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Malaria: Minimal Role Of Climate Change

Dr David Whitehouse

A lot has been written about the relationship between malaria and climate change to the extent that many articles about the topic were in little doubt that as the global temperature increases so would the extent and severity of the disease. This possibility makes the interrelationship between malaria and climate change a major, global public health issue.

The debate has been contentious. Some commentators have drawn simple conclusions and predicted a malaria-ridden world from what is obviously a complicated situation. The relationship between climate and mosquito populations is highly complex. There are over 3,500 species of mosquito and all breed, feed and behave differently.

It has been suggested that increased temperatures, higher rainfall and humidity can affect the incidence and spread of vector borne diseases such as malaria by creating more breeding pools for vectors and allowing them to develop more quickly. The complexity of the situation is shown by the fact that in some areas the opposite can occur, higher rainfall can wash out breeding pools, thereby decreasing mosquito populations. Also, a 1935 malaria epidemic in Sri Lanka, which killed an estimated 100 000 people, came after two exceptionally dry years.

Mosquitoes, however, are highly adaptable and use highly effective survival strategies to protect themselves against extreme heat and cold, even to the extent of changing feeding routines and delaying breeding during adverse conditions. They have been known to survive winters with temperatures as low as -10 deg C and Anopheles gambiae survives temperatures of more than 55 deg C in the Sudan.

Today malaria is primarily associated with tropical countries, yet until the early 1950s, it was widespread throughout Europe and North America. Indeed, infection and mortality rates remained constant during the 16th through mid-19th centuries. It was due to changing practices in agriculture (separating humans from livestock), lifestyle (protecting against mosquitos), and availability of treatment, meant that the strain of plasmodium falciparum died out in these areas. Use of the DDT helped to eliminate malaria from Europe after World War II

Even when a link between malaria and current climate change was postulated it has not been conclusively substantiated. Recent research into climate change and malaria in East Africa showed that claimed associations between local malaria resurgences and regional changes in climate were overly simplistic. Economic, social and political factors more readily explained recent resurgences in malaria and other mosquito-borne diseases better than climate change.

A new paper to be published in the journal Nature shows that when considered globally the issue has been grossly overplayed and that any worldwide changes in the spread of malaria die to climate change will be minor.

Peter Gething and colleagues from the Malaria Atlas Project at the University of Oxford, look at historical data to see how malaria has spread over the past 100 years or so as the world warmed during that period. They conclude that claims that malaria’s spread and intensity will increase are at odds with its currently decreasing endemicity and global extent.

They conclude that any change due to increasing temperatures will be at least ten times smaller than the changes already seen since 1900, and will be 100 times smaller than the change in affected area brought about by better management of the disease achieved during the 20th century.

The histories of malaria, yellow fever and dengue fever  show that climate has rarely been the principal determinant of their prevalence or range. Human activities and their impact on local ecology have generally been much more significant. This will remain the case even if the world warms significantly this century.