Improved limit of detection for zoonotic Plasmodium knowlesi and P. cynomolgi surveillance using reverse transcription for total nucleic acid preserved samples or dried blood spots

Background

Zoonotic P. knowlesi and P. cynomolgi symptomatic and asymptomatic infections occur across endemic areas of Southeast Asia. Most infections are low-parasitemia, with an unknown proportion below routine microscopy detection thresholds. Molecular surveillance tools optimizing the limit of detection (LOD) would allow more accurate estimates of zoonotic malaria prevalence.

Methods

An established ultra-sensitive Plasmodium genus quantitative-PCR (qPCR) assay targeting the 18S rRNA gene underwent LOD evaluation with and without reverse transcription (RT) for P. knowlesi, P. cynomolgi and P. vivax using total nucleic acid preserved (DNA/RNA Shield™) isolates and archived dried blood spots (DBS). LODs for selected P. knowlesi-specific assays, and reference P. vivax- and P. cynomolgi-specific assays were determined with RT. Assay specificities were assessed using clinical malaria samples and malaria-negative controls.

Results

The use of reverse transcription improved Plasmodium species detection by up to 10,000-fold (Plasmodium genus), 2759-fold (P. knowlesi), 1000-fold (P. vivax) and 10-fold (P. cynomolgi). The median LOD with RT for the Kamau et al. Plasmodium genus RT-qPCR assay was ≤0.0002 parasites/μL for P. knowlesi and 0.002 parasites/μL for both P. cynomolgi and P. vivax. The LODs with RT for P. knowlesi-specific PCRs were: Imwong et al. 18S rRNA (0.0007 parasites/μL); Divis et al. real-time 18S rRNA (0.0002 parasites/μL); Lubis et al. hemi-nested SICAvar (1.1 parasites/μL) and Lee et al. nested 18S rRNA (11 parasites/μL). The LOD for P. vivax- and P. cynomolgi-specific assays with RT were 0.02 and 0.20 parasites/μL respectively. For DBS P. knowlesi samples the median LOD for the Plasmodium genus qPCR with RT was 0.08, and without RT was 19.89 parasites/uL (249-fold change); no LOD improvement was demonstrated in DBS archived beyond 6 years. The Plasmodium genus and P. knowlesi-assays were 100% specific for Plasmodium species and P. knowlesi detection, respectively, from 190 clinical infections and 48 healthy controls. Reference P. vivax-specific primers demonstrated known cross-reactivity with P. cynomolgi.

Conclusion

Our findings support the use of an 18S rRNA Plasmodium genus qPCR and species-specific nested PCR protocol with RT for highly-sensitive surveillance of zoonotic and human Plasmodium species infections.

Non-human primate malaria in travellers

Emerging zoonotic malaria caused by Plasmodium knowlesi, Plasmodium cynomolgi and Plasmodium inui occurred in humans amid a decline of malaria prevalence in Thailand during the past decade. Diagnosis of non-human primate malaria in ecotourists is challenging. Molecular diagnosis seems to be mandatory such as species-specific polymerase chain reaction (PCR) and targeted amplicon deep sequencing.

The threat of increased transmission of non-knowlesi zoonotic malaria in humans: a systematic review

Of the 5 human malarial parasites, Plasmodium falciparum and Plasmodium vivax are the most prevalent species globally, while Plasmodium malariae, Plasmodium ovale curtisi and Plasmodium ovale wallikeri are less prevalent and typically occur as mixed-infections. Plasmodium knowlesi, previously considered a non-human primate (NHP) infecting species, is now a cause of human malaria in Malaysia. The other NHP Plasmodium species, Plasmodium cynomolgi, Plasmodium brasilianum, Plasmodium inui, Plasmodium simium, Plasmodium coatneyi and Plasmodium fieldi cause malaria in primates, which are mainly reported in southeast Asia and South America. The non-knowlesi NHP Plasmodium species also emerged and were found to cross-transmit from their natural hosts (NHP) – to human hosts in natural settings. Here we have reviewed and collated data from the literature on the NHPs-to-human-transmitting non-knowlesi Plasmodium species. It was observed that the natural transmission of these NHP parasites to humans had been reported from 2010 onwards. This study shows that: (1) the majority of the non-knowlesi NHP Plasmodium mixed species infecting human cases were from Yala province of Thailand; (2) mono/mixed P. cynomolgi infections with other human-infecting Plasmodium species were prevalent in Malaysia and Thailand and (3) P. brasilianum and P. simium were found in Central and South America.

