Evaluation of three rapid diagnostic tests for the detection of human infections with Plasmodium knowlesi

Detection of Plasmodium knowlesi in whole blood samples with sandwich enzyme-linked immunosorbent assay (ELISA) using rhoptry-associated protein 1 specific polyclonal antibodies

BACKGROUND OBJECTIVES

Plasmodium knowlesi, a simian malaria species, is now known to infect humans. Due to disadvantages in the current diagnosis methods, many efforts have been placed into developing new methods to diagnose the disease. This study assessed the ability of the PkRAP-1 sandwich enzyme-linked immunosorbent (ELISA) to detect P knowlesi antigens in whole blood specimens.

METHODS

Western blot assay was conducted to evaluate the ability of raised mouse and rabbit anti-PkRAP-1 polyclonal antibodies to bind to the native proteins in P. knowlesi lysate. The polyclonal antibodies were then used in sandwich ELISA to detect P. knowlesi. In the sandwich ELISA, mouse and rabbit polyclonal antibodies were used as the capture and detection antibodies, respectively. The limit of detection (LOD) of the assay was determined using P. knowlesi A1H1 culture and purified recombinant PkRAP-1.

RESULTS

Western blot results showed positive reactions towards the proteins in P. knowlesi lysate. The LOD of the assay from three technical replicates was 0.068% parasitaemia. The assay performance in detecting P. knowlesi was 83% sensitivity and 70% specificity with positive and negative predictive values of 74% and 80%, respectively. The anti-PkRAP-1 polyclonal antibodies did not cross-react with P. falciparum and healthy samples, but P. vivax by detecting all 12 samples.

INTERPRETATION CONCLUSION

PkRAP-1 has the potential as a biomarker for the development of a new diagnostic tool for P. knowlesi detection. Further studies need to be conducted to establish the full potential of the usage of anti-PkRAP-1 antibodies for P. knowlesi detection.

Clinical characteristics and outcome of severe malaria in Kapit, Sarawak, Malaysian Borneo

Background & objectives

Severe malaria is a medical emergency and can lead to severe complications and death if not treated promptly and appropriately. Along with Plasmodium falciparum, P. knowlesi is increasingly recognised as a significant cause of fatal and severe malaria.

Methods

We performed a retrospective review on 54 cases of severe malaria in a district hospital in Kapit, Sarawak, from January 2018 to May 2019. The patients' demographics, clinical features, complications based on organ involvement, and treatment outcomes were examined.

Results

There were 54 cases of severe malaria, with the majority being male (70%) and between the ages of 40 and 49 (26%). All patients with severe malaria were febrile or had a history of pyrexia except for one patient. P. knowlesi (81.5%) was the most common species causing severe malaria in our study, followed by P. falciparum (13%), and P. vivax (5.5%). There were no cases of severe malaria caused by P. ovale or P. malariae. Hyperparasitaemia was present in 76% of patients and the median parasitemia value at hospital admission was 33,944 parasites/μL (interquartile range: 19,920-113,285 parasites/μL). Circulatory shock was observed in 17 patients (31.5%). There were eight patients with acute renal failure and six patients with respiratory distress. One patient died as a result of severe malaria with multiorgan involvement (1.9% fatality rate).

Interpretation & conclusion

P. knowlesi is the most common cause of severe malaria in Kapit, Sarawak, Malaysia. Recognizing symptoms of severe malaria and prompt administration of antimalarial are critical for good clinical outcomes.

Primate malaria of human importance

Nonhuman primate (NHP) malaria poses a major threat to the malaria control programs. The last two decades have witnessed a paradigm shift in our understanding of the malaria caused by species other than the traditionally known human Plasmodium species - Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale. The emergence of the malaria parasite of long-tailed macaque monkeys, Plasmodium knowlesi, as the fifth malaria species of humans has made the scientific community consider the risk of other zoonotic malaria, such as Plasmodium cynomolgi, Plasmodium simium, Plasmodium inui, and others, to humans. The development of knowledge about P. knowlesi as a pathogen which was earlier only known to experimentally cause malaria in humans and rarely cause natural infection, toward its acknowledgment as a significant cause of human malaria and a threat of malaria control programs has been made possible by the use of advanced molecular techniques such as polymerase chain reaction and gene sequencing. This review explores the various aspects of NHP malaria, and the association of various factors with their emergence and potential to cause human malaria which are important to understand to be able to control these emerging infections.

Diagnostic accuracy and limit of detection of ten malaria parasite lactate dehydrogenase-based rapid tests for Plasmodium knowlesi and P. falciparum

Background

Plasmodium knowlesi causes zoonotic malaria across Southeast Asia. First-line diagnostic microscopy cannot reliably differentiate P. knowlesi from other human malaria species. Rapid diagnostic tests (RDTs) designed for P. falciparum and P. vivax are used routinely in P. knowlesi co-endemic areas despite potential cross-reactivity for species-specific antibody targets.

Methods

Ten RDTs were evaluated: nine to detect clinical P. knowlesi infections from Malaysia, and nine assessing limit of detection (LoD) for P. knowlesi (PkA1-H.1) and P. falciparum (Pf3D7) cultures. Targets included Plasmodium-genus parasite lactate dehydrogenase (pan-pLDH) and P. vivax (Pv)-pLDH.

Results

Samples were collected prior to antimalarial treatment from 127 patients with microscopy-positive PCR-confirmed P. knowlesi mono-infections. Median parasitaemia was 788/µL (IQR 247-5,565/µL). Pan-pLDH sensitivities ranged from 50.6% (95% CI 39.6–61.5) (SD BIOLINE) to 87.0% (95% CI 75.1–94.6) (First Response^® and CareStart™ PAN) compared to reference PCR. Pv-pLDH RDTs detected P. knowlesi with up to 92.0% (95% CI 84.3-96.7%) sensitivity (Biocredit™). For parasite counts ≥200/µL, pan-pLDH (Standard Q) and Pv-pLDH RDTs exceeded 95% sensitivity. Specificity of RDTs against 26 PCR-confirmed negative controls was 100%. Sensitivity of six highest performing RDTs were not significantly different when comparing samples taken before and after (median 3 hours) antimalarial treatment. Parasite ring stages were present in 30% of pre-treatment samples, with ring stage proportions (mean 1.9%) demonstrating inverse correlation with test positivity of Biocredit™ and two CareStart™ RDTs.

For cultured P. knowlesi, CareStart™ PAN demonstrated the lowest LoD at 25 parasites/µL; LoDs of other pan-pLDH ranged from 98 to >2000 parasites/µL. Pv-pLDH LoD for P. knowlesi was 49 parasites/µL. No false-positive results were observed in either P. falciparum-pLDH or histidine-rich-protein-2 channels.

Conclusion

Selected RDTs demonstrate sufficient performance for detection of major human malaria species including P. knowlesi in co-endemic areas where microscopy is not available, particularly for higher parasite counts, although cannot reliably differentiate among non-falciparum malaria.

Non-Human Primate Malaria Infections: A Review on the Epidemiology in Malaysia

Malaria remains a public health problem in many parts of the world, including Malaysia. Although Malaysia has been recognized as one of the countries free from indigenous human malaria since 2018, the rising trend of zoonotic malaria, particularly Plasmodium knowlesi cases, poses a threat to public health and is of great concern to the country’s healthcare system. We reviewed previously scattered information on zoonotic malaria infections in both Peninsular Malaysia and Malaysian Borneo to determine the epidemiology and distribution of emerging zoonotic malaria infections. Given the high prevalence of zoonotic malaria in Malaysia, efforts should be made to detect zoonotic malaria in humans, mosquito vectors, and natural hosts to ensure the success of the National Malaria Elimination Strategic Plan.

Comparison of two malaria multiplex immunoassays that enable quantification of malaria antigens

Background

Immunoassay platforms that simultaneously detect malaria antigens including histidine-rich protein 2 (HRP2)/HRP3 and Plasmodium lactate dehydrogenase (pLDH), are useful epidemiological tools for rapid diagnostic test evaluation. This study presents the comparative evaluation of two multiplex platforms in identifying Plasmodium falciparum with presence or absence of HRP2/HRP3 expression as being indicative of hrp2/hrp3 deletions and other Plasmodium species. Moreover, correlation between the malaria antigen measurements performed at these platforms is assessed after calibrating with either assay standards or international standards and the cross-reactivity among Plasmodium species is examined.

Methods

A 77-member panel of specimens composed of the World Health Organization (WHO) international Plasmodium antigen standards, cultured parasites for P. falciparum and Plasmodium knowlesi, and clinical specimens with mono-infections for P. falciparum, Plasmodium vivax, and Plasmodium malariae was generated as both whole blood and dried blood spot (DBS) specimens. Assays for HRP2, P. falciparum–specific pLDH (PfLDH), P. vivax–specific pLDH (PvLDH), and all human Plasmodium species Pan malaria pLDH (PanLDH) on the Human Malaria Array Q-Plex and the xMAP platforms were evaluated with these panels.

