Malaria rapid diagnostic tests in elimination settings--can they find the last parasite?

Loop mediated isothermal amplification (LAMP) accurately detects malaria DNA from filter paper blood samples of low density parasitaemias

Background

Loop mediated isothermal amplification (LAMP) provides an opportunity for improved, field-friendly detection of malaria infections in endemic areas. However data on the diagnostic accuracy of LAMP for active case detection, particularly low-density parasitaemias, are lacking. We therefore evaluated the performance of a new LAMP kit compared with PCR using DNA from filter paper blood spots.

Methods and Findings

Samples from 865 fever patients and 465 asymptomatic individuals collected in Zanzibar were analysed for Pan (all species) and Pf (P. falciparum) DNA with the Loopamp MALARIA Pan/Pf kit. Samples were amplified at 65°C for 40 minutes in a real-time turbidimeter and results were compared with nested PCR. Samples with discordant results between LAMP and nested PCR were analysed with real-time PCR. The real-time PCR corrected nested PCR result was defined as gold standard. Among the 117 (13.5%) PCR detected P. falciparum infections from fever patients (mean parasite density 7491/µL, range 6–782,400) 115, 115 and 111 were positive by Pan-LAMP, Pf-LAMP and nested PCR, respectively. The sensitivities were 98.3% (95%CI 94–99.8) for both Pan and Pf-LAMP. Among the 54 (11.6%) PCR positive samples from asymptomatic individuals (mean parasite density 10/µL, range 0–4972) Pf-LAMP had a sensitivity of 92.7% (95%CI 80.1–98.5) for detection of the 41 P. falciparum infections. Pan-LAMP had sensitivities of 97% (95%CI 84.2–99.9) and 76.9% (95%CI 46.2–95) for detection of P. falciparum and P. malariae, respectively. The specificities for both Pan and Pf-LAMP were 100% (95%CI 99.1–100) in both study groups.

Conclusion

Both components of the Loopamp MALARIA Pan/Pf detection kit revealed high diagnostic accuracy for parasite detection among fever patients and importantly also among asymptomatic individuals of low parasite densities from minute blood volumes preserved on filter paper. These data support LAMPs potential role for improved detection of low-density malaria infections in pre-elimination settings.

Plasmodium falciparum infection and clinical indicators in relation to net coverage in central Côte d'Ivoire

BACKGROUND

Sleeping under a net, particularly a long-lasting insecticidal net (LLIN), is associated with reduced malaria morbidity and mortality, but requires high coverage and adherence. In this study, parasitologically confirmed Plasmodium falciparum infection and a clinical indicator (i.e. fever) were measured among children in three villages of central Côte d'Ivoire (Bozi, N'Dakonankro and Yoho) and associations with net coverage explored. In Bozi and Yoho, LLINs were provided by the national malaria control programme, prior to the study and an additional catch-up coverage was carried out in Bozi. In N'Dakonankro, no net intervention was conducted.

METHODS

Three cross-sectional surveys were carried out; two in the dry season (February 2010 and November 2011) and one in the rainy season (May 2012). Among 897 children aged <15 years, P. falciparum infection was determined by microscopy and a rapid diagnostic test (RDT). Fever was defined as an axillary temperature ≥37.5°C. A questionnaire was administered to obtain demographic data and net usage.

RESULTS

The proportion of children infected with P. falciparum according to microscopy in the third survey was 74%, 81% and 82% in Yoho, N'Dakonankro and Bozi, respectively. Meanwhile, 46% of the children in N'Dakonankro, 44% in Bozi and 33% in Yoho slept under a net. The risk of P. falciparum infection did not differ between net-sleepers and non-net-sleepers. Fewer children had parasitaemia ≥1,000 parasites/μl of blood in Bozi in the third compared to the first survey. Fever was poorly correlated with P. falciparum infection. The risk of P. falciparum infection did not depend on the village of residence, presence of fever or sleeping under LLIN the night before the survey. Conversely, it was higher in the rainy season and among older children.

CONCLUSIONS

In an area where P. falciparum is highly prevalent, the use of nets was associated with significantly lower levels of parasitaemia. The apparent lack of effect on P. falciparum infection and fever might be explained by the relatively low net coverage in Bozi and Yoho and the relatively short period (<2 years) during which the impact of nets was measured.

