How epidemics begin with human actions in nature

It's an astonishing thought.

We normally think of infectious diseases such as AIDS, West Nile virus, SARS, Ebola, Lyme disease and others as foreign invaders attacking us. But it turns out that we've largely been responsible for inflicting them upon ourselves.

At least that's the new paradigm in a field that looks at the "ecology of disease" -- or, how our interactions with the environment unleash viruses from wildlife upon human society.

According to this field, which was the topic of a recent New York Times op-ed piece, how do these viruses from wildlife get introduced into the human population and what do human actions in nature have to do with it? Also, what is being done to prevent it?

How human acts trigger disease: five stories

While infectious diseases have long found their way out of the woods into human society (think plague and malaria), the number of such diseases has quadrupled in the last 50 years. Sixty percent of emerging infectious diseases originate in animals, and more than two-thirds of those originate in wildlife.

The culprits: human encroachment into wildlife habitat, particularly in tropical regions that harbor many diseases, and increased global interconnectedness from both air travel and wildlife trafficking.

Here are several examples of how our interactions with the environment have brought more wildlife diseases to our doorstep:

AIDS: This is origin story is probably the most well-known. AIDS originated in chimpanzees but most likely entered the human population when African bush-meat hunters killed an infected chimp.
Nipah virus: This South Asian virus originated in flying foxes (commonly called fruit bats). They usually hang upside down and chew on fruit pulp while spitting out the juices and seeds. A bat infected with the virus experiences little more than a cold from it. But in humans, it is much more lethal. In 1999, the virus broke out in humans in rural Malaysia, killing 106 of the 276 infected people and giving survivors permanent neurological disorders. How did it jump into humans? Most likely, an infected bat spit out a piece of chewed fruit onto a piggery. The pigs became infected, and the virus then was introduced to humans.
Hendra virus: Related to the Nipah virus, but present in Australia, the Hendra virus has entered human populations in a different manner: suburbanization of their habitat means that they no longer live in forests but in backyards and pastures. Additionally, outbreaks of the virus are more likely to afflict flying foxes living in urban and suburban areas. Biologist Raina K. Plowright at Pennsylvania State University who studies the ecology of disease hypothesizes that urbanized bats are more sedentary and less frequently exposed to the virus. So, the bats that are more likely to have the virus are also the ones living in closer proximity to people.
West Nile virus: Though the West Nile virus came from Africa, its spread across the United States had to do with a very un-African animal: the American robin. Basically, mosquitoes spread West Nile virus, and mosquitoes love robins, which in turn thrive in the human habitats of lawns and agricultural fields. The fact that West Nile virus was carried by a species that does well around humans meant that it spread easily and quickly.
Lyme disease: What made Lyme disease so common on the East Coast was human development of forests. Fragmenting and shrinking those ecosystems disrupted them so that predators such as wolves, foxes, owls and hawks were less common. And that meant that the population of prey such as the white-footed mouse was able to quintuple. And what do these animals often harbor? Lyme bacteria.

For more on other infectious diseases that came to us from the wild, check out this beautiful infographic from The Times.

Preventing a $3 trillion epidemic

These infectious diseases obviously harm human health: More than two million people a year die from diseases that come from wild and domestic animals, according to a study released earlier this month by the International Livestock Research Institute.

Another huge impact is cost: According to the World Bank, a severe flu epidemic would cost the world as much as $3 trillion.

In order to prevent both negative consequences, several groups are working to better understand the ecology of disease.

Predict

Predict, a project funded by the United States Agency for International Development, is working to assess where diseases are likely to emerge based on how people are altering the landscape -- for instance, where they are building a road or farm. The organization is hoping to identify them before disease emerge and before they can spread. The Times reports:

They are gathering blood, saliva and other samples from high-risk wildlife species to create a library of viruses so that if one does infect humans, it can be more quickly identified. And they are studying ways of managing forests, wildlife and livestock to prevent diseases from leaving the woods and becoming the next pandemic.

EcoHealth

EcoHealth Alliance and its partners -- the University of California at Davis, the Wildlife Conservation Society, the Smithsonian Institution and Global Viral Forecasting -- are building a virus library of wildlife-originating diseases from the tropics. The group also talks with people who live or work on the edges of forests.

Dr. Peter Daszak, EcoHealth's president, told The Times, "So we're going to the edge of villages, we're going to places where mines have just opened up, areas where new roads are being built. We are going to talk to people who live within these zones and saying, 'what you are doing is potentially a risk.'"

The group may talk with the villagers about how they butcher and eat bush meat or how they handle items that attract species, such as bat, that are likely to carry viruses unsafe for humans. For instance, in Bangladesh, which experienced several Nipah outbreaks, they discovered the culprit were containers collecting date palm sap. They were for people to drink from, but they attracted bats. The group placed bamboo screens over the collectors, preventing bats from contaminating them.

EcoHealth also monitors imported wildlife in airport luggage and packages, and it warns consumers about exotic pets that come from forests known as disease hot spots.

One Health Initiative

One Health Initiative is a worldwide program that brings together more than 600 scientists and other professionals to promote the idea that human, animal and ecological health are inextricably tied and should be studied and managed holistically.