Deforestation in the Amazon basin is strongly linked to an increase in malaria cases, a large geospatial review has revealed – but there’s an upside. Sort of.

The review, conducted by Andrew MacDonald of Stanford University and Erin Mordecai of the University of Florida, both in the US, also found that as the malaria burden climbs the rate of land-clearing starts to fall.

The effect, the researchers write in the journal PNAS, can be described as “bidirectional socioecological feedback”. To put it in less technical terms: cutting down trees makes people sick, and sick people are no good at cutting down trees.

To make their finding, MacDonald and Mordecai looked at geospatial data for 795 municipalities across the Amazon basin covering the years 2003 to 2015. They report that the numbers “show deforestation has a strong positive effect on malaria incidence”. {%recommended 5977%}

The pair are not the first investigators to look at the relationship between forest clearing and vector-borne disease transmission, but earlier studies returned results that were unclear and ambivalent.

This, they suggest, was because it failed to detect that cause and effect in the Amazon runs in both directions.

“Our results suggest a 10% increase in deforestation leads to a 3.3% increase in malaria incidence,” they write.

In 2008, for instance, they found that 1567 square kilometres of land was cleared across the basin. In the same period, and in the same localities, there was an increase of 9980 malaria cases.

These results only became clear, however, once the researchers looked for, and found, a counter-effect.

“We estimate a 1% increase in malaria incidence results in a 1.4% decrease in forest area cleared,” they state.

Interestingly, the mechanisms involved in the seesawing tree-cutting and disease scenario are quite different. The increase in malaria cases is the result of ecological drivers, while the decrease in clearing is because of socioeconomic factors.

The effect, too, is not universal, but very much influenced by local conditions. MacDonald and Mordecai found that the numbers, in both directions, were highest in the Amazon interior but absent in the outer states, “where little forest remains”.

The uptick in malaria cases, they note, is another example of the unintended consequences of large-scale land conversion. Factoring such things into planning and policy is critical to achieving sustainable ecological, agricultural and public health outcomes.

“Ignoring this statistically well supported feedback leads to substantial underestimates of the true effect of deforestation on malaria transmission,” they warn.