Background
Malaria parasites in Peninsular Malaysian monkeys were first reported in 1908 [
1], but only gained prominence in the 1960's after the accidental discovery [
2] that
Plasmodium cynomolgi could be transmitted to humans via mosquito bites in the laboratory. This stimulated interest at a time when the Malaria Eradication Programme was initiated by the World Health Organization [
3] and it was important to determine if malaria was a zoonosis. Therefore, extensive studies were carried out in Peninsular Malaysia to determine the distribution, prevalence and species of malaria parasites in monkeys and apes and the natural vectors of monkey malaria parasites [
4‐
7]. Instead of uncovering human cynomolgi malaria infections,
Plasmodium knowlesi was the first simian malaria parasite found to be infecting humans in nature. The first case was reported in 1965 from the state of Pahang [
8], Peninsular Malaysia, followed by a second case five years later acquired from Johore, Peninsular Malaysia [
9]. It was postulated that
P. knowlesi could be transmitted from monkeys to man and laboratory studies proved that it was possible [
10]. However, a large scale study that was initiated in Pahang to investigate whether malaria was a zoonosis, by a group of American and local researchers based at the Institute for Medical Research in Kuala Lumpur, Peninsular Malaysia, concluded that simian malaria in humans was an extremely rare event [
11,
12]. This was based on their studies in which they collected blood samples from more than 1,100 local residents, pooled the samples and injected them into rhesus monkeys and none of the monkeys contracted malaria. However, in 2004 a large focus of human
P. knowlesi infection was reported in the Kapit Division of Sarawak [
13]. In that study 71.6% (101/141) of human malaria cases at Kapit Hospital which had been identified by microscopy as single
Plasmodium malariae infections were actually
P. knowlesi and other non-
P. malariae species by nested polymerase chain reaction (PCR) assays.
Plasmodium knowlesi is naturally found in long-tailed macaques (
Macaca fascicularis), pig-tailed macaques (
Macaca nemestrina) [
14] and banded leaf monkeys (
Presbytis malalophos) [
15,
16]. Since transmission of this zoonotic parasite to humans is occurring in the Kapit Division of Sarawak, Malaysian Borneo, it is important to identify the vectors so that appropriate measures can be planned and initiated to control the spread of simian malaria in humans.
Numerous studies on vectors of human malaria have been carried out in Sarawak [
17‐
20] but none of these have been undertaken in the Kapit District of Sarawak. Several anopheline species incriminated or suspected to transmit monkey malaria parasites in Peninsular Malaysia, such as
Anopheles latens,
Anopheles balabacensis are also present in Sarawak [
21]. Hence, the vectorial status of the anopheline species present in the Kapit Division needs to be ascertained in order to determine if they are competent vectors of simian malaria parasites. Furthermore, the epidemiological data of
P. knowlesi infection in humans revealed that infections occur primarily in adults and no clustering of cases occurred within communities that live in communal longhouses, which suggests transmission of
P. knowlesi to humans occurred away from the vicinity of the longhouses [
13].
Thus the objectives of this study were to determine the vectors of
P. knowlesi and other simian malaria parasites in the Kapit Division of Sarawak; and to study the dynamics of these vectors in different ecological sites in order to elucidate the most likely place where transmission was taking place. Preliminary results of this study were reported where
An. latens was incriminated as the vector for
P. knowlesi [
22]. Here detailed results of this eleven-month study on vectors of malaria and their bionomics in the Kapit district of Sarawak, Malaysian Borneo is presented.
Discussion
In the 1960's, it was suggested that mosquitoes of the
Anopheles leucosphyrus group, may provide a link between humans and monkeys and that if simian malaria is transmitted to man in nature, it is likely to be in areas where these mosquitoes are common [
31]. This has been confirmed by the current study where
An. latens has been incriminated as the vector of
P. knowlesi in nature in the Kapit division [
22] where a large focus of naturally acquired
P. knowlesi infection in humans was reported [
13]. Aside from being the most predominant anopheline mosquitoes caught biting humans in Kapit,
An. latens was observed to be attracted to both human and monkey hosts. In the monkey-baited net traps, it was also observed that a significant number of
An. latens were collected at the higher levels compared to those caught at ground level showing an acrodendrophilic behaviour. This is important as the natural monkey host is arboreal in nature. In order for this malaria parasite to be maintained in nature and for transmission to man to occur, the vector needs to be highly simio-anthropophagic in nature. If the vector for
P. knowlesi is highly simiophagic, as in the case of
Anopheles hackeri in Peninsular Malaysia [
4], transmission in the natural hosts may be maintained, but transmission to humans will be rare [
32].