Comparison of three PCR-based assays for the non-invasive diagnosis of malaria: detection of Plasmodium parasites in blood and saliva

The conventional molecular diagnosis of malaria uses 18S rRNA-based PCR assay employing blood samples. This assay presents limitation in terms of long turnaround time and increased chances of false-positive results. Here, we evaluated one-step singleplex or multiplex PCR assay based on high copy species-specific consensus repeat sequences (CRS) along with standard 18S rRNA nested PCR (18S n-PCR) assay to detect P. falciparum and P. vivax infection using blood and saliva samples from Indian febrile patients. Out of 327 patients, 187 were found to be positive for malaria parasites by microscopic examination of peripheral blood smears. Among these 130 were P. vivax and 57 were P. falciparum cases. The 18S n-PCR assay and CRS PCR assay identified 186 out of 187 cases (99.4 %). Multiplex CRS PCR assay detected Plasmodium in 176 out of 187 cases (94.1 %). Both singleplex and multiplex CRS PCR assay identified 6 mixed infection cases, while 18S n-PCR assay detected 10 mixed infection cases of P. vivax and P. falciparum, which were not recognized by microscopy. Non-invasive Plasmodium detection rate with DNA derived from saliva samples was highest for 18S n-PCR (87.36 %), followed by singleplex CRS (81 %) and multiplex CRS PCR assay (70.5 %). Specificity for P. vivax and P. falciparum detection for all assays was 98.48 % and 100 % respectively. Detection rate for P. vivax in saliva correlated with parasite density for CRS target-based assays. The species-specific CRS PCR, either as a singleplex or multiplex assay, can have an impact on diagnosis and epidemiological studies in malaria.

Field evaluation of a real-time fluorescence loop-mediated isothermal amplification assay, RealAmp, for the diagnosis of malaria in Thailand and India

BACKGROUND

To eliminate malaria, surveillance for submicroscopic infections is needed. Molecular methods can detect submicroscopic infections but have not hitherto been amenable to implementation in surveillance programs. A portable loop-mediated isothermal amplification assay called RealAmp was assessed in 2 areas of low malaria transmission.

METHODS

RealAmp was evaluated in 141 patients from health clinics in India (passive surveillance) and in 127 asymptomatic persons in Thailand (active surveillance). The diagnostic validity, precision, and predictive value of RealAmp were determined using polymerase chain reaction (PCR) as the reference method. A pilot study of RealAmp was also performed on samples from patients presenting at a Thai health center.

RESULTS

A total of 96 and 7 positive cases were detected in India and Thailand, respectively, via PCR. In comparison with nested PCR, the sensitivity and specificity of RealAmp in India were 94.8% (95% confidence interval [CI], 88.3%-98.3%) and 100% (95% CI, 92.1%-100%), respectively, with correct identification of all 5 Plasmodium vivax cases. In Thailand, compared with pooled real-time PCR, RealAmp demonstrated 100% sensitivity (95% CI, 59.0%-100%) and 96.7% specificity (95% CI, 91.7%-99.1%). Testing at the health center demonstrated RealAmp's potential to serve as a point-of-care test with results available in 30-75 minutes.

CONCLUSION

RealAmp was comparable to PCR in detecting malaria parasites and shows promise as a tool to detect submicroscopic infections in malaria control and elimination programs worldwide.

Malaria prevalence, anemia and baseline intervention coverage prior to mass net distributions in Abia and Plateau States, Nigeria

Background

Nigeria suffers the world’s largest malaria burden, with approximately 51 million cases and 207,000 deaths annually. As part of the country’s aim to reduce by 50% malaria-related morbidity and mortality by 2013, it embarked on mass distribution of free long-lasting insecticidal nets (LLINs).

Methods

Prior to net distribution campaigns in Abia and Plateau States, Nigeria, a modified malaria indicator survey was conducted in September 2010 to determine baseline state-level estimates of Plasmodium prevalence, childhood anemia, indoor residual spraying (IRS) coverage and bednet ownership and utilization.

Results

Overall age-adjusted prevalence of Plasmodium infection by microscopy was similar between Abia (36.1%, 95% CI: 32.3%–40.1%; n = 2,936) and Plateau (36.6%, 95% CI: 31.3%–42.3%; n = 4,209), with prevalence highest among children 5-9 years. P. malariae accounted for 32.0% of infections in Abia, but only 1.4% of infections in Plateau. More than half of children ≤10 years were anemic, with anemia significantly higher in Abia (76.9%, 95% CI: 72.1%–81.0%) versus Plateau (57.1%, 95% CI: 50.6%–63.4%). Less than 1% of households in Abia (n = 1,305) or Plateau (n = 1,335) received IRS in the 12 months prior to survey. Household ownership of at least one bednet of any type was 10.1% (95% CI: 7.5%–13.4%) in Abia and 35.1% (95% CI: 29.2%-41.5%) in Plateau. Ownership of two or more bednets was 2.1% (95% CI: 1.2%–3.7%) in Abia and 14.5% (95% CI: 10.2%–20.3%) in Plateau. Overall reported net use the night before the survey among all individuals, children <5 years, and pregnant women was 3.4%, 6.0% and 5.7%, respectively in Abia and 14.7%, 19.1% and 21.0%, respectively in Plateau. Among households owning nets, 34.4% of children <5 years and 31.6% of pregnant women in Abia used a net, compared to 52.6% of children and 62.7% of pregnant women in Plateau.