Erythrocyte tropism of malarial parasites: The reticulocyte appeal

Erythrocytes are formed from the enucleation of erythroblasts in the bone marrow, and as erythrocytes develop from immature reticulocytes into mature normocytes, they undergo extensive cellular changes through their passage in the blood. During the blood stage of the malarial parasite life cycle, the parasite sense and invade susceptible erythrocytes. However, different parasite species display varying erythrocyte tropisms (i.e., preference for either reticulocytes or normocytes). In this review, we explore the erythrocyte tropism of malarial parasites, especially their predilection to invade reticulocytes, as shown from recent studies. We also discuss possible mechanisms mediating erythrocyte tropism and the implications of specific tropisms to disease pathophysiology. Understanding these allows better insight into the role of reticulocytes in malaria and provides opportunities for targeted interventions.

The Emerging Threat of Plasmodium knowlesi Malaria Infection: A Concept Paper on the Vulnerable Factors in Human

In South-East Asia (SEA), there has been an increase in the trend of detected and reported cases of Plasmodium knowlesi malaria in the last few decades. This higher transmission rate within SEA countries is attributed to the distribution of the Macaque, banded leaf monkeys, and Anopheles mosquito in this region. This study aims to propose a concept that highlights the integration of vulnerability factors to P. knowlesi malaria infection. The relevant literature on the vulnerability factors of P. knowlesi was reviewed. Any theories and models that could be integrated to support the factors were also explored throughout this study. Exposure to P. knowlesi malaria was found to be influenced by sociodemographic, socioeconomic, environmental, social context, belief, and human behaviour factors. However, these factors were commonly discussed separately in existing studies. For better disease prevention and control, all these factors should be explored collectively, to facilitate research aimed at generating a deeper understanding of the vulnerability factors to P. knowlesi malaria from various perspectives, including the genetic, sociodemographic, socioeconomic, environmental, sociocultural beliefs, and human behaviour of the population.

Expert Consensus on a Proposed Study Framework to Explore Factors Influencing Plasmodium knowlesi Malaria Preventive Behavior: A Modified Delphi Method Protocol

The increasing incidence of P. knowlesi malaria infection among humans is a public health threat. This zoonotic disease is challenging to eliminate owing to the presence of animal reservoirs. Understanding the factors such as the community’s belief, social context, drivers, and barriers can provide insights into malaria preventive behavior. It is crucial to improve the current preventive measures. This study aims to achieve consensus among malaria experts based on evidence from literature reviews and experts’ opinions on possible factors influencing malaria preventive behavior among communities exposed to P. knowlesi malaria infection. A modified Delphi study protocol was developed to gather experts’ consensus on the study framework to explore the factors influencing preventive behavior among communities exposed to P. knowlesi malaria infection. The framework is adapted from the ideation model, and it is integrated with other relevant theories and extensive literature reviews. We will use the modified Delphi protocol to reach a consensus. The experts will respond to each questionnaire item and a related open-ended questionnaire. Consensus is predetermined at more than 70% agreement on the items. We will use descriptive statistics and thematic analysis to analyze the data. All experts will remain anonymous to maintain the characteristics of a traditional Delphi study.

Ongoing host-shift speciation in Plasmodium simium

Plasmodium simium, a malaria parasite that infects platyrrhine monkeys and humans in the New World, is nearly identical to Plasmodium vivax. Recent genomic comparative analyses of these sister species have identified elevated divergence in a gene that may underlie P. simium adaptation to non-human primates during its gradual speciation process.

Zoonotic Malaria: Non-Laverania Plasmodium Biology and Invasion Mechanisms

Malaria, which is caused by Plasmodium parasites through Anopheles mosquito transmission, remains one of the most life-threatening diseases affecting hundreds of millions of people worldwide every year. Plasmodium vivax, which accounts for the majority of cases of recurring malaria caused by the Plasmodium (non-Laverania) subgenus, is an ancient and continuing zoonosis originating from monkey hosts probably outside Africa. The emergence of other zoonotic malarias (P. knowlesi, P. cynomolgi, and P. simium) further highlights the seriousness of the disease. The severity of this epidemic disease is dependent on many factors, including the parasite characteristics, host-parasite interactions, and the pathology of the infection. Successful infection depends on the ability of the parasite to invade the host; however, little is known about the parasite invasion biology and mechanisms. The lack of this information adds to the challenges to malaria control and elimination, hence enhancing the potential for continuation of this zoonosis. Here, we review the literature describing the characteristics, distribution, and genome details of the parasites, as well as host specificity, host-parasite interactions, and parasite pathology. This information will provide the basis of a greater understanding of the epidemiology and pathogenesis of malaria to support future development of strategies for the control and prevention of this zoonotic infection.

