This page lays out what we know regarding key questions around ITN resistance (defined broadly as "any ways in which populations of mosquitoes adapt to the presence of ITNs in order to make them less effective"). We then give some general considerations in favor of not weighing this factor too heavily against ITN distribution for the time being.
Published: November 2012
In this section we primarily cite the World Health Organization's Global Plan for Insecticide Resistance Management in Malaria Vectors (though its content is consistent with that of the other source we've relied on most for this report, Ranson 2011).
"Resistance" can be used to refer to genetic properties of mosquitoes, to refer to behavioral properties of mosquitoes, or to refer directly to failures of mosquito control.1 In the context of malaria control, we've generally seen it used to refer to the first two; as discussed below, there is relatively little evidence of control failure due to these factors, so "resistance" is usually used to discuss specific risk factors for control failure.
The four types of resistance we've seen discussed in this context are:2
We've seen fairly little discussion of the second two types. The report comments:
Ranson (2011) provides a map summarizing the available information about resistance, using publications from 2000-2010. In this context, "resistance" is defined by the percentage of mosquitoes that die after a set period of exposure to insecticide under laboratory conditions; anything under 80% is considered "resistant" and anything under 97% is considered "possibly resistant." 3 This measure is intended for sensitive, early detection and does not necessarily indicate ineffectiveness of control measures.4
Resistance appears common (though far from universal) in the area of western African running from Cote d'Ivoire through Cameroon; it is less common in the countries west of that and in southeast Africa; data from other parts of Africa is thin or nonexistent.
Another map in the same document lists what is known about the spread of genes associated with knockdown resistance; detecting genes is believed to be an even more sensitive and earlier way to detect threats of resistance.5 If these genes were fully responsible for the observed resistance in laboratories, that would be good news, since resistance associated with these genes appears to us to have been most heavily studied6 and (as discussed below) appears to be associated with relatively lower risks of control failure. However, not all of the resistance observed in the map above can be attributed to these genes, and metabolic resistance (a different type, as discussed in the previous section) has been implicated in 10 studies in 8 countries.7
The above covers only target-site and metabolic resistance, which make insecticides less effective in killing mosquitoes; by the nature of the laboratory test, it does not address behavioral resistance. As discussed above, there is currently relatively little evidence regarding behavioral resistance and no clear consensus about its importance.
It has been difficult for us to reliably distinguish the increase in reports of resistance (caused by increased efforts to assess resistance) from an increase in actual prevalence of resistance; reports on resistance often do not draw clear distinctions between the two.8 We also have been unable to find a comprehensive review regarding the question of what factors appear to have contributed to the development of resistance. Ranson (2011) states that there is reason to believe that ITNs contribute to the development of resistance, but resistance may also be brought about through the use of agricultural pesticides and other factors:
It appears to us that
There are multiple reasons that ITNs may retain effectiveness even against mosquitoes that are "resistant" (in the sense of demonstrating low mortality rates in laboratory settings). In addition to the fact that ITNs provide a physical barrier, the insecticide may also repel mosquitoes (and cause them to seek out other targets) even when it does not kill or fully disable them.15 In addition, it's possible that mosquitoes are still killed by the insecticide (despite reduced susceptibility) when they have enough contact with it; that insecticide may inhibit them in other ways that stops them from transmitting malaria; that resistant mosquitoes are less fit overall or less prone to transmitting malaria; or that mosquitoes that are resistant to insecticides at young ages may become less resistant as they age (and that older mosquitoes are more relevant to malaria transmission.)16 We have come across one study that argues for the last of these phenomena.17
The World Health Organization writes that "It is broadly accepted that different resistance mechanisms have differing capacity to cause control failure, kdr [knock-down resistance] tending to be less likely than metabolic resistance (or a combination of mechanisms) to cause control failure."18
One approach to managing resistance is to periodically change the type of insecticide being used, so as to reduce selective pressure for resistance to particular insecticides. (This may reverse the spread of resistance, if resistant mosquitoes are less fit in other ways.19) However, there is only one class of insecticide (pyrethroids) approved for use on LLINs,20 so in areas where LLINs are the main form of malaria control, options are relatively limited (though in cases where resistance is confirmed to be having a major effect, other methods may be brought in specifically in order to get the benefits of using multiple insecticides).21 The World Health Organization recommends that in all circumstances - even where resistance is confirmed and is interfering with control efforts - LLINs should continue to be promoted.22
Hopefully, there will be LLINs with different insecticides that are usable in the future; research has been done toward this goal and has made preliminary progress.23
We're very concerned about insecticide resistance. It appears that relatively little is known about the extent, causes, and control implications of resistance; that resistance could substantially effect (or even negate) the effectiveness of malaria control; and that more research has great potential to improve the impact of the substantial amounts of money spent on LLIN distribution (and other forms of mosquito control). This investigation has highlighted research on insecticide resistance as a potentially outstanding giving opportunity itself, and we hope to look into it more.
