Raphael N’Guessan is a Medical Entomologist and West Africa IVCC programme manager based in Benin. His current research interests are on malaria vector control, with particular emphasis on control of resistant vectors, insecticide resistance management, and investigation of alternative strategies for its delay.
Q: Dear Raphael, please tell us what the main focus of your work is, and why this is important within the framework of malaria control and elimination.
A: My current research interests are on malaria parasite and vector control, with particular emphasis on Research and Development of new tools to delay the appearance of insecticide resistance alleles in mosquitoes and prevent their further spread across Africa.
There is a current policy to eliminate malaria from the African continent. This goal can never be achieved without reliance on effective chemical weapons such as pyrethroid-incorporated Long Lasting Insecticidal Nets (LLINs) and/or Indoor Residual Spraying (IRS). Rather worryingly, resistance to pyrethroids is presently well documented in the major vectors of malaria parasites in Africa, and could decimate the contribution that vector control can make to any successful elimination agenda over the next decade. There is a need to identify pyrethroid resistance mechanisms in the areas where these interventions are currently being scaled up, evaluate their direct impact on the efficacy of these tools and identify novel tools that might have potential as alternatives to pyrethroids and DDT for net or indoor residual treatments.
DDT (Dichlorodiphenyltrichloroethane) for IRS is cost-effective but undesirable because of its environmental impact. Other insecticides, including pyrethroids are too short-lived or too expensive to sustain. More than 2 spray rounds per year IRS in endemic situations with current insecticides becomes difficult to implement by impoverished countries. Malaria eradication in the past failed in many instances partly because of vector resistance but also when donor contributions dwindled in many parts of the poor world. An African malaria elimination goal will only be met if current Research and Development of vector control elements are tailored in a bottom up way that would ever facilitate genuine involvement and investments (as modest as possible) of African nations themselves. For IRS programmes in stable malaria areas to be financially and logistically effective, it is important to develop new long-lasting formulations of currently available classes of insecticide. Investment in this line of applied research will not only produce large cost savings and mitigate logistic but also galvanize nation-wide involvement and expand coverage rates in rural endemic areas. Also, the selective pressure generated by pyrethroids will be reduced and the pyrethroids restricted for Long Lasting Insecticide-Treated Nets only.
Q: You obtained your PhD last December. One of the propositions in your thesis reads 'We cannot eliminate malaria in Africa unless we eliminate poverty'. Please explain what you mean with this.
A: Sorry, I am reserved on this and I skip from this question to the next.
Q: With your collaborators, you have recently discovered a new potent insecticide, chlorpyrifos methyl, that appears better than DDT. Tell us more about these developments.
A: While elimination of malaria is on the agenda, the exploring of alternative insecticides to DDT for IRS has not received the attention it deserves. With a generous grant by the Gates Foundation allocated to the London School of Hygiene & Tropical Medicine (LSHTM) under the auspice of Professor Brian Greenwood, we showed that chlorpyrifos methyl is the best insecticide that meets the desirability of replacing DDT for environmental reasons and may be more cost effective than any other insecticide so far deployed for IRS.
DDT is claimed by WHO to last for at least 6 months although field data showing such evidence are scarce. We are today very evasive about the residual efficacy of a number of current best products deployed for IRS programmes in the field. DDT residual life of <3 months in our experimental hut trial in Benin calls for a systematic review of the longevity of current IRS products on substrates and the need to further re-evaluate them in multi-centric trials.
Chlorpyrifos methyl is a safe alternative to DDT and despite the terrific results with this insecticide presented in the Malaria Journal last week, the company is still not wanting to promote it partly because the IRS market is perceived as being too risky. Again, this technical constraint and market uncertainty for Dow AgroSciences is a real issue that might affect the availability and utility of such useful insecticide for vector control. Other contenders are in the pipeline and may rival chlorpyrifos methyl in the near future.
Q: If you are searching for alternative insecticides for indoor spraying or for application on nets, are you not concerned that the same, resistance, will pop up again?
A: Yes I am. Resistance to a new chemical is certainly inevitable as long as continuous exposure of mosquitoes to this new product operates during vector control programmes or from other sources of selection such as pesticide application in agricultural practices. Alternative actives to pyrethoids and DDT are suitable for mass suppression of vectors in areas where the resistance alleles are already fixed in the vector population and interventions with these classes are showing limitation. Instead, in settings where susceptibility is preserved or resistance is in its moderate form, other resistance management tactics that impose rational use of existing tools should be envisaged, otherwise the strategy to scale up LLINs and IRS use will be compromised and the present gains in malaria control will not be sustained. The research and development of long lasting combination products (of different modes of action) and their delivery formats in mixtures or spatial combination on nets and IRS within homes could be an effective way to delay the appearance of multiple resistance or further prevent their development.
Q: In your opinion, what are the critical elements of vector control that need to be improved or developed in order to make the ambitious goal of malaria elimination feasible?
A: Any elimination ambition today without safe, effective and sustainable vector control tools on board will be doomed to failure. There is a need to continue to search and develop alternative insecticides and to encourage industry to develop long lasting formulation technology for sustainable control of insecticide-resistant mosquitoes.
Also, there are two levels of malaria parasite transmission operating within communities:
a) the indoor transmission which can be tackled by current tools such as ITNs and/or IRS;
b) an outdoor transmission vehicled by selected vectors that preferably feed early on human, e.g. late cooking women in rural settings and human crowding in bars, family meetings etc…
This is an outdoor residual transmission, though small, but difficult to target during malaria control and may also explain why malaria was never wiped out in many places during the first eradication era. The well targeted project in Garki in Nigeria elucidates this well.
With new discoveries such as the Tanzanian outdoor mosquito trapping and killing system this residual transmission of malaria parasites may be absorbed. The synergy between ITN/IRS operating indoor and such a pulling/killing outdoor design of mosquitoes looks very promising.
The improvement and development of such effective outdoor killer should receive further attention.
Q: What will you be doing ten years from now?
A: Difficult to tell precisely; but I am aware that there have been very few studies that have assessed the epidemiological impact of pyrethroid resistance during intervention programmes and most of the few available are flawed by the large number of confounding factors. Over the last 5 years, we have, on behalf of LSHTM investigated a number of alternatives to pyrethroids and DDT and some of them are telling the world that they have potential for malaria vector control, especially in pyrethroid resistance areas. Where LNs are being scaled up in Africa, mainly in specific areas where the malaria vector has shown reduced vulnerability to pyrethroids, information on how malaria indicators will change, including intervention costs if IRS incorporating such long lasting formulation is applied alone or in homes containing LNs is so far unknown. Such combination may also evaluate the additional benefit to LNs of innovations such as the Tanzanian outdoor mosquito trapping and killing system in pyrethroid resistance areas.
With mentorship from Dr Mark Rowland and Prof. Steven Lindsay of LSHTM, these are the kind of trials I intend to concentrate the future on.
Q: Who would you like to nominate for the next e-interview at MalariaWorld?
A: Dr Vincent Corbel. He’s currently involved in a number of trials, including entomologic and epidemiological impact of LNs in moderate resistance area in southern benin. Would be good if he talks about these trials.