Transcription and expression of Plasmodium falciparum histidine-rich proteins in different stages and strains: implications for rapid diagnostic tests

Novel Plasmodium falciparum histidine-rich protein 2/3 repeat type in Ethiopian malaria infection: does this affect performance of HRP2-based malaria RDT?

BACKGROUND

Rapid diagnostic tests (RDTs) provide quick, easy, and convenient early diagnosis of malaria ensuring better case management particularly in resource-constrained settings. Nevertheless, the efficiency of HRP2-based RDT can be compromised by Plasmodium falciparum histidine-rich protein 2/3 gene deletion and genetic diversity. This study explored the genetic diversity of PfHRP2/3 in uncomplicated malaria cases from Ethiopia.

METHODS

A cross-sectional study was conducted from June 2022 to March 2023 at Metehara, Zenzelema and Kolla Shele health centres, Ethiopia. Finger-prick blood samples were collected for RDT testing and microscopic examination. For molecular analysis, parasite genomic DNA was extracted from venous blood. Plasmodium falciparum was confirmed using VarATS real time PCR. Additionally, PfHRP2/3 was amplified, and DNA amplicons were sequenced using Oxford Nanopore technology.

RESULTS

PfHRP2/3 sequences revealed small variations in the frequency and number of amino acid repeat types per isolate across the three health centres. Twelve and eight types of amino acid repeats were identified for PfHRP2 and PfHRP3, respectively, which had been previously characterized. Repeat type 1, 4 and 7 were present in both PfHRP2 and PfHRP3 amino acid sequences. Type 2 and 7 repeats were commonly dispersed in PfHRP2, while repeat types 16 and 17 were found only in PfHRP3. A novel 17 V repeat type variant, which has never been reported in Ethiopia, was identified in six PfHRP3 amino acid sequences. The majority of the isolates, as determined by the Baker's logistic regression model, belonged to group C, of which 86% of them were sensitive to PfHRP2-based RDT. Likewise, PfHRP2-based RDT detected 100% of the isolates in group A (product of type 2 × type 7 repeats ≥ 100) and 85.7% in group B (product of types 2 × type 7 repeats 50-99) at a parasitaemia level > 250 parasite/μl.

CONCLUSION

This study highlights the significant diversity observed in PfHRP2 and PfHRP3 among clinical isolates of Plasmodium falciparum in Ethiopia. This emphasizes the necessity for monitoring of PfHRP2- based RDT efficacy and their repeat type distribution using a large sample size and isolates from various ecological settings.

Geographical distribution and genetic characterization of pfhrp2 negative Plasmodium falciparum parasites in the Peruvian Amazon

Malaria rapid diagnostic tests (RDTs) have been evaluated in the Peruvian Amazon region and their performance has been variable. This region is known for being the first with documented evidence of wild Plasmodium falciparum parasites lacking pfhrp2 and pfhrp3 genes, leading to false-positive results with HRP2-based RDTs. In our attempt to further characterize the deletion pattern of these genes and their evolutionary relationship, 93 P. falciparum samples, collected in different communities from the Peruvian Amazon region between 2009 and 2010, were analyzed in this study. Genomic DNA was used to amplify 18S rRNA, pfmsp2 and pfglurp to confirm the diagnosis and DNA quality, respectively; pfhrp2, pfhrp3, and their flanking genes were amplified by PCR to assess the pattern of the gene deletions. In addition, microsatellite analysis were performed using seven neutral microsatellites (MS) and five microsatellite loci flanking pfhrp2. The data showed the absence of pfhrp3 gene in 53.76% (50/93) of the samples, reflecting a higher frequency than the proportion of pfhrp2 gene deletions (33.33%; 31/93). Among the flanking genes, the highest frequency of deletion was observed in the PF3D7_0831900 gene (78.49%; 73/93) for pfhrp2. MS marker analysis showed the presence of 8 P. falciparum lineages. The lineage Bv1 was the most prevalent among parasites lacking pfhrp2 and pfhrp3 genes. Additionally, using MS flanking pfhrp2 gene, the haplotypes α and δ were found to be the most abundant in this region. This study confirms the presence in this area of field isolates with deletions in either pfhrp2, pfhrp3, or both genes, along with their respective flanking regions. Our data suggest that some pfhrp2/pfhrp3 deletion haplotypes, in special the lineage Bv1, are widely dispersed within the Peruvian Amazon. The persistence of these haplotypes ensures a proportion of P.falciparum parasites lacking the pfhrp2/pfhrp3 genes in this area, which ultimately leads to false-negative results on PfHRP2-detecting malaria RDTs. However, additional studies are needed to not only confirm this hypothesis but also to further delineate the origin and genetic basis for the pfhrp2- and pfhrp3 gene deletions in wild P. falciparum parasites.

Genetic deletions and high diversity of Plasmodium falciparum histidine-rich proteins 2 and 3 genes in parasite populations in Ghana

Rapid diagnostic tests (RDTs) are used to diagnose malaria in Ghana and other malaria endemic countries. Plasmodium falciparum histidine-rich protein 2 (PFHRP2) based RDTs are widely used, however the occurrence of deletions of the pfhrp2 gene in some parasites have resulted in false negative test results. Monoclonal antibodies of PFHRP2 cross reacts with PFHRP3 because they share structural similarities and this complements the detection of the parasites by RDT. These two genes were investigated in Ghanaian P. falciparum parasite population to detect deletions and the polymorphisms in exon 2 of the pfhrp2 and pfhrp3 genes. Parasite isolates (2,540) from children ≤ 12 years with uncomplicated malaria from 2015 to 2020 transmission seasons were used. Both genes were amplified using nested PCR and negative results indicated the presence of the deletion of genes. Amplified genes were sequenced for the detection of the amino acid repeats. Deletions were observed in 30.7% (780/2,540) and 17.2% (438/2,540) of the samples for pfhrp2 and pfhrp3 respectively with increasing trends over the three time periods (χ2 -10.305, p = 0.001). A total of 1,632 amplicons were sequenced for each gene, analysis was done on 1,124 and 1,307 good quality sequences for pfhrp2 and pfhrp3 respectively. Pfhrp2 repeat polymorphisms were dominantly of types 2 (AHHAHHAAD) and 7 (AHHAAD) with large numbers of variants. A novel variant of type 14 (AHHANHATD) was seen for pfhrp2. For the pfhrp3 repeat types, 16 (AHHAAN), 17 (AHHDG) and 18 (AHHDD) were the dominant types observed. Variants of type 16 (AHHAAH) and (AHHASH) were also dominant. Repeat types 1, 2, 3, 4, 5, 6, 7, 8, 11, 13, 15, 16, and 19 were observed be shared by both genes. The haplotype diversity of both genes ranged between 0.872 and 1 indicating high diversity of the polymorphisms in the isolates. The implication of the findings of the frequencies of the pfhrp2 and pfhrp3 deletions as well as the variants of the main epitopes of the monoclonal antibodies for the RDT (types 2 and 7) in our isolates is an indication of decreased sensitivity of the RDTs in diagnosing malaria infections in Ghana.

Plasmodium falciparum histidine-rich protein 2 and 3 genes deletion in global settings (2010–2021): a systematic review and meta-analysis

Background

The usefulness of histidine-rich protein-2/3 (HRP2/3)-based rapid diagnostic tests of malaria due to Plasmodium falciparum has been threatened by the appearance of mutant PfHRP2/3 genes. This study was undertaken to determine the global pooled estimates of PfHRP2/3gene deletions.

Methods

Relevant publications were identified from electronic databases such as; PubMed, EMBASE, and MEDLINE online. Besides, all the relevant literatures were retrieved through Google and Google Scholar. STATA software was used for data analysis. The pooled estimates were calculated using random effect model. The summary estimates were presented using forest plots and tables.

Results

A total of 27 studies were included in the systematic review. However, only 24 and 17 studies were included for PfHRP2 and 3 gene deletion meta-analysis, respectively. The prevalence of PfHRP2 gene deletion across the individual studies ranged from the highest 100% to the lowest 0%. However, the meta-analysis result showed that the global pooled prevalence of PfHRP2 and PfHRP3 gene deletions were 21.30% and 34.50%, respectively. The pooled proportion of PfHRP2 gene deletion among false negative PfHRP2-based RDTs results was found to be 41.10%. The gene deletion status was higher in South America and followed by Africa. The pooled estimate of PfHRP2 gene deletion among studies, which did not follow the WHO PfHRP2/3 gene deletion analysis protocol was higher than their counter parts (21.3% vs 10.5%).

Conclusions

This review showed that there is a high pooled prevalence of PfHRP2/3 gene deletions in Plasmodium falciparum confirmed isolates and also a high proportion of their deletions among false-negative malaria cases using PfHRP2-based RDT results. Hence, malaria diagnosis based on PfHRP2-based rapid tests seems to be less sensitive and warrants further evaluation of PfHRP2/3 gene deletions.

Analysis of nucleic acids extracted from rapid diagnostic tests reveals a significant proportion of false positive test results associated with recent malaria treatment

BACKGROUND

Surveillance programmes often use malaria rapid diagnostic tests (RDTs) to determine the proportion of the population carrying parasites in their peripheral blood to assess the malaria transmission intensity. Despite an increasing number of reports on false-negative and false-positive RDT results, there is a lack of systematic quality control activities for RDTs deployed in malaria surveillance programmes.

