which faithfully simulates the initial stages of the PfSMFA in vitro. It utilizes a dual readout that individually and simultaneously reports on the functional viability of male and female mature stage V gametocytes. To validate, we screen the Medicines for Malaria Venture (MMV) Malaria Box library with the PfDGFA. Unique to this assay, we find compounds that target male gametocytes only and also compounds with reversible and irreversible activity. Most importantly, we show that compound activity in the PfDGFA accurately predicts activity in PfSMFAs, which validates and supports its adoption into the transmission-stage screening pipeline.

The prospect of malaria elimination in the Arabian Peninsula: a population genetic approach

BACKGROUND

In the Arabian Peninsula malaria control is progressing steadily, backed by adequate logistic and political support. As a result, transmission has been interrupted throughout the region, with exception of limited sites in Yemen and Saudi Arabia. Here we examined Plasmodium falciparum parasites in these sites to assess if the above success has limited diversity and gene flow.

METHODS

We examined 108 P. falciparum isolates in three sites in Yemen (Taiz, Dhamar and Hodeidah) and 91 isolates from Saudi Arabia (Jazan). Nine microsatellites were analyzed for allelic diversity, multi-locus haplotype and inter-population differentiation.

RESULTS

Diversity at each locus (unbiased heterozygosity [H]) was relatively lower in Yemen; (Hodeidah, H=0.615, Taiz, H=0.66, Dhamar, H=0.481), compared to Saudi Arabia (Jazan, H=0.76). Microsatellites were distributed widely and private alleles, detected in a single population, were rare. Pairwise comparisons revealed that parasites population in Dhamar was relatively distanced (FST=0.19). However, Taiz (Yemen) (FST=0.065) and Hodeidah (FST=0.107) populations were closer to that in Jazan (Saudi Arabia). Nonetheless, parasites in the four sites can be considered as one population.

CONCLUSION

Although malaria risk in Saudi Arabia has been cut considerably, the extent of diversity and parasite genetic structure are indicative of a large population size. Elimination strategy should target demographic factors that favor parasite dispersal and flow of imported malaria.

A qualitative study to assess community barriers to malaria mass drug administration trials in The Gambia

Background

Mass drug administration (MDA) is a strategy widely used in the control of human parasitic diseases but has been rarely attempted with malaria, the most common and dangerous parasitic disease in humans. MDA is an intervention strategy that involves simultaneously dispensing treatment to an entire population in a given geographic area. With some areas in sub-Saharan Africa documenting a decline in malaria transmission, the feasibility of MDA to further reduce malaria transmission is being considered. Understanding community perceptions of such an activity is vitally important for the design of the study and gaining the support of participants in order to maximize compliance and adherence.

Methods

A qualitative study to assess factors likely to influence community acceptance of MDA in the seasonal and low malaria transmission setting of The Gambia was conducted. Using in-depth interviews, the perceptions, knowledge and attitudes of medical personnel and community members who have undergone MDA trials in The Gambia were investigated.

Results

Several major themes emerged, namely: 1) the importance of timing of rounds of MDA doses for maximum participation; 2) the need to educate the target population with accurate information on the procedures, drug regimen, and possible side effects to enhance adherence; 3) the need for continuous sensitization meetings to maintain and increase uptake of MDA; and, 4) the importance for defining roles in the delivery and assessment of MDA, including existing healthcare structures.

Discussion

To increase the likelihood of participation in MDA trials in this setting, activities should be undertaken just before and during the rainy season when community members are less mobile. Importantly, fears regarding blood sampling and side effects of the drug regimen need to be addressed prior to the start of the trial and repeated throughout the study period. Accurate and frequent communication is essential, and village leaders should consistently be included in sensitization meetings to enhance community participation. Additionally, village healthcare workers should be included in training and implementation, with supervision by a fieldworker permanently posted in every few villages during the trial. Future collaboration with Senegal may prove important for enhanced elimination efforts in The Gambia.

NOS2 variants reveal a dual genetic control of nitric oxide levels, susceptibility to Plasmodium infection, and cerebral malaria

Nitric oxide (NO) is a proposed component of malaria pathogenesis, and the inducible nitric oxide synthase gene (NOS2) has been associated to malaria susceptibility. We analyzed the role of NOS2 polymorphisms on NO bioavailability and on susceptibility to infection, Plasmodium carrier status and clinical malaria. Two distinct West African sample collections were studied: a population-based collection of 1,168 apparently healthy individuals from the Príncipe Island and a hospital-based cohort of 269 Angolan children. We found that two NOS2 promoter single-nucleotide polymorphism (SNP) alleles associated to low NO plasma levels in noninfected individuals were also associated to reduced risk of pre-erythrocytic infection as measured anti-CSP antibody levels (6.25E-04 < P < 7.57E-04). In contrast, three SNP alleles within the NOS2 cistronic region conferring increased NO plasma levels in asymptomatic carriers were strongly associated to risk of parasite carriage (8.00E-05 < P < 7.90E-04). Notwithstanding, three SNP alleles in this region protected from cerebral malaria (7.90E-4 < P < 4.33E-02). Cohesively, the results revealed a dual regimen in the genetic control of NO bioavailability afforded by NOS2 depending on the infection status. NOS2 promoter variants operate in noninfected individuals to decrease both NO bioavailability and susceptibility to pre-erythrocytic infection. Conversely, NOS2 cistronic variants (namely, rs6505469) operate in infected individuals to increase NO bioavailability and confer increased susceptibility to unapparent infection but protect from cerebral malaria. These findings corroborate the hypothesis that NO anti-inflammatory properties impact on different steps of malaria pathogenesis, explicitly by favoring infection susceptibility and deterring severe malaria syndromes.

The interplay between drug resistance and fitness in malaria parasites

Controlling the spread of antimalarial drug resistance, especially resistance of Plasmodium falciparum to artemisinin-based combination therapies, is a high priority. Available data indicate that, as with other microorganisms, the spread of drug-resistant malaria parasites is limited by fitness costs that frequently accompany resistance. Resistance-mediating polymorphisms in malaria parasites have been identified in putative drug transporters and in target enzymes. The impacts of these polymorphisms on parasite fitness have been characterized in vitro and in animal models. Additional insights have come from analyses of samples from clinical studies, both evaluating parasites under different selective pressures and determining the clinical consequences of infection with different parasites. With some exceptions, resistance-mediating polymorphisms lead to malaria parasites that, compared with wild type, grow less well in culture and in animals, and are replaced by wild type when drug pressure diminishes in the clinical setting. In some cases, the fitness costs of resistance may be offset by compensatory mutations that increase virulence or changes that enhance malaria transmission. However, not enough is known about effects of resistance mediators on parasite fitness. A better appreciation of the costs of fitness-mediating mutations will facilitate the development of optimal guidelines for the treatment and prevention of malaria.