Climate change is defined as a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period—typically decades or longer—that may be attributed to natural internal processes, external forcing, or persistent anthropogenic changes in the composition of the atmosphere or in land use.1 Malaria, the world’s most important and deadly tropical mosquito-borne parasitic disease, kills approximately 1 million people and afflicts as many as 1 billion people in 109 countries throughout Africa, Asia, and Latin America.2 Reducing the impact of malaria will significantly enhance the efforts to achieve the Millennium Development Goals, agreed upon by every United Nations Member State.2 Variation in climatic conditions, such as temperature, rainfall patterns, and humidity, has a profound effect on the longevity of the mosquito and on the development of malaria parasites in the mosquito and, subsequently, on malaria transmission.3
The global temperature has risen significantly over the past 100 years, with an accelerated warming trend since the mid-1950s.4 Elementary modelling suggests that this increase will enhance the transmission rates of mosquito-borne disease and widen its geographical distribution,5 with an increase in malaria, in particular, being identified as a potential impact of climate change.1 While some studies report an increase in the spread of the disease in the current malaria endemic areas,6-8 or a reemergence of the disease in areas which have controlled transmission or eliminated the disease in the past,9-10 others report no association between malaria and climate change.11 Historically, malaria was endemic in Europe, including Scandinavia, but was eliminated in 1975—despite the increase in global temperature—due to better socio-economic conditions, improved irrigation and drainage, adoption of new farming methods, behavioural changes, and access to better health care.12-13
Due to the complex relationship between malaria and climate change, gaps in knowledge still exist in the mechanisms of the linkage. Climate change will increase the opportunities for malaria transmission in traditionally malarious areas, in areas the disease has been controlled, as well as in new areas which have been traditionally non-malarious. An increase in temperature, rainfall, and humidity may cause a proliferation of the malaria-carrying mosquitoes at higher altitudes, resulting in an increase in malaria transmission in areas in which it was not reported earlier.14 At lower altitudes where malaria is already a problem, warmer temperatures will alter the growth cycle of the parasite in the mosquito enabling it to develop faster, increasing transmission and thus having implications on the burden of disease.15-16
Climate change greatly influences the El Niño cycle that is known to be associated with increased risks of some diseases transmitted by mosquitoes, such as malaria, dengue, and Rift Valley fever. In dry climates, heavy rainfall can provide good breeding conditions for the mosquitoes. Increased humidity, droughts may turn rivers into strings of pools, the preferred breeding sites of mosquitos.17 In some areas, heavy rainfall can wash out the breeding sites and reduce the incidence of malaria. In Colombia and Venezuela, malaria cases increased by more than one-third following dry conditions associated with El Niño. In Sri Lanka, before the use of DDT (a synthetic agricultural pesticide used in controlling malarial life cycle), the risk of malaria increased three-fold following the failure of monsoons, which were also associated with El Niño. In Southern Africa, countries have recently experienced malaria epidemics following unusual rainfall.17 Western and north-western India recorded more malaria cases with higher rainfall during La Niña in 1996 and less rain and fewer malaria cases in the same area during El Niño in 1998.18 In short, the changes in the El Niño cycle have the ability to increase the malariogenic potential resulting in malaria epidemics.