METHODS

The diagnostic performance of field-deployed RDTs used for malaria surveys was assessed by retrospective molecular analysis of the blood retained on the tests.

RESULTS

Of the 2865 RDTs that were collected in 2018 on Bioko Island and analysed in this study, 4.7% had a false-negative result. These false-negative RDTs were associated with low parasite density infections. In 16.6% of analysed samples, masked pfhrp2 and pfhrp3 gene deletions were identified, in which at least one Plasmodium falciparum strain carried a gene deletion. Among all positive RDTs analysed, 28.4% were tested negative by qPCR and therefore considered to be false-positive. Analysing the questionnaire data collected from the participants, this high proportion of false-positive RDTs could be explained by P. falciparum histidine rich protein 2 (PfHRP2) antigen persistence after recent malaria treatment.

CONCLUSION

Malaria surveillance depending solely on RDTs needs well-integrated quality control procedures to assess the extent and impact of reduced sensitivity and specificity of RDTs on malaria control programmes.

Genetic variation of Plasmodium falciparum histidine-rich protein 2 and 3 in Assosa zone, Ethiopia: its impact on the performance of malaria rapid diagnostic tests

Background

Rapid diagnostic tests (RDT) are commonly used for the diagnosis of malaria caused by Plasmodium falciparum. However, false negative results of RDT caused by genetic variation of P. falciparum histidine-rich protein 2 and 3 genes (pfhrp2/3) threaten existing malaria case management and control efforts. The main objective of this study was to investigate the genetic variations of the pfhrp2/3 genes.

Methods

A cross-sectional study was conducted from malaria symptomatic individuals in 2018 in Assosa zone, Ethiopia. Finger-prick blood samples were collected for RDT and microscopic examination of thick and thin blood films. Dried blood spots (DBS) were used for genomic parasite DNA extraction and molecular detection. Amplification of parasite DNA was made by quantitative PCR. DNA amplicons of pfhrp2/3 were purified and sequenced.

Results

The PfHRP2 amino acid repeat type isolates were less conserved compared to the PfHRP3 repeat type. Eleven and eight previously characterized PfHRP2 and PfHRP3 amino acid repeat types were identified, respectively. Type 1, 4 and 7 repeats were shared by PfHRP2 and PfHRP3 proteins. Type 2 repeats were found only in PfHRP2, while types 16 and 17 were found only in PfHRP3 with a high frequency in all isolates. 18 novel repeat types were found in PfHRP2 and 13 novel repeat types were found in PfHRP3 in single or multiple copies per isolate. The positivity rate for PfHRP2 RDT was high, 82.9% in PfHRP2 and 84.3% in PfHRP3 sequence isolates at parasitaemia levels > 250 parasites/µl. Using the Baker model, 100% of the isolates in group A (If product of types 2 × type 7 repeats ≥ 100) and 73.7% of the isolates in group B (If product of types 2 × type 7 repeats 50–99) were predicted to be detected by PfHRP2 RDT at parasitaemia level > 250 parasite/μl.

Conclusion

The findings of this study indicate the presence of different PfHRP2 and PfHRP3 amino acid repeat including novel repeats in P. falciparum from Ethiopia. These results indicate that there is a need to closely monitor the performance of PfHRP2 RDT associated with the genetic variation of the pfhrp2 and pfhrp3 gene in P. falciparum isolates at the country-wide level.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03928-3.

Assessment of Plasmodium antigens and CRP in dried blood spots with multiplex malaria array

Dried blood spots (DBS) typically prepared on filter papers are an ideal sample type for malaria surveillance by offering easy and cost-effective methods in terms of sample collection, storage, and transport. The objective of this study was to evaluate the applicability of DBS with a commercial multiplex malaria assay, developed to concurrently measure Plasmodium antigens, histidine-rich protein 2 (HRP2), Plasmodium lactate dehydrogenase (pLDH), and a host inflammatory biomarker, C-reactive protein (CRP), in whole blood. The assay conditions were optimized for DBS, and thermal stability for measurement of Plasmodium antigens and CRP in dried blood were determined. Performance of the multiplex assay on matched DBS and whole blood pellet samples was also evaluated using the clinical samples. The results indicate the acceptable performance in multiplex antigen detection using DBS samples. At cutoff levels for DBS, with a diagnostic specificity with a lower 95% confidence bound > 92%, diagnostic sensitivities against polymerase chain reaction (PCR)–confirmed malaria for HRP2, Pf LDH, Pv LDH, and Pan LDH were 93.5%, 80.4%, 21.3%, and 55.6%, respectively. The half-life of pLDH was significantly less than that of HRP2 in thermal stability studies. Results with DBS samples collected from Peru indicate that the uncontrolled storage conditions of DBS can result in inaccurate reporting for infection with P. falciparum parasites with hrp2/3 deletions. With careful consideration that minimizing the unfavorable DBS storage environment is essential for ensuring integrity of heat-labile Plasmodium antigens, DBS samples can be used as an alternative to liquid whole blood to detect P. falciparum with hrp2/3 deletions in malaria surveillance.

Molecular characterization of Plasmodium falciparum DNA-3-methyladenine glycosylase

Background

The emergence of artemisinin-resistant malaria parasites highlights the need for novel drugs and their targets. Alkylation of purine bases can hinder DNA replication and if unresolved would eventually result in cell death. DNA-3-methyladenine glycosylase (MAG) is responsible for the repair of those alkylated bases. Plasmodium falciparum (Pf) MAG was characterized for its potential for development as an anti-malarial candidate.

Methods

Native PfMAG from crude extract of chloroquine- and pyrimethamine-resistant P. falciparum K1 strain was partially purified using three chromatographic procedures. From bio-informatics analysis, primers were designed for amplification, insertion into pBAD202/D-TOPO and heterologous expression in Escherichia coli of recombinant PfMAG. Functional and biochemical properties of the recombinant enzyme were characterized.

Results

PfMAG activity was most prominent in parasite schizont stages, with a specific activity of 147 U/mg (partially purified) protein. K1 PfMAG contained an insertion of AAT (coding for asparagine) compared to 3D7 strain and 16% similarity to the human enzyme. Recombinant PfMAG (74 kDa) was twice as large as the human enzyme, preferred double-stranded DNA substrate, and demonstrated glycosylase activity over a pH range of 4–9, optimal salt concentration of 100–200 mM NaCl but reduced activity at 250 mM NaCl, no requirement for divalent cations, which were inhibitory in a dose-dependent manner.

Conclusion

PfMAG activity increased with parasite development being highest in the schizont stages. K1 PfMAG contained an indel AAT (asparagine) not present in 3D7 strain and the recombinant enzyme was twice as large as the human enzyme. Recombinant PfMAG had a wide range of optimal pH activity, and was inhibited at high (250 mM) NaCl concentration as well as by divalent cations. The properties of PfMAG provide basic data that should be of assistance in developing anti-malarials against this potential parasite target.

Rapid diagnostic tests failing to detect infections by Plasmodium falciparum encoding pfhrp2 and pfhrp3 genes in a non-endemic setting

Background

Rapid diagnostic tests (RDTs) detecting the histidine-rich protein 2 (PfHRP2) have a central position for the management of Plasmodium falciparum infections. Yet, variable detection of certain targeted motifs, low parasitaemia, but also deletion of pfhrp2 gene or its homologue pfhrp3, may result in false-negative RDT leading to misdiagnosis and delayed treatment. This study aimed at investigating the prevalence, and understanding the possible causes, of P. falciparum RDT-negative infections at Montpellier Academic Hospital, France.

Methods

The prevalence of falsely-negative RDT results reported before and after the introduction of a loop-mediated isothermal amplification (LAMP) assay, as part as the malaria screening strategy in January 2017, was analysed. Negative P. falciparum RDT infections were screened for pfhrp2 or pfhrp3 deletion; and exons 2 were sequenced to show a putative genetic diversity impairing PfHRP2 detection.

Results

The overall prevalence of P. falciparum negative RDTs from January 2006 to December 2018 was low (3/446). Whereas no cases were reported from 2006 to 2016 (0/373), period during which the malaria diagnostic screen was based on microscopy and RDT, prevalence increased up to 4.1% (3/73) between 2017 and 2018, when molecular detection was implemented for primary screening. Neither pfhrp2/3 deletion nor major variation in the frequency of repetitive epitopes could explain these false-negative RDT results.

Conclusion

This paper demonstrates the presence of pfhrp2 and pfhrp3 genes in three P. falciparum RDT-negative infections and reviews the possible reasons for non-detection of HRP2/3 antigens in a non-endemic setting. It highlights the emergence of falsely negative rapid diagnostic tests in a non-endemic setting and draws attention on the risk of missing malaria cases with low parasitaemia infections using the RDT plus microscopy-based strategy currently recommended by French authorities. The relevance of a novel diagnostic scheme based upon a LAMP assay is discussed.