That said, this issue doesn't change our bottom line that LLIN distribution is a highly cost-effective intervention or that Against Malaria Foundation is our #1-rated charity. There is strong evidence that LLINs reduce malaria and save lives and only preliminary/suggestive/mixed evidence that insecticide resistance may reduce their impact. Importantly, it appears to us that the malaria control community has been devoting at least some attention and investigation to this issue for a long time, has developed a reasonable knowledge base (if one that has plenty of room to grow), and still recommends the use of LLINs regardless of the resistance situation (as noted in the previous section). We have previously been impressed with the thoughtfulness and data behind malaria scholars' answers to our concerns over the interpretation of macro-level data on LLIN distributions and malaria burden, the question of why universal coverage is now being pursued (as opposed to targeting children under five), the question of whether ITNs delay the development of immunity and thus merely delay malaria deaths, and the question of whether ITNs are used by recipients, and are inclined to put high weight on recommendations such as this.
Finally, we'd like to note that
|Source name used in footnotes||Link||Date that link was last accessed||Archived link (saved version in case link is down)|
|Czeher et al. 2008||Link||Nov. 5 2012||Link|
|Dabiré et al. 2006||Link||Nov. 5 2012||Link|
|Jones et al. 2012||Link||Nov. 5 2012||Link|
|Ranson 2011||Link||Nov. 5 2012||Link|
|Van Bortel et al. 2009||Link||Nov. 5 2012||Link|
|World Health Organization 1998||Link||Nov. 5 2012||Link|
|World Health Organization 2012||Link||Nov. 5 2012||Link|
"Molecular genotyping of resistance is the identification of the underlying genes that confer the inherited trait of resistance (15 ). Identification of a resistance gene provides evidence of the underlying evolutionary process. Depending on the type of resistance mechanism, this provides understanding of both the degree of resistance expressed in individual insects with the resistance gene, and the frequency of such insects in the population.
"Phenotypic resistance is the basic expression of the genetic cause of resistance, shown by a vector’s ability to resist and survive the effects of the insecticide. Phenotypic resistance is measured in a susceptibility test of vector mortality when subjected to a standard dose of the insecticide. WHO has defined phenotypic resistance as “development of an ability, in a strain of insects, to tolerate doses of toxicants, which would prove lethal to the majority of individuals in a normal population of the same species” (16 ). Phenotypic resistance is the phenomenon most commonly referred to in public health.
"Resistance leading to control failure - while phenotypic resistance provides an indication of the effects of resistance on the vector, the most informative way of looking at resistance is as an epidemiological phenomenon, in which resistance is identified as the cause of increasing malaria transmission. In the notion of resistance leading to control failure, evidence of resistant vectors is linked directly to the failure of vector control programmes in the field. Resistance leading to control failure can be defined as the 'selection of heritable characteristics in insect population that results in repeated failure of an insecticide product to provide intended level of control when used as recommended.' Resistance leading to control failure is the phenomenon mocommonly referred to in agriculture. National malaria control programmes should not, however, wait for control failure to occur before implementing strategies to manage insecticide resistance. There is no acceptable level of control failure in public health, and waiting could result in delaying action until it is too late." World Health Organization 2012, Pg 27.