Results

The xMAP showed a higher percent positive agreement for identification of hrp2-deleted P. falciparum and Plasmodium species in whole blood and DBS than the Q-Plex. For whole blood samples, there was a highly positive correlation between the two platforms for PfLDH (Pearson r = 0.9926) and PvLDH (r = 0. 9792), moderate positive correlation for HRP2 (r = 0.7432), and poor correlation for PanLDH (r = 0.6139). In Pearson correlation analysis between the two platforms on the DBS, the same assays were r = 0.9828, r = 0.7679, r = 0.6432, and r = 0.8957, respectively. The xMAP HRP2 assay appeared to cross-react with HRP3, while the Q-Plex did not. The Q-Plex PfLDH assay cross-reacted with P. malariae, while the xMAP did not. For both platforms, P. knowlesi was detected on the PvLDH assay. The WHO international standards allowed normalization across both platforms on their HRP2, PfLDH, and PvLDH assays in whole blood and DBS.

Conclusions

Q-Plex and xMAP show good agreement for identification of P. falciparum mutants with hrp2/hrp3 deletions, and other Plasmodium species. Quantitative results from both platforms, normalized into international units for HRP2, PfLDH, and PvLDH, showed good agreement and should allow comparison and analysis of results generated by either platform.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04203-9.

Fluorescence In Situ Hybridization (FISH) Tests for Identifying Protozoan and Bacterial Pathogens in Infectious Diseases

Diagnosing and treating many infectious diseases depends on correctly identifying the causative pathogen. Characterization of pathogen-specific nucleic acid sequences by PCR is the most sensitive and specific method available for this purpose, although it is restricted to laboratories that have the necessary infrastructure and finance. Microscopy, rapid immunochromatographic tests for antigens, and immunoassays for detecting pathogen-specific antibodies are alternative and useful diagnostic methods with different advantages and disadvantages. Detection of ribosomal RNA molecules in the cytoplasm of bacterial and protozoan pathogens by fluorescence in-situ hybridization (FISH) using sequence-specific fluorescently labelled DNA probes, is cheaper than PCR and requires minimal equipment and infrastructure. A LED light source attached to most laboratory light microscopes can be used in place of a fluorescence microscope with a UV lamp for FISH. A FISH test hybridization can be completed in 30 min at 37 °C and the whole test in less than two hours. FISH tests can therefore be rapidly performed in both well-equipped and poorly-resourced laboratories. Highly sensitive and specific FISH tests for identifying many bacterial and protozoan pathogens that cause disease in humans, livestock and pets are reviewed, with particular reference to parasites causing malaria and babesiosis, and mycobacteria responsible for tuberculosis.

Diagnostic performance of a 5-plex malaria immunoassay in regions co-endemic for Plasmodium falciparum, P. vivax, P. knowlesi, P. malariae and P. ovale

Commercial point-of-care tests remain insufficient for accurately detecting and differentiating low-level malaria infections in regions co-endemic with multiple non-falciparum species, including zoonotic Plasmodium knowlesi (Pk). A 5-plex chemiluminescent assay simultaneously measures pan-Plasmodium lactate dehydrogenase (pLDH), P. falciparum (Pf)-LDH, P. vivax (Pv)-LDH, Pf-histidine-rich protein-2 (HRP2), and C-reactive protein. We assessed its diagnostic performance on whole blood (WB) samples from 102 healthy controls and 306 PCR-confirmed clinical cases of Pf, Pv, Pk, P. malariae (Pm) and P. ovale (Po) mono-infections from Southeast-Asia. We confirm its excellent HRP2-based detection of Pf. Cross-reactivity of Pf-LDH with all non-falciparum species tested was observed (specificity 57.3%). Pv-LDH performance was suboptimal for Pv (93.9% sensitivity and 73.9% specificity). Poor specificity was driven by strong Pk cross-reactivity, with Pv-LDH detecting 93.9% of Pk infections. The pan-LDH-to-Pf-LDH ratio was capable of discerning Pv from Pk, and robustly differentiated Pf from Pm or Po infection, useful in regions with hrp2/3 deletions. We tested the platform's performance in plasma for the first time, with WB outperforming plasma for all analytes except Pv-LDH for Pk. The platform is a promising tool for WB malaria diagnosis, although further development is warranted to improve its utility in regions co-endemic for multiple non-falciparum species.

Plasmodium knowlesi: the game changer for malaria eradication

Plasmodium knowlesi is a zoonotic malaria parasite that has gained increasing medical interest over the past two decades. This zoonotic parasitic infection is prevalent in Southeast Asia and causes many cases with fulminant pathology. Despite several biogeographical restrictions that limit its distribution, knowlesi malaria cases have been reported in different parts of the world due to travelling and tourism activities. Here, breakthroughs and key information generated from recent (over the past five years, but not limited to) studies conducted on P. knowlesi were reviewed, and the knowledge gap in various research aspects that need to be filled was discussed. Besides, challenges and strategies required to control and eradicate human malaria with this emerging and potentially fatal zoonosis were described.

Epidemiology of the zoonotic malaria Plasmodium knowlesi in changing landscapes

Within the past two decades, incidence of human cases of the zoonotic malaria Plasmodium knowlesi has increased markedly. P. knowlesi is now the most common cause of human malaria in Malaysia and threatens to undermine malaria control programmes across Southeast Asia. The emergence of zoonotic malaria corresponds to a period of rapid deforestation within this region. These environmental changes impact the distribution and behaviour of the simian hosts, mosquito vector species and human populations, creating new opportunities for P. knowlesi transmission. Here, we review how landscape changes can drive zoonotic disease emergence, examine the extent and causes of these changes across Southeast and identify how these mechanisms may be impacting P. knowlesi dynamics. We review the current spatial epidemiology of reported P. knowlesi infections in people and assess how these demographic and environmental changes may lead to changes in transmission patterns. Finally, we identify opportunities to improve P. knowlesi surveillance and develop targeted ecological interventions within these landscapes.

Plasmodium knowlesi detection methods for human infections-Diagnosis and surveillance

Within the overlapping geographical ranges of P. knowlesi monkey hosts and vectors in Southeast Asia, an estimated 1.5 billion people are considered at risk of infection. P. knowlesi can cause severe disease and death, the latter associated with delayed treatment occurring from misdiagnosis. Although microscopy is a sufficiently sensitive first-line tool for P. knowlesi detection for most low-level symptomatic infections, misdiagnosis as other Plasmodium species is common, and the majority of asymptomatic infections remain undetected. Current point-of-care rapid diagnostic tests demonstrate insufficient sensitivity and poor specificity for differentiating P. knowlesi from other Plasmodium species. Molecular tools including nested, real-time, and single-step PCR, and loop-mediated isothermal amplification (LAMP), are sensitive for P. knowlesi detection. However, higher cost and inability to provide the timely point-of-care diagnosis needed to guide appropriate clinical management has limited their routine use in most endemic clinical settings. P. knowlesi is likely underdiagnosed across the region, and improved diagnostic and surveillance tools are required. Reference laboratory molecular testing of malaria cases for both zoonotic and non-zoonotic Plasmodium species needs to be more widely implemented by National Malaria Control Programs across Southeast Asia to accurately identify the burden of zoonotic malaria and more precisely monitor the success of human-only malaria elimination programs. The implementation of specific serological tools for P. knowlesi would assist in determining the prevalence and distribution of asymptomatic and submicroscopic infections, the absence of transmission in certain areas, and associations with underlying land use change for future spatially targeted interventions.

Validation of SYBR green I based closed-tube loop-mediated isothermal amplification (LAMP) assay for diagnosis of knowlesi malaria

BACKGROUND

As an alternative to PCR methods, LAMP is increasingly being used in the field of molecular diagnostics. Under isothermal conditions at 65 °C, the entire procedure takes approximately 30 min to complete. In this study, we establish a sensitive and visualized LAMP method in a closed-tube system for the detection of Plasmodium knowlesi.

METHODS

A total of 71 malaria microscopy positive blood samples collected in blood spots were obtained from the Sarawak State Health Department. Using 18s rRNA as the target gene, nested PCR and SYBR green I LAMP assay were performed following the DNA extraction. The colour changes of LAMP end products were observed by naked eyes.

RESULTS

LAMP assay demonstrated a detection limit of 10 copies/µL in comparison with 100 copies/µL nested PCR. Of 71 P. knowlesi blood samples collected, LAMP detected 69 microscopy-positive samples. LAMP exhibited higher sensitivity than nested PCR assay. The SYBR green I LAMP assay was 97.1% sensitive (95% CI 90.2-99.7%) and 100% specific (95% CI 83.2-100%). Without opening the cap, incorporation of SYBR green I into the inner cap of the tube enabled the direct visualization of results upon completion of amplification. The positives instantaneously turned green while the negatives remained orange.