A novel multiplex qPCR assay for detection of Plasmodium falciparum with histidine-rich protein 2 and 3 (pfhrp2 and pfhrp3) deletions in polyclonal infections

BACKGROUND

Many health facilities in malaria endemic countries are dependent on Rapid diagnostic tests (RDTs) for diagnosis and some National Health Service (NHS) hospitals without expert microscopists rely on them for diagnosis out of hours. The emergence of P. falciparum lacking the gene encoding histidine-rich protein 2 and 3 (HRP2 and HRP3) and escaping RDT detection threatens progress in malaria control and elimination. Currently, confirmation of RDT negative due to the deletion of pfhrp2 and pfhrp3, which encodes a cross-reactive protein isoform, requires a series of PCR assays. These tests have different limits of detection and many laboratories have reported difficulty in confirming the absence of the deletions with certainty.

METHODS

We developed and validated a novel and rapid multiplex real time quantitative (qPCR) assay to detect pfhrp2, pfhrp3, confirmatory parasite and human reference genes simultaneously. We also applied the assay to detect pfhrp2 and pfhrp3 deletion in 462 field samples from different endemic countries and UK travellers.

RESULTS

The qPCR assay demonstrated diagnostic sensitivity of 100% (n = 19, 95% CI= (82.3%; 100%)) and diagnostic specificity of 100% (n = 31; 95% CI= (88.8%; 100%)) in detecting pfhrp2 and pfhrp3 deletions. In addition, the assay estimates P. falciparum parasite density and accurately detects pfhrp2 and pfhrp3 deletions masked in polyclonal infections. We report pfhrp2 and pfhrp3 deletions in parasite isolates from Kenya, Tanzania and in UK travellers.

INTERPRETATION

The new qPCR is easily scalable to routine surveillance studies in countries where P. falciparum parasites lacking pfhrp2 and pfhrp3 are a threat to malaria control.

Plasmodium falciparum Merozoite Associated Armadillo Protein (PfMAAP) Is Apically Localized in Free Merozoites and Antibodies Are Associated With Reduced Risk of Malaria

Understanding the functional role of proteins expressed by Plasmodium falciparum is an important step toward unlocking potential targets for the development of therapeutic or diagnostic interventions. The armadillo (ARM) repeat protein superfamily is associated with varied functions across the eukaryotes. Therefore, it is important to understand the role of members of this protein family in Plasmodium biology. The Plasmodium falciparum armadillo repeats only (PfARO; Pf3D7_0414900) and P. falciparum merozoite organizing proteins (PfMOP; Pf3D7_0917000) are armadillo-repeat containing proteins previously characterized in P. falciparum. Here, we describe the characterization of another ARM repeat-containing protein in P. falciparum, which we have named the P. falciparum Merozoites-Associated Armadillo repeats protein (PfMAAP). Antibodies raised to three different synthetic peptides of PfMAAP show apical staining of free merozoites and those within the mature infected schizont. We also demonstrate that the antibodies raised to the PfMAAP peptides inhibited invasion of erythrocytes by merozoites from different parasite isolates. In addition, naturally acquired human antibodies to the N- and C- termini of PfMAAP are associated with a reduced risk of malaria in a prospective cohort analysis.

Quantification of malaria antigens PfHRP2 and pLDH by quantitative suspension array technology in whole blood, dried blood spot and plasma

BACKGROUND

Malaria diagnostics by rapid diagnostic test (RDT) relies primarily on the qualitative detection of Plasmodium falciparum histidine-rich protein 2 (PfHRP2) and Plasmodium spp lactate dehydrogenase (pLDH). As novel RDTs with increased sensitivity are being developed and implemented as point of care diagnostics, highly sensitive laboratory-based assays are needed for evaluating RDT performance. Here, a quantitative suspension array technology (qSAT) was developed, validated and applied for the simultaneous detection of PfHRP2 and pLDH in a variety of biological samples (whole blood, plasma and dried blood spots) from individuals living in different endemic countries.

RESULTS

The qSAT was specific for the target antigens, with analytical ranges of 6.8 to 762.8 pg/ml for PfHRP2 and 78.1 to 17076.6 pg/ml for P. falciparum LDH (Pf-LDH). The assay detected Plasmodium vivax LDH (Pv-LDH) at a lower sensitivity than Pf-LDH (analytical range of 1093.20 to 187288.5 pg/ml). Both PfHRP2 and pLDH levels determined using the qSAT showed to positively correlate with parasite densities determined by quantitative PCR (Spearman r = 0.59 and 0.75, respectively) as well as microscopy (Spearman r = 0.40 and 0.75, respectively), suggesting the assay to be a good predictor of parasite density.

CONCLUSION

This immunoassay can be used as a reference test for the detection and quantification of PfHRP2 and pLDH, and could serve for external validation of RDT performance, to determine antigen persistence after parasite clearance, as well as a complementary tool to assess malaria burden in endemic settings.

A multiplex qPCR approach for detection of pfhrp2 and pfhrp3 gene deletions in multiple strain infections of Plasmodium falciparum

The rapid and accurate diagnosis of Plasmodium falciparum malaria infection is an essential factor in malaria control. Currently, malaria diagnosis in the field depends heavily on using rapid diagnostic tests (RDTs) many of which detect circulating parasite-derived histidine-rich protein 2 antigen (PfHRP2) in capillary blood. P. falciparum strains lacking PfHRP2, due to pfhrp2 gene deletions, are an emerging threat to malaria control programs. The novel assay described here, named qHRP2/3-del, is well suited for high-throughput screening of P. falciparum isolates to identify these gene deletions. The qHRP2/3-del assay identified pfhrp2 and pfhrp3 deletion status correctly in 93.4% of samples with parasitemia levels higher than 5 parasites/µL when compared to nested PCR. The qHRP2/3-del assay can correctly identify pfhrp2 and pfhrp3 gene deletions in multiple strain co-infections, particularly prevalent in Sub-Saharan countries. Deployment of this qHRP2/3-del assay will provide rapid insight into the prevalence and potential spread of P. falciparum isolates that escape surveillance by RDTs.

Plasmodium falciparum histidine-rich protein (PfHRP2 and 3) diversity in Western and Coastal Kenya

Plasmodium falciparum histidine-rich proteins 2 (PfHRP2) based RDTs are advocated in falciparum malaria-endemic regions, particularly when quality microscopy is not available. However, diversity and any deletion in the pfhrp2 and pfhrp3 genes can affect the performance of PfHRP2-based RDTs. A total of 400 samples collected from uncomplicated malaria cases from Kenya were investigated for the amino acid repeat profiles in exon 2 of pfhrp2 and pfhrp3 genes. In addition, PfHRP2 levels were measured in 96 individuals with uncomplicated malaria. We observed a unique distribution pattern of amino acid repeats both in the PfHRP2 and PfHRP3. 228 PfHRP2 and 124 PfHRP3 different amino acid sequences were identified. Of this, 214 (94%) PfHRP2 and 81 (65%) PfHRP3 amino acid sequences occurred only once. Thirty-nine new PfHRP2 and 20 new PfHRP3 amino acid repeat types were identified. PfHRP2 levels were not correlated with parasitemia or the number of PfHRP2 repeat types. This study shows the variability of PfHRP2, PfHRP3 and PfHRP2 concentration among uncomplicated malaria cases. These findings will be useful to understand the performance of PfHRP2-based RDTs in Kenya.

Diagnostic capacity, and predictive values of rapid diagnostic tests for accurate diagnosis of Plasmodium falciparum in febrile children in Asante-Akim, Ghana

Background

This study seeks to compare the performance of HRP2 (First Response) and pLDH/HRP2 (Combo) RDTs for falciparum malaria against microscopy and PCR in acutely ill febrile children at presentation and follow-up.

Methods

This is an interventional study that recruited children < 5 years who reported to health facilities with a history of fever within the past 72 h or a documented axillary temperature of 37.5 °C. Using a longitudinal approach, recruitment and follow-up of participants was done between January and May 2012. Based on results of HRP2-RDT screening, the children were grouped into one of the following three categories: (1) tested positive for malaria using RDT and received anti-malarial treatment (group 1, n = 85); (2) tested negative for malaria using RDT and were given anti-malarial treatment by the admitting physician (group 2, n = 74); or, (3) tested negative for malaria using RDT and did not receive any anti-malarial treatment (group 3, n = 101). Independent microscopy, PCR and Combo-RDT tests were done for each sample on day 0 and all follow-up days.

Results

Mean age of the study participants was 22 months and females accounted for nearly 50%. At the time of diagnosis, the mean body temperature was 37.9 °C (range 35–40.1 °C). Microscopic parasite density ranged between 300 and 99,500 parasites/µL. With microscopy as gold standard, the sensitivity of HRP2 and Combo-RDTs were 95.1 and 96.3%, respectively. The sensitivities, specificities and predictive values for RDTs were relatively higher in microscopy-defined malaria cases than in PCR positive-defined cases. On day 0, participants who initially tested negative for HRP2 were positive by microscopy (n = 2), Combo (n = 1) and PCR (n = 17). On days 1 and 2, five of the children in this group (initially HRP2-negative) tested positive by PCR alone. On day 28, four patients who were originally HRP2-negative tested positive for microscopy (n = 2), Combo (n = 2) and PCR (n = 4).

