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

Richard Paul B Malijan,Chris Drakeley,Wilfredo E Aure,Majhalia M Torno,Brian A Bacay,Frank Mechan,Jessie C Braganza,Kimberly M Fornace,Heather M Ferguson,Ferdinand V Salazar,Fe Esperanza Espino,Kristelle Mae R Valle,Stephen J Torr

doi:10.1186/s13071-021-04853-9

Abstract

BackgroundA 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.MethodsThe 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.ResultsPreviously 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.ConclusionsThe 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 insecticide-treated nets are unlikely to provide protection against P. knowlesi vectors.Graphical abstract

Highlights

A small number of human cases of the zoonotic malaria Plasmodium knowlesi have been reported in Palawan Island, the Philippines
While potential vectors of P. knowlesi are present in Palawan, their comparatively low densities and infection rates indicate that human exposure to P. knowlesi is considerably lower in this setting than in nearby Sabah, where this parasite is the primary cause of malaria in humans
The monkey malaria P. knowlesi is the primary cause of human malaria in Malaysian Borneo; only sporadic human cases have been reported in the nearby island of Palawan

Summary

Introduction

A small number of human cases of the zoonotic malaria Plasmodium knowlesi have been reported in Palawan Island, the Philippines. Concern has been raised that the emergence of the zoonotic malaria parasite P. knowlesi as a public health problem in several Southeast Asian countries may threaten regional elimination [4, 5]. Human infections of P. knowlesi have been reported in both settings [5, 7, 9], cases have been sporadic in Palawan [6, 10, 11] whilst P. knowlesi is the leading cause of human malaria in Sabah [5, 12, 13]

Objectives

Methods

Results

Discussion

Conclusion