LLIN, new products and the impact of/ on insecticide resistance
In the past 15 years malaria mortality and morbidity rates have been halved. This owes not least to insecticide based interventions and in particular the Long Lasting Insecticidal Net (LLIN). In recent years increased findings of insecticide resistance have caused serious concerns whether these advances are threatened, although how much and how to respond remain topics for discussion.
Much of the polemic surrounding insecticide resistance of malaria mosquitoes seems to be caused by imprecise terminology and recent claims around new mosquito control products and their ability to mitigate it. But the question that we really need to ask is “what do we really want the LLIN to do?” Kill mosquitoes, prevent them from biting, or prevent malaria transmission? While resistance to the class of insecticide used in LLINs
, pyrethroids, is found in more and more places it is not shown to have impact on the LLIN as a tool against malaria transmission.
Insecticide resistance can be defined in many ways depending on the purpose and test method. IRAC (Insecticide resistance Action Committee under CropLife, the agrochemicals industry organisation) uses an often quoted operational definition: “Insecticide resistance is the selection of a heritable characteristic in an insect population resulting in repeated failure of an insecticide product to provide the intended level of control when used as recommended” (IRAC 2011)[i]. This definition is different from that of WHO, which is linked to how to measure resistance: “Place 0-3 day old female mosquitoes into a test tube lined with an insecticide treated paper and determine survival rate after 1 hr exposure” (WHO 2013)[ii].
Types of resistance
There are two general types of mechanisms of resistance to the pyrethroid class of insecticides (that include deltamethrin, alfacypermethrin and permethrin) that have been identified in Anopheles mosquitoes (malaria transmitting mosquitoes).
The first type is the Knock-Down Resistance mechanism (KDR). KDR is by far the most wide-spread type of resistance and is based on a genetic change in the receptor for pyrethroids, which causes the insecticide to be less likely to attach to the neuronal cell. In a WHO cone test used for testing LLINs (3 minute exposure), KDR mosquitoes are not knocked down and fewer are killed and the tested product will appear to have a reduced level of efficacy. Pyrethroids are also widely used in agriculture and mosquito larvae often breed in puddles that receive wash off with these insecticides from plants or soil. KDR was originally associated with cultivation of cotton (Diabate et al, 2002)[iii]. KDR however, seems to have little effect on LLIN efficacy in real use because mosquitoes do not rest just for 3 minutes on an LLIN and KDR resistant mosquitoes can rest on the LLIN much longer thus increasing the chance of picking up a lethal dose of the insecticide (Chandre et al, 2000)[iv].
The second type of mechanism is the more general metabolic resistance. It has several sub-types based on enzymes that can break down (metabolize) the insecticide. One such enzyme group are the oxidases (especially cytochrome P450) and, like KDR, this mechanism of resistance also seems to be spreading, though it is less common. Another group is glutathion transferase, which is so far much less common.
Is resistance important for malaria transmission?
WHO underlines; “the changes of resistance with age differ dramatically depending on the resistance mechanism involved, with insects sometimes becoming more susceptible over time” (WHO, 1998)[v]. The WHO guideline requires that insecticide resistance measurements are carried out with females of age 0 to 3 days. Mosquitoes of this age do not transmit malaria. Mosquitoes need to bite a malaria infected person and from then it takes about two weeks before the malaria parasite is developed in the mosquito and can be transmitted to the next person. Several studies show that mosquitoes at this age have more or less lost their insecticide resistance (Rajatileka et al, 2011; Jones et al, 2012)[vi].
LLIN use impacts malaria transmission by “personal protection” and “mass effect”. Personal protection of an LLIN is provided by the prevention of contact i.e. transmission. When a person is sleeping under a (mostly) intact LLIN, the mosquito will not be able to bite through the net or find a hole in it before it is repelled or killed by the pyrethroid on the net.
If many people in a village sleep under LLINs, the mosquito population is reduced. This is a mass effect. Since mosquitoes cannot transmit malaria before an advanced age and will have to bite people several times before getting to that age, many will already have been killed by LLINs and few mosquitoes will actually reach the age of ability to transmit malaria.