Conclusion

The HRP2/pLDH RDTs showed comparable diagnostic accuracy in children presenting with an acute febrile illness to health facilities in a hard-to-reach rural area in Ghana. Nevertheless, discordant results recorded on day 0 and follow-up visits using the recommended RDTs means improved malaria diagnostic capability in malaria-endemic regions is necessary.

High proportions of pfhrp2 gene deletion and performance of HRP2-based rapid diagnostic test in Plasmodium falciparum field isolates of Odisha

BACKGROUND

With the documentation of cases of falciparum malaria negative by rapid diagnostic tests (RDT), though at low frequency from natural isolates in a small pocket of Odisha, it became absolutely necessary to investigate the status of HRP-2 based RDT throughout the state and in different seasons of the year.

METHODS

Suspected individuals were screened for malaria infection by microscopy and RDT in 25/30 districts of Odisha, India. Discrepancies in results were confirmed by PCR. False negative RDT samples for Plasmodium falciparum mono-infection were evaluated for detection of HRP2 antigen in ELISA and genotyped for pfhrp2, pfhrp3 and their flanking genes. Multiplicity of infection was ascertained based on msp1 and msp2 genotyping and parasitaemia level was determined by microscopy.

RESULTS

Of the total 1058 patients suspected for malaria, 384 were microscopically confirmed for P. falciparum mono-infection and RDT failure was observed in 58 samples at varying proportion in different regions of the state. The failure in detection was due to undetectable level of HRP-2. Although most of these samples were screened during rainy season (45/345), significantly high proportion (9/17) of RDT negative samples were obtained during the summer compared to rainy season (P = 0.0002; OR = 7.5). PCR genotyping of pfhrp2 and pfhrp3 in RDT negative samples showed 38/58 (65.5) samples to be pfhrp2 negative and 24/58 (41.4) to be pfhrp3 negative including dual negative in 17/58 (29.3). Most of the RDT negative samples (39/58) were with single genotype infection and high proportions of pfhrp2 deletion (7/9) was observed in summer. No difference in parasitaemia level was observed between RDT positive and RDT negative patients.

CONCLUSION

High prevalence of parasites with pfhrp2 deletion including dual deletions (pfhrp2 and pfhrp3) is a serious cause of concern, as these patients could not be given a correct diagnosis and treatment. Therefore, HRP2-based RDT for diagnosing P. falciparum infection in Odisha is non-reliable and must be performed in addition to or replaced by other appropriate diagnostic tools for clinical management of the disease.

Evidence for histidine-rich protein 2 immune complex formation in symptomatic patients in Southern Zambia

Background

Rapid diagnostic tests based on histidine-rich protein 2 (HRP2) detection are the primary tools used to detect Plasmodium falciparum malaria infections. Recent conflicting reports call into question whether α-HRP2 antibodies are present in human host circulation and if resulting immune complexes could interfere with HRP2 detection on malaria RDTs. This study sought to determine the prevalence of immune-complexed HRP2 in a low-transmission region of Southern Zambia.

Methods

An ELISA was used to quantify HRP2 in patient sample DBS extracts before and after heat-based immune complex dissociation. A pull-down assay reliant on proteins A, G, and L was developed and applied for IgG and IgM capture and subsequent immunoprecipitation of any HRP2 present in immune complexed form. A total of 104 patient samples were evaluated using both methods.

Results

Immune-complexed HRP2 was detectable in 17% (18/104) of all samples evaluated and 70% (16/23) of HRP2-positive samples. A majority of the patients with samples containing immune-complexed HRP2 had P. falciparum infections (11/18) and were also positive for free HRP2 (16/18). For 72% (13/18) of patients with immune-complexed HRP2, less than 10% of the total HRP2 present was in immune-complexed form. For the remaining samples, a large proportion (≥ 20%) of total HRP2 was complexed with α-HRP2 antibodies.

Conclusions

Endogenous α-HRP2 antibodies form immune complexes with HRP2 in the symptomatic patient population of a low-transmission area in rural Southern Zambia. For the majority of patients, the percentage of HRP2 in immune complexes is low and does not affect HRP2-based malaria diagnosis. However, for some patients, a significant portion of the total HRP2 was in immune-complexed form. Future studies investigating the prevalence and proportion of immune-complexed HRP2 in asymptomatic individuals with low HRP2 levels will be required to assess whether α-HRP2 antibodies affect HRP2 detection for this portion of the transmission reservoir.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2400-8) contains supplementary material, which is available to authorized users.

Plasmodium falciparum Histidine-Rich Protein 2 Gene Variation in a Malaria-Endemic Area of Papua New Guinea

Histidine-rich protein 2 of Plasmodium falciparum (PfHRP2) forms the basis of many current malaria rapid diagnostic tests (RDTs). It is concerning that there are parasites that lack part or all of the pfhrp2 gene, and thus do not express the PfHRP2 protein; such parasites are not identifiable by PfHRP2-detecting RDTs. Very limited information is available regarding pfhrp2 genetic variation in Papua New Guinea (PNG). In the present study, this gene variation was evaluated using 169 samples previously collected from the Wosera area in East Sepik Province of PNG. Molecular diagnosis of these samples showed that 81% were infected, and P. falciparum was present in 91% of those infected samples. One hundred and twenty samples were amplified for pfhrp2 exon-2, from which 12 randomly selected amplicons were sequenced, yielding 18 sequences, all of which were unique. Baker repeat type 2 × type 7 numbers ranged from 0 to 108. Epitope mapping analysis revealed that three major epitopes, DAHHAHHA, AHHAADAHHA, and AHHAADAHH, were present in high prevalence and frequencies. These major epitopes have been shown to be recognized by the monoclonal antibodies 3A4 and PTL-3 (DAHHAHHA), C1-13 (AHHAADAHHA), and S2-5 and C2-3 (AHHAADAHH). This study provides further information on the high genetic variation of pfhrp2 and its unclear relationship with prediction of RDT detection sensitivity, and identifies major epitopes in this gene from PNG. These results could be relevant and useful to understand the genetic diversity of this gene and the performance of current and future RDTs in this malarious region of the world.

Quantification of Histidine-Rich Protein 3 of Plasmodium falciparum

Malaria is a life-threatening infectious disease and continues to be a major public health crisis in many parts of the tropical world. Plasmodium falciparum is responsible for the majority of mortality and morbidity associated with malaria. During the intraerythrocytic cycle, P. falciparum releases three proteins with high histidine content as follows: histidine-rich protein 1 (HRP1), histidine-rich protein 2 (HRP2), and histidine-rich protein 3 (HRP3). Currently, most of the diagnostic tests of P. falciparum infection target HRP2, and a number of monoclonal antibodies (mAbs) against HRP2 have been developed for use in HRP2 detection and quantification. When parasites have HRP2 deletions, the detection of HRP3 could augment the sensitivity of the detection system. The combination of both HRP2 and HRP3 mAbs in the detection system will enhance the test sensitivity. In the HRP quantitative enzyme-linked immunosorbent assay (ELISA), both HRP2 and HRP3 contribute to the result, but the relative contribution of HRP2 and HRP3 was unable to investigate, because of the nonavailability of HRP3 specific antibody ELISA. Hence an ELISA test system based on HRP3 is also essential for detection and quantification. There is not much documented in the literature on HRP3 antigen and HRP3 specific mAbs and polyclonal antibodies (pAbs). In the present study, recombinant HRP3 was expressed in Escherichia coli and purified with Ni-NTA agarose column. The purified rHRP3 was used for the generation and characterization of monoclonal and pAbs. The purification of monoclonal and pAbs was done using a mixed-mode chromatography sorbent, phenylpropylamine HyperCel™. With the purified antibodies, a sandwich ELISA was developed. The sandwich ELISA method was explored to detect and quantify HRP3 of P. falciparum in the spent medium. The generated mAbs could be potentially used for the detection and quantification of P. falciparum HRP3.

Insights into the Performance of SD Bioline Malaria Ag P.f/Pan Rapid Diagnostic Test and Plasmodium falciparum Histidine-Rich Protein 2 Gene Variation in Madagascar

Plasmodium falciparum histidine-rich protein 2 (PfHRP2) forms the basis of many current malaria rapid diagnostic tests (RDTs). However, the parasites lacking part or all of the pfhrp2 gene do not express the PfHRP2 protein and are, therefore, not identifiable by PfHRP2-detecting RDTs. We evaluated the performance of the SD Bioline Malaria Ag P.f/Pan RDT together with pfhrp2 variation in Madagascar. Genomic DNA isolated from 260 patient blood samples were polymerase chain reaction (PCR)-amplified for the parasite 18S rRNA and pfhrp2 genes. Post-PCR ligation detection reaction-fluorescent microsphere assay (LDR-FMA) was performed for the identification of parasite species. Plasmodium falciparum histidine-rich protein 2 amplicons were sequenced. Polymerase chain reaction diagnosis of patient samples showed that 29% (75/260) were infected and P. falciparum was present in 95% (71/75) of these PCR-positive samples. Comparing RDT and P. falciparum detection by LDR-FMA, eight samples were RDT negative but P. falciparum positive (false negatives), all of which were pfhrp2 positive. The sensitivity and specificity of the RDT were 87% and 90%, respectively. Seventy-three samples were amplified for pfhrp2, from which nine randomly selected amplicons were sequenced, yielding 13 sequences. Amplification of pfhrp2, combined with RDT analysis and P. falciparum detection by LDR-FMA, showed that there was no indication of pfhrp2 deletion. Sequence analysis of pfhrp2 showed that the correlation between pfhrp2 sequence structure and RDT detection rates was unclear. Although the observed absence of pfhrp2 deletion from the samples screened here is encouraging, continued monitoring of the efficacy of the SD Bioline Malaria Ag P.f/Pan RDT for malaria diagnosis in Madagascar is warranted.

