Deltoid

There were some letters written to Nature by malariologists disputing Chapin and Wasserstrom's paper that argued that agricultural use of DDT was the major factor in the resurgence of malaria in India and Central America. Before I write about the dispute I should stress what they all agreed on:

It is generally agreed among malariologists that agricultural insecticides have made a contribution to selection for insecticide resistance in mosquitoes and that such resistance has made a contribution to the resurgence of malaria in Central America and South Asia.

Furthermore none of the malariologists suggested that environmentalist pressure had anything to do with the resurgence of malaria. I doubt that anyone could have made such a claim in 1981 without being laughed at.

Where they differ is in how important the agricultural use of DDT was in causing the resurgence. C&W had a dramatic graph showing the number of malaria cases plotted against the use of DDT. However, they were careless with the data they used with the graph, not noticing that their source (Harrison) had reported recorded cases for the start of the 70s and estimated cases at the end. This had the effect of greatly overstating the increase in malaria. A version of the graph which just shows recorded cases is shown on the left. The true number of cases is certainly much more than what is shown on the graph, but this gives a better indication of how much malaria increased. The graph still shows a strong correlation, but there are clearly other factors operating.

In the reply C&W refer to the geographic pattern of DDT resistance and increases in malaria and I think an analysis of this could give an indication of the relative importance of agricultural spraying. Unfortunately, they don't present such an analysis, so they have not proved their case.

So certainly agricultural use of DDT caused some of the increase in malaria and it may have caused a major part of the increase, but the second part is unproven.