The recent eruption of COVID-19 near a "wet market" in Wuhan, China — where vendors brought a variety of live wild animals together for purchase, slaughter and consumption — calls our attention to a phenomenon captured by a word increasingly understood by the general public: zoonosis.
Zoonoses are infectious diseases — caused by bacteria, viruses, fungi or parasites — that spread from animals to humans. They can be transmitted through direct physical contact, via air or water, or through an intermediate host like an insect. Often these zoonotic pathogens do not affect the animals in which they reside, but they can represent an enormous risk to humans who have no natural immunity to them.
The COVID-19 pandemic provides a stark reminder that handling or coming into close contact with wildlife — along with their body parts and/or excretions like blood, spit and urine (the potent mix of which lends wet markets their name) — poses a risk of spillover of the pathogens they host and maintain in nature, and that can lead to zoonotic infections.
Multiple scientific studies show a link between deforestation and increased risk of zoonosis. Forest destruction leads to increased contact between humans and wild species. Trade in wild animals, often illegal and linked to logging and forest clearing, also increases the risk of disease transmission.
Global heating is changing the behavior of many species, including disease-causing organisms. Many are now spreading from the tropics into new regions, causing diseases such as Zika and dengue fever.
In 1997, clouds of smoke hung over the rainforests of Indonesia as an area roughly the size of Pennsylvania was burned to make way for agriculture, the fires exacerbated by drought. Smothered in haze, the trees couldn’t produce fruit, leaving resident fruit bats with no other option than to fly elsewhere in search of food, carrying with them a deadly disease.
Not long after the bats settled on trees in Malaysian orchards, pigs around them started to fall sick—presumably after eating fallen fruit the bats had nibbled on—as did local pig farmers. By 1999, 265 people had developed a severe brain inflammation, and 105 had died. It was the first known emergence of Nipah virus in people, which has since caused a string of recurrent outbreaks across Southeast Asia.
It’s one of many infectious diseases usually confined to wildlife that have spilled over to people in areas undergoing rapid forest clearing. Over the past two decades, a growing body of scientific evidence suggests that deforestation, by triggering a complex cascade of events, creates the conditions for a range of deadly pathogens—such as Nipah and Lassa viruses, and the parasites that cause malaria and Lyme disease—to spread to people.
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