Deforestation as a reason for spreading of diseases

The risk of disease outbreaks can be greatly

 

magnified after forests are cleared for

 

agriculture and roads.

Throughout human history, pathogens have emerged from forests. The Zika virus, for example, which is believed to be causing microencephaly, or smaller than normal heads, in newborns in Latin America, emerged from the Zika forest of Uganda in the 1940s. Dengue, Chikungunya, yellow fever, and some other mosquito-borne pathogens likely also came out of the forests of Africa.

 
Forests contain numerous pathogens that have been passed back and forth between mosquitoes and mammals for ages. Because they evolved together, these viruses often cause few or no symptoms in their hosts, providing “a protective effect from a homegrown infection,” says Richard Pollack of the T.H. Chan School Public Health at Harvard. But humans often have no such protection. 

How deforestation helps deadly 

viruses jump from animals to 

humans

The coronavirus pandemic, suspected of originating in bats and pangolins, has brought the risk of viruses that jump from wildlife to humans into stark focus.

These leaps often happen at the edges of the world’s tropical forests, where deforestation is increasingly bringing people into contact with animals’ natural habitats. Yellow fever, malaria, Venezuelan equine encephalitis, Ebola – all of these pathogens have spilled over from one species to another at the margins of forests.

As doctors and biologists specializing in infectious diseases, we have studied these and other zoonoses as they spread in Africa, Asia, and the Americas. We found that deforestation has been a common theme.

More than half of the world’s tropical deforestation is driven by four commodities: beef, soy, palm oil, and wood products. They replace mature, biodiverse tropical forests with mono-crop fields and pastures. As the forest is degraded piecemeal, animals still living in isolated fragments of natural vegetation struggle to exist. When human settlements encroach on these forests, human-wildlife contact can increase, and new opportunistic animals may also migrate in.

How deforestation can lead to more infectious diseases

The world's forests act as shields, keeping humans safe from coronaviruses and other diseases. Their destruction can unleash devastating consequences for global public health.
Scientists have been repeating the warning for at least two decades: As humans encroach upon forests, their risk of contracting viruses circulating among wild animals increases.
That's why Ana Lucia Tourinho wasn't surprised when she heard about the novel coronavirus, which was first detected in China in December and has since spread around the world. An ecologist at the Federal University of Mato Grosso in Brazil, Tourinho studies how an environmental imbalance can cause forests and societies to become sick.
"When a virus that wasn't part of our evolutionary history leaves its natural host and enters our bodies, it's chaos. The new coronavirus is rubbing that in our faces," she said.

Deforestation destroys the equilibrium

Before it infected the first humans and spread through the world by living in travelers' bodies, the novel coronavirus, officially named SARS-CoV-2, inhabited other hosts in a wild environment — most likely bats.
When such viruses are isolated and in equilibrium in their habitat, for example, a closed forest, they are not a threat to humans. The problem arises when this natural reservoir is cut down, destroyed, and occupied.
Scientific studies published before the current pandemic had already shown a connection between deforestation, the proliferation of bats in the damaged areas, and the family of coronaviruses, which includes the current lethal strain.

Zoonotic spillover

A disease that jumps from animals to humans is called zoonosis. The jump itself — the event in which a pathogen jumps from animal to human or vice versa — is called a zoonotic spillover, or simply a spillover. And it's more common than you might think.
Six out of every ten diseases in humans, and three-quarters of the world's emerging infectious diseases, are zoonotic.

Many of them are familiar: Salmonella, malaria, Lyme disease. Others, like Rocky Mountain Spotted Fever or Rift Valley Fever, are rarer. Some are benign. Others, like the coronavirus that causes COVID-19, can be deadly.

For a spillover to occur, several things need to line up.
First, a pathogen (think: virus, bacteria, fungus) needs to find a way to make the jump from animal to human. Maybe it gets eaten by a hunter bringing home food. Maybe it gets passed in a mosquito bite. Maybe it gets inhaled by a kid playing near a hollow tree.

However the pathogen arrives, its next job is to stick the landing, so to speak, warding off or evading the new body's defenses long enough for it to replicate and put down roots.

All of this can be made easier for a pathogen by a simple thing: proximity.

The closer humans are to animals (we're usually the provocateur), the greater the likelihood of interactions between them, and the greater the opportunity for zoonotic spillover.

"A numbers game"

The risk exists everywhere, but is especially high in the tropics where fast-growing human populations abut species-rich forests, says Christina Faust, an infectious disease ecologist at the Center for Infectious Disease Dynamics at Penn State University.
"It's a numbers game," she says.

Unfortunately, the tropics also happen to be a hotbed of deforestation.

The world lost a soccer-field-sized area of mature, tropical forest every six seconds last year, according to a recent report from the group Global Forest Watch.

Brazil alone accounted for more than one-third of the total loss, as wildfires and logging, largely to clear land for agriculture, tore through large patches of the Amazon.

This year is on pace to be worse. In the first four months, deforestation in the Amazon was up 55 percent compared to last year, according to satellite data from Brazil's space research 

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The country's president, Jair Bolsonaro, has pushed to weaken environmental protections to boost the country's economy. Last month, his environment minister was reportedly caught on camera calling for more deregulation while the press was distracted with the coronavirus.

Faust has spent much of her career studying spillover between animals and humans and trying to understand what drives those events. She says there's a relationship between deforestation and the emergence of zoonotic disease, but it's not entirely clear why.
"We don't know if that's because we're losing biodiversity that would otherwise... help dilute that pathogen, or if it's because we have more humans coming into the area and doing risky behaviors," she says.