Malaria surveys using rapid diagnostic tests and validation of results using post hoc quantification of Plasmodium falciparum histidine-rich protein 2

BACKGROUND

Rapid diagnostic test (RDT) positivity is supplanting microscopy as the standard measure of malaria burden at the population level. However, there is currently no standard for externally validating RDT results from field surveys.

METHODS

Individuals' blood concentration of the Plasmodium falciparum histidine rich protein 2 (HRP2) protein were compared to results of HRP2-detecting RDTs in participants from field surveys in Angola, Mozambique, Haiti, and Senegal. A logistic regression model was used to estimate the HRP2 concentrations corresponding to the 50 and 90% level of detection (LOD) specific for each survey.

RESULTS

There was a sigmoidal dose-response relationship between HRP2 concentration and RDT positivity for all surveys. Variation was noted in estimates for field RDT sensitivity, with the 50% LOD ranging between 0.076 and 6.1 ng/mL and the 90% LOD ranging between 1.1 and 53 ng/mL. Surveys conducted in two different provinces of Angola using the same brand of RDT and same study methodology showed a threefold difference in LOD.

CONCLUSIONS

Measures of malaria prevalence estimated using population RDT positivity should be interpreted in the context of potentially large variation in RDT LODs between, and even within, surveys. Surveys based on RDT positivity would benefit from external validation of field RDT results by comparing RDT positivity and antigen concentration.

Plasmodium falciparum parasites with histidine-rich protein 2 (pfhrp2) and pfhrp3 gene deletions in two endemic regions of Kenya

Deletions of the Plasmodium falciparum hrp2 and hrp3 genes can affect the performance of HRP2-based malaria rapid diagnostic tests (RDTs). Such deletions have been reported from South America, India and Eritrea. Whether these parasites are widespread in East Africa is unknown. A total of 274 samples from asymptomatic children in Mbita, western Kenya, and 61 genomic  data from Kilifi, eastern Kenya, were available for analysis. PCR-confirmed samples were investigated for the presence of pfhrp2 and pfhrp3 genes. In samples with evidence of deletion, parasite presence was confirmed by amplifying three independent genes. We failed to amplify pfhrp2 from 25 of 131 (19.1%) PCR-confirmed samples. Of these, only 8 (10%) samples were microscopic positive and were classified as pfhrp2-deleted. Eight microscopically-confirmed pfhrp2-deleted samples with intact pfhrp3 locus were positive by HRP2-based RDT. In addition, one PCR-confirmed infection showed a deletion at the pfhrp3 locus. One genomic sample lacked pfhrp2 and one lacked pfhrp3. No sample harbored parasites lacking both genes. Parasites lacking pfhrp2 are present in Kenya, but may be detectable by HRP-based RDT at higher parasitaemia, possibly due to the presence of intact pfhrp3. These findings warrant further systematic study to establish prevalence and diagnostic significance.

Case Report: A Case of Plasmodium falciparum hrp2 and hrp3 Gene Mutation in Bangladesh

Several species of Plasmodium are responsible for causing malaria in humans. Proper diagnoses are crucial to case management, because severity and treatment varies between species. Diagnoses can be made using rapid diagnostic tests (RDTs), which detect Plasmodium proteins. Plasmodium falciparum causes the most virulent cases of malaria, and P. falciparum histidine-rich protein 2 (PfHRP2) is a common target of falciparum malaria RDTs. Here we report a case in which a falciparum malaria patient in Bangladesh tested negative on PfHRP2-based RDTs. The negative results can be attributed to a deletion of part of the pfhrp2 gene and frameshift mutations in both pfhrp2 and pfhrp3 gene. This finding may have implications for malaria diagnostics and case management in Bangladesh and other regions of South Asia.

Plasmodium falciparum Histidine-Rich Protein-2 Plasma Concentrations Are Higher in Retinopathy-Negative Cerebral Malaria Than in Severe Malarial Anemia

Background

Malaria retinopathy has been proposed as marker of “true” cerebral malaria (CM), ie, coma due to Plasmodium falciparum vs coma due to other causes, with incidental P falciparum parasitemia. Plasma P falciparum histidine-rich protein-2 (PfHRP2) concentrations distinguish retinopathy-positive (RP) from retinopathy-negative (RN) CM but have not been compared between RN CM and other forms of severe malaria or asymptomatic parasitemia (AP).

Methods

We compared plasma PfHRP2 concentrations in 260 children with CM (247 examined for retinopathy), 228 children with severe malarial anemia (SMA), and 30 community children with AP.

Results

Plasmodium falciparum HRP2 concentrations were higher in children with RP CM than RN CM (P = .006), with an area under the receiver operating characteristic curve of 0.61 (95% confidence interval, 0.53–0.68). Plasmodium falciparum HRP2 concentrations and sequestered parasite biomass were higher in RN CM than SMA (both P < .03) or AP (both P < .001).

Conclusions

Plasmodium falciparum HRP2 concentrations are higher in children with RN CM than in children with SMA or AP, suggesting that P falciparum is involved in disease pathogenesis in children with CM. Plasmodium falciparum HRP2 concentrations may provide a more feasible and consistent assessment of the contribution of P falciparum to severe disease than malaria retinopathy.

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.

Bead-based immunoassay allows sub-picogram detection of histidine-rich protein 2 from Plasmodium falciparum and estimates reliability of malaria rapid diagnostic tests

Detection of histidine-rich protein 2 (HRP2) from the malaria parasite Plasmodium falciparum provides evidence for active or recent infection, and is utilized for both diagnostic and surveillance purposes, but current laboratory immunoassays for HRP2 are hindered by low sensitivities and high costs. Here we present a new HRP2 immunoassay based on antigen capture through a bead-based system capable of detecting HRP2 at sub-picogram levels. The assay is highly specific and cost-effective, allowing fast processing and screening of large numbers of samples. We utilized the assay to assess results of HRP2-based rapid diagnostic tests (RDTs) in different P. falciparum transmission settings, generating estimates for true performance in the field. Through this method of external validation, HRP2 RDTs were found to perform well in the high-endemic areas of Mozambique and Angola with 86.4% and 73.9% of persons with HRP2 in their blood testing positive by RDTs, respectively, and false-positive rates of 4.3% and 0.5%. However, in the low-endemic setting of Haiti, only 14.5% of persons found to be HRP2 positive by the bead assay were RDT positive. Additionally, 62.5% of Haitians showing a positive RDT test had no detectable HRP2 by the bead assay, likely indicating that these were false positive tests. In addition to RDT validation, HRP2 biomass was assessed for the populations in these different settings, and may provide an additional metric by which to estimate P. falciparum transmission intensity and measure the impact of interventions.

Genetic Characterisation of Plasmodium falciparum Isolates with Deletion of the pfhrp2 and/or pfhrp3 Genes in Colombia: The Amazon Region, a Challenge for Malaria Diagnosis and Control

Most Plasmodium falciparum-detecting rapid diagnostic tests (RDTs) target histidine-rich protein 2 (PfHRP2). However, P. falciparum isolates with deletion of the pfhrp2 gene and its homolog gene, pfhrp3, have been detected. We carried out an extensive investigation on 365 P. falciparum dried blood samples collected from seven P. falciparum endemic sites in Colombia between 2003 and 2012 to genetically characterise and geographically map pfhrp2- and/or pfhrp3-negative P. falciparum parasites in the country. We found a high proportion of pfhrp2-negative parasites only in Amazonas (15/39; 38.5%), and these parasites were also pfhrp3-negative. These parasites were collected between 2008 and 2009 in Amazonas, while pfhrp3-negative parasites (157/365, 43%) were found in all the sites and from each of the sample collection years evaluated (2003 to 2012). We also found that all pfhrp2- and/or pfhrp3-negative parasites were also negative for one or both flanking genes. Six sub-population clusters were established with 93.3% (14/15) of the pfhrp2-negative parasites grouped in the same cluster and sharing the same haplotype. This haplotype corresponded with the genetic lineage B_V1, a multidrug resistant strain that caused two outbreaks reported in Peru between 2010 and 2013. We found this B_V1 lineage in the Colombian Amazon as early as 2006. Two new clonal lineages were identified in these parasites from Colombia: the genetic lineages E_V1 and F. PfHRP2 sequence analysis revealed high genetic diversity at the amino acid level, with 17 unique sequences identified among 53 PfHRP2 sequences analysed. The use of PfHRP2-based RDTs is not recommended in Amazonas because of the high proportion of parasites with pfhrp2 deletion (38.5%), and implementation of new strategies for malaria diagnosis and control in Amazonas must be prioritised. Moreover, studies to monitor and genetically characterise pfhrp2-negative P. falciparum parasites in the Americas are warranted, given the extensive human migration occurring in the region.

