The effect of holes in long-lasting insecticidal nets on malaria in Malawi: results from a case-control study.

BACKGROUND: Long-lasting insecticidal nets (LLINs) are a cornerstone of malaria prevention. Holes develop in LLINs over time and compromise their physical integrity, but how holes affect malaria transmission risk is not well known. METHODS: After a nationwide mass LLIN distribution in July 2012, a study was conducted to assess the relationship between LLIN damage and malaria. From March to September 2013, febrile children ages 6-59 months who consistently slept under LLINs (every night for 2 weeks before illness onset) were enrolled in a case-control study at Machinga District Hospital outpatient department. Cases were positive for Plasmodium falciparum asexual parasites by microscopy while controls were negative. Digital photographs of participants' LLINs were analysed using an image-processing programme to measure holes. Total hole area was classified by quartiles and according to the World Health Organization's proportionate hole index (pHI) cut-offs [< 79 cm2 (good), 80-789 cm2 (damaged), and > 790 cm2 (too torn)]. Number of holes by location and size, and total hole area, were compared between case and control LLINs using non-parametric analyses and logistic regression. RESULTS: Of 248 LLINs analysed, 97 (39%) were from cases. Overall, 86% of LLINs had at least one hole. The median number of holes of any size was 9 [interquartile range (IQR) 3, 22], and most holes were located in the lower halves of the nets [median 7 (IQR 2, 16)]. There were no differences in number or location of holes between LLINs used by cases and controls. The median total hole area was 10 cm2 (IQR 2, 125) for control LLINs and 8 cm2 (IQR 2, 47) for case LLINs (p = 0.10). Based on pHI, 109 (72%) control LLINs and 83 (86%) case LLINs were in "good" condition. Multivariable modeling showed no association between total hole area and malaria, controlling for child age, caregiver education, and iron versus thatched roof houses. CONCLUSIONS: LLIN holes were not associated with increased odds of malaria in this study. However, most of the LLINs were in relatively good condition 1 year after distribution. Future studies should examine associations between LLIN holes and malaria risk with more damaged nets.

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.

Pooled PCR testing strategy and prevalence estimation of submicroscopic infections using Bayesian latent class models in pregnant women receiving intermittent preventive treatment at Machinga District Hospital, Malawi, 2010.

BACKGROUND: Low malaria parasite densities in pregnancy are a diagnostic challenge. PCR provides high sensitivity and specificity in detecting low density of parasites, but cost and technical requirements limit its application in resources-limited settings. Pooling samples for PCR detection was explored to estimate prevalence of submicroscopic malaria infection in pregnant women at delivery. Previous work uses gold-standard based methods to calculate sensitivity and specificity of tests, creating a challenge when newer methodologies are substantially more sensitive than the gold standard. Thus prevalence was estimated using Bayesian latent class models (LCMs) in this study. METHODS: Nested PCR (nPCR) for the 18S rRNA gene subunit of Plasmodium falciparum was conducted to detect malaria infection in microscopy-negative Malawian women on IPTp. Two-step sample pooling used dried blood spot samples (DBSs) collected from placenta or periphery at delivery. Results from nPCR and histology as well as previously published data were used to construct LCMs to estimate assay sensitivity and specificity. Theoretical confidence intervals for prevalence of infection were calculated for two-step and one-step pooling strategies. RESULTS: Of 617 microscopy-negative Malawian women, 39 (6.3%) were identified as actively infected by histology while 52 (8.4%) were positive by nPCR. One hundred forty (22.7%) individuals had past infection assessed by histology. With histology as a reference, 72% of women in the active infection group, 7.1% in the past infection group and 3.2% in histology-negative group were nPCR positive. Using latent class models without a gold standard, histology had a median sensitivity of 49.7% and specificity of 97.6% for active infection while PCR had a median sensitivity of 96.0% and specificity of 99.1%. The true prevalence of active infection was estimated at 8.0% (CI: 5.8-10.5%) from PCR. PCR also had similar sensitivity for detecting either peripheral or placental malaria for submicroscopic infections. One-step pooling would give similar confidence intervals for pool sizes less than 20 while reducing the number of tests performed. CONCLUSIONS: Pooled nPCR testing was a sensitive and resource-efficient strategy and LCMs provided precise prevalence estimates of submicroscopic infections. Compared to two-step pooling, one-step pooling could provide similar prevalence estimates at population levels with many fewer tests required.