Natural Plasmodium inui Infections in Humans and Anopheles cracens Mosquito, Malaysia

Plasmodium cynomolgi: potential emergence of new zoonotic malaria in Southeast Asia

The reported cases of Plasmodium cynomolgi in Southeast Asia pose a significant public health concern. Sporadic reports of human Plasmodium cynomolgi infections have increased in the past few years, raising attention regarding its potential impact on human populations. Further compounding this issue are the morphological similarities between P. cynomolgi and the human malaria parasite Plasmodium vivax, which may lead to misdiagnosis and underreporting of P. cynomolgi infections. Both in vitro and in vivo studies have shown that P. cynomolgi can effectively invade human reticulocytes using mechanisms like those employed by P. vivax, underscoring its capacity to infect human hosts if given the opportunity. These studies collectively highlight the parasite's potential to establish infections in humans and emphasize the need for molecular diagnostic tools to accurately detect P. cynomolgi. Additionally, challenges in accurate diagnosis and surveillance systems may underestimate the true extent of their impact, making it imperative for healthcare authorities to bolster monitoring efforts and deploy targeted interventions. Strengthening surveillance, improving diagnostic capabilities, and developing targeted vector control strategies are crucial to mitigating the risk of P. cynomolgi becoming a major zoonotic disease like its counterpart, Plasmodium knowlesi. Thus, this review aims to highlight the current understanding of P. cynomolgi infections in human, vector, and macaque hosts based on collated data from previous studies while underscoring the urgent need for enhanced surveillance, accurate diagnostic tools, and effective vector control strategies to mitigate its potential as a significant zoonotic threat in Southeast Asia.

Unveiling the Diversity and Zoonotic Potential of Plasmodium inui: a Comprehensive Review of Insights from Indonesia and Southeast Asia

PURPOSE

Plasmodium inui is a type of malaria that is endemic in simian populations in Southeast Asia, primarily infecting nonhuman primates, one of which is Macaca fascicularis, or the long-tailed macaque. Plasmodium inui, a malaria parasite endemic to simian populations in Southeast Asia, predominantly targets non-human primates. The escalating rates of deforestation and urban expansion, which facilitate increased interactions between humans and primates, have intensified concerns regarding its zoonotic potential. Despite receiving comparatively less scholarly attention than P. knowlesi, P. inui is distinguished by its substantial strain diversity and capacity to infect various macaque species. This review investigates the potential for crossspecies transmission of P. inui to humans, concentrating on the regions of Indonesia and Southeast Asia.

METHODS

We evaluate the ecological and epidemiological determinants influencing the distribution and transmission dynamics of P. inui among macaques while also considering the implications for human infection based on a literature review obtained from PubMed, Google Scholar, and Scopus.

RESULTS

Although no documented human cases have emerged in Indonesia, cases in humans have only been detected in Malaysia and Thailand, the review underscores the zoonotic risk associated with P. inui, drawing comparisons to other simian malaria species that have successfully infiltrated human populations. The lack of systematic surveillance and detailed molecular investigations concerning P. inui in these regions accentuates the imperative for further scholarly inquiry.

CONCLUSION

This review emphasizes the need for ongoing monitoring and research to enhance the understanding of zoonotic threats associated with P. inui, and informs future public health initiatives in Southeast Asia through a comprehensive evaluation of the genetic diversity of the parasite and its potential implications for public health.

Deforestation and non-human primate malarias will be a threat to malaria elimination in the future: Insights from Southeast Asia

Malaria continues to be a global public health problem although it has been eliminated from many countries. Sri Lanka and China are two countries that recently achieved malaria elimination status, and many countries in Southeast Asia are currently in the pipeline for achieving the same goal by 2030. However, Plasmodium knowlesi, a non-human primate malaria parasite continues to pose a threat to public health in this region, infecting many humans in all countries in Southeast Asia except for Timor-Leste. Besides, other non-human primate malaria parasite such as Plasmodium cynomolgi and Plasmodium inui are infecting humans in the region. The non-human primates, the long-tailed and pig-tailed macaques which harbour these parasites are now increasingly prevalent in farms and forest fringes close by to the villages. Additionally, the Anopheles mosquitoes belonging to the Lecuosphyrus Group are also present in these areas which makes them ideal for transmitting the non-human primate malaria parasites. With changing landscape and deforestation, non-human primate malaria parasites will affect more humans in the coming years with the elimination of human malaria. Perhaps due to loss of immunity, more humans will be infected as currently being demonstrated in Malaysia. Thus, control measures need to be instituted rapidly to achieve the malaria elimination status by 2030. However, the zoonotic origin of the parasite and the changes of the vectors behaviour to early biting seems to be the stumbling block to the malaria elimination efforts in this region. In this review, we discuss the challenges faced in malaria elimination due to deforestation and the serious threat posed by non-human primate malaria parasites.