Identification of Plasmodium falciparum apicoplast-targeted tRNA-guanine transglycosylase and its potential inhibitors using comparative genomics, molecular modelling, docking and simulation studies

tRNA modifications play an important role in the proper folding of tRNA and thereby determine its functionality as an adaptor molecule. Notwithstanding the centrality of this basic process in translation, a major gap in the genomics of Plasmodium falciparum is unambiguous identification of enzymes catalysing the various tRNA modifications. In this study, tRNA-modifying enzymes of P. falciparum were annotated using homology-based approach. Based on the presence of these identified enzymes, the modifications were compared with those of prokaryotic and eukaryotic organisms. Through sequence comparison and phylogenetic analysis, we have identified P. falciparum apicoplast tRNA-guanine 34 transglycosylase (TGT, EC: 2.4.2.29), which shows evidence of its prokaryotic origin. The docking analysis of the modelled TGT structures revealed that binding of quinazolinone derivatives is more favourable with P. falciparum apicoplast TGT as compared to human TGT. Molecular dynamic simulation and molecular mechanics/generalized Born surface area analysis of the complex confirmed the greater binding affinity of the ligand in the binding pocket of P. falciparum TGT protein. Further, evolutionary patterning analysis identified the amino acids of P. falciparum apicoplast TGT that are under purifying selection pressure and hence can be good inhibitor-targeting sites. Based on these computational studies, we suggest that P. falciparum apicoplast tRNA-guanine 34 transglycosylase can be a promising drug target.

Circulating antibodies against Plasmodium falciparum histidine-rich proteins 2 interfere with antigen detection by rapid diagnostic tests

BACKGROUND

Rapid diagnostic tests (RDTs) for detection of Plasmodium falciparum infection that target P. falciparum histidine-rich protein 2 (PfHRP2), a protein that circulates in the blood of patients infected with this species of malaria, are widely used to guide case management. Understanding determinants of PfHRP2 availability in circulation is therefore essential to understanding the performance of PfHRP2-detecting RDTs.

METHODS

The possibility that pre-formed host anti-PfHRP2 antibodies may block target antigen detection, thereby causing false negative test results was investigated in this study.

RESULTS

Anti-PfHRP2 antibodies were detected in 19/75 (25%) of plasma samples collected from patients with acute malaria from Cambodia, Nigeria and the Philippines, as well as in 3/28 (10.7%) asymptomatic Solomon Islands residents. Pre-incubation of plasma samples from subjects with high-titre anti-PfHRP2 antibodies with soluble PfHRP2 blocked the detection of the target antigen on two of the three brands of RDTs tested, leading to false negative results. Pre-incubation of the plasma with intact parasitized erythrocytes resulted in a reduction of band intensity at the highest parasite density, and a reduction of lower detection threshold by ten-fold on all three brands of RDTs tested.

CONCLUSIONS

These observations indicate possible reduced sensitivity for diagnosis of P. falciparum malaria using PfHRP2-detecting RDTs among people with high levels of specific antibodies and low density infection, as well as possible interference with tests configured to detect soluble PfHRP2 in saliva or urine samples. Further investigations are required to assess the impact of pre-formed anti-PfHRP2 antibodies on RDT performance in different transmission settings.

Plasma concentration of parasite DNA as a measure of disease severity in falciparum malaria

In malaria-endemic areas, Plasmodium falciparum parasitemia is common in apparently healthy children and severe malaria is commonly misdiagnosed in patients with incidental parasitemia. We assessed whether the plasma Plasmodium falciparum DNA concentration is a useful datum for distinguishing uncomplicated from severe malaria in African children and Asian adults. P. falciparum DNA concentrations were measured by real-time polymerase chain reaction (PCR) in 224 African children (111 with uncomplicated malaria and 113 with severe malaria) and 211 Asian adults (100 with uncomplicated malaria and 111 with severe malaria) presenting with acute falciparum malaria. The diagnostic accuracy of plasma P. falciparum DNA concentrations in identifying severe malaria was 0.834 for children and 0.788 for adults, similar to that of plasma P. falciparum HRP2 levels and substantially superior to that of parasite densities (P < .0001). The diagnostic accuracy of plasma P. falciparum DNA concentrations plus plasma P. falciparum HRP2 concentrations was significantly greater than that of plasma P. falciparum HRP2 concentrations alone (0.904 for children [P = .004] and 0.847 for adults [P = .003]). Quantitative real-time PCR measurement of parasite DNA in plasma is a useful method for diagnosing severe falciparum malaria on fresh or archived plasma samples.

Fatty acid synthesis and pyruvate metabolism pathways remain active in dihydroartemisinin-induced dormant ring stages of Plasmodium falciparum

Artemisinin (ART)-based combination therapy (ACT) is used as the first-line treatment of uncomplicated falciparum malaria worldwide. However, despite high potency and rapid action, there is a high rate of recrudescence associated with ART monotherapy or ACT long before the recent emergence of ART resistance. ART-induced ring-stage dormancy and recovery have been implicated as possible causes of recrudescence; however, little is known about the characteristics of dormant parasites, including whether dormant parasites are metabolically active. We investigated the transcription of 12 genes encoding key enzymes in various metabolic pathways in P. falciparum during dihydroartemisinin (DHA)-induced dormancy and recovery. Transcription analysis showed an immediate downregulation for 10 genes following exposure to DHA but continued transcription of 2 genes encoding apicoplast and mitochondrial proteins. Transcription of several additional genes encoding apicoplast and mitochondrial proteins, particularly of genes encoding enzymes in pyruvate metabolism and fatty acid synthesis pathways, was also maintained. Additions of inhibitors for biotin acetyl-coenzyme A (CoA) carboxylase and enoyl-acyl carrier reductase of the fatty acid synthesis pathways delayed the recovery of dormant parasites by 6 and 4 days, respectively, following DHA treatment. Our results demonstrate that most metabolic pathways are downregulated in DHA-induced dormant parasites. In contrast, fatty acid and pyruvate metabolic pathways remain active. These findings highlight new targets to interrupt recovery of parasites from ART-induced dormancy and to reduce the rate of recrudescence following ART treatment.

Development, purification, and characterization of monoclonal antibodies against recombinant histidine-rich protein 3 of Plasmodium falciparum

In the present study, monoclonal antibodies (MAbs) against recombinant histidine-rich protein (rHRP3) were developed and assessed for their potential in detection of Plasmodium falciparum HRP3. Hybridomas were obtained by fusion of Sp2/0 mouse myeloma cells and spleen cells from the mouse immunized with purified rHRP3. Three MAbs (IgG1 isotype) specific to rHRP3 were established and characterized by enzyme-linked immunosorbent assay (ELISA) and immunoblotting for sensitivity and specificity. Purification of MAbs from hybridoma cell culture supernatant and PAbs from rabbit anti-serum were carried out using Phenylpropylamine (PPA) HyperCel(™) sorbent. The MAbs were able to detect rHRP3 and the HRP3 from P. falciparum spent medium. Sandwich ELISA was developed to quantify HRP3 in the spent medium of P. falciparum culture. The generated MAbs could be potentially used in immuno-based diagnostic systems for the detection of P. falciparum HRP.

High performance of histidine-rich protein 2 based rapid diagnostic tests in French Guiana are explained by the absence of pfhrp2 gene deletion in P. falciparum

Background

Care for malaria patients in endemic areas has been improved through the increasing use of Rapid Diagnostic Tests (RDTs). Most RDTs target the histidine-rich protein-2 antigen (PfHRP₂) to detect P. falciparum, as it is abundant and shows great heat stability. However, their use in South America has been widely questioned following a recent publication that pinpoints the high prevalence of Peruvian field isolates lacking the gene encoding this protein. In the remote rural health centers of French Guiana, RDTs are the main diagnosis tools. Therefore, a study of PfHRP2 RDT performances and pfhrp2 genotyping was conducted to determine whether a replacement of the current pLDH-based kit could be considered.

Methods

The performance study compared the SD Malaria Ag test P.f/Pan® kit with the current gold standard diagnosis by microscopy. The prevalence of pfhrp2 and pfhrp3 deletions were evaluated from 221 P. falciparum isolates collected between 2009 and 2011 in French Guiana.

Results

Between January 2010 and August 2011, 960 suspected cases of malaria were analyzed using microscopy and RDTs. The sensitivity of the SD Malaria Ag test P.f/Pan® for detection of P. falciparum was 96.8% (95% CI: 90.9–99.3), and 86.0% (95% CI: 78.9–91.5) for the detection of P. vivax. No isolates (95% CI: 0–4.5) lacking either exon of the pfhrp₂ gene were identified among the 221 P. falciparum isolates analyzed, but 7.4% (95% CI: 2.8–15.4) lacked the exon 2 part of the pfhrp₃ gene.

Conclusions

Field isolates lacking either exon of the pfhrp₂ gene are absent in this western part of South America. Despite its sensibility to detect P. vivax, the SD Malaria Ag test P.f/Pan® kit is a satisfying alternative to microscopy in remote health centers, where it is difficult to provide highly skilled microscopists and to maintain the necessary equipment.

