Impact of outdoor residual spraying on the biting rate of malaria vectors: A pilot study in four villages in Kayin state, Myanmar

Victor Chaumeau,Muesuwa Trakoolchengkaew,Ladda Kajeechiwa,Gilles Delmas,Tee Dah,Naw Yu Lee,Monthicha Phanaphadungtham,Mavuto Mukaka,Prasan Kankew,Jacher Wiladphaingern,François Nosten,Yanada Akararungrot,Praphan Wasisakun,Saw Nay Hsel,Aritsara Inta,Sunisa Sawasdichai,Thithiworada Kulabkeeree,Ramesh Vishwakarma ,Kyi Oo

doi:10.1371/journal.pone.0240598

Abstract

Outdoor and early mosquito biters challenge the efficacy of bed-nets and indoor residual spraying on the Thailand-Myanmar border. Outdoor residual spraying is proposed for the control of exophilic mosquito species. The objective of this study was to assess the impact of outdoor residual spraying on the biting rate of malaria vectors in Kayin state, Myanmar. Outdoor residual spraying using lambda-cyhalothrin was carried out in two villages in December 2016 (beginning of the dry season) and two villages were used as a control. Malaria mosquitoes were captured at baseline and monthly for four months after the intervention using human-landing catch and cow-baited trap collection methods. The impact of outdoor residual spraying on human-biting rate was estimated with propensity score adjusted generalized linear mixed-effect regressions. At baseline, mean indoor and outdoor human-biting rate estimates ranged between 2.12 and 29.16 bites /person /night, and between 0.20 and 1.72 bites /person /night in the intervention and control villages respectively. Using model output, we estimated that human-biting rate was reduced by 91% (95%CI = 88–96, P <0.0001) immediately after outdoor residual spraying. Human-biting rate remained low in all sprayed villages for 3 months after the intervention. Malaria vector populations rose at month 4 in the intervention villages but not in the controls. This coincided with the expected end of insecticide mist residual effects, thereby suggesting that residual effects are important determinants of intervention outcome. We conclude that outdoor residual spraying with a capsule suspension of lambda-cyhalothrin rapidly reduced the biting rate malaria vectors in this area where pyrethroid resistance has been documented.

Highlights

The Thailand-Myanmar border is an area where malaria transmission is low, seasonal and unstable [1]
This insecticide was chosen because of the better toxicity profile of pyrethroids when compared to other insecticide classes, it is registered for outdoor use and it gives long-lasting insecticidal effects when applied to outdoor vegetation [32]
In addition to domestic animals, we identified that aquatic organisms, wild mammals, amphibians, reptiles and terrestrial arthropods were the groups of non-target organisms most likely to be affected be the intervention because they are present in the sprayed area and lambda-cyhalothrin is toxic to those taxa

Summary

Introduction

The Thailand-Myanmar border is an area where malaria transmission is low, seasonal and unstable [1]. The endemicity of vivax malaria has declined in recent years [3], it is more difficult to tackle than falciparum malaria because of some features in the biology of P. vivax [4,5,6]. In this area, the primary mosquito vectors are Anopheles minimus (s.s.) (Minimus Complex, Funestus Group), An. maculatus (s.s.), An. sawadwongporni (Maculatus Group), An. dirus Biting rate can be very high, thereby playing a disproportionate role in driving transmission intensity in this setting where Plasmodium-infection rates in mosquito populations are low [7, 10]. Several vector species multiply in a variety of biotopes and at different times of the year, adding another layer of complexity to the dynamics of entomological indices

Objectives

Methods

Results

Conclusion