The fact that An. latens is attracted to monkeys in the canopy and humans on the ground indicates that both humans and monkeys could be exposed to infection from each other. Data from this study area indicate that the zoonotic parasite, P. knowlesi, is being transmitted to both humans and macaques by An. latens. Thus, in Kapit, humans get the infection when their forest associated activities, such as farming, logging, or hunting exposes them to An. latens. Throughout the course of this study infected mosquitoes were not obtained from the longhouse and the number of anophelines caught there were also small.
Like all members of the Leucosphyrus group,
An. latens is mainly a forest breeding mosquito associated with dense jungle and forest fringes [
21,
33]. In the current study, nearly 90% of the total
An. latens were collected at either the farm (40%) or the forest (50%) and were consistent with earlier studies conducted in other areas of Sarawak. In addition, only 126
An. latens (10%) were collected in the longhouse, and of these 71% were collected outdoors. Entomological studies conducted in other areas in Sarawak [
18,
19] showed that
An. latens were commonly found in farming zones that were located at the forest fringe rather than in villages, and the density of
An. latens decreases in relation to distance away from the jungle.
It is known that multiple factors determine the prevalence of malaria and one of these is the intensity of malaria transmission which is defined as the rate at which people get inoculated with malaria parasites from mosquitoes [
34]. In the current study, only
An. latens were found to be infective, and the average number of infectious bites by these mosquitoes in both the farm and the forest during the entire duration of the study was 11.98 and 14.1 per year respectively, which is higher compared to the rates in many Asian countries [
34]. When considering the risk of acquiring
P. knowlesi in Kapit, the intensity of transmission has been shown to be comparable for both ecotypes, hence the risk of acquiring
P. knowlesi in either the forest or the farm is the same. In Sarawak, the forests and their surrounding areas are recognized focal points for malaria transmission whenever
An. latens is present in large numbers [
20].
Although
An. latens has been incriminated as the vector of
P. knowlesi in Kapit, the role of other anopheline species as a possible vector for
P. knowlesi in Kapit needs further assessment. This is especially true for
An. pujutensis,
An. introlatus, and
An. macarthuri, all of which were caught in small numbers and were not dissected. They have been reported to be simiophagic and have either been suspected or been incriminated as vectors of simian malaria parasites in other localities in Peninsular Malaysia [
32,
35,
36]. To determine if they are vectors of
P. knowlesi, intensive entomological surveys have to be carried out in many parts of the forest and in other parts of the Kapit Division. With the advancement of molecular techniques it is possible to identify the sporozoites to species level and thus vectorial status can be determined.
Anopheles watsonii, a forest species, was caught in large numbers, but only in the forest, where it was found to be attracted to both human and monkey hosts. Previously, a small numbers of this species were also obtained in hill forest in Peninsular Malaysia at night with human and monkey- baited traps [
35]. They obtained more mosquitoes in the canopy than at ground level, but all were negative by dissection. Thus earlier workers did not consider
An. watsonii as a vector of human malaria. In the current study, oocysts were found by dissection and the numbers in each gut ranged from 7–90. However, none were positive for sporozoites by dissection. Since they are attracted to humans and non human primates, further studies should be conducted on this species before any definitive conclusions can be inferred about their vectorial status.
With large tracts of forest being cut down, non-human primates are coming close to the human environment. Human cases of
P. knowlesi have been reported not only in Sarawak[
13], but also in Thailand [
37], Manymar [
38] and in Peninsular Malaysia [
8,
9,
39]. Thus control strategies for malaria in Southeast Asia may have to change to include this zoonotic aspect of the transmission.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
IV and BS conceived the study. IV, CHT, BS were responsible for preparation of manuscript. IV, CHT, STC and AM were responsible for field collection, supervision, identification and processing of mosquitoes. BS obtained permits to collect and undertake research on biological specimens from Sarawak Biodiversity Council and permits to trap and keep monkeys in captivity from Sarawak Forestry Department. CHT conducted molecular work. All authors have read and approved the manuscript.