Plasmodium malariae and Plasmodium ovale--the "bashful" malaria parasites

Plasmodium ovale: a case of not-so-benign tertian malaria

Severe malaria is most commonly associated with Plasmodium falciparum. Plasmodium vivax is increasingly recognized as being capable of causing severe disease. In contrast, Plasmodium ovale is considered as a cause of benign disease and evidence supporting the occurrence of severe or complicated ovale infection is rare. This report describes a case of severe P. ovale infection in a patient presenting with jaundice, respiratory distress, severe thrombocytopenia, petechiae, and hypotension. He had no apparent underlying risk factors for severe disease.

A morphology-based method for the diagnosis of red blood cells parasitized by Plasmodium malariae and Plasmodium ovale

BACKGROUND

The morphology of red blood cells (RBCs) is altered significantly during the maturation stages of malaria parasites, which include ring, trophozoite, and schizont. There is dissimilarity in terms of the morphological characteristics of parasitized RBCs infected by the 4 species of Plasmodium, including falciparum, vivax, malariae, and ovale. This makes the process of diagnosis very difficult, which may lead to a wrong treatment method and substantial damage to the health of the patient. An innovative technique in introduced that accurately defines the shape of parasitized RBCs at each stage of infection as a potential method of diagnosis.

METHODS

Giemsa-stained thin blood films were prepared using blood samples collected from healthy donors as well as patients infected with P. malariae and P. ovale. The diameter and thickness of healthy and infected RBCs at each stage of infection were measured from their optical images using Olysia and Scanning Probe Image Processor (SPIP) software, respectively. A shape equation was fitted based on the morphological characteristics of RBCs, and their relative 2-dimensional shapes were plotted using Wolfram Mathematica.

RESULTS

At the ring stage, the thicknesses of RBCs parasitized by P. malariae (Pm-RBCs) and P. ovale (Po-RBCs) increased by 42% and 51%, respectively. Both Pm-RBCs and Po-RBCs remained nearly biconcave throughout parasite development even though their volumes increased.

CONCLUSIONS

It is proposed that the morphology-based characterization technique introduced here could be used to intensify the accuracy of the Giemsa staining diagnosis method for the detection of the Plasmodium genus and infection stage. Based on the significant morphological alterations induced by different Plasmodium species, the results may also find practical use for faster prediction and treatment of human malaria.

A 20-year longitudinal study of Plasmodium ovale and Plasmodium malariae prevalence and morbidity in a West African population

BACKGROUND

Plasmodium ovale and Plasmodium malariae have long been reported to be widely distributed in tropical Africa and in other major malaria-endemic areas of the world. However, little is known about the burden caused by these two malaria species.

METHODS AND FINDINGS

We did a longitudinal study of the inhabitants of Dielmo village, Senegal, between June, 1990, and December, 2010. We monitored the inhabitants for fever during this period and performed quarterly measurements of parasitemia. We analyzed parasitological and clinical data in a random-effect logistic regression model to investigate the relationship between the level of parasitemia and the risk of fever and to establish diagnostic criteria for P. ovale and P. malariae clinical attacks. The prevalence of P. ovale and P. malariae infections in asymptomatic individuals were high during the first years of the project but decreased after 2004 and almost disappeared in 2010 in relation to changes in malaria control policies. The average incidence densities of P. ovale and P. malariae clinical attacks were 0.053 and 0.093 attacks per person per year in children <15 years and 0.024 and 0.009 attacks per person per year in adults ≥ 15 years, respectively. These two malaria species represented together 5.9% of the malaria burden.

CONCLUSIONS

P. ovale and P. malariae were a common cause of morbidity in Dielmo villagers until the recent dramatic decrease of malaria that followed the introduction of new malaria control policies. P. ovale and P. malariae may constitute an important cause of morbidity in many areas of tropical Africa.

Genetic marker suitable for identification and genotyping of Plasmodium ovale curtisi and Plasmodium ovale wallikeri

We present a seminested PCR method that specifically discriminates between Plasmodium ovale curtisi and P. ovale wallikeri with high sensitivity. The test is based on species-specific amplification of a size-polymorphic fragment of the tryptophan-rich antigen gene, potra, which also permits discrimination of intraspecific sequence variants at this locus.

Major burden of severe anemia from non-falciparum malaria species in Southern Papua: a hospital-based surveillance study

BACKGROUND

The burden of anemia attributable to non-falciparum malarias in regions with Plasmodium co-endemicity is poorly documented. We compared the hematological profile of patients with and without malaria in southern Papua, Indonesia.

METHODS AND FINDINGS

Clinical and laboratory data were linked for all patients presenting to a referral hospital between April 2004 and December 2012. Data were available on patient demographics, malaria diagnosis, hemoglobin concentration, and clinical outcome, but other potential causes of anemia could not be identified reliably. Of 922,120 patient episodes (837,989 as outpatients and 84,131 as inpatients), a total of 219,845 (23.8%) were associated with a hemoglobin measurement, of whom 67,696 (30.8%) had malaria. Patients with P. malariae infection had the lowest hemoglobin concentration (n = 1,608, mean = 8.93 [95% CI 8.81-9.06]), followed by those with mixed species infections (n = 8,645, mean = 9.22 [95% CI 9.16-9.28]), P. falciparum (n = 37,554, mean = 9.47 [95% CI 9.44-9.50]), and P. vivax (n = 19,858, mean = 9.53 [95% CI 9.49-9.57]); p-value for all comparisons <0.001. Severe anemia (hemoglobin <5 g/dl) was present in 8,151 (3.7%) patients. Compared to patients without malaria, those with mixed Plasmodium infection were at greatest risk of severe anemia (adjusted odds ratio [AOR] 3.25 [95% CI 2.99-3.54]); AORs for severe anaemia associated with P. falciparum, P. vivax, and P. malariae were 2.11 (95% CI 2.00-2.23), 1.87 (95% CI 1.74-2.01), and 2.18 (95% CI 1.76-2.67), respectively, p<0.001. Overall, 12.2% (95% CI 11.2%-13.3%) of severe anemia was attributable to non-falciparum infections compared with 15.1% (95% CI 13.9%-16.3%) for P. falciparum monoinfections. Patients with severe anemia had an increased risk of death (AOR = 5.80 [95% CI 5.17-6.50]; p<0.001). Not all patients had a hemoglobin measurement, thus limitations of the study include the potential for selection bias, and possible residual confounding in multivariable analyses.

