Scenarios predict expansion of malaria in South America by 2070

Study uses IPCC's projections to simulate the future distribution of Anopheles darlingi and the protozoans Plasmodium falciparum and P. vivax.

By Peter Moon  |  Agência FAPESP – The primary malaria vector in South America is the mosquito Anopheles darlingi, which was responsible for approximately 276,000 confirmed cases of malaria in Brazil in 2012. The vast majority (99%) occurred in the Amazon region. A. darlingi is very well adapted to life in the Amazon forest, where rainfall is abundant year-round, and the average temperatures are high and within a relatively narrow annual range (i.e., little difference in temperature between the warmest and coolest months of the year).

Amazonia is expected to become a very different place in the medium term because of climate change and global warming. The tropical rainforest may be largely replaced over the next 50 years by Brazilian savanna (cerrado), with a warmer climate and a much broader temperature range.

Under such conditions, A. darlingi may disappear, but this would not mean the end of malaria. South America is home to nine species of mosquitoes belonging to the albitarsis complex. Most of these have not adapted well to Amazonia because they prefer a drier and warmer climate. All of them transmit the protozoans Plasmodium falciparum and P. vivax, the agents that cause malaria. Seven of them are present in Brazil and may become important vectors in the future, thereby expanding the area in which malaria is transmitted by 2070.

This warning is conveyed by biologist Gabriel Laporta, from the Federal University of the ABC (UFABC) in São Paulo State, Brazil, and co-authors in the article “Malaria vectors in South America: current and future scenarios” published by the journal Parasites & Vectors. Laporta’s contribution was part of his postdoctoral research at the University of São Paulo’s School of Public Health (FSP-USP), with FAPESP’s support. His research continues with a Young Investigator Grant from FAPESP under its BIOTA Program.

During the last ice age, approximately 10,000 years ago, the climate and vegetation in Amazonia were very different. The region was drier, and instead of tropical forest, there was a vast savanna. “It could happen again in the future,” Laporta said.

Global warming is poised to affect the Amazon’s climate and vegetation. Plants that prefer a drier and warmer climate than today’s, such as those of the cerrado, will proliferate, whereas species typical of the tropical rainforest are unlikely to survive when the climate becomes drier and will probably disappear or migrate to islands of humidity, forming far smaller tropical forest areas than today’s.

Under this scenario A. darlingi’s habitat will shrink, but the expansion of the cerrado will accelerate the proliferation of the albitarsis complex.

“My research focuses on probabilistic modeling of spatial distributions and ecological niches,” Laporta said. His models are based on variables such as topography, climate and biome, all of which vary considerably across South America, as well as the current distribution of P. falciparum, A. darlingi and the nine species of the albitarsis complex.

He applied these variables to the worst-case scenario computed by the Intergovernmental Panel on Climate Change (IPCC): this scenario represents a situation in which human behavior relating to climate change stays unchanged for the foreseeable future.

Laporta simulated two scenarios for the climate of South America in 2070. Under the first scenario, the annual average temperature rises 2-3°C, and precipitation falls 6-8 mm in the driest month and 12 mm in the wettest. South American biomes remain relatively unchanged under this scenario.

Future scenario 2 is more drastic, with an average increase of 4°C in the highest temperature and a higher annual range of precipitation, rising 1-5 mm in the wettest month and falling 15-17 mm in the driest. The Amazonian biome is highly likely to change significantly under this second scenario.

A. darlingi’s distribution shrinks to 11% of South America in this case, from 22% currently. As for the species in the albitarsis complex for which high levels of precipitation are important, their distribution diminishes under both scenarios, whereas other species stand to benefit from higher temperatures or alterations to the biomes, expanding in terms of geographical distribution.

In aggregate, the models predict that P. falciparum, which currently occurs in 25% of South America, is set to cover 37% of the territory under scenario 1 and 46% under scenario 2.

“To obtain these results, I considered only climate change and the mosquitoes’ habitats,” Laporta said. “I didn’t consider human efforts to eradicate malaria and control its vectors.” Thus, the models behind the scenarios that predict dramatically expanding distribution of malaria vectors in South America are reasonable but far from definitive.

Amazonia is highly likely to become warmer and drier. This change will evidently affect the region’s flora and fauna. However, the scenarios should not be construed deterministically as far as the incidence of malaria is concerned. “Malaria is a disease of the poor,” Laporta said. “It’s epidemic in countries with deficient housing, health and sanitation infrastructures. I’m an optimist. I don’t believe Brazilians will allow their country’s infrastructure to remain deficient until 2070 in the areas where malaria occurs.”

The article “Malaria vectors in South America: current and future scenarios” (doi:10.1186/s13071-015-1038-4) by Laporta et al., published in Parasites & Vectors, can be read at www.parasitesandvectors.com/content/8/1/426.