"Target-site resistance occurs when the site of action of an insecticide (typically within the nervous system) is modified in resistant strains, such that the insecticide no longer binds effectively and the insect is therefore unaffected, or less affected, by the insecticide. Resistance mutations, known as knock-down resistance (kdr ) mutations, can affect acetylcholinesterase, which is the molecular target of organophosphates and carbamates, or voltage-gated sodium channels (for pyrethroids and DDT) ( 15 , 17 ).
"Metabolic resistance is related to the enzyme systems that all insects possess to detoxify foreign materials. It occurs when increased or modified activities of an enzyme system prevent the insecticide from reaching its intended site of action. The three main enzyme systems are: esterases, mono-oxygenases and glutathione S-transferases. While metabolic resistance is important for all four insecticide classes, different enzymes affect different classes ( 15 , 17 ).
"Although most resistance mechanisms (especially kdr resistance) have been studied for decades in previous cases of resistance, the detailed study of mono-oxygenase metabolic resistance is relatively new, and our understanding of it is fairly limited. Indeed, cases of mono-oxygenase resistance in mosquitoes were unknown before its identification in South Africa in 2000–2001 (see section 1.2.3 for details).
"As described below, metabolic and target site resistance can both occur in the same vector population and sometimes within the same individual mosquito. The two types of resistance appear to have different capacities to reduce the effectiveness of insecticide-based vector control interventions, with metabolic resistance being the stronger and more worrying mechanism (see section 1.2.3 for details).
"Behavioural resistance is any modification in insect behaviour that helps it to avoid the lethal effects of insecticides. Several publications have suggested the existence of behavioural resistance and described changes in vectors’ feeding or resting behaviour to minimize contact with insecticides. Studies in New Guinea and the Solomon Islands showed that Anopheles farauti vectors stopped biting later in the night (23:00–03:00) after the introduction of indoor DDT spraying and instead bit only in the earlier part of the evening, before humans were protected by sleeping in a sprayed room ( 18 ). In most cases, however, there are insufficient data to assess whether behavioural avoidance traits are genetic or adaptive; genetic traits could have major implications for the types of vector control interventions needed. All behavioural traits, however, may not be negative, as they could lead mosquitoes to feed on non-human animals. It is also possible to initially mistake the decline of a vector species as behavioural resistance.
"Cuticular resistance is reduced uptake of insecticide due to modifications in the insect cuticle that prevent or slow the absorption or penetration of insecticides. Examples of reduced penetration mechanisms are extremely limited and only one study has suggested correlation between cuticle thickness and pyrethroid resistance in An. funestus (19). Microarray experiments have identified two genes that encode cuticular proteins that are up-regulated in pyrethroid-resistant strains of Anopheles mosquitoes. Experience with other insects suggests that if cuticular resistance emerges in mosquitoes it could have a significant impact when combined with other resistance mechanisms." World Health Organization 2012, Pg 27-28.
Where <95% mortality occurs in tests that have been conducted under optimum conditions with a sample size of >100 mosquitoes then resistance can be strongly suspected." World Health Organization 1998, Pg 17.
"Where knock down resistance (kdr) is involved, KD rate [knock down rate] is a sensitive indicator for early detection of pyrethroid resistance." World Health Organization 1998, Pg 16.
