Seasonal and spatial dynamics of the primary vector of plasmodium knowlesi within a major transmission focus in Sabah, Malaysia

Background The simian malaria parasite Plasmodium knowlesi is emerging as a public health problem in Southeast Asia, particularly in Malaysian Borneo where it now accounts for the greatest burden of malaria cases and deaths. Control is hindered by limited understanding of the ecology of potential vector species. Methodology/Principal Findings We conducted a one year longitudinal study of P. knowlesi vectors in three sites within an endemic area of Sabah, Malaysia. All mosquitoes were captured using human landing catch. Anopheles mosquitoes were dissected to determine, oocyst, sporozoites and parous rate. Anopheles balabacensis is confirmed as the primary vector of. P. knowlesi (using nested PCR) in Sabah for the first time. Vector densities were significantly higher and more seasonally variable in the village than forest or small scale farming site. However An. balabacensis survival and P. knowlesi infection rates were highest in forest and small scale farm sites. Anopheles balabacensis mostly bites humans outdoors in the early evening between 1800 to 2000hrs. Conclusions/Significance This study indicates transmission is unlikely to be prevented by bednets. This combined with its high vectorial capacity poses a threat to malaria elimination programmes within the region. Author Summary The first natural infection of Plasmodium knowlesi was reported 40 years ago. At that time it was perceived that the infection would not affect humans. However, now P. knowlesi is the predominant malaria species (38% of the cases) infecting people in Malaysia and is a notable obstacle to malaria elimination in the country. Plasmodium knowlesi has also been reported from all countries in Southeast Asia with the exception of Lao PDR and Timor Leste. In Sabah, Malaysian Borneo cases of human P. knowlesi are increasing. Thus, a comprehensive understanding of the bionomics of the vectors is required so as to enable proper control strategies. Here, we conducted a longitudinal study in Kudat district, Sabah, to determine and characterize the vectors of P. knowlesi within this transmission foci. Anopheles balabacensis was the predominant mosquito in all study sites and is confirmed as vector for P. knowlesi and other simian malaria parasites. The peak biting time was in the early part of the evening between1800 to 2000. Thus, breaking the chain of transmission is an extremely challenging task for the malaria elimination programme

Generalised linear mixed model fitting of the data.

<p>The model used is of the form “glmm<-glmmadmb (parameter ~ locality+(1|month), zero Inflation = T, data = data,family = "pdf")”. KP = Kampung Paradason, LL = Limbuak Laut, TD = Timbang Dayang. AIC <i>=</i> Akaike information criterion. Means with different superscript letters indicate they are significantly different.</p

Parous rates (with 95% confidence interval) in three study sites.

<p>Alphabet denotes study sites: ^aLimbuak Laut; ^bTimbang Dayang; ^cKg Paradason.</p

Bites/Man/Night of <i>An</i>. <i>balabacensis</i> measured each month in three study sites over time.

<p>a. Limbuak Laut, b. Timbang Dyang, c. Kg Paradason. Error bars are 95%CI.</p

Species of <i>Plasmodium</i> identified from mid gut and salivary glands of the <i>An</i>. <i>balabacnesis</i>.

<p>Key: Pk = <i>P</i>. <i>knowlesi</i>; Pcy = <i>P</i>. <i>cynomolgi</i>; Pct = <i>P</i>. <i>coatneyi</i>; Pin = <i>P</i>. <i>inui</i>; 14 <i>An</i>. <i>balabacensis</i> has salivary glands positive; 18 <i>An</i>. <i>balabacensis</i> had midgut positive and 13 had both midgut and salivary gland positive. Thus total 58.</p><p>Species of <i>Plasmodium</i> identified from mid gut and salivary glands of the <i>An</i>. <i>balabacnesis</i>.</p

Annual infection rates, man-biting rate, entomological inoculation rate, parous rate, probability of daily survival, life expectancy and vectorial capacity of <i>An</i>. <i>balabacensis</i> in study sites.

<p>¹The probability of daily survival (p) was taken as ³√P (P = percentage parous) (25).</p><p>²life expenctancy = p10/-log_e p (days) (26).</p><p>³Vectorial capacity (VC) = ma²pⁿ/-log^ep (27).</p><p>p¹⁰ Percentage of population expected to live long enough to become infective with an extrinsic cycle of 10 days based on <i>P</i>. <i>knowlesi</i> extrinsic incubation period</p><p>Annual infection rates, man-biting rate, entomological inoculation rate, parous rate, probability of daily survival, life expectancy and vectorial capacity of <i>An</i>. <i>balabacensis</i> in study sites.</p

Map showing study sites in Kudat and Banggi Island.

<p>Map showing study sites in Kudat and Banggi Island.</p