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

Abstract

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 2000 hrs.

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.

Fig 1. Map showing study sites in Kudat and Banggi Island.

Fig 2. Bites/Man/Night of An. balabacensis measured each month in three study sites over time.

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

Fig 3. Percentage of An. balabacensis (out of the total collected) that were captured biting at each hour of the nightly sampling period (18:00–6:00hrs)in each study sites (pooled across all months of collection).

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

Fig 4. Parous rates (with 95% confidence interval) in three study sites.

Alphabet denotes study sites: ^aLimbuak Laut; ^bTimbang Dayang; ^cKg Paradason.

Fig 5. Infection rates and entomological inoculation rate (EIR) of An. balabacensis in the study sites.

The solid blue line connects the points of EIR for An. balabacensis. Bars indicate the infection rates, which are sporozoite (dark grey with 95% confidence interval) and oocysts rate (light grey with 95% confidence interval). Alphabet denotes the study sites: ^a Limbuak Laut; ^bTimbang Dayang; ^cKg Paradason.