Blaming malaria rise on climate change is simplistic



The resurgence of Plasmodium falciparum malaria in four East African highland areas has not been caused by long-term climate change, according to an international team of researchers led by Oxford University epidemiologist Simon Hay (Nature 2002;415:905-9).

"The finding has important implications on the ground," says Hay. "If climate change is not responsible for the increase in malaria in these areas, government and public health officials should be focusing on the other possible causes, such as deteriorating public health systems and drug resistance."

For years, climate scientists have speculated that global warming, caused by the build-up of insulating "greenhouse gases" in the lower atmosphere, will trigger changes in the occurrence of various diseases worldwide, including vector-borne maladies such as malaria and dengue. Mathematical models suggest that shifts in temperature, rainfall and humidity will not only lengthen transmission seasons in some areas where malaria is endemic, but allow the P. falciparum parasite, and the Anopheles mosquitoes that carry it, to survive at higher altitudes and latitudes.

Hay and his colleagues set out to see if such global warming-related effects were already occurring in the East African highlands. There has been a surge in malaria cases in these areas since the 1970s. And the 2001 report on the Intergovernmental Panel on Climate Change (IPCC) noted a 0.7 oC warming over most of the African continent during the past century.

To see if the two phenomena were related, Hay's team analysed meteorological data from 1901 to 1995 for four highland sites where malaria cases are on the rise — Kericho in western Kenya, Kabale in south-western Uganda, Gikonko in southern Rwanda, and Muhanga in northern Burundi. For each month, the researchers determined the average temperature, as well as the average minimum and maximum temperature, rainfall and vapour pressure. They also noted months when average temperature was greater than 15 oC and rainfall exceeded 152mm — meteorological conditions suitable for P. falciparum transmission if they continued for two consecutive months.

After analysing the 95-years' worth of data, the team found no significant shifts in temperature or vapour pressure at any of the four sites. Rainfall had increased at only one, Muhanga. And Kabale was the only area in which the malaria season had got longer. None of these areas had had significant climate changes after 1970, when malaria was on the rise.

The findings came as a surprise. "We were expecting to find some evidence of significant trends," says Hay. "Our guess is that future climate changes may have an impact on malaria and other diseases, but the evidence just isn't there yet."

In addition to finding no evidence of long-term change, the researchers found that reconciling short-term climate fluctuations of the last century with malaria epidemics was also problematic. For example, they say that although in Kericho there is evidence of some slight warming and increased rainfall from 1971 to 1995, and it does coincide with the recent resurgence of malaria, historical data from the site show a series of very severe malaria epidemics in the 1940s — a decade that was significantly cooler and drier than average. Similar inconsistencies in attributing recent epidemiological changes to climate show up for the highlands of Uganda, the United Republic of Tanzania and Madagascar, the scientists say.

If the climate has not changed at the four study sites, other factors must have been responsible for the increases in malaria, the researchers say. Those conclusions are in line with other reports which also indicate that the IPCC's claimed 0.7 oC warming of Africa cannot be blamed for the continent's resurgence of malaria, says Pierre Guillet, a vector control expert at WHO. So what are the real causes? Possibilities include: resistance to chloroquine and other drugs; population growth rates that outpace health services; changes in land use; and short-term wobbles in weather such as those resulting from El Niño and La Niña events in the Pacific, as a paper in the Journal of the American Mosquito Control Association argued in 1998 (14:121-30).

However, the recent findings shed no light on the future. "Since [it] reports no evidence of climate trends at the sites studied," says epidemiologist Tony McMichael who co-edited the chapter on the health aspects of climate change in IPCC's 2001 report, "the study says nothing about the potential impact of any future climate change on malaria transmission."

Just how dramatically global warming will influence malaria will continue to be hotly debated. According to the IPCC, there are scenarios for the year 2080 which show malaria moving into some areas but out of others, so the net number of people living in malarial regions remains fairly stable. Other scenarios, however, are much more dire, with malarial zones vastly expanded,putting 260—320 million more people at risk.

Charlene Crabb, Paris

World Health Organization Genebra - Genebra - Switzerland