Conclusions

These results reveal high Plasmodium prevalence and childhood anemia in both states, low baseline coverage of IRS and LLINs, and sub-optimal net use—especially among age groups with highest observed malaria burden.

Loop-mediated isothermal amplification assay for rapid diagnosis of malaria infections in an area of endemicity in Thailand

The loop-mediated isothermal amplification (LAMP) method, developed by our group for diagnosis of four human malaria parasites, was evaluated on a large scale at a remote clinic in Thailand where malaria is endemic. A total of 899 febrile patients were analyzed in this study. LAMP was first evaluated in 219 patients, and the result was compared to those of two histidine-rich protein (HRP)-2 rapid diagnostic tests (RDTs) and microscopy as a gold standard. LAMP DNA extraction was conducted by a simple boiling method, and the test results were assessed visually. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were 95.7%, 100%, 100%, and 98%, respectively, for LAMP and 98.6%, 98%, 95.8%, and 99.3%, respectively, for RDTs. Since RDT-positive results were based on one out of two RDTs, the sensitivity of RDTs was slightly higher than that of LAMP. However, LAMP tended to be more specific than RDTs. LAMP next was evaluated in 680 patients, and the result was compared to that of microscopy as a gold standard. Sensitivity, specificity, PPV, NPV, and diagnostic accuracy of LAMP were 88.9%, 96.9%, 92.2%, 95.5%, and 94.6%, respectively. Nested PCR was used to confirm the discrepant results. Malaria LAMP in a remote clinic in Thailand achieved an acceptable result, indicating that LAMP malaria diagnosis is feasible in a field setting with limited technical resources. Additionally, the rapid boiling method for extracting DNA from dried blood spots proved to be simple, fast, and suitable for use in the field.

Cytometry in malaria--a practical replacement for microscopy?

Malaria, caused by protozoan Plasmodium parasites, kills ~800,000 people each year. Exact figures are uncertain because presumptive diagnoses are often made without identifying parasites in patients' blood either by microscopy, using Giemsa's century-old stain, or by simpler tests that are ultimately dependent on microscopy for quality control. Microscopy itself relies on trained observers' ability to detect subtle morphological features of parasitized red blood cells, only a few of which may be present on a slide. Quantitative and objective flow cytometric measurements of cellular constituents such as DNA, RNA, and the malaria pigment hemozoin are now useful in research in malaria biology and pharmacology, and can provide more reliable identification of parasite species and developmental stages and better detection of low-density parasitemia than could microscopy. The same measurements can now be implemented in much smaller, simpler, cheaper imaging cytometers, potentially providing a more accurate and precise diagnostic modality.

Lessons learned from the use of HRP-2 based rapid diagnostic test in community-wide screening and treatment of asymptomatic carriers of Plasmodium falciparum in Burkina Faso

Background

Rapid diagnostic tests (RDTs) are immune chromatographic tests targeting antigens of one or more Plasmodium species and offer the potential to extend accurate malaria diagnosis in endemic areas. In this study, the performance of Plasmodium falciparum-specific histidine-rich protein-2 (PfHRP-2) RDT in the detection of asymptomatic carriers from a hyperendemic region of Burkina Faso was compared with microscopy to gain further insight on its relevance in community-based interventions.

Methods

The performance of HRP-2 test was evaluated in terms of sensitivity, specificity, positive and negative predictive values, discordant values, likelihood ratios, accuracy, and precision using microscopy as the 'gold standard’. This analysis was carried out in a controlled, parallel, cluster-randomized (18 clusters; 1:1) study in children and adults. The effect of systematic treatment of P. falciparum asymptomatic carriers during three consecutive monthly community screening campaigns on the incidence of symptomatic malaria episodes over a 12-month period was compared with no treatment of asymptomatic carriers.

Results

Sensitivity of HRP-2 test in asymptomatic carriers was higher in campaign 1 (92.4%) when compared to campaign 2 (84.0%) and campaign 3 (77.8%). The sensitivity of HRP-2 test increased as parasite density increased across all the age groups. Highest sensitivity (≥97.0%) was recorded at parasite densities of 1,000-4,999/μl, except for children aged 10 to 14 years. The specificity of HRP-2 test was comparable across age groups and highest in campaign 3 (95.9%). The negative predictive values were high across the three campaigns (≥92.7%) while the positive predictive values ranged from 23.2 to 73.8%. False-positive and false-negative rates were high in campaign 1 and campaign 3, respectively.