Overview of Plasmodium spp. and Animal Models in Malaria Research

Malaria is a parasitic disease caused by protozoan species of the genus Plasmodium and transmitted by female mosquitos of the genus Anopheles and other Culicidae. Most of the parasites of the genus Plasmodium are highly species specific with more than 200 species described affecting different species of mammals, birds, and reptiles. Plasmodium species strictly affecting humans are P. falciparum, P. vivax, P. ovale, and P. malariae. More recently, P. knowlesi and other nonhuman primate plasmodia were found to naturally infect humans. Currently, malaria occurs mostly in poor tropical and subtropical areas of the world, and in many of these countries it is the leading cause of illness and death. For more than 100 y, animal models, have played a major role in our understanding of malaria biology. Avian Plasmodium species were the first to be used as models to study human malaria. Malaria parasite biology and immunity were first studied using mainly P. gallinaceum and P. relictum. Rodent malarias, particularly P. berghei and P. yoelii, have been used extensively as models to study malaria in mammals. Several species of Plasmodium from nonhuman primates have been used as surrogate models to study human malaria immunology, pathogenesis, candidate vaccines, and treatments. Plasmodium cynomolgi, P. simiovale, and P. fieldi are important models for studying malaria produced by P. vivax and P. ovale, while P. coatneyi is used as a model for study- ing severe malaria. Other nonhuman primate malarias used in research are P. fragile, P. inui, P. knowlesi, P. simium, and P. brasilianum. Very few nonhuman primate species can develop an infection with human malarias. Macaques in general are resistant to infection with P. falciparum, P. vivax, P. malariae, and P. ovale. Only apes and a few species of New World monkeys can support infection with human malarias. Herein we review the most common, and some less common, avian, reptile, and mammal plasmodia species used as models to study human malaria.

Real-time fluorescence loop-mediated isothermal amplification assays for detection of zoonotic malaria Plasmodium parasites

BACKGROUND

Natural human infections by Plasmodium cynomolgi and P. inui have been reported recently and gain the substantial attention from Southeast Asian countries. Zoonotic transmission of non-human malaria parasites to humans from macaque monkeys occurred through the bites of the infected mosquitoes. The objective of this study is to establish real-time fluorescence loop-mediated isothermal amplification (LAMP) assays for the detection of zoonotic malaria parasites by combining real-time fluorescent technology with the isothermal amplification technique.

METHODS

By using 18S rRNA as the target gene, the primers for P. cynomolgi, P. coatneyi and P. inui were newly designed in the present study. Four novel real-time fluorescence LAMP assays were developed for the detection of P. cynomolgi, P. coatneyi, P. inui and P. knowlesi. The entire amplification process was completed in 60 min, with the assays performed at 65 °C. By using SYTO-9 as the nucleic acid intercalating dye, the reaction was monitored via real-time fluorescence signal.

RESULTS

There was no observed cross-reactivity among the primers from different species. All 70 field-collected monkey samples were successfully amplified by real-time fluorescence LAMP assays. The detection limit for P. cynomolgi, P. coatneyi and P. knowlesi was 5 × 109 copies/µL. Meanwhile, the detection limit of P. inui was 5 × 1010 copies/µL.

CONCLUSION

This is the first report of the detection of four zoonotic malaria parasites by real-time fluorescence LAMP approaches. It is an effective, rapid and simple-to-use technique. This presented platform exhibits considerable potential as an alternative detection for zoonotic malaria parasites.

The prevalence of simian malaria in wild long-tailed macaques throughout Peninsular Malaysia

The parasite Plasmodium knowlesi has been the sole cause of malaria in Malaysia from 2018 to 2022. The persistence of this zoonotic species has hampered Malaysia's progress towards achieving the malaria-free status awarded by the World Health Organisation (WHO). Due to the zoonotic nature of P. knowlesi infections, it is important to study the prevalence of the parasite in the macaque host, the long-tailed macaque (Macaca fascicularis). Apart from P. knowlesi, the long-tailed macaque is also able to harbour Plasmodium cynomolgi, Plasmodium inui, Plasmodium caotneyi and Plasmodium fieldi. Here we report the prevalence of the 5 simian malaria parasites in the wild long-tailed macaque population in 12 out of the 13 states in Peninsular Malaysia using a nested PCR approach targeting the 18s ribosomal RNA (18s rRNA) gene. It was found that all five Plasmodium species were widely distributed throughout Peninsular Malaysia except for states with major cities such as Kuala Lumpur and Putrajaya. Of note, Pahang reported a malaria prevalence of 100% in the long-tailed macaque population, identifying it as a potential hotspot for zoonotic transmission. Overall, this study shows the distribution of the 5 simian malaria parasite species throughout Peninsular Malaysia, the data of which could be used to guide future malaria control interventions to target zoonotic malaria.

Molecular detection and identification of Plasmodium spp. isolated from captive-bred cynomolgus monkeys in Bogor, Indonesia

Background and Aim

Asian macaques are natural hosts of several Plasmodium species. Some monkey malaria parasites may infect humans and cause zoonotic infections. This study was conducted to estimate the prevalence of monkey malaria parasites in Bogor, Indonesia, based on molecular detection and identification, particularly in cynomolgus monkeys, which have a wide geographic distribution and share extensive habitats with humans. These data are needed to evaluate the status of simian malaria among macaques in Bogor and to study the potential risks to human health. These updated data will provide sufficient information for implementing malaria control strategies in the future and for developing a potential malaria vaccine using monkeys as an animal model.

Materials and Methods

Blood samples of 274 cynomolgus monkeys (Macaca fascicularis) were collected and identified using microscopy. DNA was extracted from positive blood samples and analyzed using polymerase chain reaction (PCR) to amplify the small subunit ribosomal RNA (SSU rRNA) target gene using consensus primers for Plasmodium species. The PCR-positive samples were then nucleotide-sequenced using commercial sequencing services, analyzed using the BioEdit program, and aligned using Basic Local Alignment Search Tool from the National Center for Biotechnology Information. Phylogenetic trees were constructed using MEGA 11.0 and the neighbor-joining (NJ) method to determine the kinship of Plasmodium. Bootstrapping was performed using 500 replicates to assess the robustness of tree topologies.

