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

BackgroundA small number of human cases of the zoonotic malariaPlasmodium knowlesihave 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.MethodsThe abundance, diversity and biting behavior of human-bitingAnophelesmosquitoes 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 potentialAnophelesvectors 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 femaleAnophelesmosquitoes were tested for the presence ofPlasmodiumparasites by PCR.ResultsPreviously incriminated vectorsAnopheles balabacensisandAn. flavirostrisaccounted 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 ofAn. balabacensiswere highest in the forest edge, whileAn. flavirostriswas most abundant in the agricultural area.The abundance ofAn. balabacensisandAn. flavirostriswas significantly higher in HLC than in MBT. None of the 357 femaleAnophelesmosquitoes tested forPlasmodiuminfection were positive.ConclusionsThe relatively low density and lack of malaria infection inAnophelesmosquitoes sampled here indicates that exposure toP. knowlesiin 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. balabacensisandAn. flavirostriswere 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 insecticide-treated nets are unlikely to provide protection againstP. knowlesivectors.Graphical abstract