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.

Background: Vector surveillance is essential in determining the geographical distribution of mosquito vectors and understanding the dynamics of malaria transmission. With the elimination of human malaria cases, knowlesi malaria cases in humans are increasing in Malaysia. This necessitates intensive vector studies using safer trapping methods which are both field efficient and able to attract the local vector populations. Thus, this study evaluated the potential of Mosquito Magnet as a collection tool for Anopheles mosquito vectors of simian malaria along with other known collection methods. Methods: A randomized 4 × 4 Latin square designed experiment was conducted to compare the efficiency of the Mosquito Magnet against three other common trapping methods: human landing catch (HLC), CDC light trap and human baited trap (HBT). The experiment was conducted over six replicates where sampling within each replicate was carried out for 4 consecutive nights. An additional 4 nights of sampling was used to further evaluate the Mosquito Magnet against the “gold standard” HLC. The abundance of Anopheles sampled by different methods was compared and evaluated with focus on the Anopheles from the Leucosphyrus group, the vectors of knowlesi malaria. Results: The Latin square designed experiment showed HLC caught the greatest number of Anopheles mosquitoes (n = 321) compared to the HBT (n = 87), Mosquito Magnet (n = 58) and CDC light trap (n = 13). The GLMM analysis showed that the HLC method caught significantly more Anopheles mosquitoes compared to Mosquito Magnet (P = 0.049). However, there was no significant difference in mean nightly catch of Anopheles mosquitoes between Mosquito Magnet and the other two trapping methods, HBT (P = 0.646) and CDC light traps (P = 0.197). The mean nightly catch for both An. introlatus (9.33 ± 4.341) and An. cracens (4.00 ± 2.273) caught using HLC was higher than that of Mosquito Magnet, though the differences were not statistically significant (P > 0.05). This is in contrast to the mean nightly catch of An. sinensis (15.75 ± 5.640) and An. maculatus (15.78 ± 3.479) where HLC showed significantly more mosquito catches compared to Mosquito Magnet (P < 0.05). Conclusions: Mosquito Magnet has a promising ability to catch An. introlatus and An. cracens, the important vectors of knowlesi and other simian malarias in Peninsular Malaysia. The ability of Mosquito Magnet to catch some of the Anopheles mosquito species is comparable to HLC and makes it an ethical and safer alternative. Graphical

The increasing burden of non-human primates (NHP) malaria, driven primarily by Plasmodium knowlesi, poses a growing public health threat in many countries across Southeast Asia. Compounding this challenge, the emergence of other NHP Plasmodium species infecting humans, including P. cynomolgi, P. inui, and P. fieldi, introduces additional complexity to malaria elimination efforts, particularly in countries like Malaysia, Thailand, and Vietnam. A complex interplay among human populations, vector dynamics, and environmental factors influences the transmission and prevalence of this disease. This narrative review delves into the current vectors of NHP malaria in Southeast Asia, highlighting the crucial role of environmental determinants in shaping the bionomics of Anopheles mosquito vectors. Key environmental factors, such as temperature fluctuations, relative humidity, elevation, precipitation patterns, seasonality, and land use, play a pivotal role in shaping vector abundance and survival, ultimately influencing the transmission intensity of zoonotic malaria. Adopting a One Health approach, which recognizes the interconnections between human, animal, and environmental health, is crucial for unravelling these complex dynamics. Advancing this integrated framework will require continued research and understanding of vector ecology across diverse environmental settings and geographical regions. This review provides comprehensive information on vector bionomics in relation to the changing environmental factors, besides highlighting the importance of a multidisciplinary strategy that integrates vector surveillance, sustainable land management, and targeted public health interventions to inform effective, evidence-based malaria control efforts in Southeast Asia.

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.

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. Graphical abstract