Results

Thirty-eight of the 274 microscopically positive samples for Plasmodium spp. were also positive using PCR, resulting in a 1640 bp amplicon. Further, analysis using nucleotide sequencing confirmed that these positive samples were Plasmodium inui with more than 99% nucleotide identity compared to GenBank sequences. Phylogenetic tree analysis of the SSU rRNA partial gene showed that all our isolates clustered and were closely related to a P. inui strain isolated from cynomolgus macaques in South China in 2011.

Conclusion

P. inui is the predominant malaria parasite in cynomolgus monkeys from Bogor.

Simian malaria: a narrative review on emergence, epidemiology and threat to global malaria elimination

Simian malaria from wild non-human primate populations is increasingly recognised as a public health threat and is now the main cause of human malaria in Malaysia and some regions of Brazil. In 2022, Malaysia became the first country not to achieve malaria elimination due to zoonotic simian malaria. We review the global distribution and drivers of simian malaria and identify priorities for diagnosis, treatment, surveillance, and control. Environmental change is driving closer interactions between humans and wildlife, with malaria parasites from non-human primates spilling over into human populations and human malaria parasites spilling back into wild non-human primate populations. These complex transmission cycles require new molecular and epidemiological approaches to track parasite spread. Current methods of malaria control are ineffective, with wildlife reservoirs and primarily outdoor-biting mosquito vectors urgently requiring the development of novel control strategies. Without these, simian malaria has the potential to undermine malaria elimination globally.

Using phylogeographic link-prediction in primates to prioritize human parasite screening

Objectives

The ongoing risk of emerging infectious disease has renewed calls for understanding the origins of zoonoses and identifying future zoonotic disease threats. Given their close phylogenetic relatedness and geographic overlap with humans, non-human primates (NHPs) have been the source of many infectious diseases throughout human evolution. NHPs harbor diverse parasites, with some infecting only a single host species while others infect species from multiple families.

Materials and Methods

We applied a novel link-prediction method to predict undocumented instances of parasite sharing between humans and NHPs. Our model makes predictions based on phylogenetic distances and geographic overlap among NHPs and humans in six countries with high NHP diversity: Columbia, Brazil, Democratic Republic of Congo, Madagascar, China and Indonesia.

Results

Of the 899 human parasites documented in the Global Infectious Diseases and Epidemiology Network (GIDEON) database for these countries, 12% were shared with at least one other NHP species. The link prediction model identified an additional 54 parasites that are likely to infect humans but were not reported in GIDEON. These parasites were mostly host generalists, yet their phylogenetic host breadth varied substantially.

Discussion

As human activities and populations encroach on NHP habitats, opportunities for parasite sharing between human and non-human primates will continue to increase. Our study identifies specific infectious organisms to monitor in countries with high NHP diversity, while the comparative analysis of host generalism, parasite taxonomy, and transmission mode provides insights to types of parasites that represent high zoonotic risk.

Zoonotic simian malaria parasites in free-ranging Macaca fascicularis macaques and human malaria patients in Thailand, with a note on genetic characterization of recent isolates

Despite the natural occurrences of human infections by Plasmodium knowlesi, P. cynomolgi, P. inui, and P. fieldi in Thailand, investigating the prevalence and genetic diversity of the zoonotic simian malaria parasites in macaque populations has been limited to certain areas. To address this gap, a total of 560 long-tailed macaques (Macaca fascicularis) and 20 southern pig-tailed macaques (M. nemestrina) were captured from 15 locations across 10 provinces throughout Thailand between 2018 and 2021 for investigation of malaria, as were 15 human samples residing in two simian-malaria endemic provinces, namely Songkhla and Satun, who exhibited malaria-like symptoms. Using PCR techniques targeting the mitochondrial cytb and cox1 genes coupled with DNA sequencing, 40 long-tailed macaques inhabiting five locations had mono-infections with one of the three simian malaria species. Most of the positive cases of macaque were infected with P. inui (32/40), while infections with P. cynomolgi (6/40) and P. knowlesi (2/40) were less common and confined to specific macaque populations. Interestingly, all 15 human cases were mono-infected with P. knowlesi, with one of them residing in an area with two P. knowlesi-infected macaques. Nucleotide sequence analysis showed a high level of genetic diversity in P. inui, while P. cynomolgi and P. knowlesi displayed limited genetic diversity. Phylogenetic and haplotype network analyses revealed that P. inui in this study was closely related to simian and Anopheles isolates from Peninsular Malaysia, while P. cynomolgi clustered with simian and human isolates from Asian countries. P. knowlesi, which was found in both macaques and humans in this study, was closely related to isolates from macaques, humans, and An. hackeri in Peninsular Malaysia, suggesting a sylvatic transmission cycle extending across these endemic regions. This study highlights the current hotspots for zoonotic simian malaria and sheds light on the genetic characteristics of recent isolates in both macaques and human residents in Thailand.

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.