Conclusion

The performance of HRP-2 test in detecting asymptomatic carriers of P. falciparum varied by age and parasite density. Although the use of HRP-2 test is beneficial for the diagnosis of acute malaria, its low sensitivity in screening asymptomatic carriers may limit its utility in pre-elimination interventional settings. The use of a practical and more sensitive test such as loop-mediated isothermal amplification in combination with a cost effective HRP-2 test may be worth exploring in such settings.

Malaria diagnostics in clinical trials

Malaria diagnostics are widely used in epidemiologic studies to investigate natural history of disease and in drug and vaccine clinical trials to exclude participants or evaluate efficacy. The Malaria Laboratory Network (MLN), managed by the Office of HIV/AIDS Network Coordination, is an international working group with mutual interests in malaria disease and diagnosis and in human immunodeficiency virus/acquired immunodeficiency syndrome clinical trials. The MLN considered and studied the wide array of available malaria diagnostic tests for their suitability for screening trial participants and/or obtaining study endpoints for malaria clinical trials, including studies of HIV/malaria co-infection and other malaria natural history studies. The MLN provides recommendations on microscopy, rapid diagnostic tests, serologic tests, and molecular assays to guide selection of the most appropriate test(s) for specific research objectives. In addition, this report provides recommendations regarding quality management to ensure reproducibility across sites in clinical trials. Performance evaluation, quality control, and external quality assessment are critical processes that must be implemented in all clinical trials using malaria tests.

Usefulness of Plasmodium falciparum-specific rapid diagnostic tests for assessment of parasite clearance and detection of recurrent infections after artemisinin-based combination therapy

Background

Rapid diagnostic test (RDT) is an important tool for parasite-based malaria diagnosis. High specificity of RDTs to distinguish an active Plasmodium falciparum infection from residual antigens from a previous infection is crucial in endemic areas where residents are repeatedly exposed to malaria. The efficiency of two RDTs based on histidine-rich protein 2 (HRP2) and lactate dehydrogenase (LDH) antigens were studied and compared with two microscopy techniques (Giemsa and acridine orange-stained blood smears) and real-time polymerase chain reaction (PCR) for assessment of initial clearance and detection of recurrent P. falciparum infections after artemisinin-based combination therapy (ACT) in a moderately high endemic area of rural Tanzania.

Methods

In this exploratory study 53 children < five years with uncomplicated P. falciparum malaria infection were followed up on nine occasions, i.e., day 1, 2, 3, 7, 14, 21, 28, 35 and 42, after initiation of artemether-lumefantrine treatment. At each visit capillary blood samples was collected for the HRP2 and LDH-based RDTs, Giemsa and acridine orange-stained blood smears for microscopy and real-time PCR. Assessment of clearance times and detection of recurrent P. falciparum infections were done for all diagnostic methods.

Results

The median clearance times were 28 (range seven to >42) and seven (two to 14) days for HRP2 and LDH-based RDTs, two (one to seven) and two (one to 14) days for Giemsa and acridine orange-stained blood smear and two (one to 28) days for real-time PCR. RDT specificity against Giemsa-stained blood smear microscopy was 21% for HRP2 on day 14, reaching 87% on day 42, and ≥96% from day 14 to 42 for LDH. There was no significant correlation between parasite density at enrolment and duration of HRP2 positivity (r = 0.13, p = 0.34). Recurrent malaria infections occurred in ten (19%) children. The HRP2 and LDH-based RDTs did not detect eight and two of the recurrent infections, respectively.

Conclusion

The LDH-based RDT was superior to HRP2-based for monitoring of treatment outcome and detection of recurrent infections after ACT in this moderately high transmission setting. The results may have implications for the choice of RDT devices in similar transmission settings for improved malaria case management.

Trial registration

Clinicaltrials.gov, NCT01843764

The usefulness of rapid diagnostic tests in the new context of low malaria transmission in Zanzibar

Background

We assessed if histidine-rich-protein-2 (HRP2) based rapid diagnostic test (RDT) remains an efficient tool for Plasmodium falciparum case detection among fever patients in Zanzibar and if primary health care workers continue to adhere to RDT results in the new epidemiological context of low malaria transmission. Further, we evaluated the performance of RDT within the newly adopted integrated management of childhood illness (IMCI) algorithm in Zanzibar.