CONCLUSIONS

These results indicate that SYBR green I LAMP assay is a convenient diagnosis tool for the detection of P. knowlesi in remote settings.

Malaria elimination in Malaysia and the rising threat of Plasmodium knowlesi

Background

Malaria is a major public-health problem, with over 40% of the world’s population (more than 3.3 billion people) at risk from the disease. Malaysia has committed to eliminate indigenous human malaria transmission by 2020. The objective of this descriptive study is to understand the epidemiology of malaria in Malaysia from 2000 through 2018 and to highlight the threat posed by zoonotic malaria to the National Malaria Elimination Strategic Plan.

Methods

Malaria is a notifiable infection in Malaysia. The data used in this study were extracted from the Disease Control Division, Ministry of Health Malaysia, contributed by the hospitals and health clinics throughout Malaysia. The population data used in this study was extracted from the Department of Statistics Malaysia. Data analyses were performed using Microsoft Excel. Data used for mapping are available at EPSG:4326 WGS84 CRS (Coordinate Reference System). Shapefile was obtained from igismap. Mapping and plotting of the map were performed using QGIS.

Results

Between 2000 and 2007, human malaria contributed 100% of reported malaria and 18–46 deaths per year in Malaysia. Between 2008 and 2017, indigenous malaria cases decreased from 6071 to 85 (98.6% reduction), while during the same period, zoonotic Plasmodium knowlesi cases increased from 376 to 3614 cases (an 861% increase). The year 2018 marked the first year that Malaysia did not report any indigenous cases of malaria caused by human malaria parasites. However, there was an increasing trend of P. knowlesi cases, with a total of 4131 cases reported in that year. Although the increased incidence of P. knowlesi cases can be attributed to various factors including improved diagnostic capacity, reduction in human malaria cases, and increase in awareness of P. knowlesi, more than 50% of P. knowlesi cases were associated with agriculture and plantation activities, with a large remainder proportion linked to forest-related activities.

Conclusions

Malaysia has entered the elimination phase of malaria control. Zoonotic malaria, however, is increasing exponentially and becoming a significant public health problem. Improved inter-sectoral collaboration is required in order to develop a more integrated effort to control zoonotic malaria. Local political commitment and the provision of technical support from the World Health Organization will help to create focused and concerted efforts towards ensuring the success of the National Malaria Elimination Strategic Plan.

Malaria distribution and performance of malaria diagnostic methods in Malaysia (1980-2019): a systematic review

BACKGROUND

Malaysia has already achieved remarkable accomplishments in reaching zero indigenous human malaria cases in 2018. Prompt malaria diagnosis, surveillance and treatment played a key role in the country's elimination success. Looking at the dynamics of malaria distribution during the last decades might provide important information regarding the potential challenges of such an elimination strategy. This study was performed to gather all data available in term of prevalence or incidence on Plasmodium infections in Malaysia over the last four decades.

METHODS

A systematic review of the published English literature was conducted to identify malaria distribution from 1980 to June 2019 in Malaysia. Two investigators independently extracted data from PubMed, Scopus, Web of Science and Elsevier databases for original papers.

RESULTS

The review identified 46 epidemiological studies in Malaysia over the 39-year study period, on which sufficient information was available. The majority of studies were conducted in Malaysia Borneo (31/46; 67.4%), followed by Peninsular Malaysia (13/46; 28.3%) and in both areas (2/46; 4.3%). More than half of all studies (28/46; 60.9%) were assessed by both microscopy and PCR. Furthermore, there was a clear trend of decreases of all human malaria species with increasing Plasmodium knowlesi incidence rate throughout the year of sampling period. The summary estimates of sensitivity were higher for P. knowlesi than other Plasmodium species for both microscopy and PCR. Nevertheless, the specificities of summary estimates were similar for microscopy (40-43%), but varied for PCR (2-34%).

CONCLUSIONS

This study outlined the epidemiological changes in Plasmodium species distribution in Malaysia. Malaria cases shifted from predominantly caused by human malaria parasites to simian malaria parasites, which accounted for the majority of indigenous cases particularly in Malaysia Borneo. Therefore, malaria case notification and prompt malaria diagnosis in regions where health services are limited in Malaysia should be strengthened and reinforced to achieving the final goal of malaria elimination in the country.

Rapid diagnostic tests for Plasmodium vivax malaria in endemic countries

BACKGROUND

Plasmodium vivax (P vivax) is a focus of malaria elimination. It is important because P vivax and Plasmodium falciparum infection are co-endemic in some areas. There are asymptomatic carriers of P vivax, and the treatment for P vivax and Plasmodium ovale malaria differs from that used in other types of malaria. Rapid diagnostic tests (RDTs) will help distinguish P vivax from other malaria species to help treatment and elimination. There are RDTs available that detect P vivax parasitaemia through the detection of P vivax-specific lactate dehydrogenase (LDH) antigens.

OBJECTIVES

To assess the diagnostic accuracy of RDTs for detecting P vivax malaria infection in people living in malaria-endemic areas who present to ambulatory healthcare facilities with symptoms suggestive of malaria; and to identify which types and brands of commercial tests best detect P vivax malaria.

SEARCH METHODS

We undertook a comprehensive search of the following databases up to 30 July 2019: Cochrane Infectious Diseases Group Specialized Register; Central Register of Controlled Trials (CENTRAL), published in the Cochrane Library; MEDLINE (PubMed); Embase (OVID); Science Citation Index Expanded (SCI-EXPANDED) and Conference Proceedings Citation Index-Science (CPCI-S), both in the Web of Science.

SELECTION CRITERIA

Studies comparing RDTs with a reference standard (microscopy or polymerase chain reaction (PCR)) in blood samples from patients attending ambulatory health facilities with symptoms suggestive of malaria in P vivax-endemic areas.

DATA COLLECTION AND ANALYSIS

For each included study, two review authors independently extracted data using a pre-piloted data extraction form. The methodological quality of the studies were assessed using a tailored Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) tool. We grouped studies according to commercial brand of the RDT and performed meta-analysis when appropriate. The results given by the index tests were based on the antibody affinity (referred to as the strength of the bond between an antibody and an antigen) and avidity (referred to as the strength of the overall bond between a multivalent antibody and multiple antigens). All analyses were stratified by the type of reference standard. The bivariate model was used to estimate the pooled sensitivity and specificity with 95% confidence intervals (CIs), this model was simplified when studies were few. We assessed the certainty of the evidence using the GRADE approach.

MAIN RESULTS

We included 10 studies that assessed the accuracy of six different RDT brands (CareStart Malaria Pf/Pv Combo test, Falcivax Device Rapid test, Immuno-Rapid Malaria Pf/Pv test, SD Bioline Malaria Ag Pf/Pv test, OnSite Pf/Pv test and Test Malaria Pf/Pv rapid test) for detecting P vivax malaria. One study directly compared the accuracy of two RDT brands. Of the 10 studies, six used microscopy, one used PCR, two used both microscopy and PCR separately and one used microscopy corrected by PCR as the reference standard. Four of the studies were conducted in Ethiopia, two in India, and one each in Bangladesh, Brazil, Colombia and Sudan. The studies often did not report how patients were selected. In the patient selection domain, we judged the risk of bias as unclear for nine studies. We judged all studies to be of unclear applicability concern. In the index test domain, we judged most studies to be at low risk of bias, but we judged nine studies to be of unclear applicability concern. There was poor reporting on lot testing, how the RDTs were stored, and background parasitaemia density (a key variable determining diagnostic accuracy of RDTs). Only half of the included studies were judged to be at low risk of bias in the reference standard domain, Studies often did not report whether the results of the reference standard could classify the target condition or whether investigators knew the results of the RDT when interpreting the results of the reference standard. All 10 studies were judged to be at low risk of bias in the flow and timing domain. Only two brands were evaluated by more than one study. Four studies evaluated the CareStart Malaria Pf/Pv Combo test against microscopy and two studies evaluated the Falcivax Device Rapid test against microscopy. The pooled sensitivity and specificity were 99% (95% CI 94% to 100%; 251 patients, moderate-certainty evidence) and 99% (95% CI 99% to 100%; 2147 patients, moderate-certainty evidence) for CareStart Malaria Pf/Pv Combo test. For a prevalence of 20%, about 206 people will have a positive CareStart Malaria Pf/Pv Combo test result and the remaining 794 people will have a negative result. Of the 206 people with positive results, eight will be incorrect (false positives), and of the 794 people with a negative result, two would be incorrect (false negative). For the Falcivax Device Rapid test, the pooled sensitivity was 77% (95% CI: 53% to 91%, 89 patients, low-certainty evidence) and the pooled specificity was 99% (95% CI: 98% to 100%, 621 patients, moderate-certainty evidence), respectively. For a prevalence of 20%, about 162 people will have a positive Falcivax Device Rapid test result and the remaining 838 people will have a negative result. Of the 162 people with positive results, eight will be incorrect (false positives), and of the 838 people with a negative result, 46 would be incorrect (false negative).

