Case Report: Case Series of Human Plasmodium knowlesi Infection on the Southern Border of Thailand

Human-to-Anopheles dirus mosquito transmission of the anthropozoonotic malaria parasite, Plasmodium knowlesi

BACKGROUND

Plasmodium knowlesi, identified as the fifth human malaria parasite, has rapidly spread across various Southeast Asian countries, yet uncertainties persist regarding its human-mosquito-human transmission. Therefore, this study aims to explore the transmission potential of P. knowlesi from human blood to mosquitoes.

METHODS

A direct membrane-feeding assay was conducted by infecting laboratory-reared female Anopheles dirus mosquitoes with P. knowlesi-infected human blood from a single patient presenting with febrile malaria. Mosquitoes were dissected 7 days post-infection under a stereomicroscope to detect oocysts in the midgut, stained with mercurochrome. Salivary glands were examined 14 days post-infection for the presence of sporozoites. Malaria diagnosis employed microscopy by expert microscopists and nested PCR assays.

RESULTS

Upon dissecting 745 out of 1439 blood-fed An. dirus mosquitoes on day 7 post-infection, two oocysts were identified in the midguts of two mosquitoes (0.27%). An additional 694 mosquitoes were dissected for salivary glands on day 14 post-infection, with three mosquitoes (0.43%) exhibiting sporozoites. Further confirmation by nested-PCR assay verified these sporozoites as belonging to the P. knowlesi species.

CONCLUSIONS

The findings underscore the potential transmission of P. knowlesi from human blood to mosquitoes. The significance of these findings necessitates further investigation, such as repeating similar experiments among natural vectors, to gain deeper insights into the transmission dynamics of P. knowlesi in Southeast Asia.

Declining Genetic Polymorphisms of the C-terminus Merozoite Surface Protein-1 Amidst Increased Plasmodium knowlesi Transmission in Thailand

Recent reports from Thailand reveal a substantial surge in Plasmodium knowlesi cases over the past decade, with a more than eightfold increase in incidence by 2023 compared to 2018. This study investigates temporal changes in genetic polymorphism associated with the escalating transmission of P. knowlesi malaria in Thailand over the past two decades. Twenty-five P. knowlesi samples collected in 2018-2023 were sequenced for the 42-kDa region of pkmsp1 and compared with 24 samples collected in 2000-2009, focusing on nucleotide diversity, natural selection, recombination rate, and population differentiation. Seven unique haplotypes were identified in recent samples, compared to 15 in earlier samples. Nucleotide and haplotype diversities were lower in recent samples (π = 0.016, Hd = 0.817) than in earlier samples (π = 0.018, Hd = 0.942). Significantly higher synonymous substitution rates were observed in both sample sets (dS - dN = 2.77 and 2.43, p < 0.05), indicating purifying selection and reduced genetic diversity over time. Additionally, 8 out of 17 mutation points were located on B-cell epitopes, suggesting an adaptive response by the parasites to evade immune recognition. Population differentiation analysis using the fixation index (Fst) revealed high genetic differentiation between parasite populations in central and southern Thailand or Malaysia. Conversely, the relatively lower Fst value between southern Thailand and Malaysia suggests a closer genetic relationship, possibly reflecting historical gene flow. In conclusion, our findings highlight a decline in genetic diversity and evidence of purifying selection associated with the recently increased incidence of P. knowlesi malaria in Thailand. The minor genetic differentiation between P. knowlesi populations from southern Thailand and Malaysia suggests a shared recent ancestry of these parasites and underscores the need for coordinated efforts between the two countries for the elimination of P. knowlesi.

