The commonly-held view that climate change can only increase the burden of infectious diseases has been challenged — provoking a debate that could ripple out to health professionals, conservationists and policymakers.
In recent years thousands of papers have been published projecting that increased temperatures will extend the range of disease vectors, increasing the amount of disease.
But now an ecologist argues that the effects of climate change will be more complex than has been acknowledged — and that there may even be a reduction in the incidence of some infectious diseases.
Kevin Lafferty, a research ecologist for the United States Geological Survey at the University of California Santa Barbara, makes his argument in the April issue of Ecology. The journal has published five articles in response, representing the "extreme and contrasting views Lafferty's paper elicited in its reviewers," according to Kenneth Wilson of the UK-based Lancaster University, who has written a further, discussion article for the journal.
Lafferty's paper "looks set to spark another heated debate among ecologists" and further afield "because of the funding implications and political fallout that might be generated by questioning the association between climate change and infectious diseases," says Wilson.
In his paper, Lafferty argues that temperature increases due to climate change are just one factor among many socioeconomic and environmental influences affecting diseases. Climate change is more likely to shift, than expand, the range of disease-causing bugs — and some areas might experience a decrease in disease, he writes. The discipline of ecology is essential for untangling these complexities.
"[The effect of climate change] might be relatively minor, particularly in many human infectious diseases where other factors such as economics are known to play a large role," he told SciDev.Net.
Higher-latitude, richer nations have the resources to control and treat malaria, for example, so just because their environment becomes more suitable for mosquitoes doesn't necessarily mean an increase in the number of malaria infections.
Factors such as day length — a seasonal rather than climatic factor — can affect rates of disease transmission, while precipitation can play a complex role in changing habitats for disease vectors. Human action such as land-use change and disease control methods also impact on the number of cases.
Lafferty says that simple summaries of the topic often conclude with a one-directional outcome: that infectious disease is going to increase in general.
"In no way am I saying we shouldn't be studying this issue, what I'm interested in doing is laying out a framework through which we can study it more effectively," he says. "I think we ought to be worried by overemphasising climate at the expense of other really important effects that we can do things about, like economics."
But in a response paper, Mercedes Pascual of the University of Michigan, United States, and Menno Bouma of the UK-based University of London, argue that even if the geographical range of, for example, malaria-carrying mosquitoes does not increase, it may extend into more populated and vulnerable areas. Populations in malarial countries tend to concentrate at low-risk, higher altitudes, they say, so the shifting of malaria uphill could increase the number of cases.
Pascual and Bouma are also concerned that Lafferty's conclusions could be interpreted as meaning that climate change does not play a role in changing the patterns of infectious diseases.
And some authors point out that there is already compelling evidence that climate change has increased some human diseases and caused dramatic disease outbreaks in amphibians, shellfish and corals.
But Sarah Randolph of Oxford University, United Kingdom, writes in her Ecology paper that "exaggerated simplistic rhetoric" about climate change's role "is morally indefensible if it distracts public health agencies from more effective ameliorative action targeted at the real causes".