Warming up the disease debate

It’s a commonly held belief that climate change is causing an increase in infectious vector-borne diseases such as malaria, dengue fever and Lyme disease. But according to Duane Gubler of the University of Hawaii, speaking at the American Association for the Advancement of Science (AAAS) annual meeting in Boston, US, last month, the situation is much more complicated than that.

“There is no solid scientific evidence to date that global warming has been a major driving force of the twentieth century increase in epidemic infectious diseases,” he said. “The lay community would have you believe that it’s just temperature change [affecting the incidence]. But these are very complex systems.”

Instead, Gubler believes the increase in such diseases may be attributed to factors such as lack of investment in public health, social changes including urbanization and globalization, and new types of land use and agricultural practice.

For example, the increase of Lyme disease in the US, which has been linked by some to climate change, is more likely to be attributed to the rise in deer population that accompanied reforestation of the North East, as well as increasing suburbanization bringing more people into contact with deer habitat. Deer carry ticks that act as the vector to transmit the bacteria that cause Lyme disease to people.

Similarly, the prevalence of tick-borne encephalitis has increased throughout Europe – Sweden is often quoted as an example. But the emergence in Sweden began at least five years before temperatures began to rise. And in many eastern European countries the rise in incidence coincided with independence from communist rule and associated changes in farming and leisure practices that led to people entering forests more.

On the mosquito-borne diseases side, malaria was rampant 100 years ago in tropical highlands, but a programme of insecticide use and public health measures brought it under control in the 1950s and 1960s. Declines in public health policy since then, rather than climate change, could well be the main cause for the disease’s recent increase in tropical highlands.

“Global warming has become the ideal excuse for public health officials and policymakers to explain why we are having repeated epidemics of diseases that were once effectively controlled,” said Gubler. But in reality, the problem may be a lack of investment in public health infrastructure.

And Gubler says that, while malaria-bearing mosquitoes might move north as a result of climate change, “we’re not going to see epidemics in the US unless we allow living and public health standards to decline.”

In almost all cases where the incidence of an epidemic infectious disease has increased, other factors appear to have had more of an impact than climate change, whether that’s demographic changes, urbanization, globalization, deforestation, or changes in land use, agricultural practices or animal husbandry.

But as temperature and precipitation levels continue to alter, that situation may change. As Gubler put it: “Many infectious diseases are climate sensitive – there are likely to be both positive and negative effects of climate change”. So it looks like there is plenty to keep scientists busy.

Or at least do your homework.