Prevalence and genetic diversity of simian malaria in wild macaque populations across Thailand: Implications for human health

Over the past year, P. falciparum infections have declined in Thailand, yet nonhuman primate malaria infections have correspondingly increased, including Plasmodium knowlesi and P. cynomolgi. Nevertheless, little is known about simian malaria in its natural macaque hosts, Macaca mulatta and Macaca fascicularis. This study aims to address several research questions, including the prevalence and distribution of simian malaria in these two Thai wild macaque species, variations in infection between different macaque species and between M. fascicularis subspecies, and the genetic composition of these pathogens. Blood samples were collected from 82 M. mulatta and 690 M. fascicularis across 15 locations in Thailand, as well as two locations in Vietnam and Myanmar. We employed quantitative real-time PCR targeting the Plasmodium genus-specific 18S ribosomal RNA (rRNA) gene to detect malaria infection, with a limit of detection set at 1,215.98 parasites per mL. We genotyped eight microsatellite markers, and the P. cynomolgi dihydrofolate reductase gene (DHFR) was sequenced (N = 29). In total, 100 of 772 samples (13 %) tested positive for malaria, including 45 (13 %) for P. cynomolgi, 37 (13 %) for P. inui, 16 (5 %) for P. coatneyi, and 2 (0.25 %) for Hepatocystis sp. in Saraburi, central and Ranong, southern Thailand. Notably, simian malaria infection was observed exclusively in M. fascicularis and not in M. mulatta (P = 0.0002). Particularly, P. cynomolgi was detected in 21.7 % (45/207) of M. f. fascicularis living in Wat Tham Phrapothisat, Saraburi Province. The infection with simian malaria was statistically different between M. fascicularis and M. mulatta (P = 0.0002) but not within M. fascicularis subspecies (P = 0.78). A haplotype network analysis revealed that P. cynomolgi shares a lineage with reference strains obtained from macaques. No mutation in the predicted binding pocket of PcyDHFR to pyrimethamine was observed. This study reveals a significant prevalence of simian malaria infection in M. fascicularis. The clonal genotypes of P. cynomolgi suggest in-reservoir breeding. These findings raise concerns about the potential spread of nonhuman primate malaria to humans and underscore the need for preventive measures.

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.

Plasmodium knowlesi (Pk) Malaria: A Review & Proposal of Therapeutically Rational Exchange (T-REX) of Pk-Resistant Red Blood Cells

Plasmodium knowlesi (Pk) causes zoonotic malaria and is known as the "fifth human malaria parasite". Pk malaria is an emerging threat because infections are increasing and can be fatal. While most infections are in Southeast Asia (SEA), especially Malaysia, travelers frequently visit this region and can present with Pk malaria around the world. So, clinicians need to know (1) patients who present with fever after recent travel to SEA might be infected with Pk and (2) Pk is often misdiagnosed as P. malariae (which typically causes less severe malaria). Here we review the history, pathophysiology, clinical features, diagnosis, and treatment of Pk malaria. Severe disease is most common in adults. Signs and symptoms can include fever, abdominal pain, jaundice, acute kidney injury, acute respiratory distress syndrome, hyponatremia, hyperparasitemia, and thrombocytopenia. Dengue is one of the diseases to be considered in the differential. Regarding pathophysiologic mechanisms, when Pk parasites invade mature red blood cells (RBCs, i.e., normocytes) and reticulocytes, changes in the red blood cell (RBC) surface can result in life-threatening cytoadherence, sequestration, and reduced RBC deformability. Since molecular mechanisms involving the erythrocytic stage are responsible for onset of severe disease and lethal outcomes, it is biologically plausible that manual exchange transfusion (ET) or automated RBC exchange (RBCX) could be highly beneficial by replacing "sticky" parasitized RBCs with uninfected, deformable, healthy donor RBCs. Here we suggest use of special Pk-resistant donor RBCs to optimize adjunctive manual ET/RBCX for malaria. "Therapeutically-rational exchange transfusion" (T-REX) is proposed in which Pk-resistant RBCs are transfused (instead of disease-promoting RBCs). Because expression of the Duffy antigen on the surface of human RBCs is essential for parasite invasion, T-REX of Duffy-negative RBCs-also known as Fy(a-b-) RBCs-could replace the majority of the patient's circulating normocytes with Pk invasion-resistant RBCs (in a single procedure lasting about 2 h). When sequestered or non-sequestered iRBCs rupture-in a 24 h Pk asexual life cycle-the released merozoites cannot invade Fy(a-b-) RBCs. When Fy(a-b-) RBC units are scarce (e.g., in Malaysia), clinicians can consider the risks and benefits of transfusing plausibly Pk-resistant RBCs, such as glucose-6-phosphate dehydrogenase deficient (G6PDd) RBCs and Southeast Asian ovalocytes (SAO). Patients typically require a very short recovery time (<1 h) after the procedure. Fy(a-b-) RBCs should have a normal lifespan, while SAO and G6PDd RBCs may have mildly reduced half-lives. Because SAO and G6PDd RBCs come from screened blood donors who are healthy and not anemic, these RBCs have a low-risk for hemolysis and do not need to be removed after the patient recovers from malaria. T-REX could be especially useful if (1) antimalarial medications are not readily available, (2) patients are likely to progress to severe disease, or (3) drug-resistant strains emerge. In conclusion, T-REX is a proposed optimization of manual ET/RBCX that has not yet been utilized but can be considered by physicians to treat Pk malaria patients.

