The primary objective was to determine whether the pyrethroid lamda-cyhalothrin bound within a polymer resin could improve the wash fastness of the insecticide on nets made from a range of synthetic polymers, natural fibres and dye finishes. To complement the study, controlled comparison was made with a standard pyrethroid CS treatment that lacked the binder. The second objective was to take the treatments through the WHO LLIN evaluation process to determine which insecticide treated substrate would withstand 15-20 washes and potentially achieve WHO recommendation. The third objective was to introduce new types of bioassay to compare against the cone test to test their potential. The fourth was to consider some currently neglected contexts badly in need of vector borne disease control and consider whether insecticide-binder treated substrates could provide a solution.
According to WHO LLIN evaluation guides, polyethylene, cotton and nylon treated with ICON Maxx met the tunnel test criteria of >80% mortality after 20 washes in Phase I, which is a recognized surrogate for 3 years of pyrethroid durability on household LLIN . Polyester white (undyed), the positive control, and polyester blue fell short of the WHO tunnel criteria in Phase I tests but elsewhere in other studies they did achieve [16, 17, 20] emphasizing the importance of multiple trials before coming to a consensus conclusion. Polyethylene also met the required criterion of >80% mortality in cone tests and was the best performing polymer of the four tested. In all bioassays except the tunnel test, cotton and nylon netting did not reach the WHO threshold. Most of the textiles performed well in one or more types of bioassay and none can be ruled out as a suitable substrate for vector control treatment. Even polyester white, the positive control, which failed to meet the required threshold in the Phase I, fared well in Phase II (experimental hut) trials and Phase III (field trials of insecticide durability) in the same locality [12, 13, 16]. Therefore, the Phase I tests reported here are best viewed comparatively, one textile versus another, rather than as pass or fail.
By contrast, none of the nettings - polymer or natural fiber - withstood more than a few washes, at best, when treated with a standard lambda-cyhalothrin CS formulation. There is no question of the superiority of the binder formulation which is a genuine technical advance for a variety of potential malaria control substrates or contexts .
Nylon showed poorer adhesion of ICON Maxx initially and poorest wash resilience of all the materials, losing 84% of insecticide content within 5 washes and 98% within 20 washes. In cone bioassay, mortality decreased by 92% within 5 washes and yet in cylinder bioassay where most of the interior was netting-covered mortality stood at 97% after 5 washes and only decreased to low level after 10-15 washes. In tunnel test, ICON Maxx treated nylon passed the WHO criterion of >80% mortality at 20 washes. Of all the materials tested, nylon was the most unpredictable. While its efficacy in tunnel was encouraging nylon failed as a substrate of preferred choice for ICON Maxx treatment due to the poorer absorption, adhesion and wash-resilience. If there is a choice, the better option would be substitution of nylon with a better adhering or wash-tolerant polymer. On ICON Maxx treated nylon net curtains, as a barrier to Aedes and prevention of Aedes borne arboviruses, it may have potential and further studies are warranted.
Owing to the high absorptive property of cotton, the cotton samples contained the highest loading dose of ICON Maxx initially. However, as on nylon, adhesion and retention of the insecticide was poor, content decreasing by 75% after 5 washes and by 96% after 15 washes. Cone bioassay recorded only 40% mortality at zero and 4% at 20 washes but, as was the case with nylon, mortality in cylinders was high between zero and 10 washes and only decreased sharply at 15 washes. As with nylon, cotton passed the tunnel test criterion for LLIN at 20 washes. The presence of a high dose of lambda-cyhalothrin together with a low insecticidal activity suggests that bioavailability on the surface of cotton netting fibers is low, that is, most of the insecticide remained locked within the cotton fibers and failed to make contact with mosquito tarsi. This is not the case with synthetic fabrics such as polyester and polyethylene on which the insecticide is readily bio-available on the surface of fibres. Other studies have also reported the low insecticidal property of pyrethroids on cotton as compared to other fabrics [21, 22]. However, with the tunnel test, results with cotton netting exceeded 80% mortality after twenty washes, so bringing cotton into line with WHO criteria for recommendation .