Current challenges in travelers' malaria

Travel health providers are often confronted with complex scenarios when advising travelers on malaria prevention. Current challenges in prevention include malaria risk assessment, where a detailed itinerary and knowledge of malaria epidemiology are needed. Up-to-date information on the correct use, limitations, and drug interactions of current priority chemoprophylaxis agents (atovaquone/proguanil, mefloquine, doxycycline) is key. Another challenge is to identify and reach travelers who are most at risk of malaria, such as the traveler visiting friends and relatives. Posttravel, delays in presentation, diagnosis, and inappropriate treatment of malaria are key risk factors leading to death. Treatment of malaria is an emergency requiring expert in-patient management and referral to a center with adequate expertise. Artemisinin combination therapies are the drugs of choice for uncomplicated malaria. Complicated malaria is treated preferably with intravenous artesunate, and the supply and quality of this life-saving antimalarial in some settings can pose one of the most urgent challenges in travelers' malaria.

Artemisinin based combination therapy in travel medicine

A steadily increasing number of Western travellers are exposed to malaria. Also, numbers of migrants from malarious areas are increasing. Fast and effective treatment options are needed to ensure effective malaria treatment in these groups in the future. Artemisinin combinations are well tolerated and have shown high efficacy in malaria endemic areas. Since 2001, 42 malaria endemic countries, 23 of them in Africa, have adopted artemisinin based combination therapies recommended by WHO. An additional 14 countries are in the process of changing their malaria treatment policy. Studies in non-immune travellers confirm a rapid parasite clearance time and very low rate of side effects. Outpatient clinics and hospitals in non-endemic countries should have standard operating procedures for diagnosing and managing patients with malaria. In this setting, artemisinin combinations should be available for treatment of uncomplicated malaria as they are clearly superior to any other oral antimalarial in their fast reduction of parasite biomass and in decreasing clinical symptoms. Also, they are the drugs of choice for travellers who are advised to carry stand-by emergency treatment during their journey.

Malaria rapid diagnostic tests in endemic settings

Malaria rapid diagnostic tests (RDTs) are instrument-free tests that provide results within 20 min and can be used by community health workers. RDTs detect antigens produced by the Plasmodium parasite such as Plasmodium falciparum histidine-rich protein-2 (PfHPR2) and Plasmodium lactate dehydrogenase (pLDH). The accuracy of RDTs for the diagnosis of uncomplicated P. falciparum infection is equal or superior to routine microscopy (but inferior to expert microscopy). Sensitivity for Plasmodium vivax is 75-100%; for Plasmodium ovale and Plasmodium malariae, diagnostic performance is poor. Design limitations of RDTs include poor sensitivity at low parasite densities, susceptibility to the prozone effect (PfHRP2-detecting RDTs), false-negative results due to PfHRP2 deficiency in the case of pfhrp2 gene deletions (PfHRP2-detecting RDTs), cross-reactions between Plasmodium antigens and detection antibodies, false-positive results by other infections and susceptibility to heat and humidity. End-user's errors relate to safety, procedure (delayed reading, incorrect sample and buffer volumes) and interpretation (not recognizing invalid test results, disregarding faint test lines). Withholding antimalarial treatment in the case of negative RDT results tends to be infrequent and tendencies towards over-prescription of antibiotics have been noted. Numerous shortcomings in RDT kits' labelling, instructions for use (correctness and readability) and contents have been observed. The World Health Organization and partners actively address quality assurance of RDTs by comparative testing of RDTs, inspections of manufacturing sites, lot testing and training tools but no formal external quality assessment programme of end-user performance exists. Elimination of malaria requires RDTs with lower detection limits, for which nucleic acid amplification tests are under development.

Pfhrp2 and pfhrp3 polymorphisms in Plasmodium falciparum isolates from Dakar, Senegal: impact on rapid malaria diagnostic tests

Background

An accurate diagnosis is essential for the rapid and appropriate treatment of malaria. The accuracy of the histidine-rich protein 2 (PfHRP2)-based rapid diagnostic test (RDT) Palutop+4® was assessed here. One possible factor contributing to the failure to detect malaria by this test is the diversity of the parasite PfHRP2 antigens.

Methods

PfHRP2 detection with the Palutop+4® RDT was carried out. The pfhrp2 and pfhrp3 genes were amplified and sequenced from 136 isolates of Plasmodium falciparum that were collected in Dakar, Senegal from 2009 to 2011. The DNA sequences were determined and statistical analyses of the variation observed between these two genes were conducted. The potential impact of PfHRP2 and PfHRP3 sequence variation on malaria diagnosis was examined.

Results

Seven P. falciparum isolates (5.9% of the total isolates, regardless of the parasitaemia; 10.7% of the isolates with parasitaemia ≤0.005% or ≤250 parasites/μl) were undetected by the PfHRP2 Palutop+4® RDT. Low parasite density is not sufficient to explain the PfHRP2 detection failure. Three of these seven samples showed pfhrp2 deletion (2.4%). The pfhrp3 gene was deleted in 12.8%. Of the 122 PfHRP2 sequences, 120 unique sequences were identified. Of the 109 PfHRP3 sequences, 64 unique sequences were identified. Using the Baker’s regression model, at least 7.4% of the P. falciparum isolates in Dakar were likely to be undetected by PfHRP2 at a parasite density of ≤250 parasites/μl (slightly lower than the evaluated prevalence of 10.7%). This predictive prevalence increased significantly between 2009 and 2011 (P = 0.0046).

Conclusion

In the present work, 10.7% of the isolates with a parasitaemia ≤0.005% (≤250 parasites/μl) were undetected by the PfHRP2 Palutop+4® RDT (7.4% by the predictive Baker’model). In addition, all of the parasites with pfhrp2 deletion (2.4% of the total samples) and 2.1% of the parasites with parasitaemia >0.005% and presence of pfhrp2 were not detected by PfHRP2 RDT. PfHRP2 is highly polymorphic in Senegal. Efforts should be made to more accurately determine the prevalence of non-sensitive parasites to pfHRP2.

Prevalence of PCR detectable malaria infection among febrile patients with a negative Plasmodium falciparum specific rapid diagnostic test in Zanzibar

We screened for malaria in 594 blood samples from febrile patients who tested negative by a Plasmodium falciparum-specific histidine-rich protein-2-based rapid diagnostic test at 12 health facilities in Zanzibar districts North A and Micheweni, from May to August 2010. Screening was with microscopy, polymerase chain reaction (PCR) targeting the cytochrome b gene (cytbPCR) of the four major human malaria species, and quantitative PCR (qPCR). The prevalence of cytbPCR-detectable malaria infection was 2% (12 of 594), including 8 P. falciparum, 3 Plasmodium malariae, and 1 Plasmodium vivax infections. Microscopy identified 4 of 8 P. falciparum infections. Parasite density as estimated by microscopy or qPCR was > 4,000 parasites/μL in 5 of 8 cytbPCR-detectable P. falciparum infections. The infections that were missed by the rapid diagnostic test represent a particular challenge in malaria elimination settings and highlight the need for more sensitive point-of-care diagnostic tools to improve case detection of all human malaria species in febrile patients.

Management of imported malaria in Europe

In this position paper, the European Society for Clinical Microbiology and Infectious Diseases, Study Group on Clinical Parasitology, summarizes main issues regarding the management of imported malaria cases. Malaria is a rare diagnosis in Europe, but it is a medical emergency. A travel history is the key to suspecting malaria and is mandatory in patients with fever. There are no specific clinical signs or symptoms of malaria although fever is seen in almost all non-immune patients. Migrants from malaria endemic areas may have few symptoms.Malaria diagnostics should be performed immediately on suspicion of malaria and the gold- standard is microscopy of Giemsa-stained thick and thin blood films. A Rapid Diagnostic Test (RDT) may be used as an initial screening tool, but does not replace urgent microscopy which should be done in parallel. Delays in microscopy, however, should not lead to delayed initiation of appropriate treatment. Patients diagnosed with malaria should usually be hospitalized. If outpatient management is preferred, as is the practice in some European centres, patients must usually be followed closely (at least daily) until clinical and parasitological cure. Treatment of uncomplicated Plasmodium falciparum malaria is either with oral artemisinin combination therapy (ACT) or with the combination atovaquone/proguanil. Two forms of ACT are available in Europe: artemether/lumefantrine and dihydroartemisinin/piperaquine. ACT is also effective against Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium knowlesi, but these species can be treated with chloroquine. Treatment of persistent liver forms in P. vivax and P. ovale with primaquine is indicated after excluding glucose 6 phosphate dehydrogenase deficiency. There are modified schedules and drug options for the treatment of malaria in special patient groups, such as children and pregnant women. The potential for drug interactions and the role of food in the absorption of anti-malarials are important considerations in the choice of treatment.Complicated malaria is treated with intravenous artesunate resulting in a much more rapid decrease in parasite density compared to quinine. Patients treated with intravenous artesunate should be closely monitored for haemolysis for four weeks after treatment. There is a concern in some countries about the lack of artesunate produced according to Good Manufacturing Practice (GMP).

