How environmental destruction sprang Ebola

This piece originally appeared in the Baltimore Sun.

By Robert Cohen
December 26, 2014

In 2003, Ebola virus killed around 5,500 gorillas in the Lossi Sanctuary of the Republic of Congo and reduced its population there by over 90 percent — the virus’ deadliest incursion in any species until the current outbreak in West Africa. That gorilla outbreak, however, was just the next step along a trajectory that appeared to begin in 1976, when Ebola was first diagnosed in humans, but which had actually been underway for decades due to the convergence of several insidious forces. Yet, the forces that brought Ebola upon us have only intensified since then while receiving little public attention and even less amelioration. Absent their remedy, Ebola’s worsening trajectory will continue.

What are these forces? In short: human overpopulation, decline of animal populations, viral ecology, lack of public understanding, and human inability to counter slow-motion emergencies.

It’s not just Ebola these conditions have wrought. Ebola is but one of many scary new diseases to emerge recently from animals and threaten humans. Such diseases, called zoonoses, have been detailed brilliantly by David Quammen in “Spillover: Animal Infections and the Next Human Pandemic.” The most-well known, HIV, has killed 39 million people since 1980. SARS, Nipah, MERS-CoV and bird flu are among many others that have recently jumped species to infect humans and scare the public and public health scientists. It must not escape our focus to ask: Why so many? Why now?

Here’s what we know about Ebola. Ebola and its family of viruses, the filoviruses, have existed for millions of years, and during that time they have innocuously integrated parts of their genome into those of small mammals, including bats and some rodents. In approximately 750 A.D. the Ebola genus diverged into at least five known species, four in Africa and one in the Philippines. As humans increased in population from one billion in 1800 to seven billion today, pressure on the natural reservoirs of Ebola viruses — almost certainly bats — increased. Prior to 1970, according to genetic mapping, all Ebola viruses faced a genetic bottleneck as their reservoir host populations declined due to hunting and habitat destruction, worsening as bats came under greater threat.

Faced with extinction, Ebola mutated. Just as bacteria mutate under a constant barrage of antibiotics to emerge resistant and more pathogenic — think MRSA or extensively drug-resistant tuberculosis — so too did Ebola change under stress to become fitter, more resistant to animal immune systems and, most importantly, capable of infecting primates. Genetic evidence tells us that in approximately 1970, a particularly strong variant emerged for each of the three major Ebola species, including both main African species from Congo and Sudan as well as the one in the Philippines. These variants, because of their greater fitness, quickly spread through bat populations, occasionally spilling over into primates, including tragically in 2013 in Guinea. All but two known Ebola outbreaks, which are from rarer Ebola species and may be the result of the same process, are descended from these three dominant variants.

The fact that the same unusual bottleneck occurred in disparate locations and to different Ebola species implies causation from forces that would stress the bats of both Africa and the Philippines. These are clear. The human population of sub-Saharan Africa quadrupled from 230 million in 1960 to 930 million today. The combined population of Guinea, Sierra Leone and Liberia alone rose from 7 million to 22 million, while the population of the Philippines increased from 26 million to 98 million. It is estimated that humans in the Congo basin consume 5 million metric tons of bushmeat each year, and bat hunting is common in the Philippines. The spread of Africa’s forests has decreased by 50 percent from its natural peak, while in the Philippines the forest cover decreased from 70 percent to 20 percent during the 20th century.

Lack of human understanding compounds the problem. For example, a colleague in Sierra Leone recently described the burial of a religious leader who died of Ebola. His body was washed in water, and by tradition, children from his village drank this water. Many of these children and their families subsequently died.

Other forms of human lack of awareness, while more subtle, have also insidiously amplified the problem. Continued hunting and burning of forests in Africa and Southeast Asia, and continued importing of timber and burning carbon dioxide in America and China, make human contact with stressed and infected bats ever more common. And overpopulation makes Ebola’s spread much easier.

Amid all the crises we face today, we must also make substantial progress against the more dangerous slow-motion emergencies that underlie them. Else, we are doomed to soon suffer the next phase of the ever-worsening zoonotic trajectory, which will catch us by surprise even though it was all too predictable.

 

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