CONCLUSIONS

In Papua P. vivax is the dominant cause of severe anemia in early infancy, mixed P. vivax/P. falciparum infections are associated with a greater hematological impairment than either species alone, and in adulthood P. malariae, although rare, is associated with the lowest hemoglobin concentration. These findings highlight the public health importance of integrated genus-wide malaria control strategies in areas of Plasmodium co-endemicity.

Recent advances in detection of Plasmodium ovale: implications of separation into the two species Plasmodium ovale wallikeri and Plasmodium ovale curtisi

Recent molecular studies indicate that Plasmodium ovale malaria is caused by two closely related species of protozoan parasites, thereby imposing new challenges for detection and species differentiation. This minireview explores the potential value of innovative methods for the molecular diagnosis of malaria with a strong emphasis on the discrimination and genotyping of P. ovale wallikeri and P. ovale curtisi as well as tools for the simultaneous detection of P. ovale sp. An update for the widely used NP-1993 to NP-2005 (SSU rRNA) protocols for all human malaria parasites is discussed.

Plasmodium ovale curtisi and Plasmodium ovale wallikeri in North-West Ethiopia

BACKGROUND

In Ethiopia Plasmodium falciparum and Plasmodium vivax are the dominant species accounting for roughly 60 and 40% of malaria cases, respectively. Recently a major shift from P. falciparum to P. vivax has been observed in various parts of the country but the epidemiology of the other human malaria species, Plasmodium ovale spp. and Plasmodium malariae remains poorly understood. The aim of this study was to assess P. ovale curtisi and wallikeri infection in north-west Ethiopia by using microscopy and nested PCR.

METHODS

A health institution-based survey using non-probability sampling techniques was conducted at Maksegnet, Enfranze and Kola Diba health centres and Metema hospital in North Gondar. Three-hundred patients with signs and symptoms consistent with malaria were included in this study and capillary blood was collected for microscopic examination and molecular analysis of Plasmodium species. Samples were collected on Whatman 903 filter papers, stored in small plastic bags with desiccant and transported to Vienna (Austria) for molecular analysis. Data from study participants were entered and analysed by SPSS 20 software.

RESULTS

Out of 300 study participants (167 males and 133 females), 184 samples were classified positive for malaria (133 P. falciparum and 51 P. vivax) by microscopy. By species-specific PCR 233 Plasmodium spp (95% CI: 72.6-82) were detected and the majority 155 (66.5%, 95% CI: 60.2-72.3) were P. falciparum followed by P. vivax 69 (29.6%, 95% CI; 24.1-35.8) and 9 (3.9%, 95% CI: 2-7.2) samples were positive for P. ovale. Seven of P. ovale parasites were confirmed as P. ovale wallikeri and two were confirmed as P. ovale curtisi. None of the samples tested positive for P. malariae. During microscopic examination there were high (16.3%) false negative reports and all mixed infections and P. ovale cases were missed or misclassified.

CONCLUSION

This study indicates that P. ovale malaria is under-reported in Ethiopia and provides the first known evidence of the sympatric distribution of indigenous P. ovale wallikeri and P. ovale curtisi in Ethiopia. Therefore, further studies assessing the prevalence of the rare species P. ovale and P. malariae are urgently needed to better understand the species distribution and to adapt malaria control strategies.

Use of qPCR and genomic sequencing to diagnose Plasmodium ovale wallikeri malaria in a returned soldier in the setting of a negative rapid diagnostic assay

Plasmodium ovale is one of several clinically relevant malaria species known to cause disease in humans. However, in contrast to Plasmodium falciparum and Plasmodium vivax, which are responsible for most cases of human malaria, P. ovale has a wide distribution but low prevalence in tropical regions. Here, we report the case of a soldier returning from Liberia with P. ovale wallikeri malaria. This case highlights the limitations of both microscopy and the malaria rapid diagnostic test for diagnosing infection with P. ovale and for distinguishing P. ovale wallikeri from P. ovale curtisi. To our knowledge, this is the first case report in which quantitative real-time polymerase chain reaction amplification using the Cytochrome B gene, coupled with genomic sequencing of the potra locus, was used for definitive diagnosis of P. ovale wallikeri malaria.

Misclassification of Plasmodium infections by conventional microscopy and the impact of remedial training on the proficiency of laboratory technicians in species identification

BACKGROUND

Malaria diagnosis is largely dependent on the demonstration of parasites in stained blood films by conventional microscopy. Accurate identification of the infecting Plasmodium species relies on detailed examination of parasite morphological characteristics, such as size, shape, pigment granules, besides the size and shape of the parasitized red blood cells and presence of cell inclusions. This work explores misclassifications of four Plasmodium species by conventional microscopy relative to the proficiency of microscopists and morphological characteristics of the parasites on Giemsa-stained blood films.

CASE DESCRIPTION

Ten-day malaria microscopy remedial courses on parasite detection, species identification and parasite counting were conducted for public health and research laboratory personnel. Proficiency in species identification was assessed at the start (pre) and the end (post) of each course using known blood films of Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale and Plasmodium vivax infections with densities ranging from 1,000 to 30,000 parasites/μL. Outcomes were categorized as false negative, positive without speciation, P. falciparum, P. malariae, P. ovale, P. vivax and mixed infections.

DISCUSSION AND EVALUATION

Reported findings are based on 1,878 P. falciparum, 483 P. malariae, 581 P. ovale and 438 P. vivax cumulative results collated from 2008 to 2010 remedial courses. Pre-training false negative and positive misclassifications without speciation were significantly lower on P. falciparum infections compared to non-falciparum infections (p < 0.0001). Post-training misclassifications decreased significantly compared to pre- training misclassifications which in turn led to significant improvements in the identification of the four species. However, P. falciparum infections were highly misclassified as mixed infections, P. ovale misclassified as P. vivax and P. vivax similarly misclassified as P. ovale (p < 0.05).

