Pouring gasoline on the insecticide resistance fire
the wrong approach to resistance management
Recently there was another review of the mosquito net that is being marketed as having been developed to overcome insecticide resistance, which may undermine the efficacy of long lasting insecticidal nets (LLIN) that are a central element in the ongoing fight against malaria. The resistance claims that were made by the manufacturer of this product have been unequivocally rejected before, both in peer reviewed articles and it the WHOPES report[i].
The product in review was Permanet 3.0 and the article Efficacy of PermaNet(R) 2.0 and PermaNet(R) 3.0 against insecticide-resistant Anopheles gambiae in experimental huts in Cote d'Ivoire by Hemingway et al [ii] and it ends with a potentially dangerous conclusion that may be read as an endorsement of the product for resistance prone areas.
The authors of the article arrive at more or less the same efficacy data as other teams of researchers before them, but they do not, explicitly conclude that the product is not a resistance tool[iii]. The authors find when comparing the unwashed nets, the roof of the Permanet3.0 performs best with the highest mortality followed by the sides of the Permanet 3.0 and then the Permanet 2.0.
This obviously sounds interesting even if the data are for unwashed nets. At 20 washes this advantage is gone but a reader may wonder if some of the effect will stick around for some of the following washes?
Permanet 3.0 is a new product in that it has sides made of one type of material, polyester (like Permanet 2.0), and a roof made of another material, polyethylene. In the side netting it has an insecticide dosage of 2.8g / kg deltamethrin and in the roof it has 4.0 g/ kg deltamethrin and 25 g/ kg of the synergist piperonyl butoxide (PBO) [iv]. The theory behind this product is based on the ability of PBO to block the pyrethroid resistance mechanism in the mosquito and that the presence of PBO thus confers the added toxicity in resistant strains of mosquitoes[v]. The theory is sound, but so far, no one has been able to demonstrate this desired effect of the Permanet 3.0 roof.
So then, what is it we see in the unwashed nets but we don’t see in the 20 times washed nets in the Hemingway et al article? Is it the effect of the deltamethrin/ PBO combination as is a likely interpretation of the article? The answer is most likely no.
Instead, what is most likely being demonstrated is a difference in activity levels due to the different concentrations of deltamethrin; and therefore, a good relation between insecticide concentration and mortality. Since resistance, functionally, is not a "yes / no" property but a sliding scale, a decrease in susceptibility can to an extent (and for a while) be countered with an increase in the dose of the insecticide.
In line with the above, the authors write in their discussion "The high dose of deltamethrin alone in the side panels of unwashed PermaNet® 3.0 could explain the significant difference in mortality of resistant An. gambiae s.s with this net compared to the other treatment arms". So why not extend that same logic to the roof of Permanet 3.0?
The answer appears to be a very unfortunate oversight. The roof of Permanet 3.0 is not characterized in the article - and the fact that it contains the highest level of deltamethrin of all of the materials tested which appears to have been overlooked. The roof of Permanet 3.0 contains 4 g/kg of the insecticide deltamethrin, as compared to the sides of Permanet 3.0 which contains 2.8 g/kg of deltamethrin. Permanet 2.0 only contains 1.8 g/kg of deltamethrin[vi]. This explains well the incremental improvement in mortality as a function of deltamethrin content and should likely have been added to that paragraph in the discussion.
Why may Permanet 2.0 perform as well as Permanet 3.0 once both products are washed? As noted in the article, the retention of deltamethrin on the Permanet 2.0 is quite good, as cited in literature. However, the study did not encompass chemical analysis of the used nets and does not offer insight in active levels of the Permanet 3.0 after washing.
From the WHOPES review of the product, we learn that Overall AI retention for PermaNet 3.0 after 20 washes was 13% and 74% for deltamethrin on the side panels and roof respectively, and 92% for PBO on the roof (Pigeon 2008f).[vii] The loss rate may explain why efficacy of the two products aligns after wash.
Another interesting point of information from the WHOPES report is the high retention of PBO. At 92% it would appear that very little of the synergist actually migrates to the surface where it should be available to function in synergy with the deltamethrin, and from where it would inevitably be washed off were it present. With a loss rate of only 8% and 92% remaining inside the polyethylene fiber it is therefore unlikely that any of the PBO ever participated in the biological activity of the product.
This again points to the likely conclusion that the relatively higher efficacy of the Permanet 3.0 roof can be accounted for by the higher level of deltamethrin found here. It is regrettable that the manufacturer chose to combine the presence of PBO in a net, which is technically an interesting idea, with a much higher level of deltamethrin as the reality of the function of the net may elude researchers.
In a recent article that describes the potential of PBO / pyrethroid combinations by Helen Pates Jamet (Vestergaard-Frandsen) et al it is demonstrated how well the deltamethrin and PBO may work against resistant mosquitoes when, in a laboratory setting, the mixture is made available to mosquitoes. The authors showed that when very resistant mosquitoes were subject to a baseline dose of deltamethrin mortality and KD were 1% and 3.5% respectively, doubling the dose improved the results to 5.3% and 9.7% but adding PBO to the baseline dose brought mortality and KD all the way up to 98% and 99.5% respectively[viii]. This is in line with literature in general, that increase in bioefficacy is counted in multiples and that doubling the dose of the insecticide alone only brings marginal improvement as seen in the different cited reviews of Permanet 3.0. The article by Helen Pates Jamet et al further supports the idea that Permanet 3.0 fails as resistance tool because PBO is simply not bio-available.
This scientific puzzle could remain of academic interest only if it wasn’t for a more problematic circumstance; doubling the dose of insecticide is one of the best ways to exacerbate resistance. In a document issued by IRAC (Insecticide Resistance Action Committee) and curiously reviewed by one the article’s authors, Janet Hemingway, the document states that; Increasing the dosages in an attempt to maintain efficacy is not a recommended option... because the resistance genes can be driven to even higher frequencies[ix] . In other words, increasing the pyrethroid content of an LLIN will not bring about a resistance management tool; it is more like adding gasoline to a fire.
One may wish that Janet Hemingway, who is generally regarded as one of the pre-eminent leaders on vector resistance, and whose name will give weight to the risky allusions in the conclusion of this article when considered by malaria campaign managers, would address the concerns raised and that the ambiguities in the article are not left generally pointing towards Permanet 3.0 as being a resistance control tool – especially if the opposite is true.
[vii]http://whqlibdoc.who.int/hq/2009/WHO_HTM_NTD_WHOPES_2009_1_eng.pdf page 52 in the pdf
[viii] Can piperonyl butoxide enhance the efficacy of pyrethroids against pyrethroid-resistant Aedes aegypti?, Georgina Bingham, Clare Strode, Lien Tran, Pham Thi Khoa and Helen Pates Jamet, Tropical Medicine & International Health, Volume 16, Issue 4, pages 492–500, April 2011