Net age, but not integrity, may be associated with decreased protection against Plasmodium falciparum infection in southern Malawi

BACKGROUND: Distribution campaigns for insecticide-treated nets (ITN) have increased the use of ITNs in Malawi, but malaria prevalence remains high even among those using the nets. Previous studies have addressed ITN ownership, insecticide resistance, and frequency of ITN use as possible contributing factors to the high prevalence of malaria infection despite high ITN coverage, but have rarely considered whether the condition of the ITN, or how many people use it, impacts efficacy. This study assessed how ITN integrity, ITN age, and the number of persons sharing a net might mitigate or reduce protective efficacy among self-identified ITN users in Malawi. METHODS: From 2012 to 2014, six cross-sectional surveys were conducted in both the rainy and dry seasons in southern Malawi. Data were collected on ITN use, integrity (number and size of holes), and age. Blood samples for detecting Plasmodium falciparum infection were obtained from reported ITN users over 6 months of age. Generalized linear mixed models were used to account for clustering at the household and community level. The final model controlled for gender, household eaves, and community-level infection prevalence during the rainy season. RESULTS: There were 9646 ITN users with blood samples across six surveys, 15% of whom tested positive for P. falciparum infection. Among children under 5 years old, there was a 50% increased odds of P. falciparum infection among those sleeping under an ITN older than two years, compared to those using an ITN less than 2 years old (OR = 1.50; 95% CI 1.07–2.08). ITN integrity and number of individuals sharing an ITN were not associated with P. falciparum infection. CONCLUSIONS: Older ITNs were associated with higher rates of P. falciparum in young children, which may indicate that insecticide concentrations play a larger role in infection prevention than the physical barrier of an ITN. ITN use was self-reported and the integrity measures lacked the precision of newer methods, suggesting a need for objective measures of ITN use and more precise assessment of ITN integrity.