Another epidemiological transition?

A literature review by Montira Pongsiri of the US Environmental Protection Agency and colleagues suggests we are moving towards the next major shift in global disease patterns — this time marked by a rise in outbreaks of new or re-emerging zoonotic diseases. The paper was covered in a news article [PDF] first published by the Emerging Health Threats Forum in December (written by my colleague Holly Else), and picked up by the Independent and Telegraph in early January this year.

Pongsiri and her team looked at evidence from the genetic to the ecological scale for five of these diseases, and concluded that major and distinct changes to the environment are driving a rise in human infections with pathogens that some time ago spread only among animals. They mention changes in the global climate, land use, and farming practices as forces behind newly emerging diseases that can, in an interconnected world, spread far and wide.

The last epidemiological transition was marked by a drop in rates of infectious disease and malnutrition at a time when health problems linked to lifestyle and polluted environments were surfacing in developed countries during the Industrial Revolution. Economic development brought on a change in risk factors for disease alongside improvements in health care and hygiene.

The news stories mention specific pathogens, including the flu virus, for which evidence points to a recent change in how widely they circulate. But there’s also a cautionary note about whether signs of a new shift amount to scientific proof. There are at least 40 diseases, beyond the five that the authors looked at, which have jumped from animals to humans in recent decades.

For me, it’s also important to acknowledge that geography comes into the picture. The first epidemiological transition occurred in the 18th and 19th centuries in the western world. But for many countries industrialisation has arrived much later, with some facing a similar transition just now. In this case, a rise in ‘modern’ health problems like cancer and chemical toxicity is unlikely to overshadow more ‘traditional’ infectious diseases. For many people the benefits of economic development simply don’t translate into healthier environments. The picture then looks more like overlapping waves than a transition, with a double burden shouldered mostly by poor and disadvantaged people. The same, I suspect, will be true if zoonoses do make up a third wave. These transitions are best understood with geographic differences in mind.