Methods and Findings

We enrolled 3890 patients aged ≥2 months with uncomplicated febrile illness in this health facility based observational study conducted in 12 primary health care facilities in Zanzibar, between May-July 2010. One patient had an inconclusive RDT result. Overall 121/3889 (3.1%) patients were RDT positive. The highest RDT positivity rate, 32/528 (6.1%), was found in children aged 5–14 years. RDT sensitivity and specificity against PCR was 76.5% (95% CI 69.0–83.9%) and 99.9% (95% CI 99.7–100%), and against blood smear microscopy 78.6% (95% CI 70.8–85.1%) and 99.7% (95% CI 99.6–99.9%), respectively. All RDT positive, but only 3/3768 RDT negative patients received anti-malarial treatment. Adherence to RDT results was thus 3887/3889 (99.9%). RDT performed well in the IMCI algorithm with equally high adherence among children <5 years as compared with other age groups.

Conclusions

The sensitivity of HRP-2 based RDT in the hands of health care workers compared with both PCR and microscopy for P. falciparum case detection was relatively low, whereas adherence to test results with anti-malarial treatment was excellent. Moreover, the results provide evidence that RDT can be reliably integrated in IMCI as a tool for improved childhood fever management. However, the relatively low RDT sensitivity highlights the need for improved quality control of RDT use in primary health care facilities, but also for more sensitive point-of-care malaria diagnostic tools in the new epidemiological context of low malaria transmission in Zanzibar.

Trial registration

ClinicalTrials.gov NCT01002066

Research gaps for three main tropical diseases in the People's Republic of China

This scoping review analyzes the research gaps of three diseases: schistosomiasis japonica, malaria and echinococcosis. Based on available data in the P.R. China, we highlight the gaps between control capacity and prevalence levels, and between diagnostic/drug development and population need for treatment at different stages of the national control programme. After reviewing the literature from 848 original studies and consultations with experts in the field, the gaps were identified as follows. Firstly, the malaria research gaps include (i) deficiency of active testing in the public community and no appropriate technique to evaluate elimination, (ii) lack of sensitive diagnostic tools for asymptomatic patients, (iii) lack of safe drugs for mass administration. Secondly, gaps in research of schistosomiasis include (i) incongruent policy in the implementation of integrated control strategy for schistosomiasis, (ii) lack of effective tools for Oncomelania sp. snail control, (iii) lack of a more sensitive and cheaper diagnostic test for large population samples, (iv) lack of new drugs in addition to praziquantel. Thirdly, gaps in research of echinococcosis include (i) low capacity in field epidemiology studies, (ii) lack of sanitation improvement studies in epidemic areas, (iii) lack of a sensitivity test for early diagnosis, (iv) lack of more effective drugs for short-term treatment. We believe these three diseases can eventually be eliminated in mainland China if all the research gaps are abridged in a short period of time.

Laboratory Diagnosis of Malaria: Conventional and Rapid Diagnostic Methods

Context.—The global control of malaria is more challenging than that of many other infectious diseases: malaria is vector borne, it is caused by 5 species of Plasmodium with different geographic distributions, infection is widespread in many regions, drug resistance is common, and the disease overlaps clinically with other infectious diseases. Therefore, malaria control programs, in addition to diagnosis and testing, must also target limiting spread of the disease through vector control. Although malaria control efforts have been successful in some regions, malaria remains one of the most important causes of death in sub-Saharan Africa, particularly in women and children.

Objective.—To review the current literature regarding diagnostic methods available to detect clinical malaria, with an emphasis on comparing the strengths and limitations of each method.

Data Sources.—Current World Health Organization malaria control report and other information, recent meta-analyses of diagnostic tests, primary literature concerning the performance characteristics of different tests, and primary literature concerning how diagnostic tests are used in daily practice.

Conclusions.—The most commonly used method for identifying cases of malaria remains microscopic examination of peripheral blood, but there is growing use of malaria rapid diagnostic tests in many regions. One of the most important findings in the recent literature is that despite the widespread use of diagnostic tests, treatment is too often based on clinical findings rather than on results of diagnostic tests.

Malaria elimination: surveillance and response

In the last decade, substantial progress has been made in reducing malaria-associated morbidity and mortality across the globe. Nevertheless, sustained malaria control is essential to continue this downward trend. In some countries, where aggressive malaria control has reduced malaria to a low burden level, elimination, either nationally or subnationally, is now the aim. As countries or areas with a low malaria burden move towards elimination, there is a transition away from programs of universal coverage towards a strategy of localized detection and response to individual malaria cases. To do so and succeed, it is imperative that a strong surveillance and response system is supported, that community cadres are trained to provide appropriate diagnostics and treatment, and that field diagnostics are further developed such that their sensitivity allows for the detection and subsequent treatment of malaria reservoirs in low prevalence environments. To be certain, there are big challenges on the road to elimination, notably the development of drug and insecticide resistance. Nevertheless, countries like Zambia are making great strides towards implementing systems that support malaria elimination in target areas. Continued development of new diagnostics and antimalarial therapies is needed to support progress in malaria control and elimination.