AUTHORS' CONCLUSIONS

The CareStart Malaria Pf/Pv Combo test was found to be highly sensitive and specific in comparison to microscopy for detecting P vivax in ambulatory healthcare in endemic settings, with moderate-certainty evidence. The number of studies included in this review was limited to 10 studies and we were able to estimate the accuracy of 2 out of 6 RDT brands included, the CareStart Malaria Pf/Pv Combo test and the Falcivax Device Rapid test. Thus, the differences in sensitivity and specificity between all the RDT brands could not be assessed. More high-quality studies in endemic field settings are needed to assess and compare the accuracy of RDTs designed to detect P vivax.

Comparative evaluation of two commercial real-time PCR kits (QuantiFast™ and abTES™) for the detection of Plasmodium knowlesi and other Plasmodium species in Sabah, Malaysia

Background

The monkey parasite Plasmodium knowlesi is an emerging public health issue in Southeast Asia. In Sabah, Malaysia, P. knowlesi is now the dominant cause of human malaria. Molecular detection methods for P. knowlesi are essential for accurate diagnosis and in monitoring progress towards malaria elimination of other Plasmodium species. However, recent commercially available PCR malaria kits have unpublished P. knowlesi gene targets or have not been evaluated against clinical samples.

Methods

Two real-time PCR methods currently used in Sabah for confirmatory malaria diagnosis and surveillance reporting were evaluated: the QuantiFast™ Multiplex PCR kit (Qiagen, Germany) targeting the P. knowlesi 18S SSU rRNA; and the abTES™ Malaria 5 qPCR II kit (AITbiotech, Singapore), with an undisclosed P. knowlesi gene target. Diagnostic accuracy was evaluated using 52 P. knowlesi, 25 Plasmodium vivax, 21 Plasmodium falciparum, and 10 Plasmodium malariae clinical isolates, and 26 malaria negative controls, and compared against a validated reference nested PCR assay. The limit of detection (LOD) for each PCR method and Plasmodium species was also evaluated.

Results

The sensitivity of the QuantiFast™ and abTES™ assays for detecting P. knowlesi was comparable at 98.1% (95% CI 89.7–100) and 100% (95% CI 93.2–100), respectively. Specificity of the QuantiFast™ and abTES™ for P. knowlesi was high at 98.8% (95% CI 93.4–100) for both assays. The QuantiFast™ assay demonstrated falsely-positive mixed Plasmodium species at low parasitaemias in both the primary and LOD analysis. Diagnostic accuracy of both PCR kits for detecting P. vivax, P. falciparum, and P. malariae was comparable to P. knowlesi. The abTES™ assay demonstrated a lower LOD for P. knowlesi of ≤ 0.125 parasites/µL compared to QuantiFast™ with a LOD of 20 parasites/µL. Hospital microscopy demonstrated a sensitivity of 78.8% (95% CI 65.3–88.9) and specificity of 80.4% (95% CI 67.6–89.8) compared to reference PCR for detecting P. knowlesi.

Conclusion

The QuantiFast™ and abTES™ commercial PCR kits performed well for the accurate detection of P. knowlesi infections. Although the QuantiFast™ kit is cheaper, the abTES™ kit demonstrated a lower LOD, supporting its use as a second-line referral-laboratory diagnostic tool in Sabah, Malaysia.

Point-of-care tests for malaria: speeding up the diagnostics at the bedside and challenges in malaria cases detection

Malaria remains as one of the major public health problems worldwide. About 228 million cases occurred in 2018 only, with Africa bearing about 93% of the cases. Asymptomatic population carrying the various forms of the parasite Plasmodium in endemic areas plays an important role in the spread of the disease. To tackle this battle, more sensitive and precise detection kits for malaria are crucial to better control the number of new malaria cases. In this review, we not only discuss some of the available approaches to rapidly detect new malaria cases in endemic areas but also shed light on parallel problems that may affect the detection of individuals infected with the parasite, covering kelch 13 mutation, glucose 6-phosphate dehydrogenase deficiency, and hemoglobin disorders. Available approaches for malaria detection covered in this review are focused on point-of-care tests, including portable polymerase chain reaction and aptamers.

The role of oxygen depletion and subsequent radioprotective effects during irradiation of mosquito pupae in water

BACKGROUND

Radiation induced sterility is the basis of the Sterile Insect Technique, by which a target insect pest population is suppressed by releasing artificially reared sterile males of the pest species in overflooding numbers over a target site. In order for the sterile males to be of high biological quality, effective standard irradiation protocols are required. Following studies investigating the effects of mosquito pupae irradiation in water versus in air, there is a need to investigate the oxy-regulatory behavior of mosquito pupae in water to better understand the consequences of irradiation in hypoxic versus normoxic conditions.

METHODS

Pupae of Aedes aegypti, Ae. albopictus, and Anopheles arabiensis were submerged in water inside air-tight 2 ml glass vials at a density of 100 pupae/ml and the dissolved oxygen (DO) levels in the water were measured and plotted over time. In addition, male pupae of Ae. aegypti (aged 40-44 h), Ae. albopictus (aged 40-44 h) and An. arabiensis (aged 20-24 h) were irradiated in a gammacell220 at increasing doses in either hypoxic (water with < 0.5% O2 content) or normoxic (in air) conditions. The males were then mated to virgin females and resulting eggs were checked for induced sterility.

RESULTS

All three species depleted the water of DO to levels under 0.5% within 30 minutes, with An. arabiensis consuming oxygen the fastest at under 10 minutes. Following irradiation, the protective effect of hypoxia was observed across species and doses (P < 0.0001), increasing at higher doses. This effect was most pronounced in An. arabiensis.

CONCLUSIONS

The consumption of dissolved oxygen by pupae submerged in water was significantly different between species, indicating that their oxy-regulatory capacity seems to have possibly evolved according to their preferred breeding site characteristics. This needs to be considered when sterilizing male mosquitoes at pupal stage in water. Depending on species, their DO consumption rates and their density, irradiation doses needed to achieve full sterility may vary significantly. Further assessments are required to ascertain optimal conditions in terms of ambient atmosphere during pupal irradiation to produce competitive sterile males, and temperature and density dependent effects are expected.

A Systematic Review: Performance of Rapid Diagnostic Tests for the Detection of Plasmodium knowlesi, Plasmodium malariae, and Plasmodium ovale Monoinfections in Human Blood

Background

Despite the increased use and worldwide distribution of malaria rapid diagnostic tests (RDTs) that distinguish between Plasmodium falciparum and non-falciparum species, little is known about their performance detecting Plasmodium knowlesi (Pk), Plasmodium malariae (Pm), and Plasmodium ovale (Po). This review seeks to analyze the results of published studies evaluating the diagnostic accuracy of malaria RDTs in detecting Pk, Pm, and Po monoinfections.

Methods

MEDLINE, EMBASE, Web of Science, and CENTRAL databases were systematically searched to identify studies that reported the performance of RDTs in detecting Pk, Pm, and Po monoinfections.

Results

Among 40 studies included in the review, 3 reported on Pk, 8 on Pm, 5 on Po, 1 on Pk and Pm, and 23 on Pm and Po infections. In the meta-analysis, estimates of sensitivities of RDTs in detecting Pk infections ranged 2%-48%. Test performances for Pm and Po infections were less accurate and highly heterogeneous, mainly because of the small number of samples tested.

Conclusions

Limited data available suggest that malaria RDTs show suboptimal performance for detecting Pk, Pm, and Po infections. New improved RDTs and appropriately designed cross-sectional studies to demonstrate the usefulness of RDTs in the detection of neglected Plasmodium species are urgently needed.

Defining the ecological and evolutionary drivers of Plasmodium knowlesi transmission within a multi-scale framework

Plasmodium knowlesi is a zoonotic malaria parasite normally residing in long-tailed and pig-tailed macaques (Macaca fascicularis and Macaca nemestrina, respectively) found throughout Southeast Asia. Recently, knowlesi malaria has become the predominant malaria affecting humans in Malaysian Borneo, being responsible for approximately 70% of reported cases. Largely as a result of anthropogenic land use changes in Borneo, vectors which transmit the parasite, along with macaque hosts, are both now frequently found in disturbed forest habitats, or at the forest fringes, thus having more frequent contact with humans. Having access to human hosts provides the parasite with the opportunity to further its adaption to the human immune system. The ecological drivers of the transmission and spread of P. knowlesi are operating over many different spatial (and, therefore, temporal) scales, from the molecular to the continental. Strategies to prevent and manage zoonoses, such as P. knowlesi malaria require interdisciplinary research exploring the impact of land use change and biodiversity loss on the evolving relationship between parasite, reservoir hosts, vectors, and humans over multiple spatial scales.

