Protective effects of Olyset® Net on Plasmodium falciparum infection after three years of distribution in western Kenya

Long-lasting insecticide-treated nets (LLINs) are effective in reducing malaria morbidity and mortality [1‐3] and are widely accepted as an important tool to control malaria parasite infection [4]. As of 2019, the World Health Organization (WHO) listed 20 LLIN brands for procurement by international agencies and countries [5]. The pyrethroid insecticides used for these LLINs are deltamethrin, alpha-cypermethrin, and permethrin [5]. The insecticides act differently against anopheline mosquitoes; for instance, a laboratory study showed that fabric treated with deltamethrin has better killing effects than one with permethrin; however, the latter shows stronger repellent effects [6‐8]. Although both types of pyrethroids reduce human contact with anophelines by killing and repelling, the question remains which mode of action or insecticide is better for reducing infection risk. To counter anophelines resistant to pyrethroids and dichlorodiphenyltrichloroethane (DDT), five LLIN brands in the list are treated with piperonyl butoxide (PBO) in addition to the pyrethroids. A randomized control trial showed that LLINs treated with PBO and permethrin are more effective in reducing Plasmodium falciparum parasite infection than nets with insecticide only [9].

The difference in insecticide treating technology and LLIN materials may also affect their performance. One treating technology is called incorporation technology; specifically, an insecticide is incorporated into polyethylene-based fibres of the net [10], such as the Olyset® Net (Sumitomo Chemical, Tokyo, Japan). The insecticide migrates from the inside of the fibres to the surface so that the amount of the active content is maintained for several years. Coating is a second technology, in which the polyester-based multifilaments are coated with insecticide using a resin-based polymer [11] that serves as a reservoir for replacement of insecticide lost from the surface. Polyethylene and polyester are the two major materials used for LLINs. The difference in treatment technology and netting material may alter the durability of nets and the availability of insecticides on the fibre surfaces and, consequently, affect infection risks [12, 13].

A meta-analysis showed that among three different insecticides the differences in effectiveness on infection risk were not statistically significant [3]. A large population-based cross-sectional study using data from 21 countries across sub-Saharan Africa also found little variability on infection of children by LLIN brands [14]. However, when data were examined separately for each surveyed population, the effects of LLINs varied among LLIN brands for some populations [14, 15]. Local environmental conditions likely vary across large geographical malarial areas and may influence the performance of LLINs. Local vector species compositions and their insecticide resistance status can also contribute to the variability of LLIN performance [16‐20].

The mean child age was 8.4 (SD = 3.5) years old, and the difference was not statistically significant between DawaPlus® 2.0 users and Olyset® users (Table 1). The differences in gender ratio and sleeping location were also not statistically significant between the two groups. The PHI on the side of Olyset® was significantly greater than that of DawaPlus® 2.0 while the difference in PHI on the roof was not significant. The proportion of children sharing the same net with two or more persons was significantly higher for DawaPlus® 2.0 users. The proportions of mud wall and open eaves were significantly higher for the rooms where DawaPlus® 2.0 users slept, and the SES of DawaPlus® 2.0 users was significantly lower. A total of 1365 anopheline mosquitoes were collected from 118 rooms where 254 target children slept. Although the mean density of anophelines in the rooms with Olyset® was almost half compared to those with DawaPlus® 2.0, the difference was not statistically significant.

This study demonstrated that the pfPR of children differed between users of Olyset® and DawaPlus® 2.0. The odds ratio indicated that use of Olyset® for a child reduced by 33% the likelihood of being infected (Table 2). The study also found differences in some variables between two groups (Table 1). The households of Olyset® users had significantly higher SES, and a decrease of infection risk is often associated with an increase of SES [46]. Households with low income cannot easily afford extra protection such as insecticide spray and drugs. Moreover, parents of households with high SES tend to have a better health knowledge, which may also lower the risk [47, 48]. It is plausible that households with high SES have better house construction, with glass or screened windows and closed eaves to prevent mosquitoes from entering [49]. While most of the houses in the study area were constructed with a mud wall and corrugated iron roofs that have open eaves, the higher proportion of the rooms with Olyset® had closed eaves and non-mud walls, such as concrete. Although the difference was not statistically significant, the density of anophelines in the rooms of Olyset® users was nearly 50% less than in those of DawaPlus® 2.0 users, suggesting that Olyset® users might have lower infection risks because of higher SES (Additional file 3: Fig. S3, Additional file 4: Fig. S4).

In addition to freely distributed LLINs, households with higher SES might be able to buy extra nets. Although the WHO guideline suggests ‘one net for every two people’ to achieve universal coverage, the goal has not been reached in several areas [50], where one net is often shared by more than two persons. The number of people sharing a net is significantly higher among DawaPlus® 2.0 users compared to Olyset® users. A recent study confirmed that the risk increases with an increase in the number of people sharing one LLIN, because the condition may increase the chance of their body being exposed outside the net [30].

