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The effect of repeated washing of long-lasting insecticide-treated nets (LLINs) on the feeding success and survival rates of Anopheles gambiae

Francis K Atieli*, Stephen O Munga, Ayub V Ofulla and John M Vulule

Malaria Journal 2010, 9:304  doi:10.1186/1475-2875-9-304

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Critique of method and discussion

Ole Skovmand   (2010-11-11 13:52)  Intelligent Insect Control email

The article of Francis Atieli et al bases its conclusions on methods that are not adequate to establish whether LLINs nets (LLIN) in practice give the protection they should The authors state that their work shows that when nets are washed according to local wash method with washes every 3 days, two polyethylene (PE) nets have a diminishing efficacy while two polyester (PES) nets have a less diminishing effect. All fail before 20 washes. However, the basis for the generalization of these data is not there. No local net-owners wash their nets every 3 day, so the study does not reflect local net washing practices as it claims. Several studies have shown that people wash their nets once or twice per year in most areas, including western Kenya. However, the method applied in this article is bucket washing as carried out locally but with time intervals of three days that no net user apply. Therefore, the practical value of the study is limited. The problem is that the conclusions in the study are presented as so far unknown problems for field performance. LLIN may have problems on large scale use but 20 washes over 60 days is not one of them.

As shown in the article "Median knock-down time as a new method for evaluating insecticide-treated textiles for mosquito control", Malaria Journal 2008:7:114, nets of the regenerating type (PE LLIN’s) takes more time to regenerate the more they are washed. Therefore, when the Atilei et al tested Netprotect and Olyset after 15 washes with 3 day intervals, the nets were not fully regenerated. Of course this problem is even more expressed after 20 washes. Had the authors taken the above mentioned article into consideration, they would know that after 20 washes and 7-8 days intervals, Netprotect kills more than 80 % with just 3 min exposure and therefore probably around 100 % with 10 min exposure time. Had this been respected, the conclusion of the Francis Atieli et al article would have been the opposite of the one presented. Since the polyester LLIN's do not regenerate, what you find after 3 days is what you will find after 10. Still, the exposure time is not field-relevant either as shown below.

This does not mean that in practice the polyethylene nets are inactive for 10 days after 20 washes. The Median Knock Down study mentioned above shows that a single wash does not remove all insecticide, but successive washes with very short intervals remove much more. Therefore, when Atieli et all washed nets with 3 days intervals after 10 washes to 15 washes, they gradually removed most of the surface insecticide and the 3 days allowed for the nets to regenerate was not sufficient. In real life in Western Kenya villages, nets are washed at most 2 or 3 times per year. A single wash does not remove all insecticide at the surface and within a week surface concentration is stabilized again.

The next problem with the study is that it applies a fixed exposure time. In real life, mosquitoes will continue to search to get to the target until it “feels” the insecticide – and then it is too late. Therefore, at a lower dosage, the mosquito will simply search for a longer time.

This phenomenon is well known and has been studied in connection with kdr resistance, a DDT and pyrethroid specific resistance mechanisms that is observed as a delayed knock down of mosquitoes with this resistance compared to those without (Chandre et al, 2000; ). This phenomenon has been used to explain why pyrethroid treated nets works in areas with kdr resistance – whereas in bioassays with short exposure, they do not.
To compensate for this phenomenon, the Pesticide Evaluation System of WHO has introduced the tunnel study for bednets. This model that mosquitoes have a choice for exposing themselves to the treated surface that hinder them to get to the target, a guinea pig or a rat – or to sit or go somewhere else for a night as in real life of a mosquito (see WHOPES guidelines for insecticide treated textiles).

Closer to real life test situation is the WHOPES II study procedure that combines washing and unlimited (for a night) exposure of mosquitoes entering a model house to bite a sleeping person protected by the insecticide treated net. Finally, if the authors wished to study effect of nets in the field for years, there are no alternatives than carrying out studies that last years. A larger 2-3 year field study showed that for polyester nets, evaporation or rubbing off of insecticide was more important than wash off for the total loss of insecticide (Kilian, 2007). This is why WHOPES III studies are field studies on large scale, since this includes all types of efficacy loss of nets, damage included.

Comment co-written by Rune Bosselmann

Competing interests

Intelligent Insect Control has been involved in the development of both polyethylene and polyester LLIN. The company currently supports a polyethylene LLIN.

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