Several new papers in the open-access journal Malaria discuss the thorny problem of insecticide resistance. I’ll give you the paper citations, abstract summaries, and a (mostly) non-technical translation in this post.
The monitoring of insecticide resistance in malaria vectors is very important to assess *potential* selection effects of insecticides, and to plan for the next step should control methods begin to fail. One of the things monitored closely in these papers is the presence of kdr mutations.
kdr stands for knock-down resistance. I’ve discussed kdr before in a long post about the mechanics of insecticide resistance to DDT. Basically, mosquitoes coming in contact with a pesticide are “knocked-down”–it takes a while for the pesticide to finish them off, but if they faint on contact (anthropomorphizing wildly here), that means they aren’t biting while they finish dying. Knock-down is also a handy measure for how well a pesticide is working.
The presence and frequency of kdr mutations, as measured by molecular biology studies in these papers, are a useful resistance marker. It provides an early warning of resistance development since the mutation arises before any effect on phenotype (resistance and control failures) can be detected in a population.
If we know that resistance is beginning in advance, we can act to try to stop it (changing practices or pesticides to reduce selective pressure for the gene), or at least plan to manage it.
In the second paper, agricultural use of pyrethroids and DDT for crop protection led to the selection of resistant individuals. Mosquitoes breed in water, and runoff and residues exert an additional selection pressure. Resistance was higher in areas where there was agriculture.
What this means is that resistance to the chemicals used in bednets could happen faster in areas where DDT and other pesticides are used. Since bednets are very effective, this could be a really bad thing.
Given that that there is a KNOWN cross-resistance between the main chemical used in bednets and DDT–and DDT strongly selects for resistance, with failures of control seen within a few years on some insects–this further convinces me that we should be extremely cautious in re-introducing that chemical into the mosquito control of malarial areas.
These studies also show that research into new chemicals for bednets, or new and tougher bednets that last longer, will need to continue, and maybe increase. Unfortunately, when dealing with a living system, constant vigilance and innovation is the rule.
Dynamics of multiple insecticide resistance in the malaria vector Anopheles gambiae in a rice growing area in south-western Burkina Faso. Malaria Journal 2008, 7:188 doi:10.1186/1475-2875-7-188
“Insecticide resistance of the main malaria vector, Anopheles gambiae, has been reported in south-western Burkina Faso, West Africa. Cross-resistance to DDT and pyrethroids was conferred by alterations at site of action in the sodium channel, the Leu-Phe kdr mutation; ….The impact of multiple resistance mechanisms in M and S populations of An. gambiae on vector control measures against malaria transmission, such as insecticide-treated nets (ITNs) and indoor residual spraying (IRS), in this area is discussed“
The second paper:
Evidence of increasing Leu-Phe knockdown resistance mutation in Anopheles gambiae from Niger following a nationwide long-lasting insecticide-treated nets implementation. Malaria Journal 2008, 7:189 doi:10.1186/1475-2875-7-189
“The distribution of the kdr-west (kdr-w) Leu-Phe mutation was studied in Anopheles gambiae s.l. populations from Niger and temporal variations were monitored following the nationwide LLIN [bg: long-lasting insecticide-treated net] implementation. This study describes the first case of kdr mutation in Anopheles gambiae populations from Niger. It is suspected that the LLIN have caused the important temporal increase of kdr-w mutation observed during this study. While the kdr mutation is still found at a low level, this rapid increase could potentially lead to high kdr frequencies within a few years.
A third paper:
A pre-intervention study of malaria vector abundance in Rio Muni, Equatorial Guinea: their role in malaria transmission and the incidence of insecticide resistance alleles. Malaria Journal 2008, 7:194 doi:10.1186/1475-2875-7-194