Given a choice, faced with a malaria-carrying mosquito heading in your direction, would you opt for an insecticide that killed it before it reached you, or one that merely repelled it?
Most of us would plump for the first option I am sure, and this is how insecticide treated bednets (ITNs) work and why they have proved so successful in those malaria endemic regions of Africa where the vector mosquito bites at night. BUT I learnt on Thursday (19/11/09), at the ASTMH 2009 meeting, that entomologists are revisiting a decades old idea of using insect repellents that do not kill. This is not to save MOSQUITO KIND: the fact is that killing mosquitoes with insecticides inevitably leads to insecticide resistance, and apparently there are only 14 safe insecticide compounds approved by the WHO for use in insect vector control, so we need to delay the development of resistance by all means possible. Exploiting non-toxic chemicals which repel, to reduce man-vector contact, adds to our vector control armoury and comes with a number of other benefits: delaying onset of resistance, application suitable for both inside & outside the home, public acceptance.
The session was entitled SPACE REPELLANTS: this was sufficient to draw me in as I had no idea what was being repelled! A model, based on observations, was presented by Edward Walker (Michigan State University) which broke down how an insect repellent worked into three parameters: contact/ absorption, elimination of the drug, followed by excitation (movement but not in a directed fashion). The combination of all 3 leads to disengagement of the insect and it turns out that it’s the excitation which has the strongest effect on disengagement (disengagement is what you want!). The model enables you to test repellent development: the ideal repellent would elicit high excitation, rapid absorption, slow elimination. Olysnet amd Permanet, 2 different insecticide-treated materials, gave different results for these parameters. Its not known if current repellants work by increasing disengagement or decreased directed movement towards the host. This model system should answer that too.
Field tests involve building huts of local design, and then treating entry points such as doors, windows and eaves, or entire interior walls with the repellent. Mosquitoes are collected at entry and exit points. Work using such huts, presented in this session, demonstrated that known insecticides such as DDT could be used at non-toxic doses as effective spatial repellents, and that treating entry points not only was highly effective , safer for the human occupants, but that the mosquitoes did not appear to decamp to an unprotected hut (Nicole Achee, USU, Bethesda).
Sarah Moore of IFAKARA & LSTMH pointed out that as more people in Tanzania acquired electricity, so their sleeping patterns changed & spent less time in bed protected by ITNs, so repellents were increasingly needed. In addition, if you can keep man-vector contact down you can decrease both transmission to the host and from the host to the vector, breaking the cycle.
Moving onto actual human interventions, Mark Rowland (LSTMH) supplied villagers in Tanzania with either DEET treated or cyphenothrin (a pyrethroid) treated bedsheets, with the longterm goal of malaria prevention in disasters. Both treatments, mosquitoes still entered the homes but they fed less (3x less with DEET) and had increased mortality (4x with DEET) . The DEET was formulated into microspheres before application onto the fabric, whihc enabled it to remain effective for many months. Even the colour, durability, washability & comfort aspects were considered in this trial: people wanted to carry on using their ITNs along with these sheets, khaki turned out to be the practical housewife’s choice of colour and woven fabric more durable. (The non-woven one rapidly filled with holes, defeating the purpose!) The hope is that following a cluster controlled RCT, these sheets (& blankets) could prove valuable in providing protection from malaria for displaced populations such as in refugee camps (ITNs being impractical), for populations in endemic areas and for the military.
Global Health and its archive have a wealth of information on insectborne diseases and the behaviour of the insect vector: I will add relevant references from these resources in the near future. Watch this space…for more on space repellants.