Serum proteomic profile of wild stump-tailed macaques (Macaca arctoides) infected with malaria parasites in Thailand

The number of patients infected with simian malaria is gradually increasing in many countries of Southeast Asia and South America. The most important risk factor for a zoonotic spillover event of malarial infection is mostly influenced by the interaction between humans, monkeys, and vectors. In this study, we determine the protein expression profile of a wild stump-tailed macaque (Macaca arctoides) from a total of 32 blood samples collected from Prachuap Kiri Khan Province, Thailand. The malarial parasite was analyzed using nested polymerase chain reaction (PCR) assays by dividing the samples into three groups: non-infected, mono-infected, and multiple-infected. The identification and differential proteomic expression profiles were determined using liquid chromatography with tandem mass spectrometry (LC-MS/MS) and bioinformatics tools. A total of 9,532 proteins (total proteins) were identified with the filter-based selection methods analysis, and a subset of 440 proteins were found to be different between each group. Within these proteins, the GhostKOALA functional enrichment analysis indicated that 142 important proteins were associated with either of the organismal system (28.87%), genetic information processing (23.24%), environmental information processing (16.20%), metabolism (13.38%), cellular processes (11.97%), or causing human disease (6.34%). Additionally, using interaction network analysis, nine potential reporter proteins were identified. Here, we report the first study on the protein profiles differentially expressed in the serum of wild stump-tailed macaques between non, mono, and multiple malarial infected living in a natural transmission environment. Our findings demonstrate that differentially expressed proteins implicated in host defense through lipid metabolism, involved with TGF pathway were suppressed, while those with the apoptosis pathway, such as cytokines and proinflammation signals were increased. Including the parasite's response via induced hemolysis and disruption of myeloid cells. A greater understanding of the fundamental processes involved in a malarial infection and host response can be crucial for developing diagnostic tools, medication development, and therapies to improve the health of those affected by the disease.

Spatial analyses of Plasmodium knowlesi vectors with reference to control interventions in Malaysia

BACKGROUND

Malaria parasites such as Plasmodium knowlesi, P. inui, and P. cynomolgi are spread from macaques to humans through the Leucosphyrus Group of Anopheles mosquitoes. It is crucial to know the distribution of these vectors to implement effective control measures for malaria elimination. Plasmodium knowlesi is the most predominant zoonotic malaria parasite infecting humans in Malaysia.

METHODS

Vector data from various sources were used to create distribution maps from 1957 to 2021. A predictive statistical model utilizing logistic regression was developed using significant environmental factors. Interpolation maps were created using the inverse distance weighted (IDW) method and overlaid with the corresponding environmental variables.

RESULTS

Based on the IDW analysis, high vector abundances were found in the southwestern part of Sarawak, the northern region of Pahang and the northwestern part of Sabah. However, most parts of Johor, Sabah, Perlis, Penang, Kelantan and Terengganu had low vector abundance. The accuracy test indicated that the model predicted sampling and non-sampling areas with 75.3% overall accuracy. The selected environmental variables were entered into the regression model based on their significant values. In addition to the presence of water bodies, elevation, temperature, forest loss and forest cover were included in the final model since these were significantly correlated. Anopheles mosquitoes were mainly distributed in Peninsular Malaysia (Titiwangsa range, central and northern parts), Sabah (Kudat, West Coast, Interior and Tawau division) and Sarawak (Kapit, Miri, and Limbang). The predicted Anopheles mosquito density was lower in the southern part of Peninsular Malaysia, the Sandakan Division of Sabah and the western region of Sarawak.

CONCLUSION

The study offers insight into the distribution of the Leucosphyrus Group of Anopheles mosquitoes in Malaysia. Additionally, the accompanying predictive vector map correlates well with cases of P. knowlesi malaria. This research is crucial in informing and supporting future efforts by healthcare professionals to develop effective malaria control interventions.

Primate malaria of human importance

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

High transmission efficiency of the simian malaria vectors and population expansion of their parasites Plasmodium cynomolgi and Plasmodium inui

BACKGROUND

The elimination of malaria in Southeast Asia has become more challenging as a result of rising knowlesi malaria cases. In addition, naturally occurring human infections with other zoonotic simian malaria caused by Plasmodium cynomolgi and Plasmodium inui adds another level of complexity in malaria elimination in this region. Unfortunately, data on vectors which are responsible for transmitting this zoonotic disease is very limited.

METHODOLOGY/PRINCIPAL FINDINGS

We conducted longitudinal studies to investigate the entomological parameters of the simian malaria vectors and to examine the genetic diversity and evolutionary pattern of their simian Plasmodium. All the captured Anopheles mosquitoes were dissected to examine for the presence of oocysts, sporozoites and to determine the parous rate. Our study revealed that the Anopheles Leucosphyrus Group mosquitoes are highly potential competent vectors, as evidenced by their high rate of parity, survival and sporozoite infections in these mosquitoes. Thus, these mosquitoes represent a risk of human infection with zoonotic simian malaria in this region. Haplotype analysis on P. cynomolgi and P. inui, found in high prevalence in the Anopheles mosquitoes from this study, had shown close relationship between simian Plasmodium from the Anopheles mosquitoes with its vertebrate hosts. This directly signifies the ongoing transmission between the vector, macaques, and humans. Furthermore, population genetic analysis showed significant negative values which suggest that both Plasmodium species are undergoing population expansion.

CONCLUSIONS/SIGNIFICANCE

With constant microevolutionary processes, there are potential for both P. inui and P. cynomolgi to emerge and spread as a major public health problem, following the similar trend of P. knowlesi. Therefore, concerted vector studies in other parts of Southeast Asia are warranted to better comprehend the transmission dynamics of this zoonotic simian malaria which eventually would aid in the implementation of effective control measures in a rapidly changing environment.