Only a few studies have evaluated the impact of resistance on malaria transmission in areas with known resistance. A meta-analysis found that even in areas with high pyrethroid resistance, LLINs continued to reduce blood feeding compared to untreated nets (Strode et al, 2014)[vii]. Measurements on malaria transmission show that LLINs with a single pyrethroid maintain efficacy against malaria transmission to under-five year old children in an area of KDR resistance in West Africa (Henry, et al, 2005)[viii] and in a cohort study in East Africa with enzyme based resistance (Lindblade et al, 2015)[ix]. There is to date still not a single study that shows that insecticide resistance reduces the effect of single pyrethroid LLINs as a malaria transmission prevention tool.
New synergist LLIN claims
Two producers of what is now known in the malaria world as “combination nets” claim that such nets are resistance management tools and/or have the ability to better control mosquitoes resistant to pyrethroid (Vestergaard, 2015; Sumitomo Chemicals, 2015). In the discussion on insecticide resistance and how to respond to it in terms of strategy, product claims have been made that “the time for a mono-insecticide LLIN is over”. This makes for an inaccurate analogy to combined artemisinin drugs (as opposed to “mono-therapy” drugs) for malaria parasite control as recommended by WHO.
Both combination nets combine a pyrethroid with piperonyl butoxide (PBO). PBO is not registered as a second insecticide, but as a synergist that reduces the resistance based on a group of enzymes P450 (described above) by letting the enzymes metabolize PBO molecules instead of the insecticide (crowding out effect that only works for as long as there is enough PBO to keep the enzymes busy). This renders the “non-mono therapy” analogy misguiding as there are not two insecticides working on two different mechanisms and the solution rather seeks to overpower the insecticide mechanism just as turning up the dosage of insecticide would. Furthermore, these enzymes are only expressed in young mosquitoes but in a mosquito at the age of malaria transmission, they provide no resistance (Rajatileka et al.2011, Jones et al. 2012) [x].
Permanet 3.0, an LLIN from Vestergaard-Frandsen (VF) with a polyethylene (PE) roof containing both deltamethrin and PBO and with increased (compared to their standard Permanet 2.0) dosages of deltamethrin on roof and sides, has been evaluated twice by WHOPES and once by a WHO Vector Control Advisory Group (VCAG). WHOPES found that a washed Permanet 3.0 had no better effect on resistant mosquitoes or even non-resistant mosquitoes than Permanet 2.0 (WHO 2008)[xi]. When VF provided new data a year after the first evaluation, WHO made a new evaluation that confirmed the former (WHO 2009)[xii]. In 2014 this net was evaluated by VCAG as a new vector control paradigm. VCAG evaluates if a new tool provides its public health benefit via a new mechanism and, if so, how this tool should be assessed. However, the committee remarked that VF had provided no studies showing it had effect on malaria transmission in areas of resistance (the would-be public health benefit). Nevertheless, the product was recognized as a new paradigm. A committee member (Dr. Lengeler) explained; “We recognised the net type as a new paradigm, not the net itself. Recognizing the product is WHOPES’ job, not ours”. Permanet 3.0 still has no recommendation or approval as a special tool for improved control of malaria in areas with insecticide resistance.
Olyset Plus, a PE net from Sumitomo Chemical (SC) with PBO and permethrin on all sides in the same dosage as the standard Olyset, is a different story. Permethrin, unlike deltamethrin or alfacypermethrin, is very sensitive to enzymatic breakdown. Therefore, in areas with oxidase based resistance, Olyset Plus should work better than Olyset, but not necessarily better than any other LLIN brand. However, Olyset Plus is also better than Olyset because of a faster release rate of permethrin (WHO evaluation report 2012)[xiii], as slow release is a known problem for Olyset (Gimnig et al, 2006)[xiv]. Cone test with Olyset Plus on resistant mosquitoes showed that Olyset Plus after 20 washed never obtained a control level of 80 % in tests in Africa and India (WHO evaluation report 2012) [xv]. The WHOPES report concludes that, it is not known if PBO plays a role in the better performance of Olyset Plus. Olyset Plus may be an improvement of Olyset, but has no recommendation or approval as a special tool for improved control of malaria in areas with insecticide resistance.