Like nylon, polyethylene treatment demonstrated a relatively low loading dosage (40 mg/m2) but in contrast to nylon and cotton, polyethylene showed better retention at 5 washes and a more regular loss rate over the course of 0 to 20 washes. Mortality in cone and cylinder bioassay was consistently high (~95%) over the course of 20 washes, and thus a completely different trajectory compared with nylon and cotton. ICON Maxx seemed to stay bound to the polyethylene which remained fully toxic whereas the binder seemed lost from nylon and cotton during washing. As with nylon and cotton, polyethylene exceeded the tunnel test criteria at 20 washes.
Polyethylene seems an ideal substrate for ICON Maxx. In some studies, polyethylene netting materials were shown to be strong and able to tolerate five years of field use [3, 23]. More recently in larger scale surveys polyethylene has shown poor durability , somewhat improved by changing the knitting weave .
The superiority of ICON Maxx on polyethylene compared to undyed polyester white was a surprise since the latter was the positive control and the polymer netting that ICON Maxx was designed for originally. While both polyester white and blue fell consistently short of polyethylene in an array of bioassay tests, ICON Maxx did attain WHO recommendation for use on polyester over 15-20 washes which is a significant increase in wash-tolerance compared to the standard formulation tested in this paper. After Phase II (experimental hut) trials and Phase III (three-year field trials of insecticide durability) ICON Maxx did attain WHO recommendation as a polyester long-lasting treatment (13). Comparing undyed and dyed polyester netting, chemical analysis indicated similar loading dosages, implying that the binder in ICON Maxx had largely overcome the poor adherence induced by the dye of earlier formulations on polyester (18). At most wash points the rate of loss of insecticide was similar between polyester blue and polyester white treated with ICON Maxx. While polyester white tended to record greater mortality than polyester blue, the differences were marginal and not consistent between all types of bioassay.
Comparison of ICON Maxx with KO-Tab 123
ICON Maxx is not the first wash-resilient formulation to be developed . K-O Tab 123 was a wash-resilient formulation of deltamethrin (25mg/m2) and binder rather than lamba-cyhalothrin (55mg/m2) and binder in ICON Maxx . Its development coincided with the development of factory produced LLIN and it was not taken forward to Phase III. Had it done so, it might have proven as effective as ICON Maxx, which did go on to Phase III evaluation and obtained WHO full recommendation for 2.5-3 years of effective field use . When compared with ICON Maxx on the same materials as tested in the present paper, it showed similarity in characteristics over 20 washes: high insecticide retention and bio-efficacy on undyed polyester and polyethylene and poorer retention and bio-efficacy on cotton and nylon .
Despite having the same 3-minute exposure, the mortality/knockdown responses differed considerably between cone and cylinder tests. The purpose of the comparison was to identify whether the cylinder should supplant the cone as the primary WHO insecticide bioassay. Both are WHO bioassays. The cone bioassay was initially designed for assessment of IRS bio-efficacy and residual activity on walls and ceilings of sprayed houses. Only later was it co-opted for use as an ITN/LLIN bioassay. The IRS bioassay exposes mosquitoes for 30 min; this gives a mortality similar to that of free-flying mosquitoes entering and exiting IRS sprayed experimental huts  and is therefore appropriate as an exposure time. For ITN exposures the cone has limitations: contact time is short and it is difficult to ‘settle’ the mosquitoes on cone netting for the prescribed 3 minutes. If the purpose of a residual bioassay is to manage undesirable variables, then control of exposure time is essential in a short exposure assay. In this respect, the cylinder is an improvement; when cylinder and cone mortality are compared, mortality is higher and less variable in the cylinder than in the cone due to low ratio of plastic to netting. But if the aim is to simulate natural host-seeking behaviour in and around the net then the overnight tunnel test is the more realistic bioassay. It may be time to seriously consider switching from cone to cylinder. It would not negate the tunnel test but for laboratories that cannot access animals for bioassay it would be a move towards reducing animal use for this purpose. The tunnel test might not be needed for most types of bioassay testing except for night-time mitochondrial-acting insecticides such as chlorfenapyr .
