Ebola and Other Diseases: The role of Climate Change and Deforestation

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This week the World Health Organisation described the current Ebola outbreak as ‘the most severe acute health emergency in modern times’, with the epidemic now killing more people than all previous Ebola outbreaks combined.

Whilst epidemics on this scale are thankfully rare, there are increasing fears that our burgeoning population, warming climate and rapid urbanisation of previously forested land has already increased the pace in which deadly diseases can infect and spread amongst humans. As deforestation and climate change look set to get worse before they get better, are we facing a future of greater risk from global disease?

The Origins of Ebola

Ebola cases in Africa. (Source)

On 6th December 2013, a little boy named Emile died in Meliandou, a small village in Guinea close to the border with Sierra Leone and Liberia. Emile is widely regarded as 'Patient Zero', the first human to contract the disease, and quickly afterwards his mother, sister, and grandmother all became ill with similar symptoms and succumbed to the same fate. Exactly how the toddler contracted the virus is unknown, but the most likely explanation is that he came into contact with a fruit bat, which researchers believe is the virus's 'natural host'.

Before Emile, there had never been a case of Ebola in West Africa. The previous outbreak to infect over 100 people was in 2007, when 249 people became infected in Congo. Just before the outbreak, a migration of fruit bats encroached on the Congolese villages and prompted local hunters to slaughter the bats in their thousands, leaving the villages "literally inundated with blood-bathed bat corpses". Considering 3.1 million hectares of Congo's rainforests were lost during the 1990s alone, it's hardly surprising that fruit bats viewed the villages that encroached on their habitat as new sources of food and shelter - inevitably resulting in more bites and more risk of disease.

The fruit bat (source)

Indeed there are many examples where deforestation and agricultural intensification are likely to have caused the onset of disease. In 1997, around 5 million hectares of Malaysian tropical forest - an area twice the size of New Jersey - were slash-and-burned for agriculture. The fruit bat population were forced onto the new orchards and pig farms for food, resulting in the deadly Nipah virus transferring to livestock. In just a few weeks, the disease had spread to the local farmers, 40% of which died of acute encephalitis. With deforestation commonplace in South Asia, Nipah virus now emerges almost every year and kills 75% of those infected.

On top of this, incidence of yellow fever, malaria, dengue fever, Lyme disease and Rocky Mountain spotted fever have all been linked to the loss of host species natural habitat. In sub-Saharan Africa, colonial expansion and deforestation in the early 20th Century allowed one particular lentivirus to jump between Cameroonian chimps and humans. The virus's new host - humans - were abundant and widespread, allowing it to evolve into HIV/AIDS, one of the most severe pandemics of modern times that has killed around 36 million people in a matter of decades.

The Role of Climate Change

Africa's meningitis belt (Source)

Unfortunately, it isn't just local urbanisation and forest loss that can drive the spread of disease. There is increasing evidence that climate and weather play a large factor in causing regional epidemics, with some diseases linked so strongly to climate that they can even be predicted. The risk of contracting malaria, for example, increases five-fold in the year following an El Nino event. Other studies have shown strong evidence of a link between cholera outbreaks and increased water temperature and salinity. Moreover, the meningitis belt of sub-Saharan Africa is also feared to grow as a result of climate change, with robust correlations linking strong winter winds and low humidity levels with the onset of meningitis.

It is possible - likely, even - that the current Ebola outbreak can, in part, be attributed to climate change. Simple bat-to-human contact alone probably isn't enough for Ebola to take hold in a population, but instead is facilitated by a "cascade of events", a bringing together of animals and humans in part because of "unique climatic conditions", according to World Health Organization expert Pierre Formenty. Ebola seems to follow heavy rains that alleviate intense drought. When rains finally come to a drought-torn land, the trees and bushes can finally produce their fruits. Disease-ridden animals and humans naturally flock to the much needed food, resulting in increased likelihood of disease spreading.

A recent report on food security in Sierra Leone, the epicentre of the current outbreak, found that climate change is resulting in "seasonal droughts, strong winds, thunderstorms, landslides, heat waves, floods, and changed rainfall patterns", all-in-all a recipe for the conditions needed for diseases like Ebola to spread and prosper.

The Future

The spread of an infectious disease can never be fully explained by a single factor; its origin and prevalence is inherently complex and unique for each case. Nevertheless, as climate change and land use change becomes an ever more serious issue, so too will infectious disease. Unfortunately, therefore, more disease is just another threat we must face in a warmer world.

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