Inter-Population Genetic Diversity and Clustering of Merozoite Surface Protein-1 (pkmsp-1) of Plasmodium knowlesi Isolates from Malaysia and Thailand

The genetic diversity of pkmsp-1 of Malaysian Plasmodium knowlesi isolates was studied recently. However, the study only included three relatively older strains from Peninsular Malaysia and focused mainly on the conserved blocks of this gene. In this study, the full-length pkmsp-1 sequence of recent P. knowlesi isolates from Peninsular Malaysia was characterized, along with Malaysian Borneo and Thailand pkmsp-1 sequences that were retrieved from GenBank. Genomic DNA of P. knowlesi was extracted from human blood specimens and the pkmsp-1 gene was PCR-amplified, cloned, and sequenced. The sequences were analysed for genetic diversity, departure from neutrality, and geographical clustering. The pkmsp-1 gene was found to be under purifying/negative selection and grouped into three clusters via a neighbour-joining tree and neighbour net inferences. Of the four polymorphic blocks in pkmsp-1, block IV, was most polymorphic, with the highest insertion-deletion (indel) sites. Two allelic families were identified in block IV, thereby highlighting the importance of this block as a promising genotyping marker for the multiplicity of infection study of P. knowlesi malaria. A single locus marker may provide an alternate, simpler method to type P. knowlesi in a population.

Transmission efficiency of Plasmodium vivax at low parasitaemia

BACKGROUND

Plasmodium vivax is responsible for much of malaria outside Africa. Although most P. vivax infections in endemic areas are asymptomatic and have low parasite densities, they are considered a potentially important source of transmission. Several studies have demonstrated that asymptomatic P. vivax carriers can transmit the parasite to mosquitoes, but the efficiency has not been well quantified. The aim of this study is to determine the relationship between parasite density and mosquito infectivity, particularly at low parasitaemia.

METHODS

Membrane feeding assays were performed using serial dilutions of P. vivax-infected blood to define the relationship between parasitaemia and mosquito infectivity.

RESULTS

The infection rate (oocyst prevalence) and intensity (oocyst load) were positively correlated with the parasite density in the blood. There was a broad case-to-case variation in parasite infectivity. The geometric mean parasite density yielding a 10% mosquito infection rate was 33 (CI 95 9-120) parasites/µl or 4 (CI 95 1-17) gametocytes/µl. The geometric mean parasite density yielding a 50% mosquito infection rate was 146 (CI 95 36-586) parasites/µl or 13 (CI 95 3-49) gametocytes/µl.

CONCLUSION

This study quantified the ability of P. vivax to infect Anopheles dirus at over a broad range of parasite densities. It provides important information about parasite infectivity at low parasitaemia common among asymptomatic P. vivax carriers.

