Risk Factors for Malaria Infection Prevalence and Household Vector Density between Mass Distribution Campaigns of Long-Lasting Insecticidal Nets in Northern Tanzania

Abstract

Background Long lasting insecticide nets (LLIN) are the intervention most widely deployed in Sub-Saharan Africa to prevent malaria. Recent reports indicate selection of pyrethroid insecticide resistance is widespread in mosquito vectors. This paper explores the risk factors associated with malaria infection prevalence and vector density between mass distribution campaigns of LLIN in Northern Tanzania. Methods A cross sectional malaria survey of 3,456 children was undertaken in 2014. Vector density was assessed using indoor light traps and outdoor tent traps. Anophelines were identified to species using PCR and tested for falciparum circumsporozoite protein. Logistic regression was used to identify household and environmental factors associated with malaria infection and vector density. Results LLIN use was 27.7%. Only 16.9% of households had sufficient nets to cover all sleeping places. Malaria infection was independently associated with access to LLIN (OR: 0.57; 95%CI 0.34 – 0.98), age group, altitude and house construction quality. LLIN less than 2 years old were more protective than older LLIN (53% vs 65% prevalence of infection); however, there was no evidence that LLIN in good condition (hole index <65) were more protective than LLIN which were more holed. Independent risk factors for vector density were consistent with malaria outcomes and included altitude, wind, livestock, house quality, open eaves and LLIN access. Indoor collections comprised 4.6% Anopheles funestus and 95.4% An. gambiae of which 4.5% were An. arabiensis and 93.5% were An. gambiae s.s . Conclusion Three years after the mass distribution campaign and despite top-ups, LLIN usage had fallen considerably. While children living in households with access to LLIN were at lower risk of malaria, infection prevalence remained high even among users of LLIN in good condition. While effort should be made to maintain high coverage between campaigns, distribution of just standard pyrethroid LLIN appears insufficient to prevent malaria transmission in this area of intense pyrethroid resistance.