Laboratory challenges of Plasmodium species identification in Aceh Province, Indonesia, a malaria elimination setting with newly discovered P. knowlesi

The discovery of the life-threatening zoonotic infection Plasmodium knowlesi has added to the challenges of prompt and accurate malaria diagnosis and surveillance. In this study from Aceh Province, Indonesia, a malaria elimination setting where P. knowlesi endemicity was not previously known, we report the laboratory investigation and difficulties encountered when using molecular detection methods for quality assurance of microscopically identified clinical cases. From 2014 to 2015, 20 (49%) P. falciparum, 16 (39%) P. vivax, 3 (7%) P. malariae, and 2 (5%) indeterminate species were identified by microscopy from four sentinel health facilities. At a provincial-level reference laboratory, loop-mediated isothermal amplification (LAMP), a field-friendly molecular method, was performed and confirmed Plasmodium in all samples though further species-identification was limited by the unavailability of non-falciparum species-specific testing with the platform used. At a national reference laboratory, several molecular methods including nested PCR (nPCR) targeting the 18 small sub-unit (18S) ribosomal RNA, nPCR targeting the cytochrome-b (cytb) gene, a P. knowlesi-specific nPCR, and finally sequencing, were necessary to ultimately classify the samples as: 19 (46%) P. knowlesi, 8 (20%) P. falciparum, 14 (34%) P. vivax. Microscopy was unable to identify or mis-classified up to 56% of confirmed cases, including all cases of P. knowlesi. With the nPCR methods targeting the four human-only species, P. knowlesi was missed (18S rRNA method) or showed cross-reactivity for P. vivax (cytb method). To facilitate diagnosis and management of potentially fatal P. knowlesi infection and surveillance for elimination of human-only malaria in Indonesia and other affected settings, new detection methods are needed for testing at the point-of-care and in local reference laboratories.

In Southeast Asia, Plasmodium knowlesi, a malaria parasite of macaques, was recently discovered to infect humans. This emerging disease is important because it has potential for causing severe disease and death, and it is a threat to malaria elimination efforts in the region. In this report from Aceh Province, Indonesia, where P. knowlesi was only recently discovered, the authors report on the laboratory challenges of distinguishing this species from other human species. Using several different molecular methods, they investigated 41 malaria cases which by microscopy, were initially reported as: P. falciparum (49%), P. vivax (39%), P. malariae (7%), and indeterminate (5%). Only after using a P. knowlesi-specific nPCR method and sequencing, did they find that nearly half were P. knowlesi. Consistent with a sparse literature, a field-friendly molecular method (genus-specific LAMP) reliably detected P. knowlesi, while use of a more standard reference laboratory molecular method (18S rRNA nPCR targeting the four human-only species) missed the infections. Also another reference laboratory molecular method (cytb nPCR) mis-classified P. knowlesi infections as P. vivax due to cross-reactivity. To address the emerging threat of P. knowlesi, new detection methods are needed for point-of-care and reference testing.

An Update on Malaria Rapid Diagnostic Tests

PURPOSE OF REVIEW

Modern advances in malaria rapid diagnostic test (RDT) technology have increased demand for low-cost, easy-to-use assays in areas endemic for malaria. Substantial developments in diagnostic sensitivity and specificity, improvements in non-falciparum RDTs, and novel biotechnological innovations are gradually aligning the performance of RDTs with reference-level diagnostics including PCR and expert microscopy gold standards.

RECENT FINDINGS

Trends have emerged in recent malaria RDT literature: (1) improvements in the sensitivity and specificity of RDTs for Plasmodium falciparum diagnosis, making them comparable to expert microscopic examination; (2) reduced false-positive and false-negative reactions with novel antibody development; (3) improved sensitivity and specificity capabilities of Plasmodium vivax-specific RDTs; (4) developing RDTs for co-endemic mixed infection differentiation; (5) significant improvements of RDTs for Plasmodium knowlesi; (6) a global push towards assessing and confronting the growing concerns of widespread pfhrp2 gene deletions; and (7) original innovation in loop-mediated isothermal amplification (LAMP) biotechnological RDT-like platforms that demonstrate promising performance characteristics for P. falciparum, P. vivax, and P. knowlesi infections. The past 5 years have been characterized by increasing demand for malaria RDTs, translating into meaningful improvements in performance and novel biotechnological innovation. Future work should facilitate the development of improved RDT platforms for Plasmodium ovale, P. knowlesi, and Plasmodium malariae, and surmount the issue of pfhrp2 gene deletions, while maintaining comparable performance to both PCR and expert microscopy reference standards.

Plasmodium knowlesi malaria: current research perspectives

Originally known to cause simian malaria, Plasmodium knowlesi is now known as the fifth human malaria species. Since the publishing of a report that largely focused on human knowlesi cases in Sarawak in 2004, many more human cases have been reported in nearly all of the countries in Southeast Asia and in travelers returning from these countries. The zoonotic nature of this infection hinders malaria elimination efforts. In order to grasp the current perspective of knowlesi malaria, this literature review explores the different aspects of the disease including risk factors, diagnosis, treatment, and molecular and functional studies. Current studies do not provide sufficient data for an effective control program. Therefore, future direction for knowlesi research is highlighted here with a final aim of controlling, if not eliminating, the parasite.

Phosphoethanolamine-N-methyltransferase is a potential biomarker for the diagnosis of P. knowlesi and P. falciparum malaria

BACKGROUND

Plasmodium knowlesi is recognised as the main cause of human malaria in Southeast Asia. The disease is often misdiagnosed as P. falciparum or P. malariae infections by microscopy, and the disease is difficult to eliminate due to its presence in both humans and monkeys. P. knowlesi infections can rapidly cause severe disease and require prompt diagnosis and treatment. No protein biomarker exists for the rapid diagnostic test (RDT) detection of P. knowlesi infections. Plasmodium knowlesi infections can be diagnosed by PCR.

METHODS AND PRINCIPAL FINDINGS

Phosphoethanolamine-N-methyltransferase (PMT) is involved in malaria lipid biosynthesis and is not found in the human host. The P. falciparum, P. vivax and P. knowlesi PMT proteins were recombinantly expressed in BL21(DE3) Escherichia coli host cells, affinity purified and used to raise antibodies in chickens. Antibodies against each recombinant PMT protein all detected all three recombinant proteins and the native 29 kDa P. falciparum PMT protein on western blots and in ELISA. Antibodies against a PMT epitope (PLENNQYTDEGVKC) common to all three PMT orthologues detected all three proteins. Antibodies against unique peptides from each orthologue of PMT, PfCEVEHKYLHENKE, PvVYSIKEYNSLKDC, PkLYPTDEYNSLKDC detected only the parent protein in western blots and P. falciparum infected red blood cell lysates or blood lysates spiked with the respective proteins. Similar concentrations of PfPMT and the control, PfLDH, were detected in the same parasite lysate. The recombinant PfPMT protein was detected by a human anti-malaria antibody pool.

CONCLUSION

PMT, like the pan-specific LDH biomarker used in RDT tests, is both soluble, present at comparable concentrations in the parasite and constitutes a promising antimalarial drug target. PMT is absent from the human proteome. PMT has the potential as a biomarker for human malaria and in particular as the first P. knowlesi specific protein with diagnostic potential for the identification of a P. knowlesi infection.

Artemether-Lumefantrine Versus Chloroquine for the Treatment of Uncomplicated Plasmodium knowlesi Malaria: An Open-Label Randomized Controlled Trial CAN KNOW

Background

Plasmodium knowlesi is reported increasingly across Southeast Asia and is the most common cause of malaria in Malaysia. No randomized trials have assessed the comparative efficacy of artemether-lumefantrine (AL) for knowlesi malaria.

Methods

A randomized controlled trial was conducted in 3 district hospitals in Sabah, Malaysia to compare the efficacy of AL against chloroquine (CQ) for uncomplicated knowlesi malaria. Participants were included if they weighed >10 kg, had a parasitemia count <20000/μL, and had a negative rapid diagnostic test result for Plasmodium falciparum histidine-rich protein 2. Diagnosis was confirmed by means of polymerase chain reaction. Patients were block randomized to AL (total target dose, 12 mg/kg for artemether and 60 mg/kg for lumefantrine) or CQ (25 mg/kg). The primary outcome was parasite clearance at 24 hours in a modified intention-to-treat analysis.

