Ebola and other emerging diseases: losing the trees before we see the forest’s connection to our health

Ebola crisis demonstrates links between the environment and human and animal health
Infographic prepared by Catherine Machalaba and William B. Karesh, EcoHealth Alliance
Sep 2014

The current Ebola outbreak highlights the links between health, global environmental change and socio-ecological systems – and shows how exploring those links is key to finding solutions.

As the saying goes, often we don’t see the forest for the trees. With the current Ebola outbreak confirmed in five West African countries, over 3,800 known cases to date – with around a 50 percent fatality rate – and the World Health Organization (WHO) estimating that 20,000 people may be infected by the end of the outbreak, attention has focused on containing, treating and preventing new cases. While the specific source of the Ebola outbreak in West Africa isn’t known, the pathogen that causes Ebola is thought to be naturally harboured in some species of bats, with initial transmission to humans via butchering or consuming bats or other infected species, and then spread through human to human transmission.

The Ebola crisis has serious impacts on public health, economies, and societies, but crucially, it’s not an isolated incident. Globally, infectious diseases account for a billion human cases per year, leading to millions of deaths. The majority of known human infectious diseases are “zoonotic”, meaning they are shared with animals. Ebola is just one of nearly 1,000 known human diseases that have originated from animals.

As the Ebola situation has demonstrated on a global scale, public health infrastructure, especially in low-income parts of the world, is often limited, and efforts to deal with disease are largely reactive.  This deficit is concerning given the huge toll that infectious diseases pose to societies, in terms of morbidity and financial losses. The SARS outbreak in 2003, which caused the death of around 800 people, cost the global economy an estimated US$30-50 billion.

Additionally, the number of new (often referred to as “emerging”) diseases seen in humans appears to be rising.The HIV/AIDS epidemic reminds us of the potential of a novel disease to cause a global, and sustained, burden. Unfortunately, our capacity to detect and prevent other diseases before they emerge in humans has not improved in much of the world. Since we don’t know what’s poised to infect us next, we risk other viruses like HIV or Ebola appearing unexpectedly. Unless we start focusing on proactive approaches to diseases before they infect humans, we will continue to be only able to adapt to their impacts on human populations.

More so, what’s striking about diseases emerging in recent decades, of which over 70 percent have come from wildlife, is that we ourselves are driving their appearance in humans. The more we expand the footprint of human populations, encroaching upon and altering natural habitats in order to extract resources, intensify food production, and move animals and people – and the pathogens they carry – the greater the potential for infection and the spread of pathogens novel to humans. Climate change may also create newly suitable habitats for species and their pathogens. As an example, projections have suggested that warming temperatures could alter the range for Flying Fox bats, which carry the deadly Nipah virus. The drivers of disease emergence and spreading are also the drivers fundamentally changing our environment.

Of course, environmental change isn’t just posing infectious disease risks. We’re also seeing increased respiratory diseases, weather-related injuries and deaths, chronic diseases, and toxicity from environmental contaminants. Environmental determinants of health are likely to increase disease burden as we lose health-promoting ecosystem services and have more contact with sources of disease.  

In the devastating Ebola crisis, there may be a silver lining to salvage. While Ebola fortunately is unlikely to become a global pandemic (due to Ebola’s infection pathway, which requires direct contact with bodily fluids, and the risk-minimizing public health advances in many parts of the world), we can harness the concern from the global community about Ebola’s potential spread to have a constructive dialogue about preventing future outbreaks. This is especially timely given other recent animal-origin diseases we’ve seen, including H7N9 and the Middle East Respiratory Syndrome (MERS-CoV), and our challenge in controlling their ongoing spread.  

In addition to essential community and policy-oriented actions around strengthening preventive public health infrastructure, science has an important role to play in prevention. Science can yield a better understanding of disease risks and identify ways to prevent pathogens from moving from animals to humans. We can expand our arsenal by targeting the source, as with the USAID Emerging Pandemic Threats PREDICT program, which has conducted novel pathogen surveillance in wildlife in settings of high risk for transmission to humans. Routine surveillance of animals may also directly benefit both wildlife conservation efforts and human health by providing sentinels for diseases of harm to humans. Detection of Ebola infection in wildlife might have prevented transmission to the first Ebola case in the Democratic Republic of the Congo (DRC) a few weeks ago, which occurred from butchering and/or consuming an infected animal, an outbreak believed to be separate from the one in West Africa.

As we look towards the Sustainable Development Goals, health stands out as a ubiquitous concern that communities worldwide will continue to grapple with. A 2012 study found that vector-borne and parasitic diseases (VBPD) can negatively affect economic development and suggested an increased VBPD burden where biodiversity decreases.

The overlapping drivers of disease and environmental change, as well as their development implications, point towards the need for, and benefits, of ‘One Health’, a more integrated view and approach to human, animal and environmental health. We can no longer afford to think of human health as separate from animal health and their environmental determinants.  “My biggest concern is the lack of understanding of the biotic and abiotic factors that lead to outbreaks such as Ebola”, says Dr. William B. Karesh from the conservation and health organization EcoHealth Alliance, who has been a leader in One Health initiatives. “Without a broad collaborative approach like Future Earth is taking, we will be severely limited in our ability to prevent future outbreaks”.

Some international groups are helping to foster synergies, including the World Health Organization’s recent conference on Climate and Health, the Convention on Biological Diversity’s initiative on Health and Biodiversity, the Lancet-Rockefeller Commission on Planetary Health, and the DIVERSITAS-Future Earth ecoHEALTH project, which brings together public and animal health, development, ecology, economics, and other sectors to investigate connections between health and environmental change to generate science and policy outputs that can inform sustainability solutions.

Central to dialogues on infectious disease is the number of pathogens that exist. A recent estimate suggests over 320,000 mammalian viruses are still to be discovered but haven’t yet emerged in humans. What we do to our planet will determine whether we can go beyond reactive responses and prevent an increasing health burden. If we don’t rapidly take a broader view of health – moving from beyond our single-species perspective, to ecosystems as a whole – we may lose the trees before we have seen the forest, and the importance it holds for our health.

Further Reading:

Morens DM, Fauci AS (2013) Emerging Infectious Diseases: Threats to Human Health and Global Stability. PLoS Pathog 9(7): e1003467. doi:10.1371/journal.ppat.1003467