Multidisciplinary Investigations of Sustained Malaria Transmission in the Greater Mekong Subregion

In the course of malaria elimination in the Greater Mekong Subregion (GMS), malaria epidemiology has experienced drastic spatiotemporal changes with residual transmission concentrated along international borders and the rising predominance of Plasmodium vivax. The emergence of Plasmodium falciparum parasites resistant to artemisinin and partner drugs renders artemisinin-based combination therapies less effective while the potential spread of multidrug-resistant parasites elicits concern. Vector behavioral changes and insecticide resistance have reduced the effectiveness of core vector control measures. In recognition of these problems, the Southeast Asian International Center of Excellence for Malaria Research (ICEMR) has been conducting multidisciplinary research to determine how human migration, antimalarial drug resistance, vector behavior, and insecticide resistance sustain malaria transmission at international borders. These efforts allow us to comprehensively understand the ecology of border malaria transmission and develop population genomics tools to identify and track parasite introduction. In addition to employing in vivo, in vitro, and molecular approaches to monitor the emergence and spread of drug-resistant parasites, we also use genomic and genetic methods to reveal novel mechanisms of antimalarial drug resistance of parasites. We also use omics and population genetics approaches to study insecticide resistance in malaria vectors and identify changes in mosquito community structure, vectorial potential, and seasonal dynamics. Collectively, the scientific findings from the ICEMR research activities offer a systematic view of the factors sustaining residual malaria transmission and identify potential solutions to these problems to accelerate malaria elimination in the GMS.

Malaria Research for Tailored Control and Elimination Strategies in the Greater Mekong Subregion

The malaria landscape in the Greater Mekong Subregion has experienced drastic changes with the ramp-up of the control efforts, revealing formidable challenges that slowed down the progress toward malaria elimination. Problems such as border malaria and cross-border malaria introduction, multidrug resistance in Plasmodium falciparum, the persistence of Plasmodium vivax, the asymptomatic parasite reservoirs, and insecticide resistance in primary vectors require integrated strategies tailored for individual nations in the region. In recognition of these challenges and the need for research, the Southeast Asian International Center of Excellence for Malaria Research has established a network of researchers and stakeholders and conducted basic and translational research to identify existing and emerging problems and develop new countermeasures. The installation of a comprehensive disease and vector surveillance system at sentinel sites in border areas with the implementation of passive/active case detection and cross-sectional surveys allowed timely detection and management of malaria cases, provided updated knowledge for effective vector control measures, and facilitated the efficacy studies of antimalarials. Incorporating sensitive molecular diagnosis to expose the significance of asymptomatic parasite reservoirs for sustaining transmission helped establish the necessary evidence to guide targeted control to eliminate residual transmission. In addition, this program has developed point-of-care diagnostics to monitor the quality of artemisinin combination therapies, delivering the needed information to the drug regulatory authorities to take measures against falsified and substandard antimalarials. To accelerate malaria elimination, this program has actively engaged with stakeholders of all levels, fostered vertical and horizontal collaborations, and enabled the effective dissemination of research findings.

Seroepidemiological surveillance, community perceptions and associated risk factors of malaria exposure among forest-goers in Northeastern Thailand

