LLINs technology was developed to address the low re-treatment rate of locally treated nets, which should be compulsorily re-treated after six months of use [17, 33]. Given the benefits of LLINs, their distribution and use have become a priority for national malaria control programs [18-21] because, according to Carnevale [19,22], net promotion will only be effective if it is part of a package of measures to improve the quality of life of the population.
The rate of use of LLINs was 73.53% in rural Kribi and 79.30% in urban Kribi. This relatively high rate of LLINs use could be correlated with the high culicid density found in some studies in the study area, which is considered malaria endemic [20, 30, 33]. Our results are consistent with those reported in Benin by Moiroux et al [31]. The latter showed a positive correlation between culicidal nuisance and effective use of LLINs by vulnerable groups. The results show that the coverage rates reported in our work are well above the minimum coverage rate recommended by WHO for universal and effective coverage. These high coverage rates are to the advantage of the study areas because, according to WHO, a large coverage of a population (>80%) with impregnated mosquito nets would provide protection even for those who do not sleep under them [33]. It would also reduce transmission, but especially the incidence rate of malaria fevers, as shown by the mass introduction of lambda-cyhalothrin-impregnated nets in the northern savannah zone of Côte d'Ivoire [25-26]. Furthermore, the coverage rate in rural Kribi was significantly higher than in urban Kribi. This result can be explained by the availability of shops in urban areas, which favours the acquisition of other means of protection such as sprays and fans by local residents. Similar results were obtained during a survey conducted in Manoka and Youpwé, rural and urban localities in the city of Douala and Mvoua, in south-Cameroon [18, 27]. In addition, the absence of some heads of households at the time of distribution could also explain this difference in coverage in the two areas.
To be effective in its protective role, a mosquito net should constitute an impenetrable physical barrier to mosquitoes. Indeed, given its architectural characteristics (polyester or polyethylene fibers, mesh size 156, i.e. 25 holes/cm2), a LLIN was designed to allow only organisms the size of an adult mosquito to pass through [23-24]. However, wear and tear over time, maintenance (frequency of washing, type of soap used, method of drying) etc. are all factors that can impact on the physical integrity of LLINs. Our study showed a positive correlation between the proportionality indices of holes (pHI) of LLINs and the frequency of washing in rural Kribi and urban Kribi. These results are consistent with those recorded in a study conducted in Ayos in the central region of Cameroon [28-29]. Indeed, the stresses placed on the net fibers by the high frequency of washing would lead to the appearance of type I holes which, and with further usage and washing, these holes will enlarge to size II, III or even IV. Similar observations have been made in neighborhoods in the city of Douala (Cameroon) [28]. The nature of the detergent used during washing could also be the cause of net deterioration. Our work shows that the hole proportionality indices were lower for LLINs washed with ordinary soap (lumpy) than those washed with corrosive detergents (powdered soap), both in rural and urban Kribi. This result is attributed to the fact that dirty LLINs are soaked in water containing detergent for several minutes or even hours before being washed. This washing process causes chemical interactions between the detergent molecules on the one hand and the polyester or polyethylene of the LLINs on the other hand; the chemical reactions thus established could contribute to the embrittlement of the LLINs fibers. In contrast, with lump soap, such a soaking process does not take place and the LLINs are washed directly. The majority of today’s polyester fibers are composed of terepthalic acid and ethylene glycol (PET) [34].
As an illustration, some nets are made from polyethylene terephthalate plastic (A) and polyester (B) with the following structures
A: Polyethylene terephthalate structure B: Polyester structure
In a soapy solution (basic medium), these esters could undergo basic hydrolysis leading to long-term degradation of the net.
A similar analysis could be undertaken to explain the fragility of LLINs spread out in the sun after washing, compared to those spread out in the shade, as observed in our study.
In our view, awareness campaigns should be organized on a regular basis by community health workers to teach people how to maintain a LLIN.
The results of insecticide tests inform us about the impact of poor LLINs maintenance on the effectiveness of the impregnating insecticide. Our results show a gradual loss of insecticide impregnation with the frequency of washing (Table 4). This was also the case for Olyset Net in Kribi-urban, PermaNet in Kribi-rural and Interceptor in rural and Kribi-urban (Table 4). The Olyset Net LLINs is a new type of net developed by Sumitomo Chemical Co Ltd (Japan), in which the insecticide (permethrin used at a concentration of 2% w/w, i.e. about 900mg/m2) is incorporated by fusion into a fiber made of polyethylene resin. Compared to impregnation by dipping, where the product is simply deposited on the fibers, the Olyset Net® manufacturing process integrates the insecticide into the support at the time of its polymerization. According to Darriet et al (2007), these nets have an efficacy period of three years and may retain their effectiveness after 30 washes. However, when washed with a corrosive detergent, there will be a loss of insecticide efficacy according to the chemical equation:
Indeed, Permethrin and Deltamethrin are very unstable in basic media so that at pH = 9, their life span is considerably reduced, in fact these molecules undergo basic hydrolysis which denatures them. In contrast, both insecticides are very stable in acidic media. It is therefore advisable to wash the net with water containing vinegar, which contains acetic acid and is less irritating in small doses (Azebaze, comm. pers.).
As for the PermaNet® brand mosquito net, it is a net in which the insecticide, in this case Deltamethrin dosed at 50mg/m2, is mixed with a resin that coats the polyester fibers. The pyrethroid attached to the substrate is gradually released by the resin, so that the net remains effective even after repeated washing. Laboratory tests on A. gambiae with WHO cones showed that after 20 washes, a three-minute contact of mosquitoes with the treated netting still induced a 100% knock-down effect (KD), while in terms of mortality, a PermaNet® washed 20 times still killed 50% of the batches of anopheles [29,32]. From the above, it can be suggested that the early decline in efficacy obtained in our results after 20 washes of polyester fiber nets is also due to washing with corrosive products and exposure to the sun after washing. Indeed, if the corrosive product hydrolyses the insecticides in question according to the chemical equations stated above, the sun's rays act on these insecticides according to the following photochemical reaction:
This equation shows that the sun has a highly damaging effect on permethrin and Deltamethrin. Beyond the fact that it causes cleavages, oxidations and cis or trans isomerization’s, it generally leads to the formation of free radicals which would be carcinogenic.