As Olyset® in the study area had been used by residents longer than DawaPlus® 2.0, the condition of the former net was worse. The PHI on the sides of Olyset® was significantly higher. Although the difference was not statistically significant, the Olyset® also had a higher PHI on the roof. As mosquitoes may enter through holes, the infection risk is expected to be higher for children sleeping under the old nets. However, the results from the present study did not agree with this notion. Even though the effects of all these confounding factors, including spatial dependency, were controlled in the multiple regression model, the difference in pfPR between Olyset® users and DawaPlus® 2.0 users still remained.

The difference in pfPR of children sleeping under these LLIN brands may be explained by the differences between the insecticides. Permethrin is incorporated in the fibres of Olyset® while DawaPlus® 2.0 is treated with deltamethrin on the filament surface. A laboratory experiment showed that An. gambiae reduced landing attempts on Olyset® and increased frequencies of flight after the first contact with the net, while landing attempts on the net treated with deltamethrin were sustained longer [7, 51]. A possible effect of disengagement behaviour associated with permethrin is loss of the ability to sense host cues, which is also known in other insects, such as Glossina austeni [52]. Because of the greater repellent effect of permethrin, Olyset® may reduce the number of anophelines by decreasing the attraction cues of the room [53, 54]. The lower density of anophelines associated with Olyset® found in the present study agrees with this notion although the lower density cannot be explained by the data from the spray catch method only.

The disengagement associated with permethrin suggests decreased interactions between mosquitoes and humans [7, 55]. This deterrence effect should be beneficial for children sleeping with bed nets having holes [54, 56], and multiple children sharing one net [30]. Mosquitoes exposed to permethrin will not seek a blood meal even under these conditions because of a loss of the ability to sense host cues. Similarly, the deterrence will be beneficial for persons who are not under an LLIN [56, 57]. Further, this effect may reduce early-hour biting activity in the room or even outdoor biting activity near a house with an Olyset® [8]. Although loss of the response to host cues may be restored within 24 h [7], as long as a mosquito repeatedly visits the net with permethrin, the effect can be sustained. Permethrin is continuously provided to the surface of the fibre from the inside via osmotic pressure [10]. The present study confirmed that Olyset® used for at least three years had sustained the efficacy.

It is known that mosquito disengagement from Olyset® reduces lethality [7, 51]. The low 24-h mortality of Olyset® found in the bioassay of the present study is also likely due to disengagement, but it is not due to net age because enough insecticide content was maintained on the surface. It has been suggested that the poor killing effect delays the development of resistance against the insecticide [16, 54, 56]. Three main vector species in western Kenya have developed resistance to pyrethroid insecticides; namely, An. gambiae sensu stricto (s.s.), Anopheles arabiensis and Anopheles funestus s.s. Specifically, An. gambiae s.s. has developed resistance associated with a point mutation (knockdown resistance mutation: kdr) in the voltage-gated sodium channel (L1014S), and the other two species have developed metabolic resistance related to one or more detoxification enzymes, such as cytochrome P450s [16, 17, 58]. Although the field-collected adults of the three species from the present study area show strong resistance to both deltamethrin and permethrin in the susceptibility test using the WHO tube test, permethrin still shows a strong repellent effect against An. arabiensis and An. funestus s.s.[8]. On the other hand, permethrin has a less repellent effect against An. gambiae with kdr [6, 8]. As An. arabiensis and An. funestus dominate the present study area after the disappearance of An. gambiae [59], the Olyset® might have become more effective. In contrast, studies in Democratic Republic of Congo as well as Benin demonstrated that use of deltamethrin-based LLINs is associated with lower pfPR than use of permethrin-based LLINs [14, 15]. This conflicting result may be due to great abundance of An. gambiae or Anopheles coluzzi with kdr in these countries [60‐63]. Although the present study excluded the potential effects of other LLIN brands in the same room, the study in Democratic Republic of Congo apparently did not exclude them because of a large-scale cross-sectional study.

This study collected data on the types and locations of LLINs. Although residents were not asked where they preferred to hang what kinds of nets, there were some important commonalities. Olyset® were more common around beds, where nets were hung in a semi-permanent fashion. DawaPlus® 2.0 were more common in community living areas, where nets must be hung, taken down and stored to make the space available for other activities during the daytime [26]. The Olyset® fabric is hard and difficult to fold to a compact size for storage possibly making it a preferred type of net in sleeping areas. The DawaPlus® 2.0, on the other hand, are softer and easier to fold on a routine basis which could explain why residents prefer to use it in living rooms and gathering spaces. This practice can explain not only the positive association of Olyset® users with sleeping in a bed, but also their negative association with pfPR. Unremoved Olyset® may strengthen the deterrence effects increasing the chance of mosquito contact with it even though no one is under the net, because human odour remaining on bedding would be still strong enough to attract mosquitoes to the net. Especially, the deterrence effects may reduce mosquito bites of residents outside of the nets in the early morning and evening. The early-hour biting is characteristic of An. arabiensis and An. funestus in this study area [64]. Moreover, simply using a net in the bed may provide more protective effects compared to sleeping on the floor, because the net spreads well with the bed frame, and its bottom end is tucked in firmly under the mattress [23].