CONCLUSION

These findings suggest that the misclassification of malaria species could be a common occurrence especially where non-falciparum infections are involved due to lack of requisite skills in microscopic diagnosis and variations in morphological characteristics within and between Plasmodium species. Remedial training might improve reliability of conventional light microscopy with respect to differentiation of Plasmodium infections.

The malaria vaccine--status quo 2013

It has been 40 years since David Clyde's landmark induction of sterile immunity against deadly falciparum malaria through immunization by exposure to 1000 irradiated mosquitoes, and the first recombinant Plasmodium falciparum vaccine, RTS,S/AS01, is now in Phase III testing. Interim reports from this largest ever Phase III pediatric trial in Africa show the malaria vaccine decreased clinical and severe disease by 56% and 47% respectively in 5-17 month olds, and by 31% and 26% respectively in infants participating in the Expanded Programme on Immunization. Final data in 2014 will more fully describe the efficacy of RTS,S/AS01 over time against all falciparum malaria cases under a variety of transmission conditions, results essential for decisions on licensure and deployment. Meanwhile, candidate components of a second-generation malaria vaccine are emerging. A field trial of the polymorphic blood stage vaccine AMA-1/AS02 demonstrated no overall efficacy (ve = 17%, P = 0.18), yet a sieve analysis revealed allele-specific efficacy (ve = 64%, P = 0.03) against the vaccine strain, suggesting AMA-1 antigens could be part of a multicomponent vaccine. Initial trials of new antigens include the highly conserved pre-erythrocytic candidate PfCelTOS, a synthetic Plasmodium vivax circumsporozoite antigen VMP-001, and sexual stage vaccines containing antigens from both P. falciparum (Pfs25) and P. vivax (Pvs25) intended to interrupt transmission. Targets for a vaccine to protect against placental malaria, the leading remediable cause of low birth weight infants in Africa, have been identified. Lastly, renewed efforts are underway to develop a practical attenuated-sporozoite vaccine to recapture the promise of David Clyde's experiment.

Declining malaria parasite prevalence and trends of asymptomatic parasitaemia in a seasonal transmission setting in North-Western Burkina Faso between 2000 and 2009-2012

Background

Malaria transmission was reported to have declined in some East African countries. However, a comparable trend has not been confirmed for West Africa. This study aims to assess the dynamics of parasite prevalence and malaria species distribution over time in an area of highly seasonal transmission in Burkina Faso. The aim was also to compare frequency of asymptomatic parasitaemia between wet and dry season by parasite density status and age group.

Methods

During the years 2009–2012, six cross-sectional studies were performed in the rural village Bourasso in the Nouna Health District in north-west Burkina Faso. In subsequent rainy and dry seasons blood samples were collected to assess the parasite prevalence, species, density and clinical parameters. In total, 1,767 children and adults were examined and compared to a baseline collected in 2000.

Results

The microscopical parasite prevalence (mainly P. falciparum) measured over the rainy seasons decreased significantly from 78.9% (2000) to 58.4%, 55.9% and 49.3%, respectively (2009–2011; p <0.001). The frequency of Plasmodium malariae infections (mono- and co-infections) decreased parallel to the overall parasite prevalence from 13.4% in 2000 to 2.1%, 4.1% and 4.7% in 2009–2011 (p <0.001). Comparing parasite-positive subjects from the rainy season versus dry season, the risk of fever was significantly reduced in the dry season adjusting for parasite density (grouped) and age group.

Conclusions

The results of this study suggest a decline of malaria transmission over the rainy seasons between 2000 and 2009–2011 in the region of Nouna, Burkina Faso. The decreased transmission intensity was associated with lower prevalence of P. malariae infections (both mono-infections and co-infections). Asymptomatic parasitaemia was more frequent in the dry season even adjusting for parasite density and age group in a multivariate regression. Possible reasons for this observation include the existence of less pathogenic Plasmodium falciparum genotypes prevailing in the dry season, or the effect of a reduced incidence density during the dry season.

Persistent detection of Plasmodium falciparum, P. malariae, P. ovale curtisi and P. ovale wallikeri after ACT treatment of asymptomatic Ghanaian school-children

Two hundred and seventy four asymptomatic Ghanaian school-children aged 5 to 17 years were screened for malaria parasites by examination of blood films. One hundred and fifty five microscopically-positive individuals were treated with dihydroartemisinin-piperaquine and followed for 3 weeks. Retrospective species-specific PCR of all 274 screened samples identified an additional 60 children with sub-patent parasitaemia, and a substantial proportion of co-infections with Plasmodium malariae, Plasmodium ovale curtisi and Plasmodium ovale wallikeri. One hundred individuals harboured at least one non-falciparum parasite species. Using standard double-read microscopy, the 21-day efficacy of treatment against Plasmodium falciparum was 91.4% among the 117 children seen at all 5 visits. Using nested PCR to test 152 visit 5 blood samples, 22 were found to be parasite-positive. Twenty individuals harboured P. falciparum, four harboured P. ovale spp. and two P. malariae, with four of these 22 isolates being mixed species infections. The persistent detection of low density Plasmodium sp. infections following antimalarial treatment suggests these may be a hitherto unrecognised obstacle to malaria elimination.

A new real-time PCR for the detection of Plasmodium ovale wallikeri

It has been proposed that ovale malaria in humans is caused by two closely related but distinct species of malaria parasites: P. ovale curtisi and P. ovale wallikeri. We have extended and optimized a Real-time PCR assay targeting the parasite’s small subunit ribosomal RNA (ssrRNA) gene to detect both these species. When the assay was applied to 31 archival blood samples from patients diagnosed with P. ovale, it was found that the infection in 20 was due to P. ovale curtisi and in the remaining 11 to P. ovale wallikeri. Thus, this assay provides a useful tool that can be applied to epidemiological investigations of the two newly recognized distinct P. ovale species, that might reveal if these species also differ in their clinical manifestation, drugs susceptibility and relapse periodicity. The results presented confirm that P. ovale wallikeri is not confined to Southeast Asia, since the majority of the patients analyzed in this study had acquired their P. ovale infection in African countries, mostly situated in West Africa.

