Will Global Warming Mean More Malaria?

Many statistical models indicate that climate change, with associated global warming, will result in increases in vector borne diseases. Such vector borne diseases include sleeping sickness which is transmitted by the tsetse fly, lyme disease caused by bites from the deer tick and malaria transmission by the Anopheles mosquito vector. Other diseases transmitted by Anopheles, Culex and Aedes mosquitoes include dengue fever, yellow fever, viral encephalitis and West Nile disease.

Malaria infections take the lead as the vector borne disease causing the greatest number of infections, the largest number of deaths and the highest amount of morbidity globally. Caused by the Plasmodium parasite, half of the world population, are at risk of malaria. Plasmodium falciparum malaria infections are estimated to cause 1 million deaths each year, with almost 90% of these deaths occurring among children under 5 in Sub-Saharan Africa. Malaria infections from all 6 Plasmodium species known to infect human can also cause significant morbidity which correlates with reductions in economic growth.

A recent model published by Gething et al. in the Journal Nature has suggested that climate change will have a minimal impact on malaria globally, and that other factors will be much more influencial. However, there is much debate on this issue, and other models have presented indications that increased malaria transmission will come with climate change. Many of these models indicate that rising temperatures, and changes in other key climate change variables, will create more permissive environments for Anopheles mosquitoes. There are predictions that in such permissive environments, particularly in areas where individuals are already infected with Plasmodium, malaria transmission will increase; meaning that these increases are predicted in regions already heavily burdened by malaria. Other models, particularly describing short-term climate changes, warn of the spread of malaria into malaria-free areas including regions which are no longer malaria endemic and areas at high elevation.

For Steve Lindsay, Professor of public health entomology at LSHTM, modelling climate change and disease risk from insect vectors plays an important first step to understanding disease risk; it can provide a “broad brush understanding” and appreciation of risk. However, Lindsay considers, it’s important “not to take models too seriously”. The second, and crucial, step for Lindsay is to “go to the field” in order to understand with real data where the mosquitoes are, and where, when and who they are likely to bite. Paul Reiter also contended in a 2008 Malaria Journal article that many climate change malaria risk models offer predictions that are analogous to “pour[ing] more water into a glass that is already full.” According to Reiter, many models “sidestep” the multitude of factors which contribute to malaria transmission such as the ecology and behaviour of humans and the ecology and behaviour of vectors, as well as individual host and host population immunity.

The history of malaria transmission in non-tropical regions provides a reminder that climate change variables, such as temperature, rainfall and humidity, cannot be considered as independent factors for malaria and other vector borne diseases. Malaria has a historical distribution both in the northern and southern hemispheres. In England, ecological changes, improvements in human living conditions and greater access to medical care were amongst the important factors which contributed to malaria eradication in the early 20^th century. In contrast, rapid reductions in malaria transmission did not occur in Soviet block countries until after the advent use of synthetic insecticide DDT (dichlorodiphenyltrichloroethane) in the 1940s, 50s and 60s. Today, many countries in the southern hemisphere are still malaria endemic, but even within malaria endemic areas disease transmission has peaks and troughs, and malaria is not transmitted all year round. So these seasonal malaria transmission patterns also serve as a reminder that climate change variables should not be considered as independent predictors for vector borne disease.

Over-emphasis on global warming in relation to malaria misses the mark on the immediate need to address persistent socio-economic and political factors which drive malaria transmission; it also overlooks the fact that malaria control measures are working. The 2010 Roll Back Malaria and UNICEF World Malaria Day report estimates that 1 million lives have been saved between 2000 and 2010 because of increased financial support for malaria interventions and research. The report shows reductions in childhood mortality as a result of increased insecticide treated bed-net usage and prompt, effective anti-malaria treatment for disease with parasite diagnosis.

Malaria remains a preventable disease and a disease intimately associated with poverty. There is compelling evidence that tackling malaria will also have a significant impact to reduce other causes of childhood mortality in Sub-Saharan Africa. So a continued commitment to combat malaria could be a win for all children under 5 in areas with high childhood mortality rates.

While some model predictions might be pessimistic, many argue that climate change won’t have marked impact on vector borne diseases. For Lindsay, climate change will be a “minor” factor defining future risk of malaria transmission. Economic development together with sustained and expanded malaria control and treatment measures for malaria endemic countries are important current and future indicators of malaria risk. Lindsay considers that urbanisation, irrigation and deforestation, rather than climate change, may be the most important environmental factors which will drive future risk of malaria and other vector borne diseases in the coming decades.

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