Malaria remains a major public health challenge in Thailand. Continuous assessment and understanding of the behavior and perceptions related to malaria exposure in the high-risk group are necessary to achieve the elimination goal. This study aimed to investigate the parasite prevalence, seroprevalence rate, knowledge, attitudes, and practices (KAP), and malaria risk factors in rural communities living close to a forested area in the northeastern part of Thailand. A community-based cross-sectional survey was conducted in three forest-goer communities (i.e., Ban Khok, Ban Koh, and Dong Yang) located in Khamcha-i district, Mukdahan Province, Thailand, from July to August 2019. Demographic, socioeconomic information and KAP data were collected using a structured questionnaire. Parasite prevalence was determined by microscopy. Seroprevalence was determined via ELISA using two Plasmodium falciparum (PfAMA-1 and PfMSP-1₁₉) and two Plasmodium vivax (PvAMA-1 and PvMSP-1₁₉) antigens. Age-adjusted antibody responses were analyzed using a reversible catalytic model to calculate seroconversion rate (SCR). Malaria parasite was not detected in any of the 345 participants. The overall malaria seroprevalence was 72.2% for PfAMA-1, 18.8% for PfMSP-1₁₉, 32.5% for PvAMA-1, and 4.4% for PvMSP-1₁₉. The proportion of seroprevalence for P. falciparum and P. vivax antigens was significantly highest in Ban Koh (35.1%, P < 0.001) and Don Yang (18.8%, P < 0.001), respectively. For all parasite antigens except PvMSP-1₁₉, the proportion of seropositive individuals significantly increased with age (P < 0.001). Based on the SCRs, there was a higher level of P. falciparum transmission than P. vivax. Regarding KAP, almost all respondents showed adequate knowledge and awareness about malaria. Nevertheless, significant effort is needed to improve positive attitudes and practices concerning malaria prevention measures. Multivariate regression analyses showed that living in Ban Koh was associated with both P. falciparum (adjusted odds ratio [aOR] 12.87, P < 0.001) and P. vivax (aOR 9.78, P < 0.001) seropositivities. We also found significant associations between age and seropositivity against P. falciparum and P. vivax antigens. The data suggest that seroepidemiological surveillance using AMA-1 and MSP-1₁₉ antigens may provide further evidence to reconstruct malaria exposure history. The absence of weak evidence of recent malaria transmission in Mukdahan Province is promising in the context of the disease elimination program.

Narrative Review of the Control and Prevention of Knowlesi Malaria

Despite the reduction in the number of cases of human malaria throughout the world, the incidence rate of knowlesi malaria is continuing to rise, especially in Southeast Asia. The conventional strategies for the prevention and control of human malaria can provide some protection against knowlesi malaria. Despite the numerous studies on the risk factors and the innovative methods that may be used to prevent and control the vectors of Plasmodium knowlesi, the incidence rate remains high. An integrated approach that includes environmental intervention should be adopted in order to ensure the successful control of zoonotic malaria. A combination of personal-level protection, vector control and environmental control may mitigate the risk of Plasmodium knowlesi transmission from macaques to humans and, ultimately, reduce the incidence rate of knowlesi malaria.

Case series of three malaria patients from Thailand infected with the simian parasite, Plasmodium cynomolgi

BACKGROUND

While human cases of Plasmodium knowlesi are now regularly recognized in Southeast Asia, infections with other simian malaria species, such as Plasmodium cynomolgi, are still rare. There has been a handful of clinical cases described, all from Malaysia, and retrospective studies of archived blood samples in Thailand and Cambodia have discovered the presence P. cynomolgi in isolates using polymerase chain reaction (PCR) assays.

CASE PRESENTATION

In Thailand, an ongoing malaria surveillance study enrolled two patients from Yala Province diagnosed with Plasmodium vivax by blood smear, but who were subsequently found to be negative by PCR. Expanded PCR testing of these isolates detected mono-infection with P. cynomolgi, the first time this has been reported in Thailand. Upon re-testing of 60 isolates collected from Yala, one other case was identified, a co-infection of P. cynomolgi and P. vivax. The clinical course for all three was relatively mild, with symptoms commonly seen in malaria: fever, chills and headaches. All infections were cured with a course of chloroquine and primaquine.

CONCLUSION

In malaria-endemic areas with macaque populations, cases of simian malaria in humans are being reported at an increasing rate, although still comprise a very small percentage of total cases. Plasmodium cynomolgi and P. vivax are challenging to distinguish by blood smear; therefore, PCR can be employed when infections are suspected or as part of systematic malaria surveillance. As Thai MoPH policy schedules regular follow-up visits after each malaria infection, identifying those with P. cynomolgi will allow for monitoring of treatment efficacy, although at this time P. cynomolgi appears to have an uncomplicated clinical course and good response to commonly used anti-malarials.