Reactive case detection for malaria elimination: real-life experience from an ongoing program in Swaziland

As countries move towards malaria elimination, methods to identify infections among populations who do not seek treatment are required. Reactive case detection, whereby individuals living in close proximity to passively detected cases are screened and treated, is one approach being used by a number of countries including Swaziland. An outstanding issue is establishing the epidemiologically and operationally optimal screening radius around each passively detected index case. Using data collected between December 2009 and June 2012 from reactive case detection (RACD) activities in Swaziland, we evaluated the effect of screening radius and other risk factors on the probability of detecting cases by reactive case detection. Using satellite imagery, we also evaluated the household coverage achieved during reactive case detection. Over the study period, 250 cases triggered RACD, which identified a further 74 cases, showing the value of RACD over passive surveillance alone. Results suggest that the odds of detecting a case within the household of the index case were significantly higher than in neighbouring households (odds ratio (OR) 13, 95% CI 3.1–54.4). Furthermore, cases were more likely to be detected when RACD was conducted within a week of the index presenting at a health facility (OR 8.7, 95% CI 1.1–66.4) and if the index household had not been sprayed with insecticide (OR sprayed vs not sprayed 0.11, 95% CI 0.03–0.46). The large number of households missed during RACD indicates that a 1 km screening radius may be impractical in such resource limited settings such as Swaziland. Future RACD in Swaziland could be made more effective by achieving high coverage amongst individuals located near to index cases and in areas where spraying has not been conducted. As well as allowing the programme to implement RACD more rapidly, this would help to more precisely define the optimal screening radius.

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

In areas with seasonal transmission, proper management of acute malaria cases that arise in the transmission season can markedly reduce the disease burden. However, asymptomatic carriage of Plasmodium falciparum sustains a long-lasting reservoir in the transmission-free dry season that seeds cyclical malaria outbreaks. Clinical trials targeting asymptomatic parasitaemia in the dry season failed to interrupt the malaria epidemics that follow annual rains. These asymptomatic infections tend to carry multiple-clones, capable of producing gametocytes and infecting Anopheles mosquitoes. Different clones within an infection fluctuate consistently, indicative of interaction between clones during the long course of asymptomatic carriage. However, the therapy-free environment that prevails in the dry season dis-advantages the drug resistant lineages and favors the wild-type parasites. This review highlights some biological and epidemiological characteristics of asymptomatic parasitaemia and calls for consideration of policies to diminish parasite exposure to drugs "therapy-free" and allow natural selection to curb drug resistance in the above setting.

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.

Molecular diagnosis of malaria by photo-induced electron transfer fluorogenic primers: PET-PCR

There is a critical need for developing new malaria diagnostic tools that are sensitive, cost effective and capable of performing large scale diagnosis. The real-time PCR methods are particularly robust for large scale screening and they can be used in malaria control and elimination programs. We have designed novel self-quenching photo-induced electron transfer (PET) fluorogenic primers for the detection of P. falciparum and the Plasmodium genus by real-time PCR. A total of 119 samples consisting of different malaria species and mixed infections were used to test the utility of the novel PET-PCR primers in the diagnosis of clinical samples. The sensitivity and specificity were calculated using a nested PCR as the gold standard and the novel primer sets demonstrated 100% sensitivity and specificity. The limits of detection for P. falciparum was shown to be 3.2 parasites/µl using both Plasmodium genus and P. falciparum-specific primers and 5.8 parasites/µl for P. ovale, 3.5 parasites/µl for P. malariae and 5 parasites/µl for P. vivax using the genus specific primer set. Moreover, the reaction can be duplexed to detect both Plasmodium spp. and P. falciparum in a single reaction. The PET-PCR assay does not require internal probes or intercalating dyes which makes it convenient to use and less expensive than other real-time PCR diagnostic formats. Further validation of this technique in the field will help to assess its utility for large scale screening in malaria control and elimination programs.

