EPIDEMICS of the most virulent form of malaria could sweep across much of Europe and parts of North America and Australia within the next few decades as a result of global warming, according to a new modelling study by scientists working for the European Commission.
Malaria kills up to 2 million people a year, most of them in Africa. It is one of the few worldwide diseases that has defied efforts to eradicate it, through both drugs and programmes to destroy the mosquitoes that carry malarial parasites.
Philippe Martin and Myriam Lefebvre of the Tropical Vegetation Monitoring Unit at the Commission鈥檚 Joint Research Centre in Ispra, Italy, say the risk of malaria spreading outside areas where people have some in-built immunity to it 鈥渋s cause for great concern鈥�. There is a risk of 鈥渆pidemics among unprepared or non-immune populations鈥�. Their findings will be published in the Swedish journal Ambio later this year.
Advertisement
Malaria is present all year round in much of the hot and wet tropics. Here, people develop some immunity and most deaths are among the very young. Major epidemics are rare. The disease does much more damage on the fringes of its terrain, where it is present for only a few months each year, for example during the monsoon season. In these regions fewer people are likely to have any immunity and the death rate is much higher.
The new modelling study, which successfully reproduced a detailed map of where malaria occurs today, suggests that areas of perennial malaria may shrink as global warming gathers pace. But areas with the more dangerous seasonal malaria will expand dramatically into both existing perennial areas and into temperate latitudes currently free of the disease.
This spread of seasonal malaria, say Martin and Lefebvre, 鈥渋s most likely to foster epidemics, causing widespread debilitation and increased mortality鈥�. David Warhurst of the London School of Hygiene and Tropical Medicine agrees. He says that when seasonal malaria spread to a new region in Madagascar in 1988, there were 100 000 cases, 20 per cent of them fatal. This tragedy is 鈥渁 model for what could happen as a result of global warming鈥�, he says.
The malaria forecast is based on predictions made by five models of the changes in temperature, rainfall and humidity that would result from a doubling of the pre-industrial atmospheric concentration of carbon dioxide, or what climatologists think the world will be like in the second half of the next century. The malaria model looks at how these changes will affect the distribution of both the malarial parasites and the mosquitoes that carry them.
According to the WHO, 60 million square kilometres of the planet, an area twice the size of Africa, currently have the right conditions for malaria, though it has been eradicated in some regions. Where it is present, up to 10 per cent of the population suffers from the disease, enduring periodic bouts of life-threatening fever.
Malaria generally requires average temperatures above 20 掳C and humidities above 55 per cent to take hold. Using the climate model produced by Britain鈥檚 Meteorological Office, the disease would extend its theoretical range by 30 per cent or 17 million square kilometres, an area almost twice the size of Europe. But seasonal malaria would increase its range by more than half, or almost 25 million square kilometres.
Plasmodium falciparum, which kills one in ten of the people it infects, is the most dangerous of four main malarial parasites. According to the Met Office model, it could spread for the first time to much of Europe, including the British Isles as far north as central Scotland, as well as most of the eastern US and Canada and parts of Australia.
The other climate models generate the same trend, though it is less marked. But each of the five models suggests that large areas of Britain will be threatened by seasonal malaria. The models also agree that large parts of Australia, Canada, Russia and the Middle East will be hit by malaria, often for the first time.
There have been other studies predicting the effect of global warming on the spread of major diseases, though none so detailed. A report on the health implications of global warming compiled by the British government鈥檚 Public Health Laboratory Service was completed in 1992 but was never published. It warned that malaria could return to Britain and recommended monitoring mosquitoes around airports for malaria parasites. A spokeswoman for the PHLS says the recommendation has not yet been acted on.