This study reveals that P. knowlesi infections were distributed across all age groups with higher prevalence among adults of the age group of 20–39 years, notably males, who likely are more active outdoors and have greater forest exposure to infection due to job opportunities . A small segment of knowlesi malaria cases was observed among children and the elderly which could be associated with limited outdoor activities and lower risk of biting by infected Anopheles mosquito.
Individuals infected with knowlesi malaria were involved in various types of occupation. However, more than half (53.96%) of them had occupations related to agriculture and forest. Nonetheless, types of occupation alone may not give a true reflection of risk of knowlesi malaria infection. Recreational activities related to forest such as bird watching, hiking, and camping may also expose an individual to infective mosquito bite. Rural and sub-urban settlements located close to forest-edge and forested areas, are mostly resided by Malay community, which suggests greater burden of P. knowlesi infection among Malays compared with other ethnic groups. Although 16.4% of the total indigenous cases were foreign citizens, the distribution of cases by nationality may not be an important factor. The most important risk factors probably are activities that expose them to bites of infective mosquitoes.
Besides, demography analysis of imported cases revealed that one-fourth of the imported cases were of Indonesian nationality. Imported cases involving Malaysians were most potentially due to international travel to knowlesi malaria endemic countries. Besides Malaysia, P. knowlesi infections have been reported throughout Southeast Asia including Indonesia , Vietnam , Thailand , and Cambodia .
In this study, Kelantan, Pahang, and Perak had consistently recorded high burden of knowlesi malaria. These states have the largest settlement of aborigine communities . Aborigine individuals made up one-tenth of the total indigenous knowlesi malaria cases. Prior to our study, human malaria parasites had been frequently reported among aborigine population in Peninsular Malaysia [28, 29, 30]. Moreover, researchers detected presence of submicroscopic P. knowlesi infections among asymptomatic individuals within these communities . Aborigine communities are considered high-risk of exposure to malaria. This is due to their settlements being located in the forest fringes and many are still dependent on forest resources for subsistence [32, 33].
Spatial analysis indicated high incidence rate of knowlesi malaria concentrated in the central-northern region of Peninsular Malaysia. Of these regions, Gua Musang and Lipis districts reported the highest incidence rate compared to other districts. Gua Musang and Lipis are neighbouring districts and most of the infected patients worked in agricultural sector, implying frequent exposure to forest, forest-edge, plantation setting, with increased probability of contact with Anopheles mosquitoes as well as macaque populations. Prior to 2015, many agriculture, logging, and quarrying activities occurred in the Gua Musang district . These activities were involved with exploitation of secondary forests and permanent forest reserves, which potentially lead to spill over of macaque population to human settlements. Natural infection of simian malaria in human tends to occur when human encroached into normal mosquito-monkey circulations in the forested area .
A decline in knowlesi malaria incidence was observed in 2015 and 2016, whereby the number of knowlesi malaria reported in Peninsular Malaysia dropped below 150 cases annually. This trend might be a transient phenomenon and might not possibly due to systematic or sustained factors. Possible explanations to this change include transient reduction in case detection activities, reduction in vector density, reduction in macaque population, or reduction in human-mosquito contacts. Malaria elimination efforts were intensified to reduce population of vectors responsible for human malaria such as P. malariae, P. vivax, and P. falciparum which these could have temporarily reduced P. knowlesi vector population as well. These measures include distribution of ITNs, larvaciding, residual spraying, and repellents to the high-risk groups in order to reduce malaria vector population and human-mosquito contacts. Otherwise, the El Nino phenomenon which led to anomalous rainfall pattern and strong drought in Southeast Asia in 2015, accompanied with severe haze episodes could have contributed to changes in vector density and reduced malaria transmission [12, 36, 37].
