Deforestation and malaria in Asia

How human activities can cause peaks and troughs in malaria cases
27 May 2021

Interview with 

Francois Rerolle, University of California San Francisco




When we think about malaria, we often instantly picture mosquitoes, which are of course the culprit vector that spread the disease. What we probably don’t picture so often are what the people are doing in areas where malaria is rife, and how those human activities are changing the environment in ways that might impact on malaria transmission. And given the massive increase in urbanisation that’s happening, especially in the tropics, knowing how human behaviours like that are affecting disease rates is critical for effective public health strategies. As Chris Smith heard, UCSF’s Francois Rerolle has been looking at what deforestation does to malaria rates in Lao People’s Democratic Republic…

Francois - There is a connection because malaria is transmitted by mosquitoes and mosquitoes might enjoy the forest environment. And so what's happening in the forest might alter what's happening to those mosquitoes and therefore might alter what's going on with the disease, with malaria.

Chris - Have we got evidence of what happens in other parts of the world when people have explored this sort of question?

Francois - Yeah, actually, and that was one of the motivations for this study. This is a research question that is pretty well studied in the Amazon. So there are some similarities between that region of the world and our region of interest around Laos.

Chris - And when people did look in the Amazon, what did they find? What was the relationship between malaria cases and what was happening with deforestation?

Francois - They led to the Frontier malaria story where deforestation is thought to increase malaria incidence in the short term, but in the long-term it's thought to decrease malaria incidence.

Chris - And why is that?

Francois - What's happening in the short term when deforestation starts, it requires humans to go in the forest, and those humans get exposed to the mosquitoes. But then in the long-term, deforestation might have such a dramatic effect on the mosquito population, you know, at one extreme you could imagine there are no mosquitoes left, and so there is no malaria left.

Chris - And is that what you found when you did the equivalent study in Asia?

Francois - So we did find some similar results in the sense that we also have a short-term increase and we also have a long term decrease in the malaria incidence following deforestation, but we find a different temporal relationship with the time at which this transition from short-term increases and long-term decreases - it was much shorter. It happened after two to three years in Asia, whereas the Amazon has this inflection point more around six to eight years.

Chris - How did you actually do this study then? Did you actually physically put boots on the ground or were you provided with all of the data that informed this?

Francois - A little bit of both. We were running some large studies and I was involved in the data collection of that study. So I went there and I had my boots on, but it's true that most of the data for that particular project was provided to us by the house ministries. Everyone that is tested for malaria is recorded in a registry and the data was given to us.

Chris - Therefore you can marry up who's getting malaria where, and when, and superimpose that on the local rates of deforestation and see what the patterns are between the two?

Francois - Yes, exactly. That's exactly how we answer that research question.

Chris - And how do you explain what you're seeing in Asia compared to what was originally documented in the Amazon with this different temporal relationship?

Francois - So human processes are for sure very different in Amazon. You have this colonisation of the forest deeper and deeper that started in the sixties where you have non-indigenous population getting deeper and deeper in the forest. Whereas in Southeast Asia, the populations that we are studying or indigenous populations that have been living in forest fringes area for not ever but for quite a long time.

Chris - But how does that explain why it takes longer in the Amazon than in Asia?

Francois - I'm not too sure. It may have to do with immunity that those people might have developed because, as in populations that have been hanging around mosquitoes and malaria for much longer, you might find some acquired immunity. And so this definitely will have a differential impact on malaria transmission in this area.

Chris - Is distance a factor as well, because if you've got a small group of people who live in one place and there's local deforestation, but they then travel farther afield to exploit that forest, you're going to see a very different dynamic than if you've got wide-scale deforestation, but wide-scale population at the same time. And that might be the difference between the two places?

Francois - Yes, that's right it might be. And actually, it's one of the points we're trying to make, because although the research question we answered was about the association between deforestation and malaria, what we were really interested in looking at was what is the impact of this population that like you said engage in forest going activities, engaging in maybe deforestation. And this type of population, this subgroup of the population, is believed to have a key impact on transmission in the region. And so we varied the spatial scale, we looked at deforestation happening further and further away from villages. And for instance, we found an impact 30 kilometres away from the villages, but not in the near vicinity of the villages, like within one or 10 kilometres. And this to us suggests that there is this subgroup in the population that is doing something in the forest, at least 30 kilometres away from the villages and that is sort of bringing back a malaria infection in the village.

Chris - Does this inform policy though, in the sense that if you've now got this data and it's much more fine grained, and you've got that geography to superimpose on the relationship, does this inform how we should approach where people are or are not allowed to explore forests? And also how we deploy resources anticipating where we might see big upticks in malaria rates because of people's activity?

Francois - Yes, so the results support the evidence around the importance of this population. And so they support any policies that will, you know, target resources and efforts to this population of forest goers. And this is really important because national control programs have set themselves a goal to eliminate malaria in the region by 2030, and focusing their resources and efforts on this key population might make the work a lot more efficient.


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