The effectiveness of long-lasting, insecticide-treated nets in a setting of pyrethroid resistance: a case-control study among febrile children 6 to 59 months of age in Machinga District, Malawi

Background: The escalating level of mosquito resistance to pyrethroid insecticides threatens the effectiveness of insecticide-treated nets (ITNs) for malaria control in Malawi. An evaluation of the effectiveness of ITNs for preventing malaria in children aged 6-59 months old, after 1 year of mass distribution of LLINs was conducted in Machinga District, Malawi, an area of moderate pyrethroid resistance. Methods: A facility-based, case-control study among children 6-59 months was conducted in an area of pyrethroid resistance between March and September 2013 in Machinga District. Cases and controls were children with fever who sought care from the same hospital and tested positive and negative, respectively, for malaria parasites by microscopy. Results: A high proportion of both cases (354 of 404 or 87.6 %) and controls (660 of 778 or 84.8 %) slept under an ITN the night before the survey. In univariable logistic regression, older age (24-59 months versus 6-23 months, p < 0.001), sleeping on the floor versus a mattress (p < 0.001), and open versus closed house eaves (p = 0.001) were associated with increased odds of malaria, whilst secondary education of the caretaker, having windows on multiple walls, and being in the least poor wealth quintile (p < 0.001 for each) reduced the odds of malaria; ITN use was not associated with malaria (p = 0.181). In multivariable analysis, older age (p < 0.001) and secondary education of the caregiver (p = 0.011) were the only factors significantly associated with malaria. Conclusion: This study did not find a significant personal protective effect of ITNs. However, high use of ITNs in the community and recent findings of lower malaria incidence in ITN users compared to bed net non-users from a cohort study in the same area suggest that ITNs provide community protection to both users and non-users alike in this area.