The rise and rise of “zoonotic” diseases: why are animals more likely than ever to make us ill?

More deadly diseases transmitted from animals are on the way, a new report shows – and they will disproportionately affect developing countries. So how do we fight back?

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It was only when I read a recent report on “Emerging Issues of Environmental Concern” that I realised how many major diseases have spread from animals to humans within the last decade. In the UK, we’re lucky to have had relatively little contact with “zoonotic” diseases – the last epidemic to affect the UK was seven years ago with swine flu, which infected 110,000 people in the last week of July 2009 alone.

Zoonotic infections originate in wildlife, and are transmitted to humans through livestock. For example, bird flu originated in wild birds and then moved to domestic poultry and subsequently humans.  

According to the report released in May by the United Nations Environment Program, 60 per cent of all infectious diseases in humans are zoonotic and on average a new infectious disease transmitted by animals emerges in humans once every four months. These statistics are terrifying, and the report predicts that they’re only going to get worse if the “critical relationship between a healthy environment and healthy people” is not maintained. 

So what is the root cause? For a start, zoonotic disease can be prompted or worsened by changes in the environment like new land use or climate change. Increasing human populations, especially urban ones in developing countries, can also contribute, as this results in an enormous upsurge in demand for milk and meat, which results in more breeding.

Finally, pathogens can evolve by over-prescription of antibiotics, more fertiliser use for intensified livestock production, and nutrient rich waste from this livestock.

Most importantly, though, these diseases tend to take hold in host populations with weakened immune systems due to stress-inducing social, environmental or economic conditions, often in under-developed countries. Such countries often have poor health systems, receive far less media attention and funding than developed countries and their citizens live in close proximity to wildlife and rely on them in their daily lives. It’s here that so-called “neglected diseases” – known pathogens that can crop up if not kept at bay – can resurge. So how do we eliminate neglected diseases in developing countries?

Malcolm Bennett, Professor of Zoonotic and Emerging Disease at Nottingham University who specialises in zoonosis, says the key is in helping the people in these countries directly: “We have neglected diseases in so-called developed countries too . . . surely our response should be directed at how we help those people rather than simply focusing on the disease."

Control systems along the food chain and targeting the disease in livestock have enabled the developed world to curb diseases in the past, and helped stop the rise of Severe Acute Respiratory Syndrome (SARS) in 2002. Ebola, on the hand, was poorly handled.

The 330 documented events of zoonosis from 1940 to 2004 threaten economic development as well as lives. The report estimates that emerging diseases have cost America over a hundred billion dollars and had they become pandemics, they could have cost up to several trillion dollars. Twenty billion dollars was spent on responding to these emerging zoonoses. Surely we could predict these disasters and prevent them?

Not according to Professor Bennett. “An important lesson to learn from past epidemics is ‘we didn’t expect that’.  The diversity of infectious agents is huge and can often evolve rapidly.  Most cause no obvious disease, and while many can jump [between] species, the probability of any individual event leading to an epidemic is very small. I’m not convinced we will ever know enough to predict which of these probably millions of individually rare events that happen every day is the one to put our efforts in to.”

Therefore, the study’s researchers suggest we need to tackle the infringement of human development on natural ecosystems, in order to protect the buffer between wildlife and humans.

Two initiatives, EcoHealth and One Health, work at the intersection of the health of humans, the environment and animals. They work to increase the diversity of species on local and national levels so that diseases cannot spread easily or dominate once spread. So far, this method has helped control the spread of rabies in the Serengeti, brucellosis in Mongolia and leishmaniasis in Tunisia.

Continued cooperation of legal, policy, effective institutions, ample financing and rapid detection then, should keep us prepared for zoonotic diseases when they strike. The ecosystem  with the health of animals, people, and nature  must be protected and prized.