Resistance is turning up in more and more places and it is reasonable to expect that this may be or become an operational threat to the vector control component of the global malaria control program. It is prudent that a principle of caution be assumed and that the toolbox of vector control products is both updated and diversified as the common good of susceptible mosquitoes requires this to be maintained. Evidence from the past 15 years do not overwhelmingly suggest that even in a world of perfect susceptibility would the LLIN ever suffice for the famously difficult last miles. This observation should also beget the question, if we are not sure of the kind of failure we think we may be seeing in regards to current generation LLIN, should we spend most of our efforts to update it? Is a new mode of action all we need or must we also consider new modes of operation? It is equally important that the discourse is guided by sound principles as it will likely lead to changes in programs that may well involve reducing the scope of coverage when newer products, like the above-mentioned combination nets, are more costly than current LLINs and the budget is not increased. Basically, the choice to reduce the number of people we protect in order better protect a smaller number requires very solid data and deliberations. It is a duty to ensure resources can be applied to maximize the public health benefit while preserving the public trust, without which all efforts will come to a stop.
[i] IRAC (The Insecticide Resistance Action Committee); Prevention and Management of Insecticide Resistance in Vectors of Public Health Importance, 2nd Ed, 2011.
[ii] WHO: Test procedures for insecticide resistance monitoring in malaria vector mosquitoes, WHO/HQ, 2013
[iii] Diabate A, Baldet T, Chandre F, Akogbeto M, Guiguemde TR, Darriet F, Brengues C, Guillet P, Hemingway J, Sall GJ, Hougard JM : The role of agricultural use of insecticides in resistance to pyrethoirds in Anopheles gambiae s.l in Burkina Faso. Am J Trop Med Hyg 67(6), 617-22, 2002.
[iv] Chandre F, Darriet F, Duchon S, Finot L, Manguin S, Carnevale P, Guillet P: Modification of pyrethroid effects associated with kdr mutations in Anopheles gambiae. Med Vet Entomol 14:81-88, 2000.
[v] WHO: Test Procedures for Insecticide Resistance monitoring in Malaria Vectors, Bioefficacy and Persistence of Inscticides on Treated Surfaces, WHO/CDC/CPC/MAL/98.12, Geneva 1998
[vi] Jones CM, Sanou A, Guelbeogo WM, Sagnong N’F, Jpohnson PCD, Ranson H: Aging partially restores the efficacy of malaria vctor control in insecticide-resistan populations of Anopheles gambiae s.l. from Burkina Faso. Mal J 2012,11:24, 2012.
[vii] Strode C, Doengan S, Garner P, Enayati AA, Hemingway J: The impact of pyrethroid resistance on the efficacy of insecticide-treated bed nets against African Anopheline mosquitoes: systematic review nd meta-analysis. PLoS Med 11: e1001619, 2014
[viii] Henry MC, Assi SB, Rogier C, Dossou-Yovo J, Chandre F,Guillet P, Carnevale P : Protective efficacy of lamdacyhalothrin treated nets in Anopheles gambiae pyrethroid resistance areas of Cote d’Ivoire. Am J Trop Med Hyg., 73(5), 859-864, 2005.
[ix] Lindblade KA, Mwandame D, Mzilahowa T, Steinhart L, Gimnig J, Shah M, Bauleni A, Wong J, Wiegand R, Howel P, Zoya J, Chiphwanya J, ,Mathanga J: A cohort study of the effectiveness of insecticide –treated bed nets to prevent malaria in an area of moederate pyrethroid resistance, Malawi. Mal J 14:31, 2015.
[x] Rajatileka S, Burhani J, Ranson H: Mosquito age and susceptibility to insecticides. Trans R Soc Trop Med Hyg 105(5):247-53. 2011
[xi] 12th WHOPES Working Group Meeting, WHO/HQ Geneva 2008.
[xii] 13th WHOPES Working Group Meeting, WHO/HQ Geneva 2009.
[xiii] 15th WHOPES Working Group Meeting, WHO/HQ Geneva 2012.
[xiv] Gimnig J,Lindblade KA, Dotson E, Hawley WA: Letter to the editors of Trop Med Int Health (as response to letter from Sumitomo bu Dr Itoh), Trop Med Int Health 11, 252-53, 2006.
[xv] 15th WHOPES Working Group Meeting, WHO/HQ Geneva 2012.