Efficient Surveillance ofPlasmodium knowlesiGenetic Subpopulations, Malaysian Borneo, 2000–2018

Abstract

T he monkey parasite Plasmodium knowlesi was discovered to be a common cause of malaria in humans in 2004, initially from investigations in the Kapit division of Sarawak state, Malaysian Borneo (1). Humans acquire infection primarily from wild long-tailed (Macaca fascicularis) and pig-tailed (M. nemestrina) macaque reservoirs (2); Anopheles mosquitoes of the leucosphyrus group are vectors (3,4). P. knowlesi malaria has been described across Southeast Asia, but most clinical cases are still reported in Malaysian Borneo (3,5-8). In 2017 and 2018, a total of 7,745 cases were reported in Malaysia, 86.8% of which were detected in Malaysian Borneo (B. Singh, unpub. data) (9). P. knowlesi infections can be asymptomatic (10,11), and clinical cases exhibit a wide spectrum of disease ranging from mild symptoms to death (3). Population genetic surveys of P. knowlesi infections in humans across Malaysia have revealed 2 divergent subpopulations of the parasite in Malaysian Borneo that are associated with the 2 macaque species locally, suggesting 2 independent zoonoses (12,13). The cluster 1 type has been associated with longtailed macaques and the cluster 2 type with pig-tailed macaques (12). The existence of 2 sympatric subpopulations also has been confirmed by whole-genome sequencing (WGS) of P. knowlesi from patients in Malaysian Borneo (13,14). In peninsular Malaysia on the Asia mainland, all cases have been caused by another subpopulation, cluster 3, that has not been detected in Malaysian Borneo (13,15). Limited WGS (14,15) and microsatellite (13) genotyping of P. knowlesi isolates derived from human and only long-tailed macaque hosts from peninsular Malaysia showed allopatric divergence for this subpopulation cluster from those of Malaysian Borneo because of geographic separation by the South China Sea. Increasing numbers of P. knowlesi malaria cases detected might be due to increased zoonotic exposure along with a reduction of endemic malaria parasite species (16). With the recent identification of different zoonotic P. knowlesi genetic subpopulations, determining whether these populations vary in frequency over space and time is important. Interactions with

Proposed Integrated Control of Zoonotic Plasmodium knowlesi in Southeast Asia Using Themes of One Health

“…It remains uncertain whether human infection with P. knowlesi has resulted from a recent host-switch event from macaques to humans, resulting in human-vector-human transmission or is currently co-existing and circulating among human and macaque populations [ 41 ]. It is speculated that both transmission dynamics are occurring simultaneously, but for now, it appears macaque-vector-human is the primary route of infection transmission [ 42 , 43 ].…”

“…This is potentially due to the ocean barrier between these locations, which has restricted the migration of the macaque hosts [ 47 ]. The P. knowlesi cluster associated with Mf dominates human infection cases in East Malaysia, exhibiting an intense selection pressure, while the cluster related to Mn is common in Sarawak [ 43 , 46 ].…”

“…The transmission dynamics of P. knowlesi at the moment appears to be primarily macaque-vector-human. Although, human-vector-human transmission is also likely occurring in communities as well, due to the movement of outdoor workers to urban areas [ 20 , 42 , 43 , 66 ]. Thus, a transdisciplinary approach through targeting vectors contact with humans must be actioned, deviating from current conventional malaria control methods.…”

“…Additionally, to support and focus on implementing control interventions, further investigations into the bionomics and blood indices of incriminated mosquito vectors are also warranted to determine the distribution and relative risk of knowlesi -malaria infection in human populations. Furthermore, monitoring the genetic subpopulations of P. knowlesi could provide an understanding of transmission dynamics occurring in urban communities and potentially ascertain whether there is an association between specific genetic subpopulations of knowelsi -malaria and clinical outcomes [ 43 , 77 ].…”

Clinical recovery of Macaca fascicularis infected with Plasmodium knowlesi

“…Plasmodium knowlesi malaria cases have been identified in 10 of the 11 countries that comprise Southeast Asia; the one exception being Timor-Leste [reviewed in 1, 2, 3]. Since its recognition in 2004 in Malaysia as an emergent zoonotic parasite species [4], P. knowlesi has in fact risen as the major cause of human clinical cases of malaria in Malaysia and threatens to upend malaria elimination in this country [1, 2, 4–8]; over 4,000 Malaysian cases of P. knowlesi malaria were reported in 2018 [9]. Plasmodium knowlesi transmission to humans has been attributed to spillover events from peri-domestic infected macaques in village settings, or from infected wild macaques in the setting of jungle trekking, foraging, farming, and logging activities [1, 5, 10–13].…”