Pharmacodynamics of antimalarial chemotherapy

Malaria chemotherapy is under constant threat from the emergence and spread of multidrug resistance of Plasmodium falciparum. Resistance has been observed to almost all currently used antimalarials. Some drugs are also limited by toxicity. A fundamental component of the strategy for malaria chemotherapy is based on prompt, effective and safe antimalarial drugs. To counter the threat of resistance of P. falciparum to existing monotherapeutic regimens, current malaria treatment is based principally on the artemisinin group of compounds, either as monotherapy or artemisinin-based combination therapies for treatment of both uncomplicated and severe falciparum malaria. Key advantages of artemisinins over the conventional antimalarials include their rapid and potent action, with good tolerability profiles. Their action also covers transmissible gametocytes, resulting in decreased disease transmission. Up to now there has been no prominent report of drug resistance to this group of compounds. Treatment of malaria in pregnant women requires special attention in light of limited treatment options caused by potential teratogenicity coupled with a paucity of safety data for the mother and fetus. Treatment of other malaria species is less problematic and chloroquine is still the drug of choice, although resistance of P. vivax to chloroquine has been reported. Multiple approaches to the identification of new antimalarial targets and promising antimalarial drugs are being pursued in order to cope with drug resistance.

Malaria: an update for physicians

Malaria remains the most important parasitic infection in humans. There have been significant advances in the treatment of both nonsevere and severe malaria with the advent of artemisinin combination therapies and parenteral artesunate, but the optimum supportive management of severe malaria is unclear. A broadly acceptable therapy for the prevention of relapses in Plasmodium vivax infection has not been discovered. Globally, the priority remains to prevent infection in the vulnerable, to move toward elimination where feasible, and to ensure that effective treatment is available to all. In developed settings, prevention of infection and its early recognition are crucial.

Plasmodium ovale in Bangladesh: genetic diversity and the first known evidence of the sympatric distribution of Plasmodium ovale curtisi and Plasmodium ovale wallikeri in southern Asia

In spite of the high prevalence of malaria in Bangladesh and other southern Asian countries, there remains a substantial shortage of knowledge about the less common human malaria parasites. Recent studies indicate that Plasmodium ovale is made up of two species, namely Plasmodium ovale wallikeri and Plasmodium ovale curtisi. Genus- and species-specific nested PCR analyses of the ssrRNA gene was used to detect P. ovale infections among 2,246 diagnostic samples. Plasmodium ovale infections were further differentiated by nested PCR of the potra gene and multilocus sequence analysis of the cox1, porbp2 and the ssrRNA genes. Both P. ovale curtisi and P. ovale wallikeri occur sympatrically in the Chittagong Hill Tracts, Bangladesh and all patients presented with a mild or asymptomatic symptom complex at the time of diagnosis. The pathogens can be differentiated by nested PCRs targeting the ssrRNA and potra genes, and display dimorphism in multilocus sequence analyses. We believe that we report the first evidence of sympatric P. ovale curtisi and P. ovale wallikeri in southern Asia within a relatively confined study area of less than 5,000 km(2). High rates of mixed infections, the emergence of "new" human malaria parasite species and the evidence of zoonotic capability call for optimised diagnostic strategies for a new era of eradication.

Comparison of molecular tests for the diagnosis of malaria in Honduras

BACKGROUND

Honduras is a tropical country with more than 70% of its population living at risk of being infected with either Plasmodium vivax or Plasmodium falciparum. Laboratory diagnosis is a very important factor for adequate treatment and management of malaria. In Honduras, malaria is diagnosed by both, microscopy and rapid diagnostic tests and to date, no molecular methods have been implemented for routine diagnosis. However, since mixed infections, and asymptomatic and low-parasitaemic cases are difficult to detect by light microscopy alone, identifying appropriate molecular tools for diagnostic applications in Honduras deserves further study. The present study investigated the utility of different molecular tests for the diagnosis of malaria in Honduras.

METHODS

A total of 138 blood samples collected as part of a clinical trial to assess the efficacy of chloroquine were used: 69 microscopically confirmed P. falciparum positive samples obtained on the day of enrollment and 69 follow-up samples obtained 28 days after chloroquine treatment and shown to be malaria negative by microscopy. Sensitivity and specificity of microscopy was compared to an 18 s ribosomal RNA gene-based nested PCR, two single-PCR reactions designed to detect Plasmodium falciparum infections, one single-PCR to detect Plasmodium vivax infections, and one multiplex one-step PCR reaction to detect both parasite species.

RESULTS

Of the 69 microscopically positive P. falciparum samples, 68 were confirmed to be P. falciparum-positive by two of the molecular tests used. The one sample not detected as P. falciparum by any of the molecular tests was shown to be P. vivax-positive by a reference molecular test indicating a misdiagnosis by microscopy. The reference molecular test detected five cases of P. vivax/P. falciparum mixed infections, which were not recognized by microscopy as mixed infections. Only two of these mixed infections were recognized by a multiplex test while a P. vivax-specific polymerase chain reaction (PCR) detected three of them. In addition, one of the day 28 samples, previously determined to be malaria negative by microscopy, was shown to be P. vivax-positive by three of the molecular tests specific for this parasite.

CONCLUSIONS

Molecular tests are valuable tools for the confirmation of Plasmodium species and in detecting mixed infections in malaria endemic regions.

Biology of human malaria plasmodia including Plasmodium knowlesi

Malaria is a vector-borne infection caused by unicellular parasite of the genus Plasmodium. Plasmodia are obligate intracellular parasites that are able to infect and replicate within the erythrocytes after a clinically silent replication phase in the liver. Four species (P.falciparum, P.malariae, P.ovale and P.vivax) are traditionally recognized as responsible of natural infection in human beings but the recent upsurge of P.knowlesi malaria in South-East Asia has led clinicians to consider it as the fifth human malaria parasite. Recent studies in wild-living apes in Africa have revealed that P.falciparum, the most deadly form of human malaria, is not only human-host restricted as previously believed and its phylogenetic lineage is much more complex with new species identified in gorilla, bonobo and chimpanzee. Although less impressive, new data on biology of P.malariae, P.ovale and P.vivax are also emerging and will be briefly discussed in this review.