Plasmodium knowlesi: the game changer for malaria eradication

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

Reimagining zoonotic malaria control in communities exposed to Plasmodium knowlesi infection

Plasmodium knowlesi malaria infection in humans has been reported throughout southeast Asia. The communities at risk are those living in areas where Macaque monkeys and Anopheles mosquito are present. Zoonotic malaria control is challenging due to the presence of the reservoir host and the possibility of human-vector-human transmission. Current control measures, including insecticide-treated nets (ITNs) and indoor residual spraying (IRS), are insufficient to address this threat due to gaps in protection associated with outdoor and early evening vector biting and social and economic activities, such as agricultural and forest work. Understanding the challenges faced by affected communities in preventing mosquito bites is important for reducing disease transmission. This opinion paper discusses opportunities to improve P. knowlesi malaria control through understanding the challenges faced by communities at risk and increasing community engagement and ownership of control measures. The paper highlights this issue by describing how the concept of reimagining malaria can be adapted to zoonotic malaria control measures including identifying current gaps in vector control, understanding interactions between environmental, economic, and human behavioral factors, and increasing community participation in and ownership of control measures.

The Role of Human Behavior in Plasmodium knowlesi Malaria Infection: A Systematic Review

Objectives: Plasmodium knowlesi is a non-human parasite that causes zoonotic disease in humans. This systematic review aims to highlight and summarize studies describing human behaviors and activities that expose humans to mosquito bites. Design: English entries in PubMed, Web of Science, and Science Direct from 2010 to 2020 were systematically perused, and the results were synthesized. Methodological quality was assessed using the Joanna Briggs Institute quality appraisal checklists. Setting: Studies that described malaria preventive measures were included. Laboratory, in vivo, in vitro, and animal studies were excluded. Primary and secondary outcome measures: The main outcome of the review was findings from studies describing the behavior that exposed a person or a group to P. knowlesi infection. Results: Twelve eligible studies were of good or medium quality. Attitude, disease misconceptions, perceived threat of disease, lack of motivation, and supernatural or traditional beliefs causing individuals to seek treatment from traditional healers influenced the exposure of individuals or communities to P. knowlesi malaria. Other factors were forestry activities (2.48, 1.45–4.23,95% CI, p = 0.0010) and sleeping outdoors (3.611, 1.48–8.85, 95% CI, p = 0.0049). Conclusions: Future studies must consider the importance of human behavior and community perspective on the infection to provide novel information to improve the current zoonotic malaria programs. Policymakers should concentrate on understanding human behavior and activities that expose individuals or communities to mosquito bites, in order to better design socially feasible interventions.

Genetic population of Plasmodium knowlesi during pre-malaria elimination in Thailand

Background

Thailand is committed to eliminating malaria by 2024. From 2013 to 2020, the total number of malaria cases have decreased, from 37,741 to 4474 (an 88.1% reduction). However, infections with Plasmodium knowlesi, a monkey malarial pathogen that can also infect humans, have been increasingly observed. This study focused on the molecular analysis of P. knowlesi parasites causing malaria in Thailand.

Methods

Under Thailand’s integrated Drug Efficacy Surveillance (iDES), which includes drug-resistance monitoring as part of routine case-based surveillance and responses, specimens were collected from malaria patients (n = 966) between 2018 and 2020. Thirty-one mono P. knowlesi infections (3.1%), most of which were from eastern and southern Thailand, were observed and confirmed by nested PCR assay and DNA sequencing. To evaluate whether these pathogens were from different lineages, cluster analysis based on seven microsatellite genotyping markers and the merozoite surface protein 1 (pkmsp1) gene was carried out. The P. knowlesi pyrimethamine resistance gene dihydrofolate reductase (pkdhfr) was sequenced and homology modelling was constructed.

Results

The results of analysing the seven microsatellite markers and pkmsp1 sequence demonstrated that P. knowlesi parasites from eastern Thailand were of the same lineage as those isolated in Cambodia, while the parasites causing malaria in southern Thailand were the same lineage as those isolated from Malaysia. The sequencing results for the pkdhfr genes indicated the presence of two mutations, Arg34Leu and a deletion at position 105. On analysis with homology modelling, the two mutations were not associated with anti-malarial drug resistance.

