In a discussion with Malaria No More, Marcia Castro, Andelot Professor of Demography and chair of the Department of Global Health and Population at Harvard T.H. Chan School of Public Health, talks about a recent Harvard School of Public Health study that found ecological changes in the Brazilian Amazon have led to a significant increase in the risk of malaria transmission.
Other studies have previously looked at the impacts of deforestation on malaria, yet you say this one is unique. How so?
CASTRO: If we go back a few decades, there was a massive of explosion of malaria cases in the Brazilian Amazon. At the time, there was a theory put forward that the environmental change resulting from large-scale agricultural colonization projects were contributing to the increase of malaria cases, a phenomenon called ‘frontier malaria,’ a term for malaria transmission in communities originating from frontier settlements in the Brazilian Amazon.
Then later in 2002, when I did my Ph.D., we revisited this concept of frontier malaria but brought a more spatial perspective to the theory. We knew there had been several studies looking at the effect of deforestation on malaria in the Amazon context. But one big limitation on those studies was that they often overlook the seasonality of the disease and were just looking at data from an annual perspective versus weekly.
As we know, malaria is very seasonal, and cases peak during certain months of the year. But much of the earlier analysis completely missed the temporal component of frontier malaria. Plus, malaria remains a serious public health risk in Brazil. In 2022 alone, 42.5 million people in Brazil were at risk of malaria, with 129 thousand cases reported. So, we really wanted to build on existing research and advance our knowledge of the impacts of deforestation.
What data did you use?
CASTRO: We started by looking at data from 2003 to 2022 and analyzing connections between malaria transmission and deforestation.
We pulled data from MapBiomas Brasil, a phenomenal platform that maps land cover and land change in Brazil at the 30m pixel since 1985. We also used daily deforestation alerts from the National Institute of Space Research to look at deforestation patterns.
What we found was a lag in the calendar due to malaria’s seasonality and the rise and fall of transmission rates. But we also found that for every 1% increase of deforestation, on average after a month, there was also 6.3% increase in malaria cases. We also extended our model to the entire Amazon, looking at various climate conditions. By using those numbers, we bring science to policymakers, because if we aren’t monitoring these things, then we don’t have the ability to react in a proactive way. So, we’ve been missing the ball.
Your study specifically looked at ecological changes, but what does your research show about climate change or any linkages?
CASTRO: A great question! One of the things we have to keep in mind is that this entire process of forest removal and environmental change in the Amazon is actually speeding up the process of climate change.
For example, the southern portion of the Amazon, where most deforestation has occurred, is already receiving less rain. That means that the length of the dry season is now longer by around one month.
Last year and this year, under the El Nino, the Amazon observed its most severe droughts. We have changes in climate and extreme events occurring much faster. So, it’s all related.
What message would you have for policymakers?
CASTRO: Overall, the message our study sends to policymakers is that we better do something, and fast. What happens in the Amazon doesn't stay in the Amazon. If things get out of control regarding malaria, we can have spillovers to other countries. So, the more we can move forward on malaria elimination across Brazil and improve the efficacy of malaria elimination programs there, everybody will be better off.
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