Droplet microfluidics platform for highly sensitive and quantitative detection of malaria-causing Plasmodium parasites based on enzyme activity measurement

We present an attractive new system for the specific and sensitive detection of the malaria-causing Plasmodium parasites. The system relies on isothermal conversion of single DNA cleavage-ligation events catalyzed specifically by the Plasmodium enzyme topoisomerase I to micrometer-sized products detectable at the single-molecule level. Combined with a droplet microfluidics lab-on-a-chip platform, this design allowed for sensitive, specific, and quantitative detection of all human-malaria-causing Plasmodium species in single drops of unprocessed blood with a detection limit of less than one parasite/μL. Moreover, the setup allowed for detection of Plasmodium parasites in noninvasive saliva samples from infected patients. During recent years malaria transmission has declined worldwide, and with this the number of patients with low-parasite density has increased. Consequently, the need for accurate detection of even a few parasites is becoming increasingly important for the continued combat against the disease. We believe that the presented droplet microfluidics platform, which has a high potential for adaptation to point-of-care setups suitable for low-resource settings, may contribute significantly to meet this demand. Moreover, potential future adaptation of the presented setup for the detection of other microorganisms may form the basis for the development of a more generic platform for diagnosis, fresh water or food quality control, or other purposes within applied or basic science.

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.

Malaria research challenges in low prevalence settings

The prevalence of malaria has reduced significantly in some areas over the past decade. These reductions have made local elimination possible and the research agenda has shifted to this new priority. However, there are critical issues that arise when studying malaria in low transmission settings, particularly identifying asymptomatic infections, accurate detection of individuals with microparasitaemic infections, and achieving a sufficient sample size to have an adequately powered study. These challenges could adversely impact the study of malaria elimination if they remain unanswered.

Direct blood PCR in combination with nucleic acid lateral flow immunoassay for detection of Plasmodium species in settings where malaria is endemic

Declining malaria transmission and known difficulties with current diagnostic tools for malaria, such as microscopy and rapid diagnostic tests (RDTs) in particular at low parasite densities, still warrant the search for sensitive diagnostic tests. Molecular tests need substantial simplification before implementation in clinical settings in countries where malaria is endemic. Direct blood PCR (db-PCR), circumventing DNA extraction, to detect Plasmodium was developed and adapted to be visualized by nucleic acid lateral flow immunoassay (NALFIA). The assay was evaluated in the laboratory against samples from confirmed Sudanese patients (n = 51), returning travelers (n = 214), samples from the Dutch Blood Bank (n = 100), and in the field in Burkina Faso (n = 283) and Thailand (n = 381) on suspected malaria cases and compared to RDT and microscopy. The sensitivity and specificity of db-PCR-NALFIA compared to the initial diagnosis in the laboratory were 94.4% (95% confidence interval [CI] = 0.909 to 0.969) and 97.4% (95% CI = 0.909 to 0.969), respectively. In Burkina Faso, the sensitivity was 94.8% (95% CI = 0.88.7 to 97.9%), and the specificity was 82.4% (95% CI = 75.4 to 87.7%) compared to microscopy and 93.3% (95% CI = 87.4 to 96.7%) and 91.4% (95% CI = 85.2 to 95.3%) compared to RDT. In Thailand, the sensitivity and specificity were 93.4% (CI = 86.4 to 97.1%) and 90.9 (95% CI = 86.7 to 93.9%), respectively, compared to microscopy and 95.6% (95% CI = 88.5 to 98.6%) and 87.1% (95% CI = 82.5 to 90.6) compared to RDT. db-PCR-NALFIA is highly sensitive and specific for easy and rapid detection of Plasmodium parasites and can be easily used in countries where malaria is endemic. The inability of the device to discriminate Plasmodium species requires further investigation.

Bayesian Latent Class Models in malaria diagnosis

Aims

The main focus of this study is to illustrate the importance of the statistical analysis in the evaluation of the accuracy of malaria diagnostic tests, without admitting a reference test, exploring a dataset (3317) collected in São Tomé and Príncipe.

Methods

Bayesian Latent Class Models (without and with constraints) are used to estimate the malaria infection prevalence, together with sensitivities, specificities, and predictive values of three diagnostic tests (RDT, Microscopy and PCR), in four subpopulations simultaneously based on a stratified analysis by age groups (, 5 years old) and fever status (febrile, afebrile).

Results

In the afebrile individuals with at least five years old, the posterior mean of the malaria infection prevalence is 3.2% with a highest posterior density interval of [2.3–4.1]. The other three subpopulations (febrile 5 years, afebrile or febrile children less than 5 years) present a higher prevalence around 10.3% [8.8–11.7]. In afebrile children under-five years old, the sensitivity of microscopy is 50.5% [37.7–63.2]. In children under-five, the estimated sensitivities/specificities of RDT are 95.4% [90.3–99.5]/93.8% [91.6–96.0] – afebrile – and 94.1% [87.5–99.4]/97.5% [95.5–99.3] – febrile. In individuals with at least five years old are 96.0% [91.5–99.7]/98.7% [98.1–99.2] – afebrile – and 97.9% [95.3–99.8]/97.7% [96.6–98.6] – febrile. The PCR yields the most reliable results in four subpopulations.