Conclusions

This report compared the genetic populations of P. knowlesi parasites in Thailand from 2018 to 2020 and have shown similar lineages as those isolated in Cambodia and Malaysia of P. knowlesi infection in Thailand and demonstrated that the P. knowlesi parasites were of the same lineages as those isolated in Cambodia and Malaysia. The parasites were also shown to be sensitive to pyrimethamine.

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.

Quantification of the misidentification of Plasmodium knowlesi as Plasmodium malariae by microscopy: an analysis of 1569 P. knowlesi cases

Background

Plasmodium knowlesi is recognized as the fifth Plasmodium species causing malaria in humans. It is morphologically similar to the human malaria parasite Plasmodium malariae, so molecular detection should be used to clearly discriminate between these Plasmodium species. This study aimed to quantify the rate at which P. knowlesi is misidentified as P. malariae by microscopy in endemic and non-endemic areas.

Methods

The protocol of this systematic review was registered in the PROSPERO International Prospective Register of Systematic Reviews (ID = CRD42020204770). Studies reporting the misidentification of P. knowlesi as P. malariae by microscopy and confirmation of this by molecular methods in MEDLINE, Web of Science and Scopus were reviewed. The risk of bias in the included studies was assessed using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS). The pooled prevalence and 95% confidence interval (CI) of the misidentification of P. knowlesi as P. malariae by microscopy were estimated using a random effects model. Subgroup analysis of the study sites was performed to demonstrate any differences in the misidentification rates in different areas. Heterogeneity across the included studies was assessed and quantified using Cochran’s Q and I² statistics, respectively. Publication bias in the included studies was assessed using the funnel plot, Egger’s test and contour-enhanced funnel plot.

Results

Among 375 reviewed studies, 11 studies with a total of 1569 confirmed P. knowlesi cases in humans were included. Overall, the pooled prevalence of the misidentification of P. knowlesi as P. malariae by microscopy was estimated at 57% (95% CI 37–77%, I²: 99.3%). Subgroup analysis demonstrated the highest rate of misidentification in Sawarak, Malaysia (87%, 95% CI 83–90%, I²: 95%), followed by Sabah, Malaysia (85%, 95% CI 79–92%, I²: 85.1%), Indonesia (16%, 95% CI 6–38%), and then Thailand (4%, 95% CI 2–9%, I²: 95%).

Conclusion

Although the World Health Organization (WHO) recommends that all P. malariae-positive diagnoses made by microscopy in P. knowlesi endemic areas be reported as P. malariae/P. knowlesi malaria, the possibility of microscopists misidentifying P. knowlesi as P. malariae is a diagnostic challenge. The use of molecular techniques in cases with malariae-like Plasmodium with high parasite density as determined by microscopy could help identify human P. knowlesi cases in non-endemic countries.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03714-1.

Plasmodium knowlesi infecting humans in Southeast Asia: What's next?

Plasmodium knowlesi, a simian malaria parasite, has been in the limelight since a large focus of human P. knowlesi infection was reported from Sarawak (Malaysian Borneo) in 2004. Although this infection is transmitted across Southeast Asia, the largest number of cases has been reported from Malaysia. The increasing number of knowlesi malaria cases has been attributed to the use of molecular tools for detection, but environmental changes including deforestation likely play a major role by increasing human exposure to vector mosquitoes, which coexist with the macaque host. In addition, with the reduction in human malaria transmission in Southeast Asia, it is possible that human populations are at a greater risk of P. knowlesi infection due to diminishing cross-species immunity. Furthermore, the possibility of increasing exposure of humans to other simian Plasmodium parasites such as Plasmodium cynomolgi and Plasmodium inui should not be ignored. We here review the current status of these parasites in humans, macaques, and mosquitoes to support necessary reorientation of malaria control and elimination in the affected areas.