Results

From November 2014 to January 2016, a total of 123 patients (including 18 children) were enrolled. At 24 hours after treatment 76% of patients administered AL (95% confidence interval [CI], 63%-86%; 44 of 58) were aparasitemic, compared with 60% administered CQ (47%-72%; 39 of 65; risk ratio, 1.3 [95% CI, 1.0-1.6]; P = .06). Overall parasite clearance was shorter after AL than after CQ (median, 18 vs 24 hours, respectively; P = .02), with all patients aparasitemic by 48 hours. By day 42 there were no treatment failures. The risk of anemia during follow-up was similar between arms. Patients treated with AL would require lower bed occupancy than those treated with CQ (2414 vs 2800 days per 1000 patients; incidence rate ratio, 0.86 [95% CI, .82-.91]; P < .001). There were no serious adverse events.

Conclusions

AL is highly efficacious for treating uncomplicated knowlesi malaria; its excellent tolerability and rapid therapeutic response allow earlier hospital discharge, and support its use as a first-line artemisinin-combination treatment policy for all Plasmodium species in Malaysia.

Clinical trials registration

NCT02001012.

Malaria in Children

Malaria is a leading cause of morbidity and mortality in endemic areas, leading to an estimated 438,000 deaths in 2015. Malaria is also an important health threat to travelers to endemic countries and should be considered in evaluation of any traveler returning from a malaria-endemic area who develops fever. Considering the diagnosis of malaria in patients with potential exposure is critical. Prompt provision of effective treatment limits the complications of malaria and can be life-saving. Understanding Plasmodium species variation, epidemiology, and drug-resistance patterns in the geographic area where infection was acquired is important for determining treatment choices.

A dual colour fluorescence in situ hybridization (FISH) assay for identifying the zoonotic malaria parasite Plasmodium knowlesi with a potential application for the specific diagnosis of knowlesi malaria in peripheral-level laboratories of Southeast Asia

Background

Plasmodium knowlesi is primarily responsible for zoonotic malaria in several Southeast Asian countries. Precise identification of the parasite in the blood of patients presently relies on an expensive and elaborate PCR procedure because microscopic examination of blood and other available field identification techniques lack adequate specificity. Therefore, the use of a simple and inexpensive dual-colour fluorescence in situ hybridization (FISH) assay, analogous to FISH assays recently described for Plasmodium falciparum and Plasmodium vivax, was investigated as a potential tool for identifying P. knowlesi.

Results

A P. knowlesi 18S rDNA sequence-based DNA probe was used to test thin blood smears of P. knowlesi by FISH, and fluorescence viewed in a light microscope fitted with a light emitting diode light source and appropriate emission and barrier filters. The limit of detection in the P. knowlesi FISH assay was 84 parasites per μl in infected monkey blood and 61 parasites per μl for P. knowlesi cultured in human blood. The P. knowlesi-specific FISH probe detected only P. knowlesi and not P. falciparum, Plasmodium malariae, Plasmodium ovale, P. vivax or a panel of other human blood-borne pathogens. A previously described Plasmodium genus-specific probe used simultaneously in the P. knowlesi FISH assay reacted with all tested Plasmodium species.

Conclusions

To our knowledge, this is the first description of a FISH assay for P. knowlesi that is potentially useful for diagnosing infections in remote laboratories in endemic countries.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2273-7) contains supplementary material, which is available to authorized users.

Update on Malaria Diagnostics and Test Utilization

Malaria is a potentially life-threatening disease requiring rapid diagnosis and treatment. Although microscopic examination of thick and thin blood films remains the gold standard for laboratory diagnosis, rapid antigen tests and nucleic acid amplification methods may also play a useful role in detection of acute infection. This review discusses the advantages and disadvantages of the commonly used diagnostic methods and provides important practice points for optimal malaria test utilization.

Clinical features and management of Plasmodium knowlesi infections in humans

Plasmodium knowlesi is a simian malaria of primarily the macaque species of South East Asia. While it was known that human infections could be induced during the years of malariotherapy, naturally occurring P. knowlesi human infections were thought to be rare. However, in 2004, knowlesi infections became recognized as an important infection amongst human populations in Sarawak, Malaysian Borneo. Since then, it has become recognized as a disease affecting people living and visiting endemic areas across South East Asia. Over the last 12 years, clinical studies have improved our understanding of this potentially fatal disease. In this review article the current literature is reviewed to give a comprehensive description of the disease and treatment.

Detection of Plasmodium knowlesi, Plasmodium falciparum and Plasmodium vivax using loop-mediated isothermal amplification (LAMP) in a co-endemic area in Malaysia

Background

Plasmodium knowlesi is the most common cause of malaria in Malaysia. However, microscopic diagnosis is inaccurate and rapid diagnostic tests (RDTs) are insufficiently sensitive. PCR is sensitive and specific but not feasible at a district level. Loop-mediated isothermal amplification (LAMP) shows potential with only basic requirements. A commercially available LAMP assay, the Eiken Loopamp™ MALARIA Pan Detection kit, is sensitive for Plasmodium falciparum and Plasmodium vivax, but has not previously been evaluated for P. knowlesi. This study aims to determine the sensitivity of this LAMP assay for detecting P. knowlesi infection.

Methods

Study participants included 73 uncomplicated malaria patients with PCR species confirmation: 50 P. knowlesi, 20 P. falciparum and 3 P. vivax. Nineteen malaria-negative, non-endemic area controls were also included. The sensitivity of the Eiken Loopamp™ MALARIA Pan Detection kit (Pan LAMP) for detecting each Plasmodium species was evaluated. Sensitivity and specificity of the Eiken Loopamp™ MALARIA Pf Detection kit (Pf LAMP) for P. falciparum were also determined. The limit of detection for each LAMP assay was evaluated, with results compared to PCR. All P. knowlesi patients were also tested by CareStart™ (Pf/VOM) and OptiMAL-IT™ (Pan/Pf) RDTs.

Results

The sensitivity of the Pan LAMP assay was 100% for P. knowlesi (95% CI 92.9–100), P. falciparum (95% CI 83.2–100), and P. vivax (95% CI 29.2–100). The Pf LAMP was 100% sensitive and specific for P. falciparum detection, with all P. knowlesi samples having a negative reaction. LAMP sensitivity was superior to both RDTs, with only 10 and 28% of P. knowlesi samples testing positive to CareStart™ and OptiMAL-IT™, respectively. Limit of detection using the Pan LAMP for both P. knowlesi and P. vivax was 2 parasites/μL, comparable to PCR. For P. falciparum both the Pan LAMP and Pf LAMP demonstrated a limit of detection of 20 parasites/μL.

Conclusions

The Eiken Loopamp™ MALARIA Pan Detection kit is sensitive for detection of P. knowlesi in low parasitaemia clinical infections, as well as P. falciparum and P. vivax. However, a P. knowlesi-specific field assay in a simpler format would assist correct species identification and initiation of optimal treatment for all malaria patients.

Malaria epidemiology in central Myanmar: identification of a multi-species asymptomatic reservoir of infection

Background

The spread of artemisinin-resistant Plasmodium falciparum is a global health concern. Myanmar stands at the frontier of artemisinin-resistant P. falciparum. Myanmar also has the highest reported malaria burden in Southeast Asia; it is integral in the World Health Organization’s plan to eliminate malaria in Southeast Asia, yet few epidemiological data exist for the general population in Myanmar.

Methods

This cross-sectional, probability household survey was conducted in Phyu township, Bago Region (central Myanmar), during the wet season of 2013. Interviewers collected clinical and behavioural data, recorded tympanic temperature and obtained dried blood spots for malaria PCR and serology. Plasmodium falciparum positive samples were tested for genetic mutations in the K13 region that may confer artemisinin resistance. Estimated type-specific malaria PCR prevalence and seroprevalence were calculated, with regression analysis to identify risk factors for seropositivity to P. falciparum. Data were weighted to account for unequal selection probabilities.

Results

1638 participants were sampled (500 households). Weighted PCR prevalence was low (n = 41, 2.5%) and most cases were afebrile (93%). Plasmodium falciparum was the most common species (n = 19. 1.1%) and five (26%) P. falciparum samples harboured K13 mutations. Plasmodium knowlesi was detected in 1.0% (n = 16) and Plasmodium vivax was detected in 0.4% (n = 7). Seroprevalence was 9.4% for P. falciparum and 3.1% for P. vivax. Seroconversion to P. falciparum was 0.003/year in the whole population, but 16-fold higher in men over 23 years old (LR test p = 0.016).