Cryptic Diversity and Demographic Expansion of Plasmodium knowlesi Malaria Vectors in Malaysia

Although Malaysia is considered free of human malaria, there has been a growing number of Plasmodium knowlesi cases. This alarming trend highlighted the need for our understanding of this parasite and its associated vectors, especially considering the role of genetic diversity in the adaptation and evolution among vectors in endemic areas, which is currently a significant knowledge gap in their fundamental biology. Thus, this study aimed to investigate the genetic diversity of Anopheles balabacensis, Anopheles cracens, Anopheles introlatus, and Anopheles latens-the vectors for P. knowlesi malaria in Malaysia. Based on cytochrome c oxidase 1 (CO1) and internal transcribed spacer 2 (ITS2) markers, the genealogic networks of An. latens showed a separation of the haplotypes between Peninsular Malaysia and Malaysia Borneo, forming two distinct clusters. Additionally, the genetic distances between these clusters were high (2.3-5.2% for CO1) and (2.3-4.7% for ITS2), indicating the likely presence of two distinct species or cryptic species within An. latens. In contrast, no distinct clusters were observed in An. cracens, An. balabacensis, or An. introlatus, implying a lack of pronounced genetic differentiation among their populations. It is worth noting that there were varying levels of polymorphism observed across the different subpopulations, highlighting some levels of genetic variation within these mosquito species. Nevertheless, further analyses revealed that all four species have undergone demographic expansion, suggesting population growth and potential range expansion for these vectors in this region.

Prevalence and proportion estimate of asymptomatic Plasmodium infection in Asia: a systematic review and meta-analysis

Asymptomatic Plasmodium infection raises a problem for the persistent transmission of malaria in low-endemic areas such as Asia. This systematic review was undertaken to estimate the prevalence and proportion of asymptomatic Plasmodium infection in Asia. The systematic review was registered at PROSPERO (ID: CRD42022373664). The research followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. A comprehensive search of five databases, Ovid, Scopus, MEDLINE, PubMed, and Embase, was conducted to identify studies of asymptomatic Plasmodium infection in Asian countries. The pooled prevalence of asymptomatic Plasmodium infection, the pooled proportion of asymptomatic Plasmodium infection among all parasitised individuals, and the associated 95% confidence intervals were estimated using a random-effects model. A total of 916 articles were retrieved, and 87 articles that met the criteria were included in the systematic review. The pooled prevalence of asymptomatic Plasmodium infection among enrolled participants in Southeast Asia, South Asia, and Western Asia was 5.8%, 9.4%, and 8.4%, respectively. The pooled proportion of asymptomatic Plasmodium infection among all parasitised individuals in Southeast Asia, South Asia, and Western Asia was 89.3%, 87.2%, and 64.8%, respectively. There was a low prevalence of asymptomatic Plasmodium infection, but there was a high proportion of asymptomatic Plasmodium infection per all parasitised individuals in different parts of Asia. These results may support and facilitate elimination and control programs for asymptomatic Plasmodium infection in Asia.

Medical Parasitology Taxonomy Update, June 2020-June 2022

The taxonomy of medically important parasites continues to evolve. This minireview provides an update of additions and updates in the field of human parasitology from June 2020 through June 2022. A list of previously reported nomenclatural changes that have not been broadly adapted by the medical community is also included.

Perspectives of vector management in the control and elimination of vector-borne zoonoses

The complex transmission profiles of vector-borne zoonoses (VZB) and vector-borne infections with animal reservoirs (VBIAR) complicate efforts to break the transmission circuit of these infections. To control and eliminate VZB and VBIAR, insecticide application may not be conducted easily in all circumstances, particularly for infections with sylvatic transmission cycle. As a result, alternative approaches have been considered in the vector management against these infections. In this review, we highlighted differences among the environmental, chemical, and biological control approaches in vector management, from the perspectives of VZB and VBIAR. Concerns and knowledge gaps pertaining to the available control approaches were discussed to better understand the prospects of integrating these vector control approaches to synergistically break the transmission of VZB and VBIAR in humans, in line with the integrated vector management (IVM) developed by the World Health Organization (WHO) since 2004.

Plasmodium cynomolgi in humans: current knowledge and future directions of an emerging zoonotic malaria parasite

Plasmodium cynomolgi (Pcy), a simian malaria parasite, is a recent perfect example of emerging zoonotic transfer in human. This review summarizes the current knowledge on the epidemiology of natural Pcy infections in humans, mosquitoes and monkeys, along with its biological, clinical and drug sensitivity patterns. Knowledge gaps and further studies on Pcy in humans are also discussed. This parasite currently seems to be geographically limited in South-East Asia (SEA) with a global prevalence in human ranging from 0 to 1.4%. The Pcy infections were reported in local SEA populations and European travelers, and range from asymptomatic carriage to mild/moderate attacks with no evidence of pathognomonic clinical and laboratory patterns but with Pcy strain-shaped clinical differences. Geographical distribution and competence of suitable mosquito vectors and non-primate hosts, globalization, climate change, and increased intrusion of humans into the habitat of monkeys are key determinants to emergence of Pcy parasites in humans, along with its expansion outside SEA. Sensitization/information campaigns coupled with training and assessment sessions of microscopists and clinicians on Pcy are greatly needed to improve data on the epidemiology and management of human Pcy infection. There is a need for development of sensitive and specific molecular tools for individual diagnosis and epidemiological studies. The development of safe and efficient anti-hypnozoite drugs is the main therapeutic challenge for controlling human relapsing malaria parasites. Experience gained from P. knowlesi malaria, development of integrated measures and strategies—ideally with components related to human, monkeys, mosquito vectors, and environment—could be very helpful to prevent emergence of Pcy malaria in humans through disruption of transmission chain from monkeys to humans and ultimately contain its expansion in SEA and potential outbreaks in a context of malaria elimination.

