While there’s been much debate in the media about the causes and effects of climate change, many scientists agree that changes in climate will impact our lives. However, demonstrating such impact is scientifically challenging because of the many different factors that are involved and the difficulty of making predictions about the future, as well as politically charged because of the implications any findings will have for public policy and human health. In a recent issue of the journal Science, Mercedes Pascual and colleagues tackled the question of whether the spread of malaria might be affected by climate change, in particular at higher (and normally, cooler) elevations [1, 2]. Their findings clearly show that in warmer years, malaria spread to higher altitudes in Colombia and in Ethiopia, suggesting that as global temperatures increase more people will be at risk for contracting this disease. With malaria being one of the oldest human diseases and one for which we have known the mechanism of transmission for a long time, why has it taken so long to demonstrate a link between climate change and disease?

 Why malaria loves the tropics

The means by which malaria is spread suggests that there would be an important role for climate in its transmission [3]. Malaria is caused by unicellular organisms of the genus Plasmodium that split their life cycle between two different hosts, a mosquito and another animal in which they cause disease – including humans as well as other animal species. Once the parasites develop within a mosquito, they migrate to the salivary glands, and because female mosquitoes feed on the blood of humans and other animals, the parasite is transmitted through the blood to a new individual and can be picked up by other uninfected mosquitoes (Figure 1).Therefore, the malaria parasite is dependent on the mosquito to spread, and the mosquito, in turn, is dependent on climate, requiring water to lay its eggs in and preferring warm, wet habitats. This explains why today malaria is mostly found in the tropics. However, the lifestyle of the mosquito is not the only way in which the malaria parasite is affected by climate. As Pascual explained in an interview in Science magazine’s March Podcast [4], the life cycle of the parasite itself is slowed down at cooler temperatures, which means that it may not mature inside the mosquito before the mosquito bites another person or dies, limiting its spread even if the mosquito is present.So warmer, wetter environments make it easier for malaria to spread.

Figure 1 ~ Lifestyle of the malaria parasite

Connecting climate change and malaria

Why then, has it been difficult to establish a relationship between global temperatures and the incidence of malaria? Detailed historical records of malaria cases exist at the sites used in the current study, but of course, climate is not the only thing that has changed over time. There are several independently varying factors that influence malaria levels over time, such as insecticide spraying to control mosquitoes, treatment of patients with the disease, development of drug resistance in the malaria parasites, and human population density at different altitudes. There also may be factors that vary along with climate change, such as rainfall patterns, that influence malaria levels more than and/or independently of any changes in temperature that may be occurring. Because it is difficult to account for the effects of these other factors, it has been a challenge to associate malaria incidence with temperature changes.

In the current study, rather than looking at temperature changes over the whole decade or more for which they had information about malaria cases in these regions, the authors focused on shorter “interannual” timescales, for example considering temperature changes between consecutive years.The use of shorter time points for comparison minimizes the impact of other factors that might change in the long-term, such as parasite drug resistance. Additionally, they repeated their analyses without the areas that had experienced insecticide spraying, and made sure they saw the same trends with and without these areas. To control for the fact that the total number of malaria cases changes from year to year, the authors looked at cumulative distributions of malaria cases by altitude, meaning that they examined the proportion of that year’s cases found in each altitude range rather than the absolute number of cases (Figure 2). And because they were studying two regions with different climates (for example, in Ethiopia rainfall increases with altitude while in Colombia it decreases), the authors could determine if the effects seen at different altitudes depended on these other variables, or whether they were actually linked to the decrease in temperature that occurs with altitude.

Figure 2 ~ Cumulative distributions of malaria cases by altitude

 The burdens of a warming world

With these and several other controls, Pascual and colleagues were able to show the impact of altitude on malaria disease burden in a couple different ways. Firstly, they took different regions and, ignoring altitude, organized them into groups that had experienced similar patterns of malaria prevalence in space and time. The resulting groups were very similar to the groups produced when, ignoring malaria patterns, regions were organized by elevation. This indicates that altitude is a good predictor of the distribution of malaria in a region. Secondly, considering data from across regions, they found that, in years with a higher mean temperature, the overall distribution of malaria cases shifted to higher altitudes. With these results, the authors argue that future increases in temperature will result in an expansion of malaria to higher altitudes and call for public health interventions to address this disease burden. For example, the authors had previously estimated that if malaria does reach higher altitudes in warmer years, a 1 degree rise in temperature would lead to 410,000 infections per year in regions not usually exposed to malaria and 2.8 million additional infections in areas already experiencing malaria in Ethiopia alone [5]. Their current study supports this prediction.

Malaria is one of several mosquito-borne diseases and one of many infectious diseases that is affected by the environment and could potentially be influenced by global climate change [6]. As research into this area will contribute to disease control efforts, it is important to ensure that studies are carefully controlled and that public health interventions are directed towards the most appropriate areas. Regardless of whether you believe that the world’s climate is being altered by human activities or changing in the long-term, it is clear that short-term changes can impact the range of a disease, and this is something we must be prepared for.

Niroshi Senaratne is a graduate student in the Biological and Biomedical Sciences program at Harvard Medical School.

References

1. Siraj AS, Santos-Vega M, Bouma MJ, Yadeta D, Ruiz Carrascal D and Pascual M (2014). Altitudinal Changes in Malaria Incidence in Highlands of Ethiopia and Colombia.Science 343(6175), 1154-1158.

2. Morelle R (2014, March 6). Malaria ‘spreading to new altitudes.’BBC World Service. http://www.bbc.com/news/health-26470755

3. World Health Organization (Fact Sheet N°94, Updated March 2014). Malaria Fact Sheet. http://www.who.int/mediacentre/factsheets/fs094/en/

4. The American Association for the Advancement of Science (2014, March 7). Science Podcast: 7 March Show. http://www.sciencemag.org/content/343/6175/1154/suppl/DC2

5. Pascual M and BoumaMJ (2009). Do rising temperatures matter.Ecology 90(4), 906-912.

6. McMichael AJ, Campbell-Lendrum DH, Corvalán CF, Ebi KL, Githeko A, Scherago JD and Woodward A (2003). Climate change and human health – risks and responses SUMMARY (published by the World Health Organization).Chapter 6 Climate change and infectious diseases. http://www.who.int/globalchange/climate/en/chapter6.pdf