The useful life of bednets for malaria control in Tanzania: Attrition, Bioefficacy, Chemistry, Durability and insecticide Resistance

Long-Lasting Insecticidal Nets (LLINs) are an important malaria control tool in sub-Saharan Africa (SSA). During 2000-2015, global malaria mortality declined by 60%, translating into 6.2 million lives saved. Insecticide treated nets contributed ca. 68% of malaria cases averted. However, as nets get older, they acquire holes, are discarded, and lose their insecticide content, which reduce the protection against mosquitoes. It is essential to understand the effective life, or durability, of current control measures to support the United Nations Sustainable Development Goals to end malaria epidemics and reduce malaria by 90% by 2030. The project collaborated with the Tanzanian National Malaria Control Program to estimate how long different brands of LLINs last when used by Tanzanian families. This information is important for procurement of the most cost-effective nets and estimating the correct timing of net distribution campaigns, which is currently set to every 3 years. This is, to date, the largest durability study, which successfully evaluated 10,600 LLINs under user conditions for direct comparison of net brands regardless of cultural, geographical or epidemiological variations. The study consisted of 1) a retrospective survey of Olyset® nets distributed by the Tanzanian Government 2-4 years earlier; and 2) a prospective study of three different LLIN products (Olyset®, PermaNet®2.0 and NetProtect®) over three years (2013-2016) across eight districts in Tanzania. The specific objectives were to assess durability of LLINs by evaluating Attrition (absence of nets due to discarding), Bioefficacy (ability to kill mosquitoes), Chemical content (insecticide in nets) and physical Degradation (hole surface area). The project also assessed insecticide Resistance in malaria mosquitoes. Socio-economic and geographical variation of these components were studied and the project supported capacity building of two Tanzanian PhD students. A cost-effectiveness analysis was also carried out. Retrospective study. Two-to-four years after mass campaigns, 35% of nets reported to have been received were no longer present. The number of holes of different sizes were counted in a sub-sample of nets. Total hole surface area calculations indicated that 39% were ?too torn? to protect against mosquito bites. Functional net survival is the proportion of nets not having too many holes and still protecting against mosquitoes out of all nets distributed and not given away to others. The functional survival of these nets was only 39%, considerably below WHO assumptions of 50% survival of theoretical 3-year nets. However, most of surviving nets retained substantial levels of insecticide and could still be useful to kill mosquitoes if their holes were repaired, highlighting the value of health education to care for and repair existing mosquito nets. Prospective study. Attrition of all three net products increased from 3.2%, 13.5% to 31% over 3 years. As time went on, most nets were thrown away or used for other purposes because of too many holes. After 3 years, 85% of sampled nets had holes and 32% were ?too torn?. A significantly higher proportion of Olyset nets were ?too torn? compared to the other two brands, regardless of the age of nets. Olyset nets were also more likely to be thrown away. Functional survival of the three brands differed significantly, with median estimated survival of 2.1 years for Olyset, 2.5 years for PermaNet and 2.6 years for NetProtect. More than 80% of PermaNet and NetProtect samples still achieved WHO thresholds for mortality and/or blood feeding inhibition, but only 75% of Olyset samples met WHO criteria after 3 years. Further, 54% of Olyset, 96% of PermNet, and 100% of Netprotect nets had insecticide content below the optimum specifications after 3 years. An economic analysis showed a 16-18% lower equivalent annual cost of a longer-lasting net product (PermaNet or NetProtect) compared to Olyset. Insecticide resistance. The project also supported Tanzania?s national insecticide resistance surveillance and management strategy. Anopheles gambiae s.l., the main malaria vector in Tanzania, was found resistant to deltamethrin, permethrin, DDT, and bendiocarb. For the first time, malaria vectors showed resistance to pirimiphos-methyl in Tanzania. This is alarming and with widespread resistance to pyrethroids, there are few effective options left for malaria vector control. General. The outcomes of this project are of significant importance to policy makers and vector control specialists in Tanzania and SSA to inform on best practices for the maintenance of high and cost-effective net coverage to maximize current advances in malaria reduction. The spread of insecticide resistance calls for urgent implementation of resistance management strategies and researching alternative effective vector control interventions. The project has contributed internationally to improve durability assessment methods.

Long-Lasting Insecticidal Nets (LLINs) are the current first line choice in malaria vector control in sub-Saharan Africa (SSA), with most countries adopting universal coverage campaigns with free or subsidised nets. However, there is only limited knowledg e from few countries of the effective life of LLINs under user conditions, an essential parameter for determining the operational and cost-effectiveness of this strategy. This study investigates LLIN effectiveness by measuring attrition, bioefficacy, chem ical content and physical degradation in Tanzania. We will use a two-stage approach: Firstly, LLINs from recent net campaigns will be evaluated retrospectively. The sampled households will then be provided with one of three leading LLIN products and follo wed up for three years in a prospective study to compare the performance of the LLIN brands in vivo. We will also develop a GIS-based network to understand potential spatial reasons for net loss and deterioration; to monitor insecticide resistance and to show malaria incidence data collected from local health centres and national surveys. We are bringing together a highly skilled multi-disciplinary team that will support Tanzanian vector control both now and in the future through the training of two Tanza nian PhD students. We are the first team to conduct a geographically representative study of LLIN use and efficacy using WHO-recommended methodologies and large sample sizes determined by robust statistical methods with independent data quality assurance. The data collected will be of importance to policy makers and vector control specialists both in Tanzania and the SSA region to inform best practice for the maintenance of high and cost-effective coverage to maximise current gains in malaria control.