Non-human primate and human malaria: past, present and future

BACKGROUND

Studies of the malaria parasites infecting various non-human primates (NHPs) have increased our understanding of the origin, biology and pathogenesis of human Plasmodium parasites.This review considers the major discoveries concerning NHP malaria parasites, highlights their relationships with human malaria and considers the impact that this may have on attempts to eradicate the disease.

RESULTS

The first description of NHP malaria parasites dates back to the early 20th century. Subsequently, experimental and fortuitous findings indicating that some NHP malaria parasites can be transmitted to humans have raised concerns about the possible impact of a zoonotic malaria reservoir on efforts to control human malaria.Advances in molecular techniques over the last 15 years have contributed greatly to our knowledge of the existence and geographical distribution of numerous Plasmodium species infecting NHPs, and extended our understanding of their close phylogenetic relationships with human malaria parasites. The clinical application of such techniques has also made it possible to document ongoing spillovers of NHP malaria parasites (Plasmodium knowlesi, P. cynomolgi, P. simium, P. brasilianum) in humans living in or near the forests of Asia and South America, thus confirming that zoonotic malaria can undermine efforts to eradicate human malaria.

CONCLUSIONS

Increasing molecular research supports the prophetic intuition of the pioneers of modern malariology who saw zoonotic malaria as a potential obstacle to the full success of malaria eradication programmes. It is, therefore, important to continue surveillance and research based on one-health approaches in order to improve our understanding of the complex interactions between NHPs, mosquito vectors and humans during a period of ongoing changes in the climate and the use of land, monitor the evolution of zoonotic malaria, identify the populations most at risk and implement appropriate preventive strategies.

Analysis of Historical Trends and Recent Elimination of Malaria from Sri Lanka and Its Applicability for Malaria Control in Other Countries

Sri Lanka is a tropical island located South of India in the Indian Ocean. Malaria has been prevalent in the island for centuries but the country succeeded in eliminating the disease in 2013. Factors governing the past endemicity of malaria and its successful elimination from Sri Lanka in 2013 are analyzed. There is evidence that malaria might have been first introduced in the thirteenth century into a dry zone area with extensive irrigation works. Regular widespread epidemics of the disease have been documented in the twentieth century. The island nature of Sri Lanka, generally low transmission rates, widespread and accessible government hospitals and clinics that provide free and readily available diagnosis and treatment for malaria, adequate financial support and commitment to the Antimalaria Campaign (AMC), national and decentralized malaria control efforts sustained over a long period by dedicated and competent AMC staff, and the absence of zoonotic malaria are recognized as key factors responsible for eliminating malaria from Sri Lanka. These factors are analyzed in the context of their relevance to the present malaria elimination efforts in other countries with the overall aim of globally eradicating the disease.

Normocyte-binding protein required for human erythrocyte invasion by the zoonotic malaria parasite Plasmodium knowlesi

The dominant cause of malaria in Malaysia is now Plasmodium knowlesi, a zoonotic parasite of cynomolgus macaque monkeys found throughout South East Asia. Comparative genomic analysis of parasites adapted to in vitro growth in either cynomolgus or human RBCs identified a genomic deletion that includes the gene encoding normocyte-binding protein Xa (NBPXa) in parasites growing in cynomolgus RBCs but not in human RBCs. Experimental deletion of the NBPXa gene in parasites adapted to growth in human RBCs (which retain the ability to grow in cynomolgus RBCs) restricted them to cynomolgus RBCs, demonstrating that this gene is selectively required for parasite multiplication and growth in human RBCs. NBPXa-null parasites could bind to human RBCs, but invasion of these cells was severely impaired. Therefore, NBPXa is identified as a key mediator of P. knowlesi human infection and may be a target for vaccine development against this emerging pathogen.