Discussion

This is the first population-based seroprevalence study from central Myanmar. Low overall prevalence was discovered. However, these data suggest endemic transmission continues, probably associated with behavioural risk factors amongst working-age men. Genetic mutations associated with P. falciparum artemisinin resistance, the presence of P. knowlesi and discrete demographic risk groups present opportunities and challenges for malaria control. Responses targeted to working-age men, capable of detecting sub-clinical infections, and considering all species will facilitate malaria elimination in this setting.

A preface on advances in diagnostics for infectious and parasitic diseases: detecting parasites of medical and veterinary importance

There are many reasons why detection of parasites of medical and veterinary importance is vital and where novel diagnostic and surveillance tools are required. From a medical perspective alone, these originate from a desire for better clinical management and rational use of medications. Diagnosis can be at the individual-level, at close to patient settings in testing a clinical suspicion or at the community-level, perhaps in front of a computer screen, in classification of endemic areas and devising appropriate control interventions. Thus diagnostics for parasitic diseases has a broad remit as parasites are not only tied with their definitive hosts but also in some cases with their vectors/intermediate hosts. Application of current diagnostic tools and decision algorithms in sustaining control programmes, or in elimination settings, can be problematic and even ill-fitting. For example in resource-limited settings, are current diagnostic tools sufficiently robust for operational use at scale or are they confounded by on-the-ground realities; are the diagnostic algorithms underlying public health interventions always understood and well-received within communities which are targeted for control? Within this Special Issue (SI) covering a variety of diseases and diagnostic settings some answers are forthcoming. An important theme, however, throughout the SI is to acknowledge that cross-talk and continuous feedback between development and application of diagnostic tests is crucial if they are to be used effectively and appropriately.

The development of malaria diagnostic techniques: a review of the approaches with focus on dielectrophoretic and magnetophoretic methods

The large number of deaths caused by malaria each year has increased interest in the development of effective malaria diagnoses. At the early-stage of infection, patients show non-specific symptoms or are asymptomatic, which makes it difficult for clinical diagnosis, especially in non-endemic areas. Alternative diagnostic methods that are timely and effective are required to identify infections, particularly in field settings. This article reviews conventional malaria diagnostic methods together with recently developed techniques for both malaria detection and infected erythrocyte separation. Although many alternative techniques have recently been proposed and studied, dielectrophoretic and magnetophoretic approaches are among the promising new techniques due to their high specificity for malaria parasite-infected red blood cells. The two approaches are discussed in detail, including their principles, types, applications and limitations. In addition, other recently developed techniques, such as cell deformability and morphology, are also overviewed in this article.

Plasmodium knowlesi: an update

There were only four species of Plasmodium that were thought to cause malaria in humans until a large number of human infections by Plasmodium knowlesi, a malaria parasite typically found in long-tailed and pig-tailed macaques, were reported in 2004 in Malaysian Borneo. Since then, cases of knowlesi malaria have been reported throughout South-east Asia and also in travellers returning from the region. This article describes the molecular, entomological and epidemiological data which indicate that P. knowlesi is an ancient parasite that is primarily zoonotic, and there are three highly divergent sub-populations. It also describes the detection methods for P. knowlesi, which is morphologicaly similar to P. malariae, and the clinical features and treatment of this malaria parasite that is potentially fatal.

Plasmodium knowlesi as a Threat to Global Public Health

Malaria is a tropical disease caused by protozoans of the Plasmodium genus. Delayed diagnosis and misdiagnosis are strongly associated with higher mortality. In recent years, a greater importance is attributed to Plasmodium knowlesi, a species found mainly in Southeast Asia. Routine parasitological diagnostics are associated with certain limitations and difficulties in unambiguous determination of the parasite species based only on microscopic image. Recently, molecular techniques have been increasingly used for predictive diagnosis. The aim of the study is to draw attention to the risk of travelling to knowlesi malaria endemic areas and to raise awareness among personnel involved in the therapeutic process.

Point-of-care molecular diagnostic devices: an overview

Point-of-care molecular diagnostic devices are a rapidly expanding market. A variety of technologies are being developed for DNA detection and amplification, mostly aiming to detect pathogens. Of the two devices for detection of human genetic variations, both focus on CYP2C19 and have obtained regulatory approval. Most other devices have not obtained US FDA approval and are still undergoing clinical trials. Most, if not all, devices in development require equipment to which disposable test cartridges are placed. Thus, they may not fulfill FDA's definition of being 'simple'. There is a clear need to develop completely disposable devices that do not require equipment maintenance, and to detect other genetic variants predictive of disease susceptibility and drug response.

Admixture in Humans of Two Divergent Plasmodium knowlesi Populations Associated with Different Macaque Host Species

Human malaria parasite species were originally acquired from other primate hosts and subsequently became endemic, then spread throughout large parts of the world. A major zoonosis is now occurring with Plasmodium knowlesi from macaques in Southeast Asia, with a recent acceleration in numbers of reported cases particularly in Malaysia. To investigate the parasite population genetics, we developed sensitive and species-specific microsatellite genotyping protocols and applied these to analysis of samples from 10 sites covering a range of >1,600 km within which most cases have occurred. Genotypic analyses of 599 P. knowlesi infections (552 in humans and 47 in wild macaques) at 10 highly polymorphic loci provide radical new insights on the emergence. Parasites from sympatric long-tailed macaques (Macaca fascicularis) and pig-tailed macaques (M. nemestrina) were very highly differentiated (FST = 0.22, and K-means clustering confirmed two host-associated subpopulations). Approximately two thirds of human P. knowlesi infections were of the long-tailed macaque type (Cluster 1), and one third were of the pig-tailed-macaque type (Cluster 2), with relative proportions varying across the different sites. Among the samples from humans, there was significant indication of genetic isolation by geographical distance overall and within Cluster 1 alone. Across the different sites, the level of multi-locus linkage disequilibrium correlated with the degree of local admixture of the two different clusters. The widespread occurrence of both types of P. knowlesi in humans enhances the potential for parasite adaptation in this zoonotic system.

Extraordinary phases of pathogen evolution may occur during an emerging zoonosis, potentially involving adaptation to human hosts, with changes in patterns of virulence and transmission. In a large population genetic survey, we show that the malaria parasite Plasmodium knowlesi in humans is an admixture of two highly divergent parasite populations, each associated with different forest-dwelling macaque reservoir host species. Most of the transmission and sexual reproduction occurs separately in each of the two parasite populations. In addition to the reservoir host-associated parasite population structure, there was also significant genetic differentiation that correlated with geographical distance. Although both P. knowlesi types co-exist in the same areas, the divergence between them is similar to or greater than that seen between sub-species in other sexually reproducing eukaryotes. This may offer particular opportunities for evolution of virulence and host-specificity, not seen with other malaria parasites, so studies of ongoing adaptation and interventions to reduce transmission are urgent priorities.

Plasmodium knowlesi - an emerging pathogen

Ten years have passed since the publication of a large focus of Plasmodium knowlesi infections in the human population. The discovery was made during a molecular investigation of atypical P. malariae cases in the Kapit Health Division, Sarawak, Malaysian Borneo. Patients were more symptomatic with higher parasite counts than expected in P. malariae infections. The investigation found only P. knowlesi DNA present in patient blood samples. Morphological similarity had allowed P. knowlesi to masquerade as P. malariae during routine diagnostic microscopy for malaria. P. knowlesi, a malaria parasite of macaque monkeys, had entered the human population. The subsequent development of P. knowlesi species-specific PCR assays soon demonstrated that the entry was not confined to the Kapit Division but extended across island and mainland Southeast Asia. Relevant clinical descriptions and guidelines for the treatment and management of patents with P. knowlesi malaria were not available. Nor was it clear whether P. knowlesi had undergone a host switch event into the human population or if infections were zoonotic. The outputs of studies on P. knowlesi malaria during the past 10 years will be summarized, highlighting major findings within the context of pathophysiology, virulence, host switch events, treatment, control and importantly malaria elimination.

Human infections with Plasmodium knowlesi--zoonotic malaria

In 2004 a large focus of Plasmodium knowlesi malaria was reported in the human population in Sarawak, Malaysian Borneo. Plasmodium knowlesi, a parasite of the South-East Asian macaques (Macaca fascicularis and Macaca nemestrina), had entered the human population. Plasmodium knowlesi is transmitted by the leucosphyrus group of Anopheline mosquitoes and transmission is largely zoonotic and restricted to the jungle setting. Humans entering jungle transmission sites are at risk. Since 2004, human cases of P. knowlesi have been continuously reported in local communities and in travellers returning from South East Asia. Plasmodium knowlesi is the most common type of indigenous malaria reported in Malaysia. Infections are most often uncomplicated but at least 10% of patients report with severe malaria and 1-2% of cases have a fatal outcome. Parasitaemia is positively associated with the clinical and laboratory markers of severe malaria. The current literature on P. knowlesi, including epidemiology, natural hosts and vectors, pathogenesis, clinical descriptions, treatment and diagnosis, is reviewed. There are many gaps in our understanding of this disease that are highlighted here with suggestions for further research to inform pre-emptive control measures that would be required to prevent a full emergence of this parasite into the human population.