The obvious use for ICON Maxx and other treat-it-yourself long lasting pyrethroid kits is to bundle the sachets with the hundreds of thousands of untreated nets that continue to be sold in retail markets, rural and urban. Conical nets, at the 'luxury’ end of the market are rarely bundled with kits, and the wholesalers and retailers of untreated conical and rectangular nets need a regular supply of kits.
This is a timely reminder for beneficiaries of free distributions of LLIN, who may know little or nothing about LLIN production, that LLIN are special because of the insecticide, and they need to treated with care and respect. Older nets can be made more protective with a top-up of insecticide, especially if the next universal coverage campaign is delayed, giving older nets a further 2-3 years of protective use. Universal campaigns are often supplemented with top-ups of new LLINs in the interval between campaigns, and if LLIN numbers are in short supply, untreated nets and older LLINs that are still serviceable would continue to provide benefit if re-treated.
Mosquito nets are not the only household product which might provide benefit from long-lasting insecticide treatment. Curtains made of nylon, polyethylene or cotton could provide family protection from Anopheles vectors of malaria and Aedes vectors of dengue, chikungunya and yellow fever. These could be immersed in ICON Maxx solution like the netting described in this article, or sprayed with deltamethrin 62 SC-PE (polymer-enhanced suspension concentrate formulation), a product specially derived from KO-Tab-123 technology as an aqueous spray formulation (K-Othrine Polyzone, Bayer Crop Sciences, Germany) .
Armed services have favored the use of permethrin on combat clothing for personal protection owing to its high repellence [30, 31]. Alphacyano-pyrethroids such as ICON Maxx might be preferred in certain locations due to its higher toxicity compared to permethrin. To prevent skin irritation the treated material might be separated by a non-treated inner layer .
Disasters and humanitarian emergencies
The same arguments apply to civilian bedding and to top-sheets and blankets treated and distributed in epidemics, disasters or emergencies [33, 34]. Standard issue in humanitarian emergencies are blankets, tents and polyethylene tarpaulins [32, 34, 35] particularly for refugee populations on the move, i.e. situations where nets are dysfunctional or where sprayable housing is absent. Acute phase emergencies are a niche which has proven difficult to supply with adequate vector control protection. The problem is compounded by sectorial nature of international aid. Blankets and tents in emergencies are administered by the shelter sector, vector control is administered by the health sector. Blankets, sheets and shelters are location-specific, and utility will depend on climate and temperature. The solution might be to coordinate the shelter and public health sectors to treat whatever shelter is provided on-site with a long-lasting insecticide or repellent formulation mixed with binder formulation and UV protectant, applied by immersion, spray pump, or treated at source during manufacture. Bespoke factory manufactured products may not justify the investment, bespoke long-lasting formulations to treat a variety of products would be.
The treatment of polyethylene tarpaulins or shade cloth with pyrethroid plus binder as used in emergency shelter has formed the basis of the insecticide treated wall liner concept of protection in the home .
Dual-AI LLIN and non-pyrethroid long-lasting treatment kits
The first Dual Active Ingredient LLINs were the PBO-synergist nets PermaNet 3.0  and Olyset Plus . While the pyrethroid in all WHO recommended LLINs should remain effective for 3 years, WHO is now referring to Dual-AI nets as ITNs because it is not clear whether the PBO component will last a full 3 years of field use . While Olyset Plus, the first in class pyrethroid–PBO net, has demonstrated effectiveness for two years, it is not yet clear in the ongoing cluster randomized trial whether the PBO will remain effective for the full 3 years. If it falls short of 3 years, there is an opportunity here to apply PBO via a PBO-binder long-lasting kit after 2 years to take it through the third year. Similarly, there is an opportunity for a PBO-binder long-lasting kit to be applied to any pyrethroid LLIN to convert those to pyrethroid-PBO LLIN. This would apply equally to other partner AI, such as pyriproxifen or chlorfenapyr which are being used with pyrethroid in other types of Dual-AI LLIN should these fall short of 3 years’ effectiveness . In environments with high pyrethroid resistance, it would be a mistake to allow Dual AI nets to revert to a pyrethroid-only LLIN in their third year as users would be only be part-protected.