The first complete genome of the simian malaria parasite Plasmodium brasilianum

Naturally occurring human infections by zoonotic Plasmodium species have been documented for P. knowlesi, P. cynomolgi, P. simium, P. simiovale, P. inui, P. inui-like, P. coatneyi, and P. brasilianum. Accurate detection of each species is complicated by their morphological similarities with other Plasmodium species. PCR-based assays offer a solution but require prior knowledge of adequate genomic targets that can distinguish the species. While whole genomes have been published for P. knowlesi, P. cynomolgi, P. simium, and P. inui, no complete genome for P. brasilianum has been available. Previously, we reported a draft genome for P. brasilianum, and here we report the completed genome for P. brasilianum. The genome is 31.4 Mb in size and comprises 14 chromosomes, the mitochondrial genome, the apicoplast genome, and 29 unplaced contigs. The chromosomes consist of 98.4% nucleotide sites that are identical to the P. malariae genome, the closest evolutionarily related species hypothesized to be the same species as P. brasilianum, with 41,125 non-synonymous SNPs (0.0722% of genome) identified between the two genomes. Furthermore, P. brasilianum had 4864 (82.1%) genes that share 80% or higher sequence similarity with 4970 (75.5%) P. malariae genes. This was demonstrated by the nearly identical genomic organization and multiple sequence alignments for the merozoite surface proteins msp3 and msp7. We observed a distinction in the repeat lengths of the circumsporozoite protein (CSP) gene sequences between P. brasilianum and P. malariae. Our results demonstrate a 97.3% pairwise identity between the P. brasilianum and the P. malariae genomes. These findings highlight the phylogenetic proximity of these two species, suggesting that P. malariae and P. brasilianum are strains of the same species, but this could not be fully evaluated with only a single genomic sequence for each species.

A survey of simian Plasmodium infections in humans in West Kalimantan, Indonesia

The simian parasite Plasmodium knowlesi is the predominant species causing human malaria infection, including hospitalisations for severe disease and death, in Malaysian Borneo. By contrast, there have been only a few case reports of knowlesi malaria from Indonesian Borneo. This situation seems paradoxical since both regions share the same natural macaque hosts and Anopheles mosquito vectors, and therefore have a similar epidemiologically estimated risk of infection. To determine whether there is a true cross-border disparity in P. knowlesi prevalence, we conducted a community-based malaria screening study using PCR in Kapuas Hulu District, West Kalimantan. Blood samples were taken between April and September 2019 from 1000 people aged 6 months to 85 years attending health care facilities at 27 study sites within or close to jungle areas. There were 16 Plasmodium positive samples by PCR, five human malarias (two Plasmodium vivax, two Plasmodium ovale and one Plasmodium malariae) and 11 in which no species could be definitively identified. These data suggest that, if present, simian malarias including P. knowlesi are rare in the Kapuas Hulu District of West Kalimantan, Indonesian Borneo compared to geographically adjacent areas of Malaysian Borneo. The reason for this discrepancy, if confirmed in other epidemiologically similar regions of Indonesian Borneo, warrants further studies targeting possible cross-border differences in human activities in forested areas, together with more detailed surveys to complement the limited data relating to monkey hosts and Anopheles mosquito vectors in Indonesian Borneo.

Cryptic Plasmodium inui and Plasmodium fieldi Infections Among Symptomatic Malaria Patients in Thailand

BACKGROUND

Some nonhuman primate Plasmodium species including P. knowlesi and P. cynomolgi can cross-transmit from macaque natural hosts to humans under natural infection. This study aims to retrospectively explore other simian Plasmodium species in the blood samples of symptomatic malaria patients in Thailand.

METHODS

A total of 5271 blood samples from acute febrile patients from 5 malaria endemic provinces and 1015 blood samples from long-tailed and pig-tailed macaques from 3 locations were examined for Plasmodium species by microscopy and species-specific polymerase chain reaction. The Plasmodium mitochondrial cytochrome oxidase 1 (COX1) gene was analyzed by amplicon deep sequencing as well as Sanger sequencing from recombinant plasmid clones to reaffirm and characterize P. inui and P. fieldi.

RESULTS

Besides human malaria, P. knowlesi, P. cynomolgi, P. inui and P. fieldi infections were diagnosed in 15, 21, 19, and 3 patients, respectively. Most P. inui and all P. fieldi infected patients had simultaneous infections with other Plasmodium species, and seemed to be responsive to chloroquine or artemisinin-mefloquine. P. inui was the most prevalent species among macaque populations. Phylogenetic analysis of the COX1 sequences from human and macaque isolates reveals the genetic diversity of P. inui and suggests that multiple parasite strains have been incriminated in human infections.

