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Debate: adults or larvae?

January 7, 2014 - 13:15 -- William Jobin

Why are there two completely opposing views about the value of direct attacks on anopheline mosquito larvae or on adults, for suppressing malaria transmission ?

In recent public and written debates, I have seen diametrically and vehemently opposed views expressed on the value of attacking larvae through eliminating breeding sites, as opposed to the current emphasis on reducing biting by anopheline adults through bednets and indoor spraying.

Of course the priority given to attacking adults and reducing their biting rates is derived theoretically from MacDonald’s mathematical expression of the transmission cycle. But in addition to theoretical calculations, I think we all have our own view from our historical perspective, perhaps while we were first learning to supress malaria.

Perhaps these opposite viewpoints are also derived from our different professional foci on historical anti-malaria efforts in the Subtropics, as opposed to the more recent use of drugs, bednets and biocides in the Tropics.

Perhaps we are fixating on one experience while ignoring the other. Maybe if we merged our understanding of these events we could accept each others view, and reach real understanding. Our differences might be analyzed in terms of groups with two different foci:

FIRST GROUP. Because anti-larval measures were about the only methods available to fight malaria at the beginning of the Twentieth Century just after development of the mosquito theory of transmission, one of the two groups in this debate sees the success of the classic examples in the Panama Canal Zone, in the southern USA, in the Pontine Marshes of Italy and in the swamps of Palestine. They note that malaria was suppressed in these places before DDT and chloroquine became available, and wonder why the anti-larval measures are no longer in fashion, and in fact have been dropped from use by international agencies..

SECOND GROUP. The other folks in this debate feel that these anti-larval measures were effective only because malaria transmission was rather light in these subtropical areas, and that they had some effect, but true suppression and even elimination occurred only after DDT and chloroquine were added to the mix of methods. Perhaps they also were impressed by the rapid suppression of malaria achieved in many countries at the beginning of the Global Malaria Eradication Program of the 1960s when DDT and chloroquine were widely used, and the classical methods of drainage and land reclamation were ignored.

An additional historic fact should be added to both of these views; the permanent elimination of malaria from these subtropical countries over the last 50 years. The US, Europe and other regions previously subjected to summertime malaria epidemics, are now completely free from local transmission, even if malaria cases are imported into these countries in the presence of efficient malaria vectors. This solid elimination of transmission is not due to DDT or chloroquine, nor is it due to new drainage projects. In fact these countries are now preserving and even re-creating wetlands to protect wildlife – also known as mosquito breeding sites. What is going on here?

Perhaps a clue is that the drainage projects of the pre-war period were permanent changes in the ecology. They did not have to be renewed or continued, the breeding sites were gone - period.

Perhaps the permanent elimination of malaria transmission in the US, Europe, and the countries on the rim of the Mediterranean Sea is also due to the winter-time hostility of their climates to the most efficient vectors, and at the same time increased protection of people living in improved housing with screens.

There is also the matter of urban pollution.

In the case of residents of Cairo and Alexandria, the excessive pollution of water and air could be holding anopheline mosquito populations in check. Thus urbanization itself could be suppressing mosquito populations in these countries, as well as improved housing. Notice that re-invasion of Egypt and the other Mediterranean countries has not happened since the Second World War, despite plenty of chances for An gambiae to return, coming from Sudan or other tropical countries.

And what of the fact that the invasion of Brasil by An arabiensis happened half a century ago, but has not happened again?

Perhaps with urban pollution, and as these countries develop industrially and provide widespread availability of affordable and reliable electricity, the anopheline mosquitoes are left out in the hostile environment, and in the dark during the hot and humid transmission season, while people sleep in closed rooms with the comfort of electric fans and even air-conditioning.

Is the Second Group justified in thinking that anti-larval measures are relatively ineffective in suppressing malaria transmission, when compared with drugs, bednets and biocides? I think they might be seeing only half the picture. It just might be that the rapid effect of DDT and chloroquine seen in the subtropical areas after the Second World War was due to the ecologic suppression of mosquito breeding already in place; drainage and land reclamation works that had been so widely applied before the War..

But the First Group is also ignoring some important facts. Quinine was in use in the early days when anti-larval work was first developed, and although expensive and not available for mass administration, the benefit of quinine was noticeably effective when used in places where the mosquito populations and the biting rates were being held down by the drainage works. Thus the satisfaction with disease suppression in the era of drainage works was partly due to the fact that cures and some transmission suppression were also being achieved with this ancient drug.

And the Second Group might be ignoring the disaster of the Global Malaria Eradication Program which quickly produced resistance in the mosquitoes and parasites due to its exclusive reliance on biocides and drugs. Perhaps if the GMEP had also attacked larval stages with the classical methods, the pressure for development of resistance would have decreased enough to avoid the collapse of the program in the early 1970’s.

So what does this tell us about our current situation in Africa? And how can we develop a common understanding of the way forward which will unite the two groups and avoid the division in our ranks? We desperately need to work together because we are coming to a brick wall soon - the Resistance Trap.

I know this is a complex argument, but would appreciate you taking time to examine it, and maybe shed some more light. We should work toward a better understanding of these two issues before the current attack on malaria in Africa falls into the Resistance Trap. I estimate we only have a year or two left to get our ducks in order.

What do you think?