Last week, WHO published a statement regarding the potential of larviciding for malaria control in Africa. This followed the circulation of a draft version of the statement in August 2011. That draft was sent to a limited group of people (how many I don't know) for comments (including myself). I attach the official version to this editorial.
After reading this document I asked myself a number of questions. First, it is abundantly clear from this document that larval control is seen as secondary if not tertiary in importance compared to the use of LLINs or IRS. Of course, the Ross-MacDonald model explains why a focus on adult control methods should be more effective than merely reducing mosquito densities at the larval stage (due to the length of the extrinsic incubation cycle and the daily survival probability, pn). But although this may be true, we are faced with some pretty hard facts in the real world of malaria control:
1) Resistance is spreading and already rampant in many places in sub-Saharan Africa. Without a new class of insecticides for bednet impregnation, and perhaps three new active ingredients coming out of the IVCC pipeline by 2020, what will we do in the meantime?
2) With a shift in vector species composition towards Anopheles arabiensis and the disappearance of An. gambiae in many places where nets/IRS have been implemented, how will we deal with outdoor biting and shifts in biting times?
3) Considering that malaria elimination/eradication is also WHO's goal, what do they propose to add to LLINs/IRS next when it has been shown in various studies that these indoor interventions on their own will not result in elimination?
Nevertheless, the overall tenure of this document is pretty negative when it gets to larval control. There is too little evidence from Africa, it may not be cost-effective (particularly in rural areas) and it requires detailed entomological surveillance and skills to be implemented properly. It is claimed that only in places where breeding sites are few, fixed, and findable, larval control may have a role to play as a supplementary tool (with nets and/or IRS).
But what is missing here is the goal when larval control is added: is it merely control or is it elimination? Take Egypt as an example: it eliminated its malaria along the Nile from the border with Sudan all the way north to Asyut during WWII. It has not seen malaria since, and no doubt, the return on investment of that elimination campaign (largely based on larval control using Paris green) is very, very positive. Was it labour intensive? Yes. Were the breeding sites few, fixed, and findable? Not really.
Would it be possible to repeat the same elimination campaign, now starting in the border town of Wadi Halfa in Sudan and move south to its capital Khartoum -by using exactly the same strategy Egypt deployed during WWII - ? No doubt it would be, as long as the capacity and financial resources allow for sufficient number of staff to be deployed and such a campaign would be well-managed.
It is beyond any doubt that many of the drier regions in the Sahelian zone and the Horn of Africa, as well as in Southern Africa have large areas that are relatively isolated and where larval control could make a huge difference. And yes, WHO's paper states that detailed knowledge of the local vectors is required and that larval control cannot be applied everywhere. But this does not mean that larval control is not a good weapon to be added to the toolbox. The fact that a larval control trial in the Gambia faltered is highlighted, even though it is well-known why this was the case - which had very little to do with the biology of the system.
In all parts of the world where malaria was eliminated larviciding did play at least a major if not significant role (in combination with larval source management). The current position paper may push policy makers further away from adopting larviciding as an add-on tool to their ongoing control efforts. NMCP managers that are not aware of the giant contributions that larval control made to malaria control (and elimination) around the world may refrain from moving beyond nets and IRS.
I have not the smallest doubt that a decade from now we will be talking a completely different language that will include the acronym AW-IPM: Area-Wide Integrated Pest Management. So well adopted in numerous agricultural pest control endeavours, yet so distant from those in the field of malaria vector control...
What do you think?