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

Abstract

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 insecticide-treated nets are unlikely to provide protection against P. knowlesi vectors.

Keywords: Anopheles balabacensis; Anopheles flavirostris; Philippines; Plasmodium knowlesi; Vector behavior.

Fig. 1

Map showing the location in Brgy. Bacungan, Puerto Princesa City, Palawan, and the sampling sites labeled AA, FA and FE

Fig. 2

Mosquito collection techniques used in the study: a human landing catch (HLC), b monkey-baited trap (MBT), c human collectors obtaining mosquitoes from the MBT, d electrocuting net trap (HEN and MEN)

Fig. 3

Predicted mean An. balabacensis bites per night from May to December 2015 (a agricultural area, b forest edge, c forest area; shaded area represents 95% CI, open circles represent observed values)

Fig. 4

Predicted mean number of An. flavirostris bites per night from May to December 2015 (a agricultural area, b forest edge; shaded area represents 95% CI, open circles represent observed values)

Fig. 5

Predicted mean number of An. balabacensis bites per hour from 18:00 to 06:00 in each site (a agricultural area, b forest edge, c forest site; shaded area represents 95% CI, open circles represent observed values)

Fig. 6

Predicted mean number of An. flavirostris bites per hour from 18:00 to 06:00 in each site (a agricultural area, b forest edge; shaded area represents 95% CI, open circles represent observed values)

Fig. 7

Predicted abundance of four Anopheles species (An. balabacensis, An. flavirostris, An. dispar, An. greeni) as sampled by different trapping methods (HLC human landing catch, MBT monkey-baited trap, HEN human-baited electrocuting net, MEN monkey-baited electrocuting net). No bar indicates no specimens of the species were collected in that trap