Plasmodium knowlesi malaria: Overview Focussing on Travel-Associated Infections

In 2004, Plasmodium knowlesi was first recognised as a relevant cause of human malaria in Southeast Asia. Since then, P. knowlesi has been described from all Southeast Asian countries except Laos and has become well-established as the fifth human malaria parasite and the first significant zoonotic Plasmodium species. As countries endemic for P. knowlesi malaria are among the most popular and most highly visited international destinations, travel medicine experts should be aware about disease and risks including prophylactic and therapeutic measures. Between 2005 and 2012, 15 cases of P. knowlesi malaria have been recognised and published in international travellers. Male gender and travel to rural/forested areas with contact to wild monkeys are risk factors for P. knowlesi infection. The present review gives an overview on current literature on the P. knowlesi parasite and summarises recent findings related to epidemiology, diagnostics, treatment and prophylaxis focussing on travellers.

Parasitic infections in travelers and immigrants: part I protozoa

ABSTRACT 

The growth in international commerce, travel and migration contribute to the global emergence of certain parasitic infections. Importation of vectors and food products may contribute to the emergence of protozoan infections in nonendemic countries. Infections such as malaria are potentially fatal, especially in nonimmune patients, and outcome depends largely on timely diagnosis and treatment. Diagnosis/management of imported parasitic infections may be complex especially as some patients may have underlying immunosuppressive conditions such as HIV infection. Major challenges concern the development of improved diagnostic techniques, safer/more effective drug therapies and identification of biological markers of progression and response to treatment. Imported parasitic diseases which may be transmitted vertically or through blood transfusion/organ donation could become a public health priority in the near future. Climate change may affect arthropod distribution and facilitate the spread of protozoan vector-borne diseases. The first part of this review focuses on protozoan infections in travelers and immigrants.

Efficacy and safety of artemisinin combination therapy (ACT) for non-falciparum malaria: a systematic review

Background

Artemisinin combination therapy (ACT) is recommended as first-line treatment for uncomplicated Plasmodium falciparum malaria, whereas chloroquine is still commonly used for the treatment of non-falciparum species (Plasmodium vivax, Plasmodium ovale and Plasmodium malariae). A more simplified, more uniform treatment approach across all malaria species is worthwhile to be considered both in endemic areas and for malaria as an imported condition alike.

Methods

A PROSPERO-registered systematic review to determine the efficacy and safety of ACT for the treatment of non-falciparum malaria was conducted, following PRISMA guidelines. Without language restrictions, Medline/PubMed, Embase, Cochrane Central Register of Controlled Trials, Web of Science, LILACS, Biosis Previews and the African Index Medicus were searched for studies published up to November 2014.

Results

The literature search identified 986 reports; 40 publications were found eligible for inclusion, all of them on non-falciparum malaria in endemic areas. Most evidence was available for P. vivax (n = 35). Five clinical trials in total were identified evaluating ACT for P. ovale, P. malariae and Plasmodium knowlesi. Most ACT presentations have high efficacy against P. vivax parasites; artemisinin-based combinations have shorter parasite and fever clearance times compared to chloroquine. ACT is as effective as chloroquine in preventing recurrent parasitaemia before day 28. Artemisinin-based combinations with long half-lives show significantly fewer recurrent parasitaemia up to day 63. The limited evidence available supports both the use of chloroquine and an ACT for P. ovale and P. malariae. ACT seems to be preferable for optimal treatment of P. knowlesi.

Conclusion

ACT is at least equivalent to chloroquine in effectively treating non-falciparum malaria. These findings may facilitate development of simplified protocols for treating all forms of malaria with ACT, including returning travellers. Obtaining comprehensive efficacy and safety data on ACT use for non-falciparum species particularly for P. ovale, P. malariae and P. knowlesi should be a research priority.

Trial registration

CRD42014009103

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-463) contains supplementary material, which is available to authorized users.

Malaria diagnostics: now and the future

Light microscopy of stained blood films is still the mainstay of malaria diagnosis in many regions, but its pre-eminence is threatened by accurate and sensitive rapid diagnostic tests (RDTs) based on immunochromatography which are now widely used in the field. In well-resourced regions, nucleic acid detection easily out performs microscopy and RDTs. This paper reviews the main in vitro methods for parasite detection and considers future trends in diagnostics, both for sophisticated laboratory settings and for field use.

Zoonotic malaria - global overview and research and policy needs

The four main Plasmodium species that cause human malaria, Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale, are transmitted between humans by mosquito vectors belonging to the genus Anopheles. It has recently become evident that Plasmodium knowlesi, a parasite that typically infects forest macaque monkeys, can be transmitted by anophelines to cause malaria in humans in Southeast Asia. Plasmodium knowlesi infections are frequently misdiagnosed microscopically as P. malariae. Direct human to human transmission of P. knowlesi by anophelines has not yet been established to occur in nature. Knowlesi malaria must therefore be presently considered a zoonotic disease. Polymerase chain reaction is now the definitive method for differentiating P. knowlesi from P. malariae and other human malaria parasites. The origin of P. falciparum and P. vivax in African apes are examples of ancient zoonoses that may be continuing at the present time with at least P. vivax, and possibly P. malariae and P. ovale. Other non-human primate malaria species, e.g., Plasmodium cynomolgi in Southeast Asia and Plasmodium brasilianum and Plasmodium simium in South America, can be transmitted to humans by mosquito vectors further emphasizing the potential for continuing zoonoses. The potential for zoonosis is influenced by human habitation and behavior as well as the adaptive capabilities of parasites and vectors. There is insufficient knowledge of the bionomics of Anopheles vector populations relevant to the cross-species transfer of malaria parasites and the real extent of malaria zoonoses. Appropriate strategies, based on more research, need to be developed for the prevention, diagnosis, and treatment of zoonotic malaria.

Combining parasite lactate dehydrogenase-based and histidine-rich protein 2-based rapid tests to improve specificity for diagnosis of malaria Due to Plasmodium knowlesi and other Plasmodium species in Sabah, Malaysia

Plasmodium knowlesi causes severe and fatal malaria in Malaysia. Microscopic misdiagnosis is common and may delay appropriate treatment. P. knowlesi can cross-react with "species-specific" parasite lactate dehydrogenase (pLDH) monoclonal antibodies used in rapid diagnostic tests (RDTs) to detect P. falciparum and P. vivax. At one tertiary-care hospital and two district hospitals in Sabah, we prospectively evaluated two combination RDTs for malaria diagnosis by using both a pan-Plasmodium-pLDH (pan-pLDH)/P. falciparum-specific-pLDH (Pf-pLDH) RDT (OptiMAL-IT) and a non-P. falciparum VOM-pLDH/Pf-HRP2 RDT (CareStart). Differential cross-reactivity among these combinations was hypothesized to differentiate P. knowlesi from other Plasmodium monoinfections. Among 323 patients with PCR-confirmed P. knowlesi (n = 193), P. falciparum (n = 93), and P. vivax (n = 37) monoinfections, the VOM-pLDH individual component had the highest sensitivity for nonsevere (35%; 95% confidence interval [CI], 27 to 43%) and severe (92%; CI, 81 to 100%) P. knowlesi malaria. CareStart demonstrated a P. knowlesi sensitivity of 42% (CI, 34 to 49%) and specificity of 74% (CI, 65 to 82%), a P. vivax sensitivity of 83% (CI, 66 to 93%) and specificity of 71% (CI, 65 to 76%), and a P. falciparum sensitivity of 97% (CI, 90 to 99%) and specificity of 99% (CI, 97 to 100%). OptiMAL-IT demonstrated a P. knowlesi sensitivity of 32% (CI, 25 to 39%) and specificity of 21% (CI, 15 to 29%), a P. vivax sensitivity of 60% (CI, 42 to 75%) and specificity of 97% (CI, 94 to 99%), and a P. falciparum sensitivity of 82% (CI, 72 to 89%) and specificity of 39% (CI, 33 to 46%). The combination of CareStart plus OptiMAL-IT for P. knowlesi using predefined criteria gave a sensitivity of 25% (CI, 19 to 32%) and specificity of 97% (CI, 92 to 99%). Combining two RDT combinations was highly specific for P. knowlesi malaria diagnosis; however, sensitivity was poor. The specificity of pLDH RDTs was decreased for P. vivax and P. falciparum because of P. knowlesi cross-reactivity and cautions against their use alone in areas where P. knowlesi malaria is endemic. Sensitive P. knowlesi-specific RDTs and/or alternative molecular diagnostic tools are needed in areas where P. knowlesi malaria is endemic.