Rapid diagnostic tests for malaria diagnosis in the Peruvian Amazon: impact of pfhrp2 gene deletions and cross-reactions

Background

In the Peruvian Amazon, Plasmodium falciparum and Plasmodium vivax malaria are endemic in rural areas, where microscopy is not available. Malaria rapid diagnostic tests (RDTs) provide quick and accurate diagnosis. However, pfhrp2 gene deletions may limit the use of histidine-rich protein-2 (PfHRP2) detecting RDTs. Further, cross-reactions of P. falciparum with P. vivax-specific test lines and vice versa may impair diagnostic specificity.

Methods

Thirteen RDT products were evaluated on 179 prospectively collected malaria positive samples. Species diagnosis was performed by microscopy and confirmed by PCR. Pfhrp2 gene deletions were assessed by PCR.

Results

Sensitivity for P. falciparum diagnosis was lower for PfHRP2 compared to P. falciparum-specific Plasmodium lactate dehydrogenase (Pf-pLDH)- detecting RDTs (71.6% vs. 98.7%, p<0.001). Most (19/21) false negative PfHRP2 results were associated with pfhrp2 gene deletions (25.7% of 74 P. falciparum samples). Diagnostic sensitivity for P. vivax (101 samples) was excellent, except for two products. In 10/12 P. vivax-detecting RDT products, cross-reactions with the PfHRP2 or Pf-pLDH line occurred at a median frequency of 2.5% (range 0%–10.9%) of P. vivax samples assessed. In two RDT products, two and one P. falciparum samples respectively cross-reacted with the Pv-pLDH line. Two Pf-pLDH/pan-pLDH-detecting RDTs showed excellent sensitivity with few (1.0%) cross-reactions but showed faint Pf-pLDH lines in 24.7% and 38.9% of P. falciparum samples.

Conclusion

PfHRP2-detecting RDTs are not suitable in the Peruvian Amazon due to pfhrp2 gene deletions. Two Pf-pLDH-detecting RDTs performed excellently and are promising RDTs for this region although faint test lines are of concern.

Sequence variation does not confound the measurement of plasma PfHRP2 concentration in African children presenting with severe malaria

Background

Plasmodium falciparum histidine-rich protein PFHRP2 measurement is used widely for diagnosis, and more recently for severity assessment in falciparum malaria. The Pfhrp2 gene is highly polymorphic, with deletion of the entire gene reported in both laboratory and field isolates. These issues potentially confound the interpretation of PFHRP2 measurements.

Methods

Studies designed to detect deletion of Pfhrp2 and its paralog Pfhrp3 were undertaken with samples from patients in seven countries contributing to the largest hospital-based severe malaria trial (AQUAMAT). The quantitative relationship between sequence polymorphism and PFHRP2 plasma concentration was examined in samples from selected sites in Mozambique and Tanzania.

Results

There was no evidence for deletion of either Pfhrp2 or Pfhrp3 in the 77 samples with lowest PFHRP2 plasma concentrations across the seven countries. Pfhrp2 sequence diversity was very high with no haplotypes shared among 66 samples sequenced. There was no correlation between Pfhrp2 sequence length or repeat type and PFHRP2 plasma concentration.

Conclusions

These findings indicate that sequence polymorphism is not a significant cause of variation in PFHRP2 concentration in plasma samples from African children. This justifies the further development of plasma PFHRP2 concentration as a method for assessing African children who may have severe falciparum malaria. The data also add to the existing evidence base supporting the use of rapid diagnostic tests based on PFHRP2 detection.

Modelling the dynamics of Plasmodium falciparum histidine-rich protein 2 in human malaria to better understand malaria rapid diagnostic test performance

BACKGROUND

Effective diagnosis of malaria is a major component of case management. Rapid diagnostic tests (RDTs) based on Plasmodium falciparumhistidine-rich protein 2 (PfHRP2) are popular for diagnosis of this most virulent malaria infection. However, concerns have been raised about the longevity of the PfHRP2 antigenaemia following curative treatment in endemic regions.

METHODS

A model of PfHRP2 production and decay was developed to mimic the kinetics of PfHRP2 antigenaemia during infections. Data from two human infection studies was used to fit the model, and to investigate PfHRP2 kinetics. Four malaria RDTs were assessed in the laboratory to determine the minimum detectable concentration of PfHRP2.

RESULTS

Fitting of the PfHRP2 dynamics model indicated that in malaria naïve hosts, P. falciparum parasites of the 3D7 strain produce 1.4 × 10⁻¹³ g of PfHRP2 per parasite per replication cycle. The four RDTs had minimum detection thresholds between 6.9 and 27.8 ng/mL. Combining these detection thresholds with the kinetics of PfHRP2, it is predicted that as few as 8 parasites/μL may be required to maintain a positive RDT in a chronic infection.

CONCLUSIONS

The results of the model indicate that good quality PfHRP2-based RDTs should be able to detect parasites on the first day of symptoms, and that the persistence of the antigen will cause the tests to remain positive for at least seven days after treatment. The duration of a positive test result following curative treatment is dependent on the duration and density of parasitaemia prior to treatment and the presence and affinity of anti-PfHRP2 antibodies.

Identification of optimal epitopes for Plasmodium falciparum rapid diagnostic tests that target histidine-rich proteins 2 and 3

Rapid diagnostic tests (RDTs) represent important tools to diagnose malaria infection. To improve understanding of the variable performance of RDTs that detect the major target in Plasmodium falciparum, namely, histidine-rich protein 2 (HRP2), and to inform the design of better tests, we undertook detailed mapping of the epitopes recognized by eight HRP-specific monoclonal antibodies (MAbs). To investigate the geographic skewing of this polymorphic protein, we analyzed the distribution of these epitopes in parasites from geographically diverse areas. To identify an ideal amino acid motif for a MAb to target in HRP2 and in the related protein HRP3, we used a purpose-designed script to perform bioinformatic analysis of 448 distinct gene sequences from pfhrp2 and from 99 sequences from the closely related gene pfhrp3. The frequency and distribution of these motifs were also compared to the MAb epitopes. Heat stability testing of MAbs immobilized on nitrocellulose membranes was also performed. Results of these experiments enabled the identification of MAbs with the most desirable characteristics for inclusion in RDTs, including copy number and coverage of target epitopes, geographic skewing, heat stability, and match with the most abundant amino acid motifs identified. This study therefore informs the selection of MAbs to include in malaria RDTs as well as in the generation of improved MAbs that should improve the performance of HRP-detecting malaria RDTs.

Diagnosing severe falciparum malaria in parasitaemic African children: a prospective evaluation of plasma PfHRP2 measurement

BACKGROUND

In African children, distinguishing severe falciparum malaria from other severe febrile illnesses with coincidental Plasmodium falciparum parasitaemia is a major challenge. P. falciparum histidine-rich protein 2 (PfHRP2) is released by mature sequestered parasites and can be used to estimate the total parasite burden. We investigated the prognostic significance of plasma PfHRP2 and used it to estimate the malaria-attributable fraction in African children diagnosed with severe malaria.

METHODS AND FINDINGS

Admission plasma PfHRP2 was measured prospectively in African children (from Mozambique, The Gambia, Kenya, Tanzania, Uganda, Rwanda, and the Democratic Republic of the Congo) aged 1 month to 15 years with severe febrile illness and a positive P. falciparum lactate dehydrogenase (pLDH)-based rapid test in a clinical trial comparing parenteral artesunate versus quinine (the AQUAMAT trial, ISRCTN 50258054). In 3,826 severely ill children, Plasmadium falciparum PfHRP2 was higher in patients with coma (p = 0.0209), acidosis (p<0.0001), and severe anaemia (p<0.0001). Admission geometric mean (95%CI) plasma PfHRP2 was 1,611 (1,350-1,922) ng/mL in fatal cases (n = 381) versus 1,046 (991-1,104) ng/mL in survivors (n = 3,445, p<0.0001), without differences in parasitaemia as assessed by microscopy. There was a U-shaped association between log(10) plasma PfHRP2 and risk of death. Mortality increased 20% per log(10) increase in PfHRP2 above 174 ng/mL (adjusted odds ratio [AOR] 1.21, 95%CI 1.05-1.39, p = 0.009). A mechanistic model assuming a PfHRP2-independent risk of death in non-malaria illness closely fitted the observed data and showed malaria-attributable mortality less than 50% with plasma PfHRP2≤174 ng/mL. The odds ratio (OR) for death in artesunate versus quinine-treated patients was 0.61 (95%CI 0.44-0.83, p = 0.0018) in the highest PfHRP2 tertile, whereas there was no difference in the lowest tertile (OR 1.05; 95%CI 0.69-1.61; p = 0.82). A limitation of the study is that some conclusions are drawn from a mechanistic model, which is inherently dependent on certain assumptions. However, a sensitivity analysis of the model indicated that the results were robust to a plausible range of parameter estimates. Further studies are needed to validate our findings.

CONCLUSIONS

Plasma PfHRP2 has prognostic significance in African children with severe falciparum malaria and provides a tool to stratify the risk of "true" severe malaria-attributable disease as opposed to other severe illnesses in parasitaemic African children.