"It provides an early warning of resistance development as the mutation arises well before any effect on phenotype can be detected in a population . Indeed, the expression of the 24 h-survival diagnostic phenotype  appear to be recessive [45,53]. Therefore, a population presenting a low kdr frequency mainly in heterozygous state is likely to show a high mortality rate during bioassays. As further evidences supporting the advantage of kdr genotyping over bioassays for emergent resistance detection, Chandre et al.  found a significant mortality reduction only when heterozygous females proportion reached 60%, and a significant increase of Knockdown time (KdT) only with 40% heterozygous females. Secondly, the kdr mutations seem to be well correlated with resistance phenotype [25,38,45,53] in both An. gambiae molecular forms, even if metabolic resistance mechanisms could also be involved in increased tolerance to pyrethroids ." Czeher et al. 2008
"Typically two major mechanisms are assumed to be responsible for insecticide resistance: changes in the target site that reduce the binding of insecticides, and increases in the rate of insecticide metabolism that lower the amount of insecticide reaching the target site. Both of these resistance mechanisms are known to contribute to pyrethroid resistance in malaria vectors and are subjects of extensive research to determine their distribution and impact, and to develop improved methods of detection. Of these, target site resistance is best understood and molecular diagnostics to detect this resistance mechanism are now integrated into insecticide resistance monitoring strategies in some malaria control programmes." Ranson 2011, Pg 2.
"The absence of simple genetic markers for metabolic resistance means that far less is known about the distribution of the responsible alleles. Biochemical assays, and in some cases microarray studies, have implicated metabolic resistance In An gambiae s.l. in Kenya [22, 108], Cameroun [109, 110], Benin , Nigeria , Ghana , Mozambique , South Africa  and Zimbabwe . Over expression of CYP6P3 and/or CYP6M2 has been found in pyrethroid-resistant An gambiae populations from Benin, Nigeria and Ghana [23, 25], mainly in co-association with the kdr L1014F allele. This co-occurrence of resistance genes may constitute an additional threat to malaria vector control as epistasis between these two types of resistance conferred extremely high level of pyrethroid resistance in Culex quinquefasciatus ." Ranson 2011, Pg 12.
For example, World Health Organization 2012 states, "There are multiple examples of insecticide resistance spreading quickly over large areas. For example, the kdr mutations known as 1014F and 1014S were first detected in West and East Africa, respectively (referred to as kdr West and kdr East mutations). They have now been detected on both sides of the African continent because of both the spread of the original mutations and because of new, independent origins of the same mutations (Figure 14). kdr West has now been detected as far East as Ethiopia, Sudan, Uganda and Zambia, while kdr East has been found in Angola and several countries in West and Central Africa (such as Benin, Burkina Faso and Côte d’Ivoire) (24 )" (pg 38). It is ambiguous here whether "spreading quickly" refers to resistance or the detection of resistance.
"On the island of Bioko on the West African coast, an IRS campaign with lambdacyalothrin failed to curtail an increase in the population density of pyrethroid resistance An gambiae M form although a modest but significant reduction in transmission index and malaria reported cases was observed [14, 116]. High frequencies of the L1014F kdr allele were observed in the local An gambiae population (M form). Only after pyrethroids were replaced with the carbamate bendiocarb did the mosquito population decline . Nevertheless, in an operational scale programme such as this, the possible contribution of other factors to the failure of pyrethroid IRS to control mosquito population density cannot be overlooked so the direct consequence of the high kdr frequency is uncertain." Ranson 2011, Pg 13
"There have been extensive randomized controlled trials (phase III) in Africa aiming at investigating the efficacy of ITNs for malaria prevention , but very few have assessed how pyrethroid resistance might affect the effectiveness of such intervention. In the Korhogo area in the north of Côte d’Ivoire where the 1014F kdr allele frequency is >90%  and malaria is endemic, lambdacyhalothrin-treated nets had a significant impact on the entomological inoculation rate (55% reduction)  and on malaria incidence in children
"In southern Benin, a randomized controlled trial was carried out in a mesoendemic area to assess the impact of long lasting ITN scale-up on malaria morbidity in children <5. In this area, where the kdr frequency is around 50 % in An gambiae, transmission increased during the rainy seasons but was not followed by a seasonal variation in parasite infection and clinical incidence, suggesting that ITNs still keep certain level of efficacy in moderate pyrethroid resistance area ." Ranson 2011, Pg 13-14
"One of the problems associated with many of these studies is that, due to the lack of molecular markers for alternative resistance mechanisms, the frequency of kdr alleles is frequently used as a proxy for resistance. This can be misleading if metabolic or other resistance mechanisms are the predominant resistance mechanism. There is an urgent need for properly controlled large-scale trials to assess the impact of pyrethroid resistance on IRS and ITNs. Such studies should use both entomological and epidemiological indices and should be conducted in areas where alternative resistance mechanisms are known to be responsible for pyrethroid resistance. Furthermore, these studies must consider the possibility of behavioural resistance and monitor for changes in key traits such as location of resting and feeding which may impact on the efficacy of current insecticide based interventions." Ranson 2011, Pg 14-15.