Cloning and heterologous expression of Plasmodium ovale dihydrofolate reductase-thymidylate synthase gene

Plasmodial bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a validated antimalarial drug target. In this study, expression of the putative dhfr-ts of Plasmodium ovale rescued the DHFR chemical knockout and a TS null bacterial strain, demonstrating its DHFR and TS catalytic functions. PoDHFR-TS was expressed in Escherichia coli BL21 (DE3) and affinity purified by Methotrexate Sepharose column. Biochemical and enzyme kinetics characterizations indicated that PoDHFR-TS is similar to other plasmodial enzymes, albeit with lower catalytic activity but better tolerance of acidic pH. Importantly, the PoDHFR from Thai isolate EU266602 remains sensitive to the antimalarials pyrimethamine and cycloguanil, in contrast to P. falciparum and P. vivax isolates where resistance to these drugs is widespread.

Activation of the hypnozoite: a part of Plasmodium vivax life cycle and survival

Background

Plasmodium vivax is the most widespread malaria parasite. It has a dormant stage in the human liver, which makes it difficult to eradicate. It is proposed that a relapse of vivax malaria, besides being genetically determined by the specific strain, is induced by the bites of uninfected vectors.

Presentation of the hypothesis

The dormant stage maximizes the possibility for the parasite to reach the vector for sexual reproduction. The advantage would increase if the parasite was able to detect the presence of a new generation of vectors. The sporozoites function both in the vector and in the human hosts. They invade the cells of the salivary gland in the vector and the hepatocytes in the human. Some of the sporozoites develop into hypnozoites in the human liver. It is suggested that the hypnozoite activates when it recognizes the same Anopheles specific protein, which it had previously recognized as a sporozoite to invade the salivary gland in the vector. Another possibility is that the hypnozoite activates upon the bodily reaction by the human on a bite by an Anopheles female.

Testing the hypothesis

The connection between the relapse and a new generation of vectors can be documented by simultaneous monitoring of both parasitaemia in humans and the presence of uninfective/infective vectors in the same area with seasonal malaria transmission. Experimental studies are needed to find the saliva components, which trigger the relapse. Although P. cynomolgi in monkeys also has hypnozoites and relapses, testing with monkeys might be problematical. These live in a reasonably stable tropical environment where relapses cannot easily be linked to vectors. The importance of the trigger increases in unpredictable variations in the vector season.

Implications of the hypothesis

Artificial triggering of hypnozoites would make the medication more effective and resistance against a protein that the parasite itself uses during its life cycle would not develop. In areas with seasonal vivax malaria it could be used locally for eradication.

Plasmodium ovale curtisi and Plasmodium ovale wallikeri circulate simultaneously in African communities

It has been proposed that ovale malaria in humans is caused by two closely related but distinct species of malaria parasite, Plasmodium ovale curtisi and Plasmodium ovale wallikeri. It was recently shown that these two parasite types are sympatric at the country level. However, it remains possible that localised geographic, temporal or ecological barriers exist within endemic countries which prevent recombination between the genomes of the two species. Here, using conventional and real-time quantitative PCR (qPCR) methods specifically designed to discriminate P. o. curtisi and P. o. wallikeri, it is shown that both species are present among clinic attendees in Congo-Brazzaville, and occur simultaneously both in lake-side and inland districts in Uganda and on Bioko Island, Equatorial Guinea. Thus P. o. curtisi and P. o. wallikeri in these localities are exactly sympatric in both time and space. These findings are consistent with the existence of a biological barrier, rather than geographical or ecological factors, preventing recombination between P. o. curtisi and P. o. wallikeri. In cross-sectional surveys carried out in Uganda and Bioko, our results show that infections with P. ovale spp. are more common than previously thought, occurring at a frequency of 1–6% in population samples, with both proposed species contributing to ovale malaria in six sites. Malaria elimination programmes in Africa need to include strategies for control of P. o. curtisi and P. o. wallikeri.

In vivo and in vitro efficacy of chloroquine against Plasmodium malariae and P. ovale in Papua, Indonesia

Reports of potential drug-resistant strains of Plasmodium malariae in western Indonesia raise concerns that chloroquine resistance may be emerging in P. malariae and P. ovale. In order to assess this, in vivo and in vitro efficacy studies were conducted in patients with monoinfection in Papua, Indonesia. Consecutive patients with uncomplicated malaria due to P. ovale or P. malariae were enrolled in a prospective clinical trial, provided with supervised chloroquine treatment, and followed for 28 days. Blood from patients with P. malariae or P. ovale parasitemia greater than 1,000 per microliter underwent in vitro antimalarial drug susceptibility testing using a modified schizont maturation assay. Of the 57 evaluable patients in the clinical study (P. malariae, n = 46; P. ovale, n = 11), none had recurrence with the same species during follow-up. The mean parasite reduction ratio at 48 h was 86 (95% confidence interval [CI], 57 to 114) for P. malariae and 150 (95% CI, 54 to 245) for P. ovale (P = 0.18). One patient infected with P. malariae, with 93% of parasites at the trophozoite stage, was still parasitemic on day 4. In vitro drug susceptibility assays were carried out successfully for 40 isolates (34 infected with P. malariae and 6 with P. ovale). The P. malariae infections at trophozoite stages had significantly higher chloroquine 50% effective concentrations (EC(50)s) (median, 127.9 nM [range, 7.9 to 2,980]) than those initially exposed at the ring stage (median, 14.0 nM [range, 3.5 to 27.0]; P = 0.01). The EC(50) for chloroquine in P. ovale was also higher in an isolate initially at the trophozoite stage (23.2 nM) than in the three isolates predominantly at ring stage (7.8 nM). Chloroquine retains adequate efficacy against P. ovale and P. malariae, but its marked stage specificity of action may account for reports of delayed parasite clearance times.