A significant rebound of knowlesi malaria transmission was observed after 2016. This may be due to drastic changes in the transmission dynamics of the parasite between human, macaques and vectors caused by agricultural expansion and forest exploration. According to the Forestry Department of Peninsular Malaysia statistical report, approximately one million hectares of total forested areas have been loss from 2016 to 2018 and permanent reserved forest area recorded decrease from 4.92 million hectares (in 2016) to 4.80 million hectares (in 2018) . Peninsular Malaysia has experienced large-scale deforestation due to intensified agricultural activities such oil palm and rubber, timber production and rapid urban expansion since 1970s . Establishment of crop plantation increases the vectors’ natural breeding site and increases human exposure to vectors’ breeding sites . Unlike in human malaria transmission, susceptible knowlesi malaria individuals are usually exposed to agricultural settings or forests instead of at home or its surrounding areas due to the exophagic nature of the vectors . In Sabah, higher P. knowlesi transmission was observed in large intact forest patches within five km of households, pulpwood plantations within three km of households, and oil palm plantations with fragmented landscapes . Malaria elimination strategies such as ITN and residual spraying applied within the vicinity of houses have proven to be effective in interrupting human malaria transmission but less protective against simian malaria vectors which feed predominantly in the forest. However, personal insecticide usage has proven to reduce risk of exposure to P. knowlesi . Although there are efforts of distributing repellents for agricultural workers, it is very dependent on the individuals’ active participation in using these repellents routinely at work.
As both Peninsular Malaysia and Malaysian Borneo share similar geographical and cultural characteristics to a certain extent, there are other comparative points from previous studies which can reflect on the transmission of P. knowlesi infections in Peninsular Malaysia. It is noted that both Peninsular Malaysia and Malaysian Borneo experienced an increase in P. knowlesi despite a dramatic decline in other human malaria parasite infections . In Sabah, higher rainfall was associated with increase in knowlesi malaria cases after three months [12, 41]. Increase in rainfall causes formation of water pockets which are ideal for mosquito breeding. Nevertheless, a combination of environmental factors including temperature, rainfall, humidity, and land use could serve as predictors for disease transmission. In addition, community-level surveillance using serological markers demonstrated that P. knowlesi exposure was positively associated with age, male sex, forest activity, and contact with macaques but negatively associated with personal insecticide practice and higher altitude [13, 42]. Further studies should be conducted to understand the environmental and exposure risks based on these factors particularly in Peninsular Malaysia.
In this study, some districts in Perak, Kelantan, Pahang, and Terengganu states were found to demonstrate knowlesi malaria spatial clustering as hotspots. These findings can guide malaria control programme in strategizing effective malaria intervention which put more focus on these districts. Moreover, the monitoring of districts clustered as hotspots is crucial because disease transmission may spill over to their neighbours due to cross-district migration of macaque populations with potential carriage of malaria parasites or long-distance spread of infective vectors. Studies in Africa suggested that some species of malaria vectors are capable of flying over hundreds of kilometers [43, 44]. Thus far, the migratory behaviours of malaria vector species in Malaysia are not fully understood and this has certainly opened a knowledge gap for further studies to determine connectivity between bionomics of vectors and malaria incidence.
This is the first study which utilized spatial data analysis to generate spatial-temporal distribution of knowlesi malaria at district scale covering the entire Peninsular Malaysia. Nonetheless, there were several limitations to this study. Firstly, the study was descriptive in nature, with no control group for comparison purposes. Therefore, we were not able to establish risk factors such as sociodemographic factors Secondly, we did not include environmental parameters in the analysis which are important to better describe the transmission dynamics of P. knowlesi in the study area. Further study could be conducted by taking into consideration how environmental variations could impact the transmission of knowlesi malaria. Climatic factors such as rainfall, relative humidity, temperature, water bodies and normalized difference vegetation index can be used to spatially assess malaria risk factors. While Malaysia is moving forward to end transmission of human malaria, the emergence of P. knowlesi could became the next challenge to the malaria programme. Advancement in our knowledge about the ecology of P. knowlesi could help policy maker to develop control strategies which are effectively in Peninsular Malaysia.