Use of Insecticide Treated Nets and Temporal Concentration of Insecticide in Olyset Nets in Dar Es Salaam, Tanzania

Abstract

All mosquito nets act as a physical barrier, preventing access by vector mosquitoes and thus providing personal protection against malaria to the individual(s) using the nets. The pyrethroid insecticides used to treat nets, have an excito-repellent effect that adds a chemical barrier to the physical one offered by the net fabric, further reducing human–vector contact and increasing the protective efficacy of treated nets. Most commonly, the insecticide kills the malaria vectors that come into contact with the insecticide-treated nets (ITNs). By reducing the vector population in this way, ITNs, when used by a majority of the target population (coverage rates > 80 %) has a mass effect by offering protection for all people in the community, including those who have no nets (Binka, 1998; Hawley, 2003). ITNs protect users against malaria in several ways; firstly, the fabric provides a physical barrier, so reducing man-mosquito contact. Secondly the impregnated insecticide knocks down and often kills mosquitoes that come into contact with the net. The insecticide dosage required for knockdown is well below the threshold level of mammalian toxicity (WHO, 2006). Thirdly impregnated insecticide is excito-repellent hence will prevent entry of mosquitoes into households with ITNs (Curtis et al., 1998, 2003). The two extra properties make ITNs advantageous over untreated nets, such that a treated net can be effective even if torn. If ITNs are used by the whole community the insecticidal effect results in reduced longevity of female Anopheles mosquitoes so that few survive for the time required for development of the malaria parasite to the sporozoite stage in the mosquito thereby reducing malaria transmission. Pyrethroids were developed after prolonged efforts to improve biological and chemical stability of the natural pyrethrins, which have long been known for their insecticidal effects. As well as causing a knockdown effect, insects encountering pyrethroids show restless behaviour and hyper excitability caused by involvement of the central nervous system. Pyrethroids may also have an anti-feeding effect on Anopheles mosquitoes (Becker et al., 2003). 2 The use of ITNs treated with synthetic pyrethroids is increasing fast in many malarial parts of the world including, sub-Saharan African countries. The efficacy and effectiveness of ITNs in terms of mortality and morbidity reduction in malaria endemic countries have been very well documented (Lengeler, 2004). The WHO currently advocates their universal use by all household members in malaria endemic areas. Previously, nets had to be re-treated every 6-12 months, more frequently if the nets were washed in between re-treatment. Nets were re-treated by dipping them in a mixture of water and insecticide and allowing them to dry in a shady place. Re-treatment kits (NGAO) were commercially available in retail shops, however not affordable by the needy majority, thus leading to low re-treatment rates (Chavasse et al., 1999; Mubyazi et al., 2002; Kolaczinski et al., 2004; Abdullah et al., 2005). The need for frequent re-treatment was a major barrier to widespread use of ITNs in endemic countries. For ITNs to be fully effective re-treatment rates need to be very high (WHO, 2003). The additional cost of the insecticide kit (NGAO) and the lack of understanding of its importance resulted in low retreatment rates in most African countries. One of the main programmatic issues affecting the conventional ITNs is the necessity for regular re-treatment. For a number of reasons (such as frequency and washing methods, wear and tear, UV light and others) the pyrethroid insecticide on net fabric wears off over time and has, therefore, to be replaced. There is convincing evidence that after 3 to 5 washes, the insecticide is removed from nets treated by conventional dipping. Under programmatic conditions, unless nets can be systematically be retreated after 3 washes as recommended by WHO (1998) the protective efficacy might be much lower than expected, especially if nets are frequently washed. While it is relatively easy to socially market mosquito nets to the population of endemic countries, the idea of a regular insecticide re-treatment has been found very difficult to implement (Minja, 2001; Armstrong-Schellenberg, 2002). 3 The barriers associated with the conventional ITNs that require re-treatment have been mitigated by the invention of long-lasting insecticidal mosquito nets (LLINs). Several companies have developed LLINs that maintain effective levels of insecticides for the life of the net fabric estimated to 5 years mitigating the need for conventional re-treatment. LLINs are treated in the factory with insecticide incorporated into or coated around the fibres, which makes them capable to resist multiple washes. LLINs are designed to survive for several years in the field without further maintenance, and to withstand for at least 20 washes without losing insecticidal activity. Application of LLINs is currently growing in many malaria endemic areas and these nets retain their insecticidal properties for their physical life in domestic use despite many washes (Tami et al., 2004). The net fabric can either be polyesters with a physical life of 3–4 years or polyethylene with a physical life of 4–5 years (WHO, 2003). Currently several insecticide and net manufactures are active in developing long lasting nets technology (WHO, 2011). Several brands of nets are available in the country including Olyset, Permanet, Mbu net, Afya net, Safi net and others. Olyset nets have been found to be popular and durable than ordinary polyester nets. Currently, Olyset® nets are locally produced in East Africa by the A-Z Textile Industry in Arusha, Tanzania. Olyset® nets have permethrin incorporated into the polyethylene material during the manufacturing process at a concentration of 20g/kg (±3g/kg), 2%w/w (1.7 – 2.3% limits). The manufacturer has reported that, over time, insecticide migrates to the surface of the yarn, replacing the one that has been removed by washing. This migration being a temperature-dependent process, exposure of nets to heat after washing (e.g. few hours to sun) can accelerate it. Olyset® nets are very strong due to their thick fibres of high density polyethylene which gives them strength of about 180 denier despite the wide mesh of 4mm. Their durability and long-lasting insecticidal properties make them suitable for use in most rural settings in malaria endemic areas in Sub-Saharan Africa.