Entomologic investigation of Plasmodium knowlesi vectors in Kuala Lipis, Pahang, Malaysia

Abstract

BackgroundThe first natural infection of Plasmodium knowlesi in humans was recorded in 1965 in peninsular Malaysia. Extensive research was then conducted and it was postulated that it was a rare incident and that simian malaria will not be easily transmitted to humans. However, at the turn of the 21st century, knowlesi malaria was prevalent throughout Southeast Asia and is life threatening. Thus, a longitudinal study was initiated to determine the vectors, their seasonal variation and preference to humans and macaques.MethodsMonthly mosquito collections were carried out in Kuala Lipis, Pahang, peninsular Malaysia, using human-landing collection and monkey-baited traps at ground and canopy levels. All mosquitoes were identified and all anopheline mosquitoes were dissected and the gut and gland examined for oocysts and sporozoites. Nested polymerase chain reaction (PCR) was conducted on positive samples, followed by sequencing of the csp gene.Results and discussionAnopheles cracens was the predominant mosquito biting humans as well as the macaques. It comprised 63.2% of the total collection and was the only species positive for sporozoites of P. knowlesi. It was exophagic and did not enter houses. Besides An. cracens, Anopheles kochi was also found in the monkey-bait trap. Both species preferred to bite monkeys at ground level compared to canopy.ConclusionAnopheles cracens, which belongs to the Dirus complex, Leucosphyrus subgroup, Leucosphyrus group of mosquitoes, has been confirmed to be the only vector for this site from Pahang during this study. It was the predominant mosquito at the study sites and with deforestation humans and villages are entering deeper in the forests, and nearer to the mosquitoes and macacques. The close association of humans with macaques and mosquitoes has led to zoonotic transmission of malaria.

Seasonal and Spatial Dynamics of the Primary Vector of Plasmodium knowlesi within a Major Transmission Focus in Sabah, Malaysia

“…Ideally parallel collections of mosquitoes attracted towards macaques would have been conducted but this was not possible due to logistical constraints and ethics regulations for working with macaques. Previous work [22, 23] showed that the P . knowlesi vectors in other areas namely An .…”

“…knowlesi make it particularly difficult to control by conventional methods: (1) it has a sizeable zoonotic reservoir in macaques, which means that even if infections are eliminated from humans there remains a risk of future spillover, and (2) current evidence indicates that previously incriminated mosquito vectors of P . knowlesi in Malaysia bite and rest outdoors where control methods such as LLINs and IRS will not be effective [19, 22, 23]. …”

“…latens in Kapit, Sarawak [22, 25], An . cracens in Kuala Lipis [14, 23] and An . introlatus in Hulu Selangor [19].…”

Human Infections and Detection of Plasmodium knowlesi

“…This mosquito species feeds mainly between 7 and 10 p.m. in the forest, is attracted to both long-tailed macaques and humans, and prefers to feed on macaques at higher elevations. A. cracens has recently been incriminated as a vector for knowlesi malaria in Pahang, Peninsular Malaysia, with peak biting times between 8 and 9 p.m. (44,84). This species is highly zoophilic and was previously found to feed on macaques at the canopy level and on humans at the ground level.…”

Plasmodium knowlesi malaria: Overview Focussing on Travel-Associated Infections

“…In Malaysian Borneo, the area with the most intense P. knowlesi transmission, several Anopheles species competent for monkey-tohuman transmission have been identified which predominantly feed between 7 and 10 p.m. [19][20][21]. Interestingly, P. knowlesi alone but also together with P. vivax and/or P. falciparum has been isolated from saliva glands of Anopheles dirus in southern Vietnam [22].…”