The seasonal dynamics and biting behavior of potential Anopheles vectors of Plasmodium knowlesi in Palawan, Philippines

Background: A small number of human cases of the zoonotic malaria Plasmodium knowlesi have been reported in

Palawan Island, the Philippines. Identification of potential vector species and their bionomics is crucial for understanding

human exposure risk in this setting. Here, we combined longitudinal surveillance with a trap-evaluation study to

address knowledge gaps about the ecology and potential for zoonotic spillover of this macaque malaria in Palawan

Island.

Methods: The abundance, diversity and biting behavior of human-biting Anopheles mosquitoes were assessed

through monthly outdoor human landing catches (HLC) in three ecotypes representing different land use (forest

edge, forest and agricultural area) across 8 months. Additionally, the host preference and biting activity of potential

Anopheles vectors were assessed through comparison of their abundance and capture time in traps baited with

humans (HLC, human-baited electrocuting net—HEN) or macaques (monkey-baited trap—MBT, monkey-baited electrocuting

net—MEN). All female Anopheles mosquitoes were tested for the presence of Plasmodium parasites by PCR.

Results: Previously incriminated vectors Anopheles balabacensis and An. flavirostris accounted for > 95% of anophelines

caught in longitudinal surveillance. However, human biting densities were relatively low (An. balabacensis:

0.34–1.20 per night, An. flavirostris: 0–2 bites per night). Biting densities of An. balabacensis were highest in the forest

edge, while An. flavirostris was most abundant in the agricultural area. The abundance of An. balabacensis and An. flavirostris

was significantly higher in HLC than in MBT. None of the 357 female Anopheles mosquitoes tested for Plasmodium

infection were positive.

Conclusions: The relatively low density and lack of malaria infection in Anopheles mosquitoes sampled here indicates

that exposure to P. knowlesi in this setting is considerably lower than in neighboring countries (i.e. Malaysia), where it

is now the primary cause of malaria in humans. Although anophelines had lower abundance in MBTs than in HLCs, An.

balabacensis and An. flavirostris were caught by both methods, suggesting they could act as bridge vectors between

humans and macaques. These species bite primarily outdoors during the early evening, confirming that insecticidetreated

nets are unlikely to provide protection against P. knowlesi vectors