CONCLUSIONS

Both P. inui and P. fieldi could establish infection in humans under natural transmission. Despite occurring at a low prevalence and mostly co-existing with other Plasmodium species, P. inui infections in humans have a wide distribution in Thailand.

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

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

Macaca fascicularis and Macaca nemestrina infected with zoonotic malaria parasites are widely distributed in Sarawak, Malaysian Borneo

Human infections with Plasmodium knowlesi, a malaria parasite of Macaca fascicularis and Macaca nemestrina (long-tailed and pig-tailed macaques respectively), occur throughout Southeast Asia, especially Malaysian Borneo. Other naturally-acquired human infections with malaria parasites from macaques in Southeast Asia are P. cynomolgi, P. inui-like, P. coatneyi and P. simiovale. In Sarawak, Malaysian Borneo, M. fascicularis and M. nemestrina from only the Kapit Division have been examined previously for malaria parasites. In order to determine the distribution of P. knowlesi and other zoonotic malaria parasites, 73 macaque blood samples derived from 7 other administrative divisions in Sarawak were studied. Of 45 blood samples from M. fascicularis and 28 from M. nemestrina tested by nested PCR assays, 23 (51.1%) M. fascicularis and 15 (53.6%) M. nemestrina samples were positive for Plasmodium DNA. Thirty-two of these macaques from 7 divisions sampled, harboured either single (n = 12), double (n = 9), triple (n = 7) or quadruple (n = 4) infections of P. knowlesi, P. inui, P. cynomolgi and P. coatneyi, while the infecting species of Plasmodium could not be identified for 6 samples. P. knowlesi was detected in 15.5% (7/45) M. fascicularis and in 7.1% (2/28) M. nemestrina sampled. Despite the small number of samples analysed from each administrative division, the current study indicates that macaques infected with the zoonotic malaria parasites P. knowlesi, P. cynomolgi, P. inui and P. coatneyi are widely distributed throughout Sarawak, Malaysian Borneo. Travelers to forested areas in Sarawak should be made aware of the potential risk of acquiring zoonotic malaria.

Exposure of Primate Reservoir Hosts to Mosquito Vectors in Malaysian Borneo

Several vector-borne pathogens of primates have potential for human spillover. An example is the simian malaria Plasmodium knowlesi which is now a major public health problem in Malaysia. Characterization of exposure to mosquito vectors is essential for assessment of the force of infection within wild simian populations, however few methods exist to do so. Here we demonstrate the use of thermal imaging and mosquito magnet independence traps (MMIT) to assess the abundance, diversity and infection rates in mosquitoes host seeking near long-tailed macaque (Macaca fasicularis) sleeping sites in the Lower Kinabatangan Wildlife Sanctuary, Malaysian Borneo. The primary Plasmodium knowlesi vector, Anopheles balabacensis, was trapped at higher abundance near sleeping sites than control trees. Although none of the An. balabacensis collected (n = 15) were positive for P. knowlesi by PCR screening, two were infected with another simian malaria Plasmodium inui. Analysis of macaque stools from sleeping sites confirmed a high prevalence of Plasmodium infection, suspected to be P. inui. Recently, natural transmission of P. inui has been detected in humans and An. cracens in Peninsular Malaysia. The presence of P. inui in An. balabacensis here and previously in human-biting collections highlight its potential for spillover from macaques to humans in Sabah. We advocate the use of MMITs for non-invasive sampling of mosquito vectors that host seek on wild simian populations.

Experimental Models to Study the Pathogenesis of Malaria-Associated Acute Respiratory Distress Syndrome

Malaria-associated acute respiratory distress syndrome (MA-ARDS) is increasingly gaining recognition as a severe malaria complication because of poor prognostic outcomes, high lethality rate, and limited therapeutic interventions. Unfortunately, invasive clinical studies are challenging to conduct and yields insufficient mechanistic insights. These limitations have led to the development of suitable MA-ARDS experimental mouse models. In patients and mice, MA-ARDS is characterized by edematous lung, along with marked infiltration of inflammatory cells and damage of the alveolar-capillary barriers. Although, the pathogenic pathways have yet to be fully understood, the use of different experimental mouse models is fundamental in the identification of mediators of pulmonary vascular damage. In this review, we discuss the current knowledge on endothelial activation, leukocyte recruitment, leukocyte induced-endothelial dysfunction, and other important findings, to better understand the pathogenesis pathways leading to endothelial pulmonary barrier lesions and increased vascular permeability. We also discuss how the advances in imaging techniques can contribute to a better understanding of the lung lesions induced during MA-ARDS, and how it could aid to monitor MA-ARDS severity.

Primate malaria: An emerging challenge of zoonotic malaria in Indonesia

The emergence of zoonotic malaria in different parts of the world, including Indonesia poses a challenge to the current malaria control and elimination program that target global malaria elimination at 2030. The reported cases in human include Plasmodium knowlesi, P. cynomolgi and P. inui, in South and Southeast Asian region and P. brazilianum and P. simium in Latin America. All are naturally found in the Old and New-world monkeys, macaques spp. This review focuses on the currently available data that may represent primate malaria as an emerging challenge of zoonotic malaria in Indonesia, the distribution of non-human primates and the malaria parasites it carries, changes in land use and deforestation that impact the habitat and intensifies interaction between the non-human primate and the human which facilitate spill-over of the pathogens. Although available data in Indonesia is very limited, a growing body of evidence indicate that the challenge of zoonotic malaria is immense and alerts to the need to conduct mitigation efforts through multidisciplinary approach involving environmental management, non-human primates conservation, disease management and vector control.

