For millions of people in the developing world, tropical diseases such as malaria are a constant presence.
As tropical diseases rely on suitable climatic conditions, the prospect of climate change has raised concerns that these diseases could spread into previously disease-free regions.
This concern is given weight by the “overwhelming evidence” on climate change contained in last month’s IPCC report.
Malaria is a Disease of Tropical Climates
Malaria is a significant global disease burden, with 1.2 billion living at high risk of malaria and over half a million dying from the disease in 2010, according to the WHO.
Malaria is a vector-borne disease, transmitted by Anopheles mosquitoes. The temperature governs both the malaria parasite and the mosquito vector’s lifecycles, so malaria cannot be transmitted in cool climates and is highly seasonal on the fringes of the tropics.
If climate change makes subtropical regions more permissive to parasite and mosquito, new populations could be placed at risk of malaria.
Modelling Malaria Epidemics: Higher Global Risk of Malaria Under Climate Change?
A study in PNAS earlier this year modelled the impact of climate change on malaria at time periods up to the 2080s. Using 5 different global climate models and 5 different malaria distribution models, the researchers found an overall increase in the population at risk of malaria under future climate change scenarios.
However, the net increased risk was primarily due to localised increased risk in the African highlands, parts of South America and south-eastern Asia. Only two of the five malaria models predicted a northward spread of malaria epidemics in to Europe, Russia, northern Asia and northern America. The authors emphasised the high level of uncertainty in their models, particularly for the later time periods.
Historical Temperature Increases Correlate With Increased Malaria Incidence
Collecting information on historical temperature changes and malaria epidemics involves less uncertainty than predictive models. Since the 1970s, malaria incidence has increased in the east African highlands, areas where cooler temperatures make malaria more seasonal than in the lowlands.
A paper in PNAS from 2006 looked at meteorological records for four sites in western Kenya, southwestern Uganda, southern Rwanda and northern Burundi, and found a 0.5°C increase since the 1970s.
When this temperature increase was used to model mosquito population dynamics, an increase in mosquito numbers of between 30-100% was predicted. While other factors such as human migration, and altered land use could also influence malarial incidence, the model points to temperature as a factor clearly worth consideration.
Malaria Moves into Higher Regions in Hotter Years
The highland areas of Ethiopia and Colombia have detailed records of both malaria and temperature from 1993-2005. Professor Mercedes Pascual from the University of Michigan used this data to determine if the geographical distribution of malaria changed year on year, rather than looking at long term trends. Writing in Science, they describe how in warmer years, malaria moved to higher altitudes. They estimate this temperature-dependent shift would result in an additional 3 million children infected annually with malaria.
Malaria Responds to Climate Change, But Also Other Factors
The science points towards climate change affecting where and when malaria will infect humans – the spatiotemporal incidence of malaria in the highlands of endemic regions. However, other factors will usually govern malaria incidence, such as local socio-economics and disease control interventions. The re-emergence of domastically-transmitted malaria in Greece in 2012 demonstrated this, after government cutbacks saw a reduction in anti-mosquito spraying schemes. Unfortunately, the areas identified to come under increasing threat during climate change are within developing countries, and as such are poorly equipped to mitigate epidemics.