The human-primate interface in the New Normal: Challenges and opportunities for primatologists in the COVID-19 era and beyond

The emergence of SARS-CoV-2 in late 2019 and human responses to the resulting COVID-19 pandemic in early 2020 have rapidly changed many aspects of human behavior, including our interactions with wildlife. In this commentary, we identify challenges and opportunities at human-primate interfaces in light of COVID-19, focusing on examples from Asia, and make recommendations for researchers working with wild primates to reduce zoonosis risk and leverage research opportunities. First, we briefly review the evidence for zoonotic origins of SARS-CoV-2 and discuss risks of zoonosis at the human-primate interface. We then identify challenges that the pandemic has caused for primates, including reduced nutrition, increased intraspecific competition, and increased poaching risk, as well as challenges facing primatologists, including lost research opportunities. Subsequently, we highlight opportunities arising from pandemic-related lockdowns and public health messaging, including opportunities to reduce the intensity of problematic human-primate interfaces, opportunities to reduce the risk of zoonosis between humans and primates, opportunities to reduce legal and illegal trade in primates, new opportunities for research on human-primate interfaces, and opportunities for community education. Finally, we recommend specific actions that primatologists should take to reduce contact and aggression between humans and primates, to reduce demand for primates as pets, to reduce risks of zoonosis in the context of field research, and to improve understanding of human-primate interfaces. Reducing the risk of zoonosis and promoting the well-being of humans and primates at our interfaces will require substantial changes from "business as usual." We encourage primatologists to help lead the way.

Plasmodium knowlesi-mediated zoonotic malaria: A challenge for elimination

Malaria, a mosquito-transmitted parasitic disease, has been targeted for elimination in many parts of the world. For many years, Plasmodium vivax, Plasmodium falciparum, Plasmodium ovale and Plasmodium malariae have been known to cause malaria in humans. Now, Plasmodium knowlesi is considered to be an important cause of malaria, especially in Southeast Asia. The emergence of P. knowlesi with zoonotic implication is a challenge in the elimination efforts of malaria in Southeast Asia. P. knowlesi is known to cause severe complicated malaria in humans. P. knowlesi parasite is transmitted between humans and wild macaque through mosquito vectors. It appears that the malaria disease severity and host immune evasion depend on antigenic variation exhibited at the surface of the infected erythrocyte. P. knowlesi is sensitive to antimalarial drug artemisinin. Identification of vector species, their biting behavior, timely correct diagnosis, and treatment are important steps in disease management and control. There is a need to identify and implement effective intervention measures to cut the chain of transmissions from animals to humans. The zoonotic malaria definitely poses a significant challenge in elimination and subsequent eradication of all types of malaria from this globe.

Malaria cross-sectional surveys identified asymptomatic infections of Plasmodium falciparum, Plasmodium vivax and Plasmodium knowlesi in Surat Thani, a southern province of Thailand

OBJECTIVES

Malaria cross-sectional surveys are rarely conducted in very low transmission settings. This study aimed to determine the prevalence and risk factors of Plasmodium infection in a near-elimination setting in southern Thailand.

METHODS

Two cross-sectional surveys were conducted in areas of active transmission in the Surat Thani province of Thailand in January and May 2019. PCR was used to detect Plasmodium infection.

RESULTS

The prevalence of Plasmodium blood infection was 0.45% and 0.61% in January and May 2019, respectively. The major parasite species was Plasmodium falciparum in January and Plasmodium vivax in May. Unexpectedly, Plasmodium knowlesi infections were also detected. Most infections, including those of Plasmodium knowlesi, were asymptomatic. Being male and staying outdoors at night-time were the only significant identified risk factors. Of people infected in January 28.0% were positive in May for the same parasite species, suggesting persistent asymptomatic infections.

CONCLUSIONS

Despite the very low incidence rate in Surat Thani, most malaria infections were asymptomatic. Outdoor mosquito biting at night-time is likely an important mode of malaria transmission. Unexpectedly, asymptomatic Plasmodium knowlesi infection was found, confirming previous reports of such infection in mainland Southeast Asia.