Conclusions

The utility of this RDT in the field seems to be relevant. However, in all subpopulations, data provide enough evidence to suggest caution with the positive predictive values of the RDT. Microscopy has poor sensitivity compared to the other tests, particularly, in the afebrile children less than 5 years. This type of findings reveals the danger of statistical analysis based on microscopy as a reference test. Bayesian Latent Class Models provide a powerful tool to evaluate malaria diagnostic tests, taking into account different groups of interest.

Diagnosis of Malaria Infection with or without Disease

The revised W.H.O. guidelines for malaria management in endemic countries recommend that treatment should be reserved to laboratory confirmed cases, both for adults and children. Currently the most widely used tools are rapid diagnostic tests (RDTs), that are accurate and reliable in diagnosing malaria infection. However, an infection is not necessarily a clinical malaria, and RDTs may give positive results in febrile patients who have another cause of fever. Excessive reliance on RDTs may cause overlooking potentially severe non malarial febrile illnesses (NMFI) in these cases. In countries or areas where transmission intensity remains very high, fever management in children (especially in the rainy season) should probably remain presumptive, as a test-based management may not be safe, nor cost effective. In contrast, in countries with low transmission, including those targeted for malaria elimination, RDTs are a key resource to limit unnecessary antimalarial prescription and to identify pockets of infected individuals. Research should focus on very sensitive tools for infection on one side, and on improved tools for clinical management on the other, including biomarkers of clinical malaria and/or of alternative causes of fever.

Operational research to inform a sub-national surveillance intervention for malaria elimination in Solomon Islands

Background

Successful reduction of malaria transmission to very low levels has made Isabel Province, Solomon Islands, a target for early elimination by 2014. High malaria transmission in neighbouring provinces and the potential for local asymptomatic infections to cause malaria resurgence highlights the need for sub-national tailoring of surveillance interventions. This study contributes to a situational analysis of malaria in Isabel Province to inform an appropriate surveillance intervention.

Methods

A mixed method study was carried out in Isabel Province in late 2009 and early 2010. The quantitative component was a population-based prevalence survey of 8,554 people from 129 villages, which were selected using a spatially stratified sampling approach to achieve uniform geographical coverage of populated areas. Diagnosis was initially based on Giemsa-stained blood slides followed by molecular analysis using polymerase chain reaction (PCR). Local perceptions and practices related to management of fever and treatment-seeking that would impact a surveillance intervention were also explored using qualitative research methods.

Results

Approximately 33% (8,554/26,221) of the population of Isabel Province participated in the survey. Only one subject was found to be infected with Plasmodium falciparum (Pf) (96 parasites/μL) using Giemsa-stained blood films, giving a prevalence of 0.01%. PCR analysis detected a further 13 cases, giving an estimated malaria prevalence of 0.51%. There was a wide geographical distribution of infected subjects. None reported having travelled outside Isabel Province in the previous three months suggesting low-level indigenous malaria transmission. The qualitative findings provide warning signs that the current community vigilance approach to surveillance will not be sufficient to achieve elimination. In addition, fever severity is being used by individuals as an indicator for malaria and a trigger for timely treatment-seeking and case reporting. In light of the finding of a low prevalence of parasitaemia, the current surveillance system may not be able to detect and prevent malaria resurgence.

Conclusion

An adaption to the malERA surveillance framework is proposed and recommendations made for a tailored provincial-level surveillance intervention, which will be essential to achieve elimination, and to maintain this status while the rest of the country catches up.

A mathematical model to predict the optimal test line location and sample volume for lateral flow immunoassays

Lateral flow immunoassay (LFIA) platform is one of the most relevant technologies for screening and diagnosing clinical conditions. However due to low sensitivity and poor repeatability of the platform it has been used only for limited and non-critical tests. Mathematical models have been used to understand the principles of capillary flow and antibody antigen based immunoreactions in nitrocellulose membrane typically seen in LFIA. The model presented in this paper predicts the optimized location of test line on LFIA strip, sample volume and total reaction time that is needed to achieve the required sensitivity for different analytes on a case to case basis. The membrane properties like capillary flow time (s/cm), concentration and affinity constants of antibodies can be varied and the corresponding effect on strip design can be found. Hence this model can be used as a design tool to optimize the LFIA strip construction and reagent development processes.