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

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.

Outdoor residual spraying for malaria vector-control in Kayin (Karen) state, Myanmar: A cluster randomized controlled trial

“…The twelve villages were selected by mean of mutual agreement between community members and the study team had no role in the final selection process. ORS was carried out in six villages between October 2 nd and October 17 th for two days per village as previously described [21]. No intervention was carried out in the villages of the control group.…”

“…No intervention was carried out in the villages of the control group. Entomological surveys were conducted as previously described [21] and yielded 25 personnights of collection indoors, 25 person-nights outdoors and 5 cow-nights per survey. Anopheles specimens were sorted macroscopically and then identified using a dichotomic identification key [22].…”

“…Anopheles specimens were sorted macroscopically and then identified using a dichotomic identification key [22]. Malaria mosquitoes captured in the cow-baited trap and still alive at the end of the survey were used to perform standard insecticide susceptibility tests with papers impregnated with 18 mg/m 2 of lambda-cyhalothrin as previously described [21]. Mosquito susceptibility to additional insecticides, including deltamethrin (18 mg of active ingredient [a.i.]…”

“…dirus, the residual effects of a capsule suspension of lambda-cyhalothrin sprayed on outdoor vegetation at a target concentration of 500 g of active ingredient per hectare lasted for several weeks to months during the rainy and dry seasons respectively [20]. Furthermore, the effects of ORS on wild populations of malaria mosquitoes are not well documented (reviewed in [21]). In a pilot study in four villages, ORS with a capsule suspension of lambda-cyhalothrin sprayed rapidly reduced the biting rate of malaria vectors but intervention impact on additional outcome measures (e.g.…”

“…In a pilot study in four villages, ORS with a capsule suspension of lambda-cyhalothrin sprayed rapidly reduced the biting rate of malaria vectors but intervention impact on additional outcome measures (e.g. vector longevity and resistance, malaria incidence) was not assessed [21]. The objective of the herein study was to further assess at scale the efficacy of ORS for malaria vector-control in Kayin state, thereby contributing to knowledge of the impact of vector-control on the entomological indices, vector resistance and vivax malaria in this region.…”