Piperonyl butoxide net utilization and durability on malaria transmission in Muhoroni sub-county, western Kenya

Abstract

Pyrethroid-treated nets have reduced malaria in endemic areas, but insecticide resistance has hindered progress, prompting WHO to recommend piperonyl butoxide (PBO) based LLINs. Kenya adopted PBO nets, but their usage and effect on malaria infection is not well documented. Additionally, their recommended three-year lifespan is challenged by fabric decay and insecticidal decay, both of which can influence malaria transmission. The current study objectives were: to determine the effect of PBO net usage on malaria infection in Muhoroni sub-county, Western Kenya, to assess the influence of PBO net integrity on malaria incidence at the household level in Muhoroni sub-county, Western Kenya and to examine the influence of PBO net bioefficacy on malaria incidence in Muhoroni sub-county, Western Kenya. A cross-sectional net usage survey was conducted on 12 randomly selected villages comprising of six villages per intervention arm (pyrethroid and PBO-LLINs) one-year post-net distribution. Using a standardized semi-structured questionnaire, 181 and 199 households in the pyrethroid and PBO-LLINs were randomly surveyed respectively. Finger-prick blood smears and dry blood spots (DBS) on filter papers were collected for microscopy and real-time PCR (RT-PCR) malaria diagnosis respectively. A repeated cross-sectional study design was employed to evaluate net integrity every six months and bioefficacy every three months, for a period of up to 18 months. Distinctive households ranging from 101-107 households/group/visit were surveyed with 116-135 nets (pyrethroid) and 137-169 nets (PBO) assessed. Inconsistently used nets were excluded from the study. Six nets per group were retrieved from households with different wall characteristics (mud plastered, mud unplastered/exposed mud, and cemented/block/brick) for residual bioefficacy testing using WHO cone bioassays. Monthly malaria active case detections were carried out by community health promoters (CHPs) in the surveyed households. The influence of net utilization on malaria infection was determined using logistic regression. A simple logistic regression model and Fisher-Freeman Halton exact test were used to determine the influence of PBO physical integrity and bioefficacy on malaria incidence respectively. Malaria parasite prevalence by qPCR was lower in the PBO-LLIN group 29% vs. 34.7%. The use of PBO net decreased the risk of a malaria infection by 39.8% [OR=0.602 (95% CI=0.373-0.971), P=0.038]. At 18 months, there was lower torn rate in the PBO-LLIN compared to pyrethroid-LLIN group (21.8% (31/142) vs. 45.2% (61/135) with pHI scores of 1618.6±1056.7 and 2494.1±1696.4 respectively. By the 18th month, bioefficacy against susceptible mosquitoes decreased for both net types (PBO: 98.1% to 18.5%, pyrethroid-LLIN: 97.6% to 18.4%), and for resistant mosquitoes as well (PBO: 55.6% to 11.8%, pyrethroid-LLIN: 36.9% to 6.8%). Malaria incidence was consistently lower (23.6 cases/1000 people/month vs. 16.2 cases/1000 people/month) among households using PBO-LLINs compared to pyrethroid-LLINs over 18 months. Compromised physical integrity (torn) and bioefficacy (not effective) of the PBO-LLINs were significantly associated with higher malaria incidences in households. To sustain PBO-LLIN effectiveness, the Ministry of Health should strengthen community education on consistent and proper net use, consider periodic assessments of fabric integrity and bioefficacy within 18 months post-distribution and integrate community-based monitoring to identify and replace ineffective nets promptly.