Will climate change lead to more disease?

Tuesday November 10 2015

Climate change is having a profound impact on animal habitats, but what disease risk does this pose for humans?

Scientists estimate that almost 75 per cent of new (and re-emerging) diseases affecting humans at the beginning of the 21st Century were transmitted through animals. Among these so-called “zoonotic” diseases are AIDS, SARS, H5N2 avian flu and H1N1, or swine flu.

Barbara Han, from the Cary Institute of Ecosystem Studies, describes bats, pigs, and birds as “mixing vats” for viruses like Ebola, Hendra, Nipah, avian and swine flus that can spread to humans. As wild animals lose their habitats through deforestation, they come into closer contact with domestic animals and people. Extreme weather events and a warmer climate are also disrupting animal habitats, breeding cycles, and migration patterns.

Vector-borne diseases like malaria, Lyme, dengue fever, West Nile and chikungunya offer the clearest examples of how a warming earth is impacting on disease spread, scientists say. As the earth heats up, pathogen-carrying mosquitoes and ticks are moving further north, spreading these disease into countries and regions not previously affected, like the northern states of the United States, Canada, Sweden and parts of Europe. Dengue fever, which currently infects around 400 million people every year, could spread to as many as five or six billion by 2080 as temperatures continue to rise, according to the Intergovernmental Panel on Climate Change.

Besides these vectors moving into areas of higher latitude and altitude, there is also evidence to show that more extreme and uneven weather patterns and catastrophic weather events such as floods can contribute to the spread of these diseases, says Han.

The World Health Organization, while acknowledging that “measuring the health effects from climate change can only be very approximate”, nevertheless predicts that in 15 years time a quarter of the 250,000 potential annual deaths from climate change could come from malaria spreading into new areas.

However, whether more people will get sick as a result of these shifting disease patterns is not so easy to determine, says Richard Ostfeld, also from the Cary Institute. “There is some evidence to show that as vector-borne diseases spread northwards, they’re also disappearing from really warm places that are getting too warm. There is also some evidence that malaria will start to decline or disappear from areas that are getting too hot or too dry,” which could mean, says Ostfeld, that “there is no net change in the number of people at risk.”

With so many variables in the equation, drawing correlations between climate change and disease poses enormous challenges for scientists. There is no earth-mimicking laboratory for control experiments to be conducted, for one thing. For another, teasing out climate change from human-driven loss of animal habitats is difficult, when the two are closely linked and often impact on each other. Not only are the clearing of forests and planting of exotic species having a profound impact on animal habitats, but increased urbanisation and development is also contributing to a warmer planet.

“It’s much more complicated than people seem to think,” says Han. “Part of the problem with communicating climate change is that when you communicate the complexity you risk losing the attention span of the public. Simplifying it creates polarising debates that plays into different agendas,” she says.

For these reasons, some researchers were wary of being interviewed. A couple were blunt about the fact that – despite media reports to the contrary – the recent Ebola outbreak in West Africa could not directly be attributed to climate change.

Some scientists have speculatedthat extreme weather patterns may have played a role in the outbreak: dry seasons followed by heavy rainfall have caused fruit to proliferate, possibly bringing bats and apes together and allowing the disease to transmit between the species. Deforestation has also steered bats ever closer to humans to find food. Another hypothesis is that climate change, by diminishing traditional food sources and causing more people to eat bush meat (one way of transmitting the disease), has helped to spread Ebola.

But biological anthropologist Peter Walsh, from Cambridge University, says there is very little data to support claims of climate change and urbanisation contributing to Ebola emergence, “and there is a lot of data that doesn’t.”

The writer filed this article for IRIN from New York