Validation of a four-primer real-time PCR as a diagnostic tool for single and mixed Plasmodium infections

Although microscopy remains the reference standard for malaria diagnosis, molecular tools are attracting increasing interest. To improve the detection of mixed infections, we developed a four-primer real-time PCR with four Plasmodium species-specific forward primers, based on the pan-primer design with universal Plasmodium primers as described previously. After validation for analytical sensitivity, specificity and reproducibility, the four-primer PCR was evaluated on 351 blood samples from patients presenting at the outpatient clinic of the Institute of Tropical Medicine (Belgium). With the four-primer PCR, we identified 188 Plasmodium falciparum (Pf), 54 Plasmodium vivax (Pv), 52 Plasmodium ovale (Po) and 13 Plasmodium malariae (Pm) single infections, 27 mixed infections (14 Pf + Pm; 12 Pf + Po; one Pv + Pm) and 17 negative specimens. We found lower cycle threshold values than with the pan-primer PCR, with a mean difference of 2.23, a higher analytical sensitivity (in asexual parasites/μL: Pf/Pv, 0.02; Po, 0.004; Pm, 0.006) and 15 extra mixed infections. As compared with microscopy, 17 extra mixed infections were detected and Plasmodium species were identified in four microscopy-positive samples in which species identification was not possible. Additionally, the PCR corrected 13 species mismatches between Po and Pv, and in 11 cases detected Pf as a second species that was not identified by microscopy and in five of them was not detected by rapid diagnostic tests (RDTs). PCR confirmed the presence of Pf in 30/46 histidine-rich protein-2-positive samples that were microscopy-negative. We conclude that the presently developed four-primer real-time PCR is complementary to standard malaria diagnostic tests in clinical laboratories, with an added value for simultaneous identification of the four Plasmodium species and the detection of mixed infections.

Ecology of malaria parasites infecting Southeast Asian macaques: evidence from cytochrome b sequences

Although malaria parasites infecting non-human primates are important models for human malaria, little is known of the ecology of infection by these parasites in the wild. We extensively sequenced cytochrome b (cytb) of malaria parasites (Apicomplexa: Haemosporida) from free-living southeast Asian monkeys Macaca nemestrina and Macaca fascicularis. The two most commonly observed taxa were Plasmodium inui and Hepatocystis sp., but certain other sequences did not cluster closely with any previously sequenced species. Most of the major clades of parasites were found in both Macaca species, and the two most commonly occurring parasite infected the two Macaca species at approximately equal levels. However, P. inui showed evidence of genetic differentiation between the populations infecting the two Macaca species, suggesting limited movement of this parasite among hosts. Moreover, coinfection with Plasmodium and Hepatocystis species occurred significantly less frequently than expected on the basis of the rates of infection with either taxon alone, suggesting the possibility of competitive exclusion. The results revealed unexpectedly complex communities of Plasmodium and Hepatocystis taxa infecting wild southeast Asian monkeys. Parasite taxa differed with respect to both the frequency of between-host movement and their frequency of coinfection.

Uncertainty in mapping malaria epidemiology: implications for control

Malaria is a location-specific, dynamic infectious disease transmitted by mosquitoes to humans and is influenced by environmental, vector, parasite, and host factors. The principal purposes of malarial epidemiology are 1) to describe the malarial distribution in space and time along with the physical, biologic, and social etiologic factors and 2) to guide control objectives for either modeling impact or measuring progress of control tactics. Mapping malaria and many of its causative factors has been achieved on many different levels from global distribution to biologic quantitative trait localization in humans, parasites, and mosquitoes. Despite these important achievements, a large degree of uncertainty still exists on the annual burden of malarial cases. Accurate, sensitive detection and treatment of asymptomatic reservoirs important to infectious transmission are additional components necessary for future control measures. Presently spurred by the leadership and funding of Bill and Melinda Gates, the malarial community is developing and implementing plans for elimination of malaria. The challenge for malariologists is to digitally integrate and map epidemiologic factors and intervention measures in space and time to target effective, sustainable control alongside research efforts.

Isolation and characterization of the MSP1 genes from Plasmodium malariae and Plasmodium ovale

The merozoite surface protein 1 (MSP1) is the principal surface antigen of the blood stage form of the Plasmodium parasite. Antibodies recognizing MSP1 are frequently detected following Plasmodium infection, making this protein a significant component of malaria vaccines and diagnostic tests. Although the MSP1 gene sequence has been reported for Plasmodium falciparum and Plasmodium vivax, this gene has not been identified for the other two major human-infectious species, Plasmodium malariae and Plasmodium ovale. MSP1 genes from these two species were isolated from Cameroon blood donor samples. The genes are similar in size to known MSP1 genes and encode proteins with interspecies conserved domains homologous to those identified in other Plasmodium species. Sequence and phylogenetic analysis of all available Plasmodium MSP1 amino acid sequences clearly shows that the Po and Pm MSP1 sequences are truly unique within the Plasmodium genus and not simply Pf or Pv variants.

The ABCs of multidrug resistance in malaria

Expanding drug resistance could become a major problem in malaria treatment, as only a limited number of effective antimalarials are available. Drug resistance has been associated with single nucleotide polymorphisms and an increased copy number of multidrug resistance protein 1 (MDR1), an ATP-binding cassette (ABC) protein family member. Many ABC transport proteins are membrane transporters that actively translocate a wide range of structurally and functionally diverse amphipathic compounds. The Plasmodium falciparum ABC family consists of 16 members and current knowledge of their physiological function and contribution to antimalarial drug resistance is limited. Here, we give an overview of the Plasmodium ABC family members with reference to their possible role in multidrug resistance.

Evaluation of three PCR-based diagnostic assays for detecting mixed Plasmodium infection

Background

One of the most commonly used molecular test for malaria diagnosis is the polymerase chain reaction (PCR)-based amplification of the 18S ribosomal DNA (rDNA) gene. Published diagnostic assays based on the 18S gene include the "gold standard" nested assay, semi-nested multiplex assay, and one tube multiplex assay. To our knowledge, no one has reported whether the two multiplex methods are better at detecting mixed Plasmodium infections compared to the nested assay using known quantities of DNA in experimentally mixed cocktails.

Findings

Here we evaluated three PCR assays (nested, semi-nested multiplex, and one-tube multiplex) for the simultaneous detection of human malaria parasites using experimentally mixed cocktails of known quantities of laboratory derived DNA. All three assays detected individual species with high sensitivity and specificity when DNA was from any one single species; however, experimentally mixed DNA cocktails with all four species present were correctly identified most consistently with the nested method. The other two methods failed to consistently identify all four species correctly, especially at lower concentrations of DNA -subclinical levels of malaria (DNA equivalent to or less than 10 parasites per microliter).