"Although there are limited options for [resistance management] with LLINs, they may retain an effect despite increased resistance to pyrethroids. Firstly, nets provide a physical barrier against biting by mosquitoes as long as they are intact ( 2). Secondly, in most vector species, resistance to pyrethroids does not completely reduce the effect of the insecticide. It has also been observed that the irritancy of pyrethroids (‘hyperexcitatory response’) may reduce mosquito blood-feeding or encourage diversion to other hosts by certain vector species that do not feed exclusively on human hosts. This effect can vary, however, by species and geographical location." World Health Organization 2012 Pg 45.
"Possible reasons for widespread insecticide resistance with no obvious impact on the effectiveness of vector control:…
World Health Organization 2012, Pg 111.
"The development of malaria parasites into the infectious sporozoite stage takes 10 or more days following a blood-meal . The older cohort of An. gambiae s.s. used in this study (17-19 days old) could potentially harbour infectious parasites but would be more susceptible to insecticide-based control. Targeting older, and arguably more epidemiologically significant mosquitoes, has been proposed as an alternative strategy for wider malaria control in which the selection on resistance to the control agent is reduced." Jones et al. 2012.
World Health Organization 2012, Pg 36.
"Resistance can probably be reversed only if the vector incurs a ‘fitness cost’ for being resistant (if the resistance gene confers some disadvantage on these vectors in comparison with susceptible populations). Once the insecticide is changed, these resistant mosquitoes will no longer have an advantage, and will die out.
"Some IRM strategies (e.g. rotations) are based on this concept — that removing selection pressure will reverse resistance, and that it may therefore be possible at some point to reintroduce the original insecticide into vector control programmes." World Health Organization 2012, Pg 14.
"Vector control is very dependant on a single class of insecticides, the pyrethroids. These insecticides are safe and fast acting and are the only class approved for use on insecticide treated materials." Ranson 2011, Pg 1.
See World Health Organization 2012, Table 5 on Pgs 83-84.
See World Health Organization 2012, Table 5 on Pgs 83-84.
"Alternative insecticide classes (e.g. pyrroles, oxadiazines) used to control agricultural pests have recently proven effective in laboratory and field trials against pyrethroid-resistant mosquitoes but unfortunately these are not currently available in suitable formulations and some of them may never be mass-produced for malaria control [132-134]. Initial results from studies combining unrelated insecticides on mosquito nets [126, 135, 136] or insecticide with synergists or repellents [137-139] to delay the appearance of resistance or further prevent its development are encouraging. At larger scale, a randomized controlled trial carried out in an area with insecticide resistance in southern Benin (REFS-MAE initiative), showed a significant reduction of transmission and malaria incidence in children <5 by combining spatially different interventions in the household (e.g. ITN plus carbamate treated plastic sheeting) compared to the control group (i.e. selective coverage of ITN to populations at risk) . Nevertheless, more work is needed on the durability of these approaches and their acceptability by populations before the additional cost can be justified to control programme managers." Ranson 2011, Pg 16.