How deadly disease outbreaks could worsen as the climate changes

The dangers posed by fruit bats and mosquitoes are rarely mentioned among the potential impacts of major environmental changes such as deforestation and climate change. But two studies this week shine a light on how environmental destruction could lead to a greater spread of deadly human diseases via animals and other organisms, with serious consequences for public health.

The 2016 Ebola outbreak in West Africa killed more than 11,000 people directly, and knock-on effects, such as diverting medical resources away from other conditions, caused thousands more deaths. Worryingly, climate change could cause an increase in Ebola rates over the next 50 years, according to UK and US researchers who created a predictive model that successfully reproduced past outbreaks of the virus.

In the worst-case warming scenarios that they looked at, the area that could be affected by “spillovers” of Ebola – when the virus jumps from an animal to a human – will increase by nearly 15 per cent compared with today. That could expose new parts of western and central Africa to the disease.

Disease carriers

“By changing the environment, we are going to directly impact our health,” says David Redding at University College London. One way in which climate change will affect the risk of diseases such as Ebola spreading is by making new areas into suitable homes for disease-carrying species. For example, if the trees that fruit bats – believed to be a reservoir for the Ebola virus – rely on can grow in a new area, the bats can follow.

Global warming isn’t the only environmental change that could increase disease risk. Clearance of the Amazon rainforest seems to be driving up the spread of malaria, suggests research by Andrew MacDonald and Erin Mordecai at Stanford University in California.

Their analysis of 13 years of malaria cases and forest satellite data for the Brazilian Amazon show that a 10 per cent increase in deforestation was associated with a 3.27 per cent increase in malaria cases – almost 10,000 additional cases every year. That is probably because more people end up settling closer to mosquito-infested areas and are more likely to get bitten, while logging creates more mosquito-friendly habitats.

The relationship also seems to hold in reverse – as malaria rises, economic activity, and hence deforestation, slows. Nonetheless, MacDonald says that counter-effect will never halt deforestation and the world needs to pay attention to greater disease risk from environmental damage.

Local factors

“I think the sorts of large environmental changes we are seeing today, including climate and land use change, have a high potential to lead to changes in health outcomes, including the transmission of infectious diseases,” says MacDonald.

“Climate change definitely has an impact on infectious diseases,” says Elke Hertig at the University of Augsburg, Germany. But she stresses that whether the impacts are good or bad will always hinge on local factors. She has found in her own research that while warmer temperatures will help malaria-carrying mosquito species spread northwards across Europe, some Mediterranean areas will become too hot and dry for them.

Daniel Bausch, part of the UK’s rapid disease response unit, says we should take the influence of environmental change on disease risk seriously. But the strength of healthcare systems remains key to determining future outbreaks of disease, he says. “We need the modelling and the ecology research to understand the high risk. But then we need to act on that.”