Conclusions

The nested PCR method remains the method of choice for the detection of mixed malaria infections and especially of sub-clinical infections. Further optimization and/or new molecular gene targets may improve the success rate of detecting multiple parasite species simultaneously using traditional PCR assays.

New innovations for an old infection: antimalarial lead discovery from marine natural products during the period 2003–2008

Malaria remains one of the most serious global infectious diseases, with an estimated 2 billion people at risk and 1 million deaths annually. Drug resistance is hampering the effectiveness of many current antimalarial therapies and resistant strains of the parasite are now known for almost all classes of antimalarial compounds. Owing to a lack of concerted drug-discovery efforts over the last 30 years, the development pipeline is limited and the identification of new antimalarial lead compounds is a pressing concern. The development of new antimalarials that exhibit novel modes of action is of critical importance if the devastating effects of malaria are to be controlled. Natural products have traditionally played an important role in antimalarial drug development and the marine environment represents an underexplored resource in this regard. This review covers developments in the field of antimalarial drug discovery from marine sources between January 2003 and December 2008 and offers a comprehensive overview of all marine-derived compounds from this period. Marine natural products represent an emerging opportunity in the development of new antimalarial lead compounds. This review provides examples of several recent lead discovery projects that show promise in this regard and presents a perspective on areas of possible future study.

Application of pharmacogenomics to malaria: a holistic approach for successful chemotherapy

Drug resistance in malaria jeopardizes the most elementary objectives of malaria control--reducing suffering and eliminating mortality. An important, and so far the only known, mechanism of drug resistance appears to be polymorphisms in the malaria parasite genes. Efforts to circumvent antimalarial drug resistance now range from the use of combination therapies with existing agents to genomics-based studies directed toward discovering novel targets and agents. However, the potential contribution of host genetic/molecular factors, particularly those associated with antimalarial drug metabolism, remains largely unexplored. Our knowledge concerning the basic mechanisms involved in the pharmacokinetics of antimalarial drugs is fragmentary. In addition, the link between antimalarial drug pharmacokinetics and treatment outcomes is generally unclear. The purpose of this article is to provide general background information on antimalarial drug resistance and associated parasite genetic factors, and subsequently highlight the aforementioned unexplored and unclear areas, with a view to stimulate much needed further research.

Chimpanzee malaria parasites related to Plasmodium ovale in Africa

Since the 1970's, the diversity of Plasmodium parasites in African great apes has been neglected. Surprisingly, P. reichenowi, a chimpanzee parasite, is the only such parasite to have been molecularly characterized. This parasite is closely phylogenetically related to P. falciparum, the principal cause of the greatest malaria burden in humans. Studies of malaria parasites from anthropoid primates may provide relevant phylogenetic information, improving our understanding of the origin and evolutionary history of human malaria species. In this study, we screened 130 DNA samples from chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) from Cameroon for Plasmodium infection, using cytochrome b molecular tools. Two chimpanzees from the subspecies Pan t. troglodytes presented single infections with Plasmodium strains molecularly related to the human malaria parasite P. ovale. These chimpanzee parasites and 13 human strains of P. ovale originated from a various sites in Africa and Asia were characterized using cytochrome b and cytochrome c oxidase 1 mitochondrial partial genes and nuclear ldh partial gene. Consistent with previous findings, two genetically distinct types of P. ovale, classical and variant, were observed in the human population from a variety of geographical locations. One chimpanzee Plasmodium strain was genetically identical, on all three markers tested, to variant P. ovale type. The other chimpanzee Plasmodium strain was different from P. ovale strains isolated from humans. This study provides the first evidence of possibility of natural cross-species exchange of P. ovale between humans and chimpanzees of the subspecies Pan t. troglodytes.

High sensitivity detection of Plasmodium species reveals positive correlations between infections of different species, shifts in age distribution and reduced local variation in Papua New Guinea

Background

When diagnosed by standard light microscopy (LM), malaria prevalence can vary significantly between sites, even at local scale, and mixed species infections are consistently less common than expect in areas co-endemic for Plasmodium falciparum, Plasmodium vivax and Plasmodium malariae. The development of a high-throughput molecular species diagnostic assay now enables routine PCR-based surveillance of malaria infections in large field and intervention studies, and improves resolution of species distribution within and between communities.

Methods

This study reports differences in the prevalence of infections with all four human malarial species and of mixed infections as diagnosed by LM and post-PCR ligase detection reaction – fluorescent microsphere (LDR-FMA) assay in 15 villages in the central Sepik area of Papua New Guinea.

Results

Significantly higher rates of infection by P. falciparum, P. vivax, P. malariae and Plasmodium ovale were observed in LDR-FMA compared to LM diagnosis (p < 0.001). Increases were particularly pronounced for P. malariae (3.9% vs 13.4%) and P. ovale (0.0% vs 4.8%). In contrast to LM diagnosis, which suggested a significant deficit of mixed species infections, a significant excess of mixed infections over expectation was detected by LDR-FMA (p < 0.001). Age of peak prevalence shifted to older age groups in LDR-FMA diagnosed infections for P. falciparum (LM: 7–9 yrs 47.5%, LDR-FMA: 10–19 yrs 74.2%) and P. vivax (LM: 4–6 yrs 24.2%, LDR-FMA: 7–9 yrs 50.9%) but not P. malariae infections (10–19 yrs, LM: 7.7% LDR-FMA: 21.6%). Significant geographical variation in prevalence was found for all species (except for LM-diagnosed P. falciparum), with the extent of this variation greater in LDR-FMA than LM diagnosed infections (overall, 84.4% vs. 37.6%). Insecticide-treated bednet (ITN) coverage was also the dominant factor linked to geographical differences in Plasmodium species infection prevalence explaining between 60.6% – 74.5% of this variation for LDR-FMA and 81.8% – 90.0% for LM (except P. falciparum), respectively.

Conclusion

The present study demonstrates that application of molecular diagnosis reveals patterns of malaria risk that are significantly different from those obtained by standard LM. Results provide insight relevant to design of malaria control and eradication strategies.

Plasmodium vivax: who cares?