Zoonotic pathogens survey in free-living long-tailed macaques in Thailand

Long-tailed macaques (Macaca fascicularis) are known to harbour a variety of infectious pathogens, including zoonotic species. Long-tailed macaques and humans coexist in Thailand, which creates potential for interspecies pathogen transmission. This study was conducted to assess the presence of B virus, Mycobacterium spp., simian foamy virus (SFV), hepatitis B virus (HBV), and Plasmodium spp. in 649 free-living Thai long-tailed macaques through polymerase-chain reaction. DNA of SFV (56.5%), HBV (0.3%), and Plasmodium spp. (2.2%) was detected in these macaques, whereas DNA of B virus and Mycobacterium spp. was absent. SFV infection in long-tailed macaques is broadly distributed in Thailand and is correlated with age. The HBV sequences in this study were similar to HBV sequences from orangutans. Plasmodium spp. DNA was identified as P. inui. Collectively, our results indicate that macaques can carry zoonotic pathogens, which have a public health impact. Surveillance and awareness of pathogen transmission between monkeys and humans are important.

Multiplex PCR Assay for the Identification of Four Species of the Anopheles Leucosphyrus Sub-Group in Malaysia

Simple Summary

Plasmodium parasites cause malaria. The bites of infected female Anopheles mosquitoes, known as “malaria vectors,” transmit the parasites to people. To prevent the spread of malaria, precise mosquito species identification is essential. This study aims to develop a quick and accurate method for identifying the Anopheles species (An. introlatus, An. latens, An. cracens, and An. balabacensis), which have been incriminated as vectors for simian malaria in Malaysia. Overall, six primers targeting the internal transcribed spacer 2 (ITS2) region of each species were designed for this assay. This study is helpful for the researchers or vector-related field workers to correctly identify the mosquitoes for control activities.

Abstract

The Leucosphyrus Group of mosquitoes are the major simian malaria vectors in Malaysia. Accurate species identification is required to help in curbing the spread of simian malaria. The aim of the study is to provide an accurate molecular method for identifying the four important Anopheles vector species found in Malaysia. Mosquito specimens were collected from various localities in Malaysia, where simian malaria cases were reported. DNA from 122 mosquito specimens was tested to develop a multiplex polymerase chain reaction (PCR) assay. The specificity of this assay was tested against other mosquito species. Molecular identification of the species was further confirmed by analysing the internal transcribed spacer 2 (ITS2) DNA region of the specimens. Anopheles balabacensis and An. latens showed two distinct clades in the phylogenetic tree. The multiplex PCR assay was developed based on the ITS2 region for the identification of Anopheles introlatus (298–299 bp), Anopheles latens (197–198 bp), Anopheles cracens (421–426 bp), and Anopheles balabacensis (224–228 bp). This method will be useful to accurately identify the major Anopheles Leucosphyrus Group species in Malaysia, which are difficult to identify morphologically, to determine the correct vector as well as its geographical distribution.

An annotated checklist of the eukaryotic parasites of humans, exclusive of fungi and algae

The classification of "parasites" in the medical field is a challenging notion, a group which historically has included all eukaryotes exclusive of fungi that invade and derive resources from the human host. Since antiquity, humans have been identifying and documenting parasitic infections, and this collective catalog of parasitic agents has expanded considerably with technology. As our understanding of species boundaries and the use of molecular tools has evolved, so has our concept of the taxonomy of human parasites. Consequently, new species have been recognized while others have been relegated to synonyms. On the other hand, the decline of expertise in classical parasitology and limited curricula have led to a loss of awareness of many rarely encountered species. Here, we provide a comprehensive checklist of all reported eukaryotic organisms (excluding fungi and allied taxa) parasitizing humans resulting in 274 genus-group taxa and 848 species-group taxa. For each species, or genus where indicated, a concise summary of geographic distribution, natural hosts, route of transmission and site within human host, and vectored pathogens are presented. Ubiquitous, human-adapted species as well as very rare, incidental zoonotic organisms are discussed in this annotated checklist. We also provide a list of 79 excluded genera and species that have been previously reported as human parasites but are not believed to be true human parasites or represent misidentifications or taxonomic changes.

Epidemiology of the zoonotic malaria Plasmodium knowlesi in changing landscapes

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

The vectors of Plasmodium knowlesi and other simian malarias Southeast Asia: challenges in malaria elimination

Plasmodium knowlesi, a simian malaria parasite of great public health concern has been reported from most countries in Southeast Asia and exported to various countries around the world. Currently P. knowlesi is the predominant species infecting humans in Malaysia. Besides this species, other simian malaria parasites such as P. cynomolgi and P. inui are also infecting humans in the region. The vectors of P. knowlesi and other Asian simian malarias belong to the Leucosphyrus Group of Anopheles mosquitoes which are generally forest dwelling species. Continual deforestation has resulted in these species moving into forest fringes, farms, plantations and human settlements along with their macaque hosts. Limited studies have shown that mosquito vectors are attracted to both humans and macaque hosts, preferring to bite outdoors and in the early part of the night. We here review the current status of simian malaria vectors and their parasites, knowledge of vector competence from experimental infections and discuss possible vector control measures. The challenges encountered in simian malaria elimination are also discussed. We highlight key knowledge gaps on vector distribution and ecology that may impede effective control strategies.