**6.** *Plasmodium knowlesi***: an additional challenge to malaria elimination**

*Plasmodium knowlesi*, a simian malaria parasite, is now considered the 5th parasite affecting humans [112]. All countries in Southeast Asia have reported cases of *P. knowlesi* with the exception of Lao PDR and Timor Leste [113]. Since most countries are now working towards malaria elimination, it is pertinent to pay serious attention to malaria cases especially in areas where malaria has been reduced to very low levels. A good example is Sabah, Malaysian Borneo where large numbers of *P. knowlesi* were diagnosed in areas where *P. falciparum* and *P. vivax* were occurring in very low numbers [114]. Malaysia is working towards malaria elimination by 2020 and currently more than 60% of the malaria cases are due to *P. knowlesi* (MOH personal communication).

Recently, an increasing number of cases of *P. knowlesi* were reported from Kalimantan and Ache in Indonesia [115, 116] where malaria was in process of being eliminated. In Northern Sumatra, Indonesia where they are working towards malaria elimination, they recorded only 614 (16.5%) positive malaria cases by microscopy out of 3731 people examined [117]. However, PCR detected malaria parasites in 1169 (31.3%) individuals. Of these, 74.9% were mono-infection and 25.1% were multiple infection. *P. falciparum* constituted 24.8%, *P. vivax* 33.9%, *P. malariae* 9.3%, and *P. knowlesi* 32% [114] of the cases. It was also found that the primers developed from the SICAvar gene were more sensitive than the SSU rRNA gene [117]. It is obvious that parasite species are being mis-identified and many people who are asymptomatic are also missed by conventional microscopy [117, 118]. Thus, it is important to develop Rapid Diagnostic Tests (RDTs) that can be used by field workers to detect accurately malaria parasite species, especially *P. knowlesi,* and also additional laboratories should be established to conduct molecular assays for malaria diagnosis in the context of malaria elimination.

Deforestation and changes in the environment are the key factors leading to a surge of *P. knowlesi* malaria [119]. This parasite occurs in *Macaca fascicularis* (long-tailed) and *Macaca nemestrina* (pigtailed) monkeys and its distribution is limited by some species of the Leucosphyrus Group of *Anopheles* mosquitoes [120]. These species are found biting in greater abundance in forest and farms compared to villages [121, 122]. However, in Sabah, Malaysian Borneo, it was found that *An*. *balabacensis* was abundant in villages as well [123], and sporozoite-positive specimens were reported in addition to farms and forest [123], while infective mosquitoes were found only in the forested sites and farms in Sarawak (Borneo) and Pahang (Peninsular), Malaysia [121, 122]. In addition, vector studies have also been conducted in Vietnam [124, 125] where the species *An. dirus* has been incriminated as the simian malaria vector in Khanh Phu—South Central Vietnam. Studies were conducted in the forest and forest-fringe areas near Nga Hai village where both human malaria parasites, *P. falciparum* and *P. vivax*, were found along with *P. knowlesi* in order to determine the potential role of *An. dirus* as bridge vectors of *Plasmodium* parasites from monkeys to humans [126]. Based on these studies, it was possible for *An. dirus* to pick up infection from humans and macaques during the mosquito's lifespan. However, since there have been no reports of epidemics of *P. knowlesi,* it is believed that humans are infected by mosquitoes which acquired infection from the macaques. Perhaps even likely given that confirmed vectors of human plasmodia in Southeast Asia also become naturally infected by the monkey malaria species [127]. A recent case control study conducted in Sabah revealed that the age group >15, predominantly males, working in farms, plantations, forested areas, and with travel history, were independently associated with the risk of acquiring knowlesi malaria [128]. It also highlighted that IRS was associated with decrease of risk [128].

from immature rubber plantations, nine from mature rubber plantations, five from secondary forests and one from the rural village [105] (Tangena Julie-Ann, personal communication).

*Plasmodium knowlesi*, a simian malaria parasite, is now considered the 5th parasite affecting humans [112]. All countries in Southeast Asia have reported cases of *P. knowlesi* with the exception of Lao PDR and Timor Leste [113]. Since most countries are now working towards malaria elimination, it is pertinent to pay serious attention to malaria cases especially in areas where malaria has been reduced to very low levels. A good example is Sabah, Malaysian Borneo where large numbers of *P. knowlesi* were diagnosed in areas where *P. falciparum* and *P. vivax* were occurring in very low numbers [114]. Malaysia is working towards malaria elimination by 2020 and currently more than 60% of the malaria cases are due to *P. knowlesi* (MOH personal communication). Recently, an increasing number of cases of *P. knowlesi* were reported from Kalimantan and Ache in Indonesia [115, 116] where malaria was in process of being eliminated. In Northern Sumatra, Indonesia where they are working towards malaria elimination, they recorded only 614 (16.5%) positive malaria cases by microscopy out of 3731 people examined [117]. However, PCR detected malaria parasites in 1169 (31.3%) individuals. Of these, 74.9% were mono-infection and 25.1% were multiple infection. *P. falciparum* constituted 24.8%, *P. vivax* 33.9%, *P. malariae* 9.3%, and *P. knowlesi* 32% [114] of the cases. It was also found that the primers developed from the SICAvar gene were more sensitive than the SSU rRNA gene [117]. It is obvious that parasite species are being mis-identified and many people who are asymptomatic are also missed by conventional microscopy [117, 118]. Thus, it is important to develop Rapid Diagnostic Tests (RDTs) that can be used by field workers to detect accurately malaria parasite species, especially *P. knowlesi,* and also additional laboratories should be established to conduct molecular assays for malaria diagnosis in the context of malaria elimination.

Deforestation and changes in the environment are the key factors leading to a surge of *P. knowlesi* malaria [119]. This parasite occurs in *Macaca fascicularis* (long-tailed) and *Macaca nemestrina* (pigtailed) monkeys and its distribution is limited by some species of the Leucosphyrus Group of *Anopheles* mosquitoes [120]. These species are found biting in greater abundance in forest and farms compared to villages [121, 122]. However, in Sabah, Malaysian Borneo, it was found that *An*. *balabacensis* was abundant in villages as well [123], and sporozoite-positive specimens were reported in addition to farms and forest [123], while infective mosquitoes were found only in the forested sites and farms in Sarawak (Borneo) and Pahang (Peninsular), Malaysia [121, 122]. In addition, vector studies have also been conducted in Vietnam [124, 125] where the species *An. dirus* has been incriminated as the simian malaria vector in Khanh Phu—South Central Vietnam. Studies were conducted in the forest and forest-fringe areas near Nga Hai village where both human malaria parasites, *P. falciparum* and *P. vivax*, were found along with *P. knowlesi* in order to determine the potential role of *An. dirus* as bridge vectors of *Plasmodium* parasites from monkeys to humans [126]. Based on these studies, it was possible for *An. dirus* to pick up infection from humans and macaques during the mosquito's lifespan. However, since there have been no reports of epidemics of *P. knowlesi,* it is believed that humans are infected by mosquitoes

**6.** *Plasmodium knowlesi***: an additional challenge to malaria** 

**elimination**

106 Towards Malaria Elimination - A Leap Forward

There are only few investigations on record in understanding bionomics of vectors transmitting *P. knowlesi* malaria. In order to implement vector control activities, the bionomics of the vectors must be understood. Based on few studies, it has been shown that the vectors are biting in the early part of the night from 18:00 h to 21:00 h and mostly outdoors [121–123, 129]. In these rural areas, people go to bed by 22:00 h and they are up by 05:00 h. The results showed that only 39.79% of *An. balabacensis* [123], 43.8% of *An. latens* [121] and 12.8% of *An. cracens* [122] were found biting during this sleeping time. Thus, current vector control measures like IRS and ITNs are not appropriate for the exophagic and exophilic vectors. The forests in Southeast Asia is providing a favorable environment with high percentage of macaques being positive for *P. knowlesi* [130–132], and with the presence of the vectors, it is going to be a daunting task to eliminate malaria. On a global scale, malaria has been reduced to low levels due to the scaling up of ITNs, IRS, ACTs, and intermittent preventive treatment to infants and pregnant women [133]. Thus, it is obvious that new tools are urgently required for successful malaria elimination.

It is known that the two human malaria species (*P. falciparum* and *P. vivax*), which infects millions of people actually were of zoonotic origin (from the African apes), which evolved thousands of years ago [134, 135]. Thus, there is always a possibility that in the future *P. knowlesi* and other simian malarias may become established in humans, especially when human malaria is eliminated. However, currently human-to-human transmission of knowlesi malaria by mosquitoes has not been established. This is crucial in the light of malaria elimination and more focused research is needed on this topic if we are to succeed with malaria elimination.

Changing landscape affects *Anopheles* distribution, mosquito density and diversity in Malaysia, and more globally Southeast Asia [105, 111, 136–138]. It has been shown that with loss of forest cover, cases of *P. knowlesi* have increased in Sabah [119]. Land use change has also led to increase of malaria cases due to various factors such as increase of macaques in small forest patches along with the colonization of the main vectors [119, 136]. It is interesting to note that *An. balabacensis,* the predominant vector of human and simian malaria, was found in great abundance in logged forest, followed by thinly logged virgin jungle reserve and was lowest in primary forest [136]. This vector was also found to be biting humans more at ground level compared to canopy level [136]. It is therefore important to include both the public health and agro-forestry sectors in controlling malaria vectors in the country. Studies from Thailand also indicate that if landscape management should be used for malaria control in northern Thailand, large-scale reduction and fragmentation of forest cover would be needed [139, 140]. Such drastic actions, however, do not align with current global objectives concerning forest and biodiversity conservation.

The vectors of simian malaria described to date were *An. hackeri* (Leucosphyrus Group) [141] recorded biting mainly the macaques and large numbers were collected resting on Nipah palm trees in Selangor in 1960s; *An. cracens* (Dirus Complex) [122] biting both macaques and humans and found mainly in the forest and farms; *An. latens* (Leucosphyrus Complex) [121] was the predominant mosquito in the forest compared to farm and village, and was biting macaques at ground level and at six meters in the canopy compared to three meters. The biting ratio of monkey *versus* human for *An*. *latens* was 1:1.3 [121]. *An. introlatus* (Leucosphyrus Complex) [142] was biting in the early part of the night from 19:00 h to 21:00 h and was the predominant mosquito in Hulu Selangor where cases of *P. knowlesi* were reported. Most recently, *An. balabacensis* (Leucosphyrus Complex) has been incriminated as vector of *P. knowlesi* in Sabah [123], as well as human malaria and Bancroftian lymphatic filariasis due to *Wuchereria bancrofti* [143–145].

vivax malaria could reduce the disease burden more than that of *P. falciparum*, because avoiding one infection will result in preventing a number of clinical episodes over several years [155].

Human and Simian Malaria in the Greater Mekong Subregion and Challenges for Elimination

http://dx.doi.org/10.5772/intechopen.76901

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Vivax malaria is diagnosed late, because infected people get ill with low parasite densities, which cannot be detected with current diagnostics, such as RDTs and microscopy. Delayed diagnosis means not only delayed treatment (hence prolonged morbidity, especially anemia) but also ability to transmit over an extended period. This is further amplified by the fact that mature gametocytes appear simultaneously with asexual forms—hence transmission occurs

As recently described [156], an effective *P. vivax* control and elimination toolbox should

**i.** Practical point-of-care G6PD deficiency diagnostics allowing wider access to safe primaquine therapy or with tafenoquine—a related single dose hypnozoitocide recently developed by GSK and Medicines for Malaria Venture (MMV); the latter has been submitted to the United States Food and Drug Administration (FDA) seeking approval of singledose tafenoquine for the radical cure (prevention of relapse) of vivax malaria in patients

**ii.** More sensitive point-of-care diagnostics for detecting intrinsically lower parasitemia, in-

**iii.** Validated strategies for relapse prevention in special population groups, i.e., pregnant women, young infants, G6PD deficient and G6PD unknowns in which 8-aminoquinoline

**iv.** Clinical care algorithms acknowledging risk of severe and threatening syndromes de-

**v.** Interventions of proven efficacy to minimize human contact with often zoophilic, ex-

In conclusion, the malaria community needs to address these challenges and create a viable strategy to achieve vivax elimination goals, providing novel solutions for overcoming critical bottlenecks. This process needs to begin now to enhance treatment practice for 8-aminoquinoline drugs based radical cure. Highlighting the benefits of radical cure for the patient and community will improve prescription practice and patient adherence [160]. Coupling this with improved access to adequate G6PD testing will pave the way for the intro-

duction of tafenoquine, with huge potential to accelerate the elimination of *P. vivax*.

**8. Socio-ecological and adaptive management of malaria ecosystem** 

WHO has recently proposed sustainable prevention and control of diseases emerging within complex, dynamic, adaptive systems, such as malaria, based on interdisciplinary and approaches addressing environmental and social health determinants holistically [161]. More

before diagnosis and treatment [157, 158].

16 years of age and older [159];

is contraindicated;

cluding sub-patent and asymptomatic infections;

spite seemingly non-threatening levels of parasitemia; and

ophagic and exophilic *Anopheles* species of great diversity.

**in areas approaching malaria elimination**

include:

Although an increased number of countries are successfully eliminating human malaria in recent years, no country has yet eliminated non-human malaria, which adds another layer of complexity to be addressed. The complex situation of malaria is Southeast Asia is very unique from the rest of the tropical countries. More effort is needed to study the host switching mechanisms between the parasites in humans, macaques and vectors. A series of review papers have been published over the years and all these have indicated the importance of addressing the problem caused by *P. knowlesi,* if malaria elimination is to be successful in the region [113, 146–151].