More attention is being focused on malaria today than any time since the world's last efforts to achieve eradication over 40 years ago. The global community is now discussing strategies aimed at dramatically reducing malarial disease burden and the eventual eradication of all types of malaria, everywhere. As a consequence, Plasmodium vivax, which has long been neglected and mistakenly considered inconsequential, is now entering into the strategic debates taking place on malaria epidemiology and control, drug resistance, pathogenesis and vaccines. Thus, contrary to the past, the malaria research community is becoming more aware and concerned about the widespread spectrum of illness and death caused by up to a couple of hundred million cases of vivax malaria each year. This review brings these issues to light and provides an overview of P. vivax vaccine development, then and now. Progress had been slow, given inherent research challenges and minimal support in the past, but prospects are looking better for making headway in the next few years. P. vivax, known to invade the youngest red blood cells, the reticulocytes, presents a strong challenge towards developing a reliable long-term culture system to facilitate needed research. The P. vivax genome was published recently, and vivax researchers now need to coordinate efforts to discover new vaccine candidates, establish new vaccine approaches, capitalize on non-human primate models for testing, and investigate the unique biological features of P. vivax, including the elusive P. vivax hypnozoites. Comparative studies on both P. falciparum and P. vivax in many areas of research will be essential to eradicate malaria. And to this end, the education and training of future generations of dedicated "malariologists" to advance our knowledge, understanding and the development of new interventions against each of the malaria species infecting humans also will be essential.

Artemisia annua as a self-reliant treatment for malaria in developing countries

Malaria is a vector-borne infectious disease caused by the protozoan Plasmodium parasites. Each year, it causes disease in approximately 515 million people and kills between one and three million people, the majority of whom are young children in sub-Saharan Africa. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Due to climate change and the gradual warming of the temperate regions the future distribution of the malaria disease might include regions which are today seen as safe. Currently, malaria control requires an integrated approach comprising of mainly prevention, including vector control and the use of effective prophylactic medicines, and treatment of infected patients with antimalarials. The antimalarial chloroquine, which was in the past a mainstay of malaria control, is now ineffective in most malaria areas and resistance to other antimalarials is also increasing rapidly. The discovery and development of artemisinins from Artemisia annua have provided a new class of highly effective antimalarials. ACTs are now generally considered as the best current treatment for uncomplicated Plasmodium falciparum malaria. This review gives a short history of the malaria disease, the people forming a high risk group and the botanical aspects of A. annua. Furthermore the review provides an insight in the use of ART and its derivatives for the treatment of malaria. Its mechanism of action and kinetics will be described as well as the possibilities for a self-reliant treatment will be revealed. This self-reliant treatment includes the local production practices of A. annua followed by the possibilities for using traditional prepared teas from A. annua as an effective treatment for malaria. Finally, HMM will be described and the advantages and disadvantages discussed.

ama1 genes of sympatric Plasmodium vivax and P. falciparum from Venezuela differ significantly in genetic diversity and recombination frequency

Background

We present the first population genetic analysis of homologous loci from two sympatric human malaria parasite populations sharing the same human hosts, using full-length sequences of ama1 genes from Plasmodium vivax and P. falciparum collected in the Venezuelan Amazon.

Methodology/Principal Findings

Significant differences between the two species were found in genetic diversity at the ama1 locus, with 18 distinct haplotypes identified among the 73 Pvama1 sequences obtained, compared to 6 unique haplotypes from 30 Pfama1 sequences, giving overall diversity estimates of h = 0.9091, and h = 0.538 respectively. Levels of recombination were also found to differ between the species, with P. falciparum exhibiting very little recombination across the 1.77kb sequence. In contrast, analysis of patterns of nucleotide substitutions provided evidence that polymorphisms in the ama1 gene of both species are maintained by balancing selection, particularly in domain I. The two distinct population structures observed are unlikely to result from different selective forces acting upon the two species, which share both human and mosquito hosts in this setting. Rather, the highly structured P. falciparum population appears to be the result of a population bottleneck, while the much less structured P. vivax population is likely to be derived from an ancient pool of diversity, as reflected in a larger estimate of effective population size for this species. Greatly reduced mosquito transmission in 1997, due to low rainfall prior to the second survey, was associated with far fewer P. falciparum infections, but an increase in P. vivax infections, probably due to hypnozoite activation.

Conclusions/Significance

The relevance of these findings to putative competitive interactions between these two important human pathogen species is discussed. These results highlight the need for future control interventions to employ strategies targeting each of the parasite species present in endemic areas.

Effect of transmission setting and mixed species infections on clinical measures of malaria in Malawi

Background

In malaria endemic regions people are commonly infected with multiple species of malaria parasites but the clinical impact of these Plasmodium co-infections is unclear. Differences in transmission seasonality and transmission intensity between endemic regions have been suggested as important factors in determining the effect of multiple species co-infections.

Principal Findings

In order to investigate the impact of multiple-species infections on clinical measures of malaria we carried out a cross-sectional community survey in Malawi, in 2002. We collected clinical and parasitological data from 2918 participants aged >6 months, and applied a questionnaire to measure malaria morbidity. We examined the effect of transmission seasonality and intensity on fever, history of fever, haemoglobin concentration ([Hb]) and parasite density, by comparing three regions: perennial transmission (PT), high intensity seasonal transmission (HIST) and low intensity seasonal transmission (LIST). These regions were defined using multi-level modelling of PCR prevalence data and spatial and geo-climatic measures. The three Plasmodium species (P. falciparum, P. malariae and P. ovale) were randomly distributed amongst all children but not adults in the LIST and PT regions. Mean parasite density in children was lower in the HIST compared with the other two regions. Mixed species infections had lower mean parasite density compared with single species infections in the PT region. Fever rates were similar between transmission regions and were unaffected by mixed species infections. A history of fever was associated with single species infections but only in the HIST region. Reduced mean [Hb] and increased anaemia was associated with perennial transmission compared to seasonal transmission. Children with mixed species infections had higher [Hb] in the HIST region.

Conclusions

Our study suggests that the interaction of Plasmodium co-infecting species can have protective effects against some clinical outcomes